Дисертації з теми "Single precursor synthesis"

There is a considerable interest in the use of tungsten oxide in the research and development of new materials and devices, such as gas sensors and as photocatalysts. In order to improve the photocatalytic properties of WO3, its combination with metals which allows the preparation of WMxOy materials are believed to be promising photocatalysts under visible light. The present work deals with the synthesis of homo- and hetero-metallic tungsten alkoxide and amide compounds using the single source precursor approach for potential chemical vapour deposition precursors of mixed-metal oxide films.

Abhijit Bera_Ph.D thesis
The work incorporated in this thesis is mainly focused on various single source metal precursors like metal thiolates and metal dithiocarbamate complexes. Herein, several simple and general methods have been developed for the synthesis of various such single source metal precursors, which comprising the main two constituents of metal chalcogenide nanocrystals (NCs), namely, the tiny inorganic metal chalcogenide complex as core and an organic molecule as shell. Specially, both binary metal thiolates and bimetallic (ternary) thiolates have been prepared and both of them turned out to be excellent precursors for the synthesis of metal sulfide/selenide NCs. The methods used to prepare metal chalcogenide NCs included a direct-heating (solvo-thermal decomposition) method or solid state grinding method. First, the large scale synthesis of various 2D molecular precursors like metal thiolates and metal dithiocarbamate complexes (M-C8DTCA) have been developed and studied their thermal decomposition to metal sulfide NCs via solution based methods. We observed that some of the metal thiolates like Pb-thiolate requires very high temperature to decompose into PbS resulting in particles bigger than their Bohr exciton radius and hence displayed poor optical properties. In the next, to reduce the decomposition temperature an active sulfur precursor called octyl ammonium octyldithiocarbamate (C8DTCA) has been utilized for the synthesis of various metal sulfide NCs (including most challenging PbS NCs, with tunable optical properties) by solution based method (hot injection) or solid state grinding method. We also show that the size of the nanocrystals could be controlled by changing the reaction temperature or metal: chalcogenide precursor ratio. Interestingly, we have also been successful in establishing that these newly developed solid state grinding methods are scalable without compromising their structural and optical properties. The binary or ternary materials synthesized by these solid state routes could be re-dispersed as desired in non-polar organic solvents allowing them to be solution processible. The optical properties of the metal chalcogenide nanocrystals could further be improved by post synthetic surface passivation.
CSIR-NCL
AcSIR

The work presented in this thesis reports the use of a series of novel thiobiuret metal complexes [M(SON(CNiPr2)2)n] (M = Cu, Ni, Fe, Zn, Cd or In; n = 2 or 3) for the first time as single source precursors for the colloidal synthesis of metal sulfide nanoparticles. Other single source precursor(s) were also used for the synthesis of CdSe, CdS, CdSe/CdS core/shell, CdSeS alloys and Cu2-xS nanoparticles in microfluidic reactors. Thermolysis experiments of [Cu(SON(CNiPr2)2)2] using only oleylamine produced Cu7S4 nanoparticles as a mixture of monoclinic and orthorhombic phases. Pure orthorhombic Cu7S4 nanoparticles were obtained when a solution of precursor in octadecene was injected into hot oleylamine whereas, Cu1.94S nanoparticles were obtained when a solution of the precursor in oleylamine was injected into hot dodecanethiol. The thermolysis of [Ni(SON(CNiPr2)2)2] gave Ni3S4 in all cases except when precursor solution in oleylamine was injected into hot octadecene which produced NiS nanoparticles. The thermolysis of [Fe(SON(CNiPr2)2)3] in oleylamine/oleylamine produced Fe7S8 nanoparticles but other combinations, in most cases, gave amorphous material. Thermolysis of [Zn(SON(CNiPr2)2)2] in oleylamine produced spherical ZnS nanoparticles. Particles with size smaller than 4.3 nm had a cubic phase, whereas the particles with size larger than 4.3 nm had a hexagonal crystal structure as suggested by the selected area electron diffraction. Powder X-Ray diffraction showed that the CdS nanoparticles obtained from the thermolysis of [Cd(SON(CNiPr2)2)2] in oleylamine were cubic under all reaction conditions except when dodecanethiol was used as an injection solvent which produced hexagonal CdS. β-In2S3 were synthesized from the thermolysis of [In(SON(CNiPr2)2)3]. Transmission electron microscopy showed that the copper, nickel and iron sulfide nanoparticles had various morphologies such as spherical, hexagonal disks, trigonal disks, rods or wires; depending on the reaction temperature, concentration of the precursor, the growth time and the solvent/capping agent combination. The zinc and cadmium sulfide nanoparticles were mostly spherical whereas the indium sulfide nanoparticles were produced in the form of ultra-thin (< 1.0 nm) nanorods or nanowires. ZnxCd1-xS and CuInS2 nanoparticles were synthesised from the 1,1,5,5-tetra-iso-propyl-4-thiobiureto complexes of Zn, Cd and Cu, In, respectively. Powder X-Ray diffraction showed that the obtained ZnxCd1-xS nanoparticles are cubic under all reaction conditions. The ZnxCd1-xS nanoparticles had an average diameter between 3.5 to 6.4 nm as shown by transmission electron microscopy. The optical properties of the ZnxCd1-xS nanoparticles were highly dependent on the ZnS to CdS precursor ratio and the solvents/capping agents. Chalcopyrite (tetragonal), wurtzite (hexagonal) or a mixture of both CuInS2 nanoparticles were obtained depending on the reaction conditions. TEM showed that the CuInS2 nanoparticles could be synthesised with different morphologies (spherical, hexagonal, trigonal or cone). Luminescent CuInS2 nanoparticles were obtained only in the absence of oleylamine. [Cd(S2CNMenHex)2], [Cd(Se2P(iPr)2)2] and [Cu(SON(CNiPr2)2)2] were used as single source precursor(s) for the synthesis of CdS, CdSe, CdSe/CdS core/shell, CdSeS alloys and Cu2-xS in microfludic reactor. The CdS nanoparticles were in size range of 5.0 to 8.0 nm whereas the CdSe nanoparticles were ultra small (ca. 2 nm) with blue luminescence. The CdSe/CdS core/shell and the CdSeS alloys were bluish green or green luminescent depending on their size. The copper sulfide nanoparticles were found to be monoclinic Cu7S4 or monoclinic Cu7S4 with minor impurities of rhombohedral Cu9S5 depending on the reaction conditions.

There is much interest in the electronic potential of ‘nano’-semiconductors. The avenue of research pursued in this project was in inorganic analogues of graphene, namely metal chalcogenides MxEy (M = metal, E = S, Se, Te, x ≠ y = integer value). Thin films of these materials have been used in solar cells, ambient thermoelectric generators and IR detectors, due to their interesting properties, such as: optoelectronics, magnetooptic, piezoelectric, thermoelectric and photovoltaic, as well as electrical conductivity. The key issues with the use of these materials are the formation of controlled films, especially in terms of stoichiometry, crystallinity and uniformity, and also the precursor system used. The aim of this research was to synthesise and isolate novel precursor compounds for use in the deposition of metal sulfide thin films (for use with molybdenum and tungsten). The potential viability of the compounds as single source precursors (ssp) was judged following ThermoGravimetric Analysis (TGA). The compounds were also subjected to analysis using NMR (1H, 13C and 31P where applicable), infrared and UV-Vis spectroscopy, as well as elemental analysis. Cadmium sulfide (CdS) is one of the key direct band gap II-VI semiconductors, having vital optoelectronic applications for laser light-emitting diodes, and optical devices based on non-linear properties. The ratio of these films should ideally be 1:1, however, during the formation of cadmium sulfide films, particularly at elevated temperatures, a common problem encountered is the production of sulfur deficient films. These films have a formula consistent with 〖Cd〗_x S_y, where x is an integer value greater than y, but the sulfur deficiency is generally no greater than 10 %. In order to correct this sulfur deficiency, it was decided to investigate deposition making use of both a ssp and an additional sulfur source, with the aim of producing uniform films with 1:1 Cd:S.Molybdenum disulfide films have been deposited previously from multi source precursors and more recently using ssp. In this project MoS2 was deposited using novel ssps in both LP and AACVD on a variety of substrates with the aim of producing uniform thin films and assessing any differences in the morphology of the deposition. This work was continued with the deposition of WS2 and MoxW1-xS2 from ssps which had not been reported previously. The films deposited were analysed using XRD, SEM, EDX (when available) and Raman spectroscopy.

