Дисертації з теми "Ag₂Te"

Parmi les matériaux thermoélectriques, le tellure possède un coefficient Seebeck positif élevé (500 μV/K) à température ambiante mais avec une conductivité thermique relativement élevée (3 W/m.K). La nanostructuration est une solution pour améliorer les performances thermoélectriques, la conductivité thermique de réseau pouvant être réduite. Dans ce travail, nous avons synthétisé des nanostructures de Te auto-supportées par électrodéposition en profitant de l'effet matrice des liquides ioniques. Des nanostructures hexagonales monocristallines ont été systématiquement obtenues avec une orientation préférentielle le long de la direction [001]. En appliquant notamment une faible densité de charge, leurs tailles ont pu être ajustées à 52 ± 11 nm de diamètre et à moins de 300 nm de long. Les nanobâtonnets de tellure ont été alors utilisés comme précurseurs d'une étape de synthèse supplémentaire. En effet, par simple réaction d'oxydo-réduction dans une solution aqueuse d'Ag(I) ou de Cu(II), les nanostructures de Te ont été transformées respectivement en nanobâtonnets monocristallins Ag2+xTe et Cu1,75Te. Dans la dernière partie du travail, nous avons tenté d'incorporer ces nanostructures ainsi que des graphène quantum dots, à conductivité électrique élevée, dans des polymères conducteurs afin de réaliser des films hybrides flexibles. Ces derniers combinent les propriétés thermoélectriques intéressantes des nanostructures