Recently there is growing interest for the production of cheap and nontoxic colloidal nanomaterials or thin films for photovoltaic applications. Iron chalcogenides are cheapest materials available for solar cell applications. The work presented here involve the synthesis of iron chalcogenide nanocrystals by colloidal methods and the deposition of thin films by aerosol assisted chemical vapour deposition (AACVD) method from single source precursors. In addition, a comprehensive literature review of iron chlacogenide nanocrystals and thin films is presented. Several new iron complexes belonging to thiocarbamato, xanthato, selenoureato and imidodiphosphinato, have been synthesised. Tris(dialkyldithiocarbamato)-iron(III) complexes of general formula [Fe(S2CNRR’)3] where R = Et, R’ = iPr; R, R’ = Hex; R = Me, R’ = Et and R, R’ = Et and tris(O-alkylxanthato)iron(III) complexes of general formula [Fe(S2COR)3] where R = Me, Et, ipr and iBu have been synthesised. The X-ray single crystal structure of [Fe(S2CNEtiPr)3], Fe(S2CNEtMe)3 and [iPrOC(S)S-S(S)COiPr] were determined. Iron complexes were used as a single source precursors for the deposition of iron sulfide nanocrystals by thermolysis in oleylamine, hexadecylamine and octadecene and thin films on silicon substrate at different temperatures. The complexes show typical paramagnetic behaviours whereas the iron sulfide nanocrystals produced show ferromagnetic behaviour. The greigite and pyrrhotite phases with hexagonal and cubic morphology were obtained by thermolysis. Pyrite and pyrrhotite phases were dominant in thin films. The complex tris(N,N-diethyl-N’-naphthoylselenoureato)iron(III) and its X-ray single crystal structure is also reported. Long rod like nanocrystals of orthorhombic ferroselite (FeSe2) obtained by thermolysis in oleylamine, dodecanthiol and in the mixture of oleylamine and dodecanthiol at 190, 240, and 290 °C. Paramagnetic behaviour was found under magnetic measurement of iron selenide nanocrystals. A very thin film of iron selenide (FeSe) phase was deposited on silicon substrate at 625 °C by AACVD method. The complexes [Fe{(SePiPr2)2N}2] and [Fe{(SePPh2)2N}2] were synthesised. The X-ray single crystal structure of [Fe{(SePPh2)2N}2] and [(SePPh2)2N)-O-(SePPh2)2N)] were also reported. A mixture of orthorhombic ferroselite (FeSe2) with rod and plate-like crystallites was obtained from the thermolysis of these complexes in oleylamine and hexadecylamine at 190, 240 and 290 °C temperatures. Also the mixed phases of iron selenide (Fe7Se8 and FeSe2) thin films having rod and sheet-like morphology were deposited at different deposition temperature (500, 550 and 600 °C) onto silicon substrates from these complexes.

The main aim of this research is to synthesize Ni(II) and Pd(II) dithiocarbamate complexes and use them as single source precursors for the synthesis of NiS and PdS nanoparticles and metal sulphides potato starch nanocomposites. Four dithiocarbamate ligands were synthesized and characterized using elemental analysis and spectroscopic techniques. The ligands were used to prepared homoleptic Ni(II) and Pd(II) complexes of the dithiocarbamate ligands. The metal complexes were characterized with elemental analysis, UV-Vis, FTIR and 1H-NMR spectroscopic techniques. Conductivity measurements indicate that all the complexes are non-electrolytes in solution and results from the electronic spectra studies confirmed the proposed 4-coordinate square planar geometry around the metal ions. The nickel complexes showed d-d transitions around 477 nm while in the palladium complexes, no d-d transitions were observed but the compounds showed strong metal to ligand charge transfer transitions. From the FTIR spectra studies, it can be confirmed that the complexes were successfully synthesised because all peaks of interest were observed at expected regions from the literature. The νC-N was observed around 1469-1495 cm-1, νC=S around 1101-1188 cm-1 and νC-S around 738-1060 cm-1 for both Ni(II) and Pd(II) complexes. νNi-S was observed around 375-543 cm-1 and νPd-S around 529-545 cm-1. The FTIR also confirmed that the dithiocarbamate ligands act as bidentate chelating ligands through the sulfur atoms. The complexes were used as single source precursors and thermolysed in hexadecylamine (HDA) at 220 °C to prepare four HDA-capped nickel sulfide nanoparticles and four palladium sulfide nanoparticles. The as-prepared nanoparticles were studied with optical absorption spectra, photoluminescence, powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The optical studies results showed that NiS have large band gaps that are greater than that of the bulk, therefore they are found to be blue shifted relative to the bulk, which shows that they have small particle size and thus confirming their quantum confinement effect. PL spectra reveal that the emission peaks are red shifted compared to the absorption band edges of the nanoparticles. The XRD patterns confirmed the formation of cubic and rhombohedral phase for NiS nanoparticles and cubic phase for PdS nanoparticles. SEM images of both NiS and PdS show uniform surface morphology at low and high magnification with different shapes. EDS analyses confirmed the presence of Ni, S, and Pd in each of the spectrum indicating that the nanoparticles were successfully synthesized. TEM images showed that the synthesised nanoparticles have uniform and narrow size distribution with no agglomeration. The sizes of the NiS nanoparticles were found to be in the range of 12-38 nm for NiS1, 8-11 nm for NiS2, 9-16 nm for NiS3 and 4-9 nm for NiS4. The TEM images for the as-prepared PdS nanoparticles showed that the average crystallite sizes are 6.94-9.62 nm for PdS1, 8-11 nm for PdS2, 9-16 nm for PdS3 and 4-9 nm for PdS4 respectively. The nanoparticles were used to prepare potato starch nanocomposites and SEM images indicate that the surface morphology of starch polymer nanocomposites compose of potato starch and few particles in between the pores of the matrix, this is due to the small ratio of nanoparticles used.

L’imagerie par fluorescence (IF) et l’imagerie de résonance magnétique (IRM) comptent parmi les outils de diagnostic les plus efficaces. Dans ce contexte, des QDs possédant à la fois des propriétés fluorescentes et magnétiques sont d’un grand intérêt en tant que sondes bimodales. Dans ce travail, des QDs Ag-In-Zn-S (AIZS) et Ag-In-Ga-Zn-S (AIGZS) ont été préparés et dopés afin de développer de nouvelles sondes de sondes bimodales pour l’IF et l’IRM. Des QDs AIZS très fluorescents ont été préparés en milieu organique à l’aide de DDT et d’OAm comme ligands. Les QDs Mn:AIZS possèdent des propriétés paramagnétiques et superparamagnétiques. Les QDs AIZS et Mn:AIZS ont également été transférés en phase aqueuse à l’aide du polymère amphiphile PMAO. Par la suite, des QDs AIZS dopés Mn, Gd ou Fe ont été préparés en milieu. Des études toxicologiques et d’imagerie ont montré une bonne biocompatibilité avec les cellules KB ainsi que le fort potentiel de ces nanocristaux pour l’IF. Dans la dernière partie de ce travail, des QDs AIGZS and Mn:AIGZS QDs ont été préparés via un procédé de décomposition thermique n’utilisant qu’un seul précurseur. Ces QDs possèdent de très bonnes propriétés optiques et magnétiques. Les QDs dopés Mn ont été transférés en phase aqueuse et ont montré un fort potentiel comme agent de contraste pour l’imagerie T1 et T2
Since fluorescence imaging (FI) and magnetic resonance imaging (MRI) are among the most effective diagnostic tools, QDs with fluorescent and magnetic properties are of great interest as dual-modal probes. In this work, undoped and doped Ag-In-Zn-S (AIZS) and Ag-In-Ga-Zn-S (AIGZS) QDs were synthesized and investigated as bimodal probes for FI and MRI. Highly fluorescent AIZS QDs were prepared in organic media using DDT and OAm as capping ligands. Mn:AIZS QDs showed paramagnetic and superparamagnetic properties. AIZS and Mn:AIZS QDs were also transferred into aqueous phase using the amphiphilic PMAO polymer. Further, Mn, Gd or Fe-doped AIZS QDs were prepared in aqueous media, showed low cytotoxicity toward KB cells, and demonstrated potential as fluorescent probes for FI. Finally, AIGZS and Mn:AIGZS QDs, synthesized via a novel single precursor thermal decomposition method, showed high fluorescence and paramagnetic/superparamagnetic properties. Mn-doped aqueous transferred AIGZS QDs increased contrast in both T1-weighted and T2-weighted images with increasing in Mn loading

xx, 153 p. : ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number.
In the research area of Group 13 hydroxide clusters, progress is often hampered by difficult and inefficient synthetic procedures. This has greatly limited the numerous potential applications of Group 13 hydroxide compounds, many of which require large amounts of material. Most relevant to this dissertation is their application as precursors for high quality amorphous metal oxide thin films. Addressing this issue, this dissertation presents a series of Group 13 containing hydroxide compounds of general formula [M 13 (μ 3 -OH) 6 (μ-OH) 18 (H 2 O) 24 ](NO 3 ) 15 which are generated through an efficient, scalable synthetic procedure. Throughout this dissertation, the compounds are generally referred to by their metal content, i.e. [Ga 13 (μ 3 -OH) 6 (μ-OH) 18 (H 2 O) 24 ](NO 3 ) 15 is designated as Ga 13 . Chapter I reviews the literature of inorganic and ligand-supported Group 13 hydroxide compounds with the aim of identifying common structural trends in metal composition and coordinating ligands. This summary is limited to clusters of aluminum, gallium, and indium. Chapter II describes in detail the synthesis and characterization of one such cluster, Al 13 . Following this in Chapter III is the description of the first heterometallic Group 13 hydroxide compound, Ga 7 In 6 , which along with Ga 13 was used as a precursor material for metal oxide thin films in collaboration with Professor Doug Keszler at Oregon State University. Chapter IV describes a series of six Ga/In compounds, as well as two Al/In compounds. Included in this chapter is an analysis of the heat-induced decomposition properties of the Ga/In clusters. Understanding such thermal decomposition is particularly relevant for the use of these compounds as precursor materials, as an annealing step is used to condense the films. Chapter V addresses the potential for post-synthetic modification of the compounds through metal and ligand exchange reactions, an area that also addresses the issue of solution stability of the structures Chapter VI describes the synthesis and characterization of related Group 13 compounds, including two infinite chain structures and additional heterometallic compounds. Lastly, Chapter VII concludes this dissertation and discusses potential areas of future research. This dissertation includes co-authored material and previously published results.
Committee in charge: Victoria DeRose, Chairperson, Chemistry; Darren Johnson, Member, Chemistry; James Hutchison, Member, Chemistry; Michael Haley, Member, Chemistry; Raghuveer Parthasarathy, Outside Member, Physics

Thesis (PhD (Chemistry and Polymer Science))--Stellenbosch University, 2008.
The successful preparation and structural characterization of a number of N,N-dialkyl-N’-benzoyl(thio)selenourea ligands is described; where the intermolecular interactions are characterized by the presence of Resonance Assisted Hydrogen Bonding (RAHB), π- π interactions between neighbouring benzene residues only being evident amongst the longer alkyl chain derivatives. The first structural characterization of an asymmetrically substituted N,N-dialkyl- N’-benzoylselenourea ligand reveals an increased stability of the Z isomer in the solid state, this being reflected by the sulfur analogue. Attempts to synthesise N,N-dicyclohexyl-N’-benzoylselenourea led to the isolation and structural characterization of a novel 1,3,5-oxaselenazine salt and dicyclohexylaminobenzoate. The first structural characterization of a “bipodal” N,N-dialkyl-N’-benzoylselenourea ligand, 3,3,3’,3’-tetrabutyl-1,1’- isophthaloylbis(selenourea), reveals RAHB in the crystal lattice similar to that exhibited by the “monopodal” analogue, N,N-dibutyl-N’-benzoylselenourea. The successful complexation of the N,N-dialkyl-N’-benzoyl(thio)selenourea ligands to a number of different transition metal ions is reported allowing the preparation of several potential single source precursors. Coordination through the O and Se/S donor atoms to Pd(II) results in the formation of square planar metal complexes, with a cis conformation, several of which could be structurally characterized. In particular, the first structural elucidation of an asymmetrically substituted N,N-dialkyl-N’-benzoylselenourea metal complex, cis-bis(N-benzyl-N-methyl-N’- benzoylselenoureato)palladium(II) indicates the increased stability of the EZ isomer in the solid state. Structural elucidation of the novel (N,N-diphenyl-N’-benzoylselenoureato)cadmium(II) reveals a bimetallic complex in the solid state, where the expected 2:1 ligand : metal ratio is maintained, and the two Cd(II) centres are 5 and 6 coordinated, with O and Se donor atoms. Multinuclear Nuclear Magnetic Resonance (NMR) Spectroscopy has been employed in the thorough characterisation of the potential single source precursors, 77Se NMR spectroscopy indicating a decreased shielding of the 77Se nucleus as the “hardness” of the central metal ion increases i.e. Pd(II) > Zn(II) > Cd(II). Use of 113Cd NMR spectroscopy indicates the preferential binding of N,N-diethyl-N’- benzoylselenourea to Cd(II) over that of its sulfur analogue, and initial studies suggest a form of chelate metathesis taking place in solution. 31P NMR spectroscopy is used to gain insight into the formation of cis-bis(N,N-diethyl-N’- benzoylselenoureato)Pt(II). Thermolysis of (N,N-diethyl-N’-benzoylselenoureato)cadmium(II) and its sulfur analogue led to the successful synthesis of CdSe and CdS quantum dots respectively, where thermolysis over a range of temperatures allows a degree of size control over the resulting nanoparticles. The effect of precursor alkyl chain length on nanoparticle morphology was investigated for both the N,N-dialkyl-N’-benzoylthio- and –selenoureas. A correlation between the two for the (N,N-dialkyl-N’-benzoylselenoureato)Cd(II) complexes is described and possible growth mechanisms are discussed. Preliminary investigations into the use of other N,N-dialkyl-N’-benzoyl(thio)selenourea metal complexes as single source precursors reveal that both (N,N-diethyl-N’-benzoylselenoureato)Zn(II) and its sulfur analogue show potential as single source precursors for the formation of ZnO and ZnS nanoparticles respectively. Initial studies into the use of N,N-dialkyl-N’-benzoyl(thio)selenourea metal complexes as single source precursors for the synthesis of core-shell nanoparticles is briefly described. The Aerosol Assisted Chemical Vapour Deposition (AACVD) of several N,N-dialkyl-N’-benzoyl(thio)selenourea metal complexes is reported, where both (N,N-diethyl-N’-benzoylselenoureato)Cd(II) and its sulfur analogue allow the deposition of crystalline CdSe and CdS respectively. The AACVD of (N,N-diethyl-N’- benzoylselenoureato)Zn(II) leads to the deposition of crystalline ZnSe, ZnS being deposited by (N,N-diethyl-N’-benzoylthioureato)Zn(II). The deposition of heazelwoodite (Ni3S2) with varying morphologies results from the AACVD of cis-bis(N,N-diethyl-N’-benzoylthioureato)Ni(II). Thermal annealing of the amorphous material deposited by the AACVD of cis-bis(N,N-diethyl-N’-benzoylthioureato)Pd(II), allows the formation of highly crystalline palladium. The deposition of metallic platinum using cis-bis(N,N-diethyl-N’-benzoylthioureato)Pt(II) is described as well as the deposition of crystalline Pd17Se15 from cis-bis(N,N-diethyl-N’-benzoylselenoureato)Pd(II). This, to the best of our knowledge, is the first time that AACVD has been performed, using the N,N-dialkyl-N’- benzoyl(thio)selenourea metal complexes as single source precursors, in addition, we believe it to be the first time that palladium selenide has been deposited using the AACVD technique.

The present work deals with the synthesis, characterization, and fabrication of Si-M-C-based ceramic nanocomposites (M = B and V). These were produced by the thermal transformation of tailor-made single-source-precursors, which were synthesized by the chemical modification of an allyl-hydrido polycarbosilane with suitable precursors (i.e., borane dimethylsulfide, allyl-functionalized carboranes, vanadium acetylacetonate and vanadium oxytriisopropoxide). The typical approach to this synthesis consists of a pyrolytic ceramization of the precursors, which converted into amorphous single-phase SiMC(O)-based materials. They are further subjected to high temperature treatment for phase separation and crystallization processes to furnish SiC-based ceramic nanocomposites. The preceramic polymer allyl-hydrido polycarbosilane (commercial name SMP-10) and derived SiC-based ceramics were thorougly investigated with respect to cross-linking behavior, polymer-to-ceramic transformation as well as high-temperature phase composition and microstructure. This knowledge served to optimize the processing of the preceramic polymeric precursor to produce dense and crack-free SiC-based monolithic ceramics by pressureless technique. The obtained ceramic monoliths have been shown to exhibits residual porosity of 15-25 vol%, which however can further be reduced by the use of polymer-infiltration and pyrolysis (PIP) to about 0.5 vol%. Boron-containing single-source-precursors were synthesized upon reactions of SMP-10 with borane dimethylsulfide complex (BMS) or with allyl-functionalized carboranes (AFC). In case of BMS-modified SMP-10 (BMS-SiBC), a detailed structural characterization has been done by means of various spectroscopic techniques. The main aspects addressed in case of BMS-modified SMP-10 (BMS-SiBC) are the fate of boron in the prepared SiBC ceramics, which was not been clarified unambiguously so far, and the role of boron in terms of densification of SiC. X-ray diffraction data, corroborated with X-ray photoelectron spectrocopy, Attenuated total reflectance-Fourier transform infrared spectroscopy, and Raman spectroscopic results indicate that in the SiBC ceramic prepared from the BMS-SiBC, boron preferably gets incorporated within the segregated carbon phase. Moreover, it was shown that the incorporation of boron has a positive effect on the densification behavior of SiC; so monolithic SiC ceramics with residual porosity below 5 vol% could be produced with pressureless processing. The SiBC material prepared from the AFC-modified SMP-10 shows a different phase composition, indicating the presence of a boron-rich boron carbide phase, which was not detected in BMS-SiBC. The results shows the crucial effect of the molecular architecture and chemism of the single-source-precursors on the phase composition and consequently properties of the resulting ceramic materials. Vanadium-containing single-source-precursors were obtained upon chemical modification of SMP-10 with vanadyl acetylacetonate or vanadium oxytriisopropoxide. High temperature treatment of the resulting single-source-precursors in argon atmosphere initially led to an amorphous single-phase ceramic (SiVCO) which was then converts into ceramic nanocomposites consisting of a non-stoichiometric vanadium carbide phase (V8C7) finely dispersed in a polycrystalline β-SiC matrix. In this context, the first investigation was carried out on biomorphic and the template-assisted processing of single-source-precursor to form porous monolithic samples. In addition, preliminary results of the catalytic activity of SiVC(O) show that the nanocomposites are active for the decomposition of the ammonia. The maximum ammonia conversion efficiency was found to be 35 % at around 650 oC which is higher than that of pure V8C7 reported in the literature (13 %). The results of this study show that the processing of ceramics starting from suitable preceramic polymer is a versatile technique for the production of SiC-based ceramic nanocomposites with tailored phase composition, microstructure, and property profiles. Moreover, the single-source-precursor technique used for the preparation of ceramic nanocomposites allows flexibility with respect to processing. Thus it is possible, starting from preceramic precursors to prepare ceramic powder, crack-free and dense monoliths as well as materials/components with tailored porosity which can be used flexibly for different applications.

Si-M-N (M=metal) ceramic nanocomposites are novel materials that combine the advantages of both ceramics and metals. Additionally, a variety of intriguing functional properties are observed in the metal-modified ceramics due to the formation of a second phase, which reveals promising applications in the fields of optics (e.g., light-emitting diodes), semiconductors, catalysis and energy technology. Until now, most of the metal-modified nanocomposites are fabricated by using traditional powder techniques, but the grain sizes of the composites are limited to the micrometer range, and the dispersion of metal particles is not homogeneous. Polymer-derived ceramics route is widely considered as a promising approach in the synthesis of novel nanocomposites, where the nanocomposites are derived from the corresponding single-source precursors and exhibit improved structural and functional properties due to the unique nanostructures. This Ph.D. thesis is focused on the synthesis of ternary Si-M-N ceramics derived from single-source precursors with tailored compositions and structures, which were synthesized via the chemical modification of polysilazane with metallic compounds. The main objective of this research is to study the chemical modification of the precursors and nanostructures of Si-M-N ceramics and to gain a better understanding of the effect caused by the modification with different metallic compounds on the structures and properties of resultant ceramic nanocomposites. In the present research, single-source precursors with varied compositions and structures were synthesized by chemical modification of perhydropolysilazane (PHPS) with transition-metal compounds. Si-Hf-N, Si-V-N(O) and Si-Fe-N(O) single-source precursors were synthesized by using TDMAH, VO(acac)2 and Fe(acac)2, respectively. Amorphous single-phase Si-M-N ceramics were prepared via the subsequent cross-linking and pyrolysis under an ammonia atmosphere. The syntheses of these preceramic polymers were investigated by means of spectroscopic techniques including FT-IR, Raman and solid MAS NMR spectroscopy, and the results indicated the formation of expected transition-metal-modified precursors. Then, the structural evolution during the polymer-to-ceramic conversion of the precursors was monitored with FT-IR measurements. The prepared materials were investigated with respect to their crystallization behaviors and phase compositions using spectroscopic techniques together with X-ray diffraction (XRD), elemental analysis (EA) and scanning/transmission electron microscopy (SEM/TEM). Annealing experiments on the Si-M-N ceramics were performed in a nitrogen atmosphere at temperatures ranging from 1100 to 1800 °C, leading to the conversion of the amorphous materials into crystalline nanocomposites. It was found that α- and β-Si3N4 were obtained in the Si-M-N composites during the high-temperature treatment and built a matrix, while the transition-metals formed different crystallites such as metal nitrides (HfN, VN and Fe2N), pure metal (α-Fe) and metal silicide (Fe3Si) depending on the intrinsic characteristics of transition-metals and sintering temperatures, and these metal-containing crystallites homogeneously dispersed in the silicon nitride matrix. The high-temperature phase separation and crystallization behaviors of the Si-M-N ceramics were intensively investigated. The focus was firstly placed on the synthesis of novel polymer-derived SiHfN ceramics. They were prepared via the pyrolysis of a single-source precursor which was synthesized by the chemical reaction between perhydropolysilazane (PHPS) and tetrakis(dimethylamido) hafnium(IV) (TDMAH). The hafnium-modified PHPS precursor convert upon heat treatment in an ammonia atmosphere at 1000 °C into an XRD amorphous single-phase Si1Hf0.056N1.32 ceramic and remained amorphous even after annealing at 1400 °C in a nitrogen atmosphere. The PHPS-derived ceramic without modification showed a composition of Si1N0.71 at 1000 °C and started to crystallize at 1300 °C. The electron microscopy investigation exhibited that the annealing of the highly homogeneous single-phase SiHfN ceramic induced a local enrichment (clustering) of hafnium, leading to amorphous HfN/SiNx nanocomposites. The modification with TDMAH not only increases the nitrogen content of the ceramic materials but also efficiently improves the high-temperature stability of the Si3N4 against crystallization greatly. Annealing in nitrogen at 1600 °C resulted in a phase separation, and crystallized HfN/Si3N4 nanocomposite was obtained. The α- to β-Si3N4 phase transformation was greatly inhibited in the SiHfN ceramics at 1800 °C. Extensive STEM characterizations of the polycrystalline nanocomposites indicated further substitutional and interstitial doping of hafnium in Si3N4. An amorphous single-phase SiVN(O) ceramic was prepared via the ammonolysis of the corresponding single-source precursor, which was synthesized by the chemical modification of PHPS with vanadyl acetylacetonate (VO(acac)2). The as-obtained SiVN(O) ceramic exhibited high-temperature resistance against crystallization up to 1400 °C. Annealing at 1600 °C caused a phase separation and intensive crystallization. As a result, the nanocomposite composed of VN, α- and β-Si3N4 was obtained. Further investigation suggested that the introduction of VO(acac)2 promoted the α- to β-Si3N4 phase transformation at 1600 °C, and a VN/β-Si3N4 nanocomposite was obtained when the sample was annealed at 1600 °C. Furthermore, mesoporous SiVN(O) ceramics with high specific surface area (SSA) were successfully prepared by using polystyrene (PS) as self-sacrificial templates via a one-pot synthesis. After cross-linking and pyrolysis in an ammonia atmosphere at 1000 °C, a mesoporous SiVN(O) ceramic with a SSA of 506 m2/g was produced. Both the specific surface area and pore size distribution of the mesoporous ceramics can be adjusted by changing the amount of PS templates in the feed. Moreover, the mesoporous SiVN(O) ceramics exhibited good structural stability up to 1400 °C (SSA maintained ca. 200 m2/g), but a total collapse of the mesoporous structures was observed at 1600 °C. A SiFeN(O) precursor was synthesized by the reaction of PHPS with iron(II) acetylacetonate (Fe(acac)2) via the formation of Si-O-Fe bonds. The pyrolysis of SiFeN(O) precursor in ammonia induced a phase separation with the formation of Fe2N at 600 °C, and then the Fe2N decomposed into α-Fe by increasing the temperature to 1000 °C. Crystalline Fe3Si was obtained when the temperature was over 1200 °C. The observations demonstrated that the modification with Fe(acac)2 had a considerable influence on the phase separation and crystallization behaviors of the ceramics. Subsequently, a SiFeN(O)-based ceramic paper with in-situ generated hierarchical micro/nano-morphology was prepared by pyrolyzing a filter paper template that was modified with a SiFeN(O) precursor. After ammonolysis at 1000 °C, the obtained SiFeN(O)-based ceramic paper decorated with crystalline α-Fe had the same morphology as that of the used paper template. Ultra-long silicon nitride nanowires with great aspect ratios (~200 nm in diameter and several millimeters in length) were in-situ formed in a large quantity both on the surface and in the pores of the ceramic paper, when the ceramic paper was further annealed in nitrogen at temperatures from 1200 to 1400 °C. The nanowires exhibited a round Fe3Si tip at the end, indicating that the growth of one-dimensional nanostructures occurred via an iron-catalyzed VLS (vapor-liquid-solid) mechanism, and the length and yield of nanowires can be controlled by adjusting the experimental conditions, including temperatures and the addition of Fe(acac)2. Therefore, the combination of the single-source precursor, catalyst-assisted pyrolysis and template method provides a convenient one-pot route for the fabrication of ceramic paper and one-dimensional structures with high yields. In summary, the present Ph.D work proved that Si-M-N single-source precursors can be synthesized via the PDC route by modifying PHPS with different metallic compounds, and amorphous Si-M-N single-phase ceramics can be obtained via pyrolysis of the corresponding precursors. Polymer-derived ceramic nanocomposites composed of X/Si3N4 (X = metal, metal nitride or metal silicide) with a homogenous microstructure can be prepared by further annealing at higher temperatures. This thesis provides some new insights into the design and synthesis of metal-modified precursors and enables the production of Si-M-N ceramic nanocomposites via the PDC approach.

Industrial and aerospace demands on future technologies have created an urgent need for new material properties that are beyond those of materials known today and that can only be fabricated by designing the respective microstructure at the nanoscale. Taking advantage of the correlation between the molecular structure of preceramic precursors and the microstructure of the derived ceramic materials, the single-source-precursor route offers possibilities to fabricate novel ceramic materials that are inaccessible by conventional synthesis.[1] The motivation of this Ph.D. work is to further develop the concepts for fabrication of novel ceramic nanocomposites with a tailor-made microstructure and versatile properties by molecular design of their precursors. With this motivation, a series of dense monolithic SiMC(N) ceramic nanocomposites (M = Hf, Ta, HfTa) were fabricated using single-source-precursor synthesis plus spark plasma sintering. The chemical synthesis, polymer-to-ceramic transformation as well as high-temperature microstructural evolution was characterized using FT-IR, MAS solid NMR, TG/MS, XRD, Elemental analysis, SEM, TEM and Raman spectroscopy. Moreover, electrical conductivity, microwave absorption capability, electromagnetic interference shielding performance and laser ablation resistance of the as-prepared dense monoliths were investigated as well. In the synthesis part, a series of M-containing single-source precursors were synthesized upon reactions between a commercially available allylhydridopolycarbosilane (SMP10) and metal compounds, including Hf(NMe2)4, Hf(NEt2)4 and Ta(NMe2)5. The polymer-to-ceramic transformation was characterized using FT-IR, 13C and 29Si MAS solid NMR as well as in situ TG/MS. The precursors synthesized using Hf(NMe2)4 and Ta(NMe2)5 lead to higher ceramic yield (≈ 80 wt.%) than that of Hf(NEt2)4 (≈ 71 wt.%), while the ceramic yield of the latter can be improved to ≈ 78 wt.% by introduction of BH3·SMe2. Several thermal stable SiMC(N) ceramic nanocomposites (powders) were prepared upon high-temperature annealing of the amorphous SiMC(N) ceramics, including SiHfC(N), boron-doped SiHfC(N), SiTaC(N), SiHf7Ta3C(N) and SiHf2Ta2C(N). XRD, Raman and TEM results reveal that the ceramic nanocomposites mainly comprise β-SiC and MCxN1-x as well as free carbon (M = Hf, Ta, HfyTa1-y). Rietveld refinement of XRD patterns and the TEM images confirm that the grain size of both β-SiC and MCxN1-x are less than 100 nm even after annealing at 1900 ºC for 5 h. The grain growth of β-SiC can be effectively suppressed by introducing M elements into the single-source precursors. Hf0.7Ta0.3CxN1-x and Hf0.2Ta0.8CxN1-x solid solutions with an expected Hf/Ta atomic ratio can be controlled precisely by adjusting the mole ratio of metal compounds during synthesis of the single-source precursors. It is worth emphasizing that a unique MCxN1-x-carbon core shell microstructure is observed within all the SiMC(N) ceramic nanocomposites, and the Hf-rich phase (e.g., HfCxN1-x and Hf0.7Ta0.3CxN1-x) seems to facilitate the formation of the carbon shell more easily. The carbon shell on the MCxN1-x core is able to hinder the coarsening of MCxN1-x grains during high-temperature processing. Thus, dense monolithic SiMC(N) ceramic nanocomposites are fabricated successfully upon spark plasma sintering of the amorphous SiMC(N) ceramics at 2200 ºC. The achieved maximum diameter is 35 mm, which is rarely reported in the literature. Laser ablation behavior of the SiHfC(N) ceramics was investigated on dense monolithic SiHfC(N) ceramic nanocomposites and Cf-reinforced SiHfC(N) ceramic matrix composites. With addition of the HfCxN1-x phase, the rim of the ablation pit is covered by Hf-containing materials (e.g., HfO2), which are able to suppress the growth of the ablation pit. The dielectric properties and microwave absorption performance of the SiHfC(N) ceramics were investigated in the X-band (8.2 ~ 12.4 GHz) at room temperature. The minimum reflection loss and the maximum effective absorption bandwidth amount to -47 dB and 3.6 GHz, respectively. Free carbon, including graphitic carbon homogeneously dispersed in the SiC-matrix and less ordered carbon deposited as a shell on HfCxN1-x nanoparticles, accounts for the unique dielectric behavior of the SiHfC(N) ceramics. Electromagnetic interference (EMI) shielding performance of the dense monolithic SiHfC(N) ceramic nanocomposites were investigated in the X-band (8.2 ~ 12.4 GHz) at temperatures up to 600 ºC. At room temperature, the SiC/C-35mm and SiC/15HfCxN1-x/C-35mm exhibit an average total shielding effectiveness (SET) of ≈ 21 dB and ≈ 42 dB, respectively, and at 600 ºC, ≈ 22.6 dB and ≈ 40.2 dB, respectively. That means that with addition of a small amount of conductive HfCxN1-x, the SET is highly improved at both room and high temperatures. In summary, the synthesis, ceramization, densification as well as microstructural evolution of SiMC(N) ceramic nanocomposites are deeply investigated in this work. With the addition of an M-containing phase, the as-prepared SiMC(N) ceramic nanocomposites exhibit enhanced electrical conductivity, microwave absorption capability, electromagnetic interference shielding performance and laser ablation resistance. Moreover, the correlations regarding to molecular design, microstructure and properties of the SiMC(N) ceramic nanocomposites are carefully discussed.

Hf-containing ultra-high-temperature ceramics (UHTCs) are being pursued for Thermal Protection Systems (TPSs) for high-temperature applications (i.e., future hypersonic vehicles) in harsh environments. Most of these ceramic composites have been prepared using traditional powder techniques; however, the grain sizes of the resulting composites are limited to the micrometer range. Furthermore, nano-sized Hf-containing materials have proven to exhibit tremendously improved structural/functional properties, even at elevated temperatures, compared with microcomposite ceramics. Single-source precursors (SSPs) have yielded promising results in the processing of ceramic nanocomposites; moreover, these composites exhibit unique properties, e.g., high-temperature stability and high-temperature oxidation and corrosion. The objective of this work was to synthesize new Hf-containing ultra-high-temperature ceramic nanocomposites (UHTC-NCs) using SSP-based methods and to investigate their behavior in harsh environments. In the research presented in this PhD thesis, focus was first placed on the synthesis of novel Hf-containing SiHfCN and SiHfBCN amorphous UHTC-NCs derived from polysilazane. Amorphous SiHfCN and SiHfBCN ceramics were prepared from commercial polysilazane (HTT1800, AZ-EM), which was modified through reactions with Hf(NEt2)4 and BH3·SMe2 and subsequently cross-linked and pyrolyzed. The prepared materials were investigated with respect to their chemical and phase compositions using spectroscopic techniques (FTIR, Raman, and MAS NMR spectroscopy) and via X-ray diffraction (XRD) and transmission electron microscopy (TEM). Annealing experiments on SiHfCN and SiHfBCN samples in inert gas atmospheres (Ar and N2) at temperatures ranging from 1300 to 1700 °C revealed the conversion of the amorphous materials into nano-structured UHTC-NCs, whose high-temperature decomposition and crystallization were also investigated. It was found that β-SiC/HfCxN1-x nanocomposites were obtained from SiHfCN upon annealing at 1500 °C. Depending on the annealing atmosphere, HfCxN1-x/HfB2/SiC (annealing in argon) and HfNxC1-x/Si3N4/SiBCN/C (annealing in nitrogen) nanocomposites were obtained from SiHfBCN annealed at 1700 °C. The results demonstrate that the conversion of single-phase SiHf(B)CN into UHTC-NCs is thermodynamically controlled and thus offer insight toward the development of nano-structured ultra-high-temperature stable materials with tunable compositions. The second focus of the present study was the development of dense Hf-containing ceramic monoliths via pressureless sintering (PLS) or spark plasma sintering (SPS) and the development of ceramic matrix composites (CMCs) via polymer infiltration and pyrolysis (PIP) methods. Dense amorphous ceramic monoliths were prepared upon annealing pyrolytic ceramics in nitrogen at 1300 °C. Dense SiHfCN- and SiHfBCN-based UHTC-NCs were successfully prepared via SPS at 1850-1950 °C using high heating rates (~450 °C/min.) and high pressures (≥ 100 MPa). The obtained UHTC-NCs were investigated via spectroscopic analyses (XRD and Raman spectroscopy) and electron microscopy (SEM and TEM) with regard to their phase evolution and microstructure. Despite the very high sintering temperatures, the microstructures of the prepared dense UHTC-NCs remained rather fine, with grain sizes varying from 165 nm down to a few tens of nm. The hardness and elastic modulus of the dense SiHfCN were found to be 26.8 and 367 GPa, respectively, whereas the SiHfBCN samples exhibited a hardness of 24.6 GPa and an elastic modulus of 284 GPa (measured by nanoindentation). Additionally, Cf/SiCN and Cf/SiHfBCN CMCs were fabricated via a simple and low-cost PIP route. Cf/SiC-SiCN and Cf/SiC-SiHfBCN materials with pyrolytic carbon coatings were synthesized using hybrid techniques (CVI and PIP). The bending strength of the prepared CMCs resulted in the observation of brittle fracture surfaces only in the Cf/SiHfBCN material, indicating strong interfacial bonding between the fibers and the matrix; the much higher values of bending strength observed for Cf/SiC-SiCN and Cf/SiC-SiHfBCN resulted from the fact that weak interfaces (pyrolytic carbon) lead to transfer loading. This finding of the present work suggests that a single-source precursor route is suitable for the preparation of a variety of (ultra)-high-temperature ceramics, such as amorphous ceramics, UHTC-NC monoliths, and CMCs. Moreover, we explored the behavior of the prepared materials in harsh environments, e.g., their high-temperature stability with respect to decomposition and crystallization and their oxidation, corrosion and ablation behavior. High-temperature annealing experiments revealed that the SiHfCN and SiHfBCN materials exhibited improved high-temperature stability with respect to decomposition compared with non-modified SiCN. The oxidation behavior of the SiCN, SiHfCN and SiHfBCN ceramic powders was studied via thermogravimetric analysis (TGA) in air at 1200-1400 °C, revealing that the modified SiHfCN and SiHfBCN ceramics exhibited poorer oxidation resistance than that of SiCN. However, parabolic oxidation kinetics of SiHfCN and SiHfBCN were observed, wherein the parabolic rate (Kp) that was obtained from the equation K_p=〖 (∆m/(S_BET×m))〗^2×t^(-1) indicated that the amorphous SiHfBCN ceramic powder exhibited enhanced oxidation resistance compared with that of the SiHfCN. Furthermore, the oxidation behavior of SiHfBCN ceramic monoliths was investigated in a tube furnace (stagnant air, up to 100-200 h). The microstructure and phase composition of the monoliths’ oxide scale was investigated via XRD and microscopy (SEM, BSE and EPMA). The results revealed that the oxidation of the SiHfBCN ceramic monoliths followed typical parabolic kinetics, indicating that the oxidation diffusion was controlled by a passive oxide layer. However, the microstructure and composition of the oxide scale were strongly dependent on temperature. A continuous oxide layer, consisting of cristobalite and hafnia (m- and t- HfO2), was observed at 1200 °C; however, at 1400 °C, it became a discontinuous oxide layer and its composition changed to cristobalite, HfO2 and HfSiO4. Thus, the wide range of Ea values (174 and 140 KJ mol-1, depending on the Hf content) obtained from the apparent or corrected oxidation kinetics indicate the complex nature of their oxidation process, which might be the result of a wide variety of oxygen-controlling mechanisms in both the inward oxygen transport into the oxide scale (borosilicate or silica, hafnia, or hafnium silicate) and the outward transport of gas produced by oxidation reactions. Additionally, an investigation of the oxidation of the prepared dense UHTC-NCs at high temperature revealed that both samples exhibited parabolic behavior. Interestingly, the parabolic oxidation rates of the SiHfCN were comparable to those of other UHTCs (e.g., HfC-20 vol% SiC), whereas the parabolic oxidation rates of the SiHfBCN were 3 to 4 orders of magnitude lower. The results obtained in this study indicate that amorphous Hf-containing Si(Hf)BCN ceramics nanocomposites and nanoscale Hf-containing UHTC-NCs are promising candidates for high-temperature applications in harsh environments. The behavior of Cf/SiCN and Cf/SiHfBCN under subcritical hydrothermal conditions was also investigated at temperatures of 150-250 °C for exposure times of 48, 96 and 240 h. The effect of the ratio between the surface area of the sample and the volume of water used (S/V ratio) on the corrosion behavior of the prepared CMCs was analyzed. For S/V ratios greater than 0.18, the exposure of the CMCs to hydrothermal conditions led to a gain in mass, whereas at lower S/V ratios, a mass loss of the samples was recorded. Because the behavior of the studied samples was representative and reliable at small S/V ratios, both investigated CMC samples were concluded to exhibit active corrosion behavior in a subcritical hydrothermal corrosive environment. Based on the corrosion experiments performed at an S/V ratio of 0.075, the data for the mass loss as a function of the corrosion time and temperature were used to rationalize the corrosion kinetics of the Cf/SiCN and Cf/SiHfBCN samples. Both materials were shown to exhibit excellent stability under subcritical hydrothermal conditions. The corrosion rate of Cf/SiHfBCN was found to be lower than that of Cf/SiCN; furthermore, an SEM investigation indicated that spallation occurred in the Cf/SiCN samples, whereas the ceramic matrix remained attached to the individual carbon fibers in Cf/SiHfBCN. The results of the present study indicate that the incorporation of Hf and B into the SiCN matrix leads to significant improvement in its hydrothermal corrosion performance. Finally, the ablation mechanism of the Cf/SiHfBCN ceramic composites after treatment in a laser ablation environment was investigated. The microstructure and ablation behavior of this composite were studied using SEM combined with EDS. The formation of porous HfO2, molten HfO2 and SixOyHfz yielded fibers with good protection from oxidation and the laser beam. Three regions with different ablation behaviors are proposed based on the temperature distribution. The ablation center exhibited bubble-like structures, corresponding to the melting of HfO2 and SiHfxOy layers that covered the ends of the carbon fibers, and moreover, eroded carbon fibers that retained their original shape were also observed. In the transition region, carbon sheets and oxidation-product particles (HfCxOy and SiO2) peeled off from the eroded fibers and the matrix because of the high vapor pressure. Additionally, the growth of SiC grains and glass with bubble structure, corresponding to SiO2 with inclusions of B2O3 and SiO gas, was observed.

Carbon nanotubes (CNTs), reduced graphene oxide (RGO) and CNTs/RGO hybrids are highly promising electromagnetic (EM) absorbents due to their intrinsic advantages including excellent electric conductivity, high specific surface area and good mechanical strength. However, the homogeneous dispersion of them has been a challenge. In this thesis, single-source-precursor route as a promising approach to achieve homogeneous dispersion, is employed to prepare nano-carbon fillers (CNTs, RGO, RGO/CNTs) modified ceramic nanocomposites as novel and prospective EM materials for EM absorption and shielding application. The microstructure of the nanocomposites is analyzed in detail and correlated with the permittivity and EM shielding performance of the resultant materials.

This thesis details the synthesis, characterization, and functionalization of transition metal phosphide nanomaterials from single source molecular precursors. The decomposition of the organometallic cluster, H2Fe3(CO) 9PtBu, yielded iron phosphide (Fe2P) nanomaterials of various morphologies depending on the surfactants used for the decomposition. Branched nanostructures were observed as a result of crystal splitting in a few of the surfactant systems. Cross-shaped structures were also observed and attributed to the twinning of two individual bundles during growth as the result of an interrupted growth process. The role of the solvents in particular the use of oleic acid for the formation of nanorods, in the formation of Fe2P nanoparticles will be discussed. Magnetic measurements taken of a variety of different morphologies of these iron phosphide nanoparticles will also be presented. Fe2P nanoparticles were also isolated via the decomposition of other clusters, including Fe3(CO) (P tBu)2, Fe2(CO)6(PHtBu) 2, Fe4(CO)11PtBu2, and Fe3(CO)10PtBu. In order to study the mechanism by which the clusters decompose, the decompositions were monitored using infrared spectroscopy. For all of the systems studied, the clusters rearranged in the surfactant solutions, ultimately resulting in Fe2(CO) 6(PHtBu)2 prior to decomposition. This rearrangement is believed to be a result of the interaction of the clusters with the surfactants employed, suppored by the finding that the solid state decomposition of H 2Fe3(CO)pPtBu was found to result in a combination of Fe 3P, Fe2P, and Fe3O4. In addition to the formation of the binary phases of transition metal phosphide nanomaterials, investigation into the formation of mixed metal phosphides of iron and manganese were also performed. For these experiments, H2 Fe3(CO)9PtBu with a manganese source, either Mn2(CO)10 or Mn(CO)5Br, were decomposed in a variety of surfactant systems. The resulting nanoparticles were only doped with manganese; pure stoichiometric phases were not isolated. Finally the functionalization of Fe2P split rods, T-shapes, and crosses with a gold shell was performed. Their optical properties were studied, and a redshift in the extinction maximum was seen as the shell thickness increased. This plasmon peak shift, as opposed to the trends seen in silica-Au core-shell structures as shell thickness increases, is attributed to the high permittivity of the Fe2P core.

M.Tech. (Chemistry, Faculty of Applied and Computer Science), Vaal University of Technology.
Complexes of alkyldithiocarbamate and thiuram have been extensively explored for various applications in the medical field. Thiuram and dithiocarbamate ligands were used to prepared complexes of cobalt and copper. The high abundance of sulfur in these ligands has resulted to be the preferred complexes for the synthesis of metal sulfide nanoparticles. All the prepared complexes were characterized using techniques such as IR and 1HNMR spectroscopy, elemental analysis, and thermogravimetric analysis. All the spectra data obtained were consistent with the coordination of the ligands through sulfur atom to the metal ion. The thermogravimetric analysis of all complexes decomposed to form metal sulfide, which really confirmed that all the complexes could be used to metal sulfide nanoparticles. All the prepared complexes were used to synthesize MxSy nanoparticles. The metal sulfide nanoparticles were successful prepared by thermal decomposition of the single-source precursor in hexadecylamine solution. The reaction parameter such as the concentration (1.0, 0.5, 0.25 and 0.125 g), reaction temperature (80, 130, 200, 250 °C) and the time (5, 10, 15, 20, 25 and 30) of the reactionwere varied to see their effect on the preparation of the nanoparticles. The prepared metal sulfide nanoparticles were characterized using techniques such as UV spectroscopy, photoluminescence spectroscopy, X-ray diffraction analysis and transmission electron microscopy. The concentration was found to have a profound effect in size and shape of the prepared nanoparticles. The nanoparticles prepared at various concentrations were dominated by sphere with an average size of 2-30 nm. The XRD pattern confirmed that the composition is not affected by the temperature. Thetemperature has a dramatic effect in size, shape and the stoichiometry of the reaction. This was confirmed by an increase in size as the temperature was increased, with the exception of cobalt sulfide nanoparticles that decrease in size while temperature was increase. The XRD pattern showed different composition as the temperature was varied. Time of the reaction was found to affect the particles size of the nanoparticle. The sizes of the nanoparticles were increase as the time of the reaction was prolonged.

This thesis details the synthesis and characterization of anisotropic cadmium and lead sulfide nanostructures from single-source molecular precursors. Six new precursors were synthesized for cadmium and lead sulfide each, by the reaction of the appropriate metal acetate with picolinic (HPic), 2,6-dipicolinic (H2dipic) or salicylic acid (H2sal) followed by the addition of thiourea (th) or thiosemicarbazide (ths). The precursors for CdS are [Cd(Hsal)2(tu)2] (Cd1a), [Cd(Hsal) 2(ths)2]·nH2O (Cd1b), [Cd(pic) 2(tu)2]·0.5H2O (Cd2a), [Cd(pic) 2(ths)2]·2H2O (Cd2b), [Cd(dipic)(tu) 2] (Cd3a) and [Cd(dipic)(ths)2(H2O)]·2H 2O (Cd3b) and the precursors for PbS are [Pb(Hsal) 2(th)2] (Pb1a), [Pb(Hsal)2(ths) 2] (Pb1b), [Pb(pic)2(th)2] ( Pb2a), [Pb(pic)2(ths)2] (Pb2b), [Pb(dipic)(th)(H2O)]2·2H2O ( Pb3a) and [Pb(dipic)(ths)2]·H2O ( Pb3b). All of the compounds were characterized spectroscopically and by elemental analysis. Cd1a, Cd2a, Cd2b, Cd3a, Cd3b, Pb2b Pb3a and Pb3b formed well-defined crystals and were characterized by single crystal X-ray diffraction. The precursors were decomposed at or around 170°C using n-cetyltrimethylammonium bromide (CTAB), sodium dodecylsulphate (SDS), ethylenediamine, oleic acid, oleylamine, trioctylamine or hexadecylamine as surfactants. Systematic variations of surfactants gave small spherical nanoparticles, micro-sized flowers, multipods and nanorods for CdS and nanocubes, truncated nanocubes, hexapods, octahedrons and dendritic stars for PbS. From XRPD studies it was found that most of the CdS nanostructures were of the stable hexagonal phase. However, in two cases the nanostructures were found to be predominantly of a metastable orthorhombic phase. For PbS system, all the decompositions yielded pure crystalline galena. For CdS system, TEM studies revealed planar defects (such as polysynthetic and multiplet twinning) in the nanocrystals, which gave an explanation for mechanism of growth. For PbS system, in order to elucidate the effect of single source precursors on the mechanism of growth of nanoparticles, the decomposition results were compared with PbS nanostructures synthesized from multiple-source precursors, lead acetate and thiourea or thiosemicarbazide. It was found that in the reactions of multiple source precursors, acidic components in the reaction mixture (oleic acid, acetic acid) led to etching and crystal splitting, which played a crucial role in the formation of anisotropic nanostructures.

碩士
國立中正大學
化學研究所
88
There are two parts in this thesis. 1. The syntheses of various copper(I) complexes are best illustrated by following examples. The compounds R3SnCu(PR’3)n, where R = Ph, Me; R’ = Me, Et, Ph and n = 3, have been synthesized by the reaction of R3SnNa with CuCl and three equivalent of the PR’3, or by the reaction of R3SnNa with CuCl(PPh3)3. All of these products were characterized by 1H NMR, 13C NMR and 31P NMR spectroscopies. There are three phosphine molecules bonding with the Cu atom. The series of Ph3Sn- complexes are thermal stable at ambient temperature, but the series of Me3Sn- complexes decompose at room temperature. All of these products are not suitable for chemical vapor deposition, because they can not be sublimated. 2. cis-Fe(CO)4(SnMe3)2 has been examined as single-source precursor for low-pressure chemical vapor deposition in cold-wall reactor. Fe/Sn alloy thin films were obtained at 240~420℃. XRD indicated only FeSn pattern at 240℃, FeSn2 pattern appeared above 270℃, FeSn pattern disappeared at 300℃ and FeSn pattern appeared gradually above 330℃. These films have been characterized and analyzed by SEM, EDS, EPMA, ESCA, AES and AA. The results indicated that these are Fe/Sn alloy thin films. The deposition rate and grain size increase with increasing temperature. The deposition rates are 40~330 Å/min. The grain sizes are 0.2~2.0 m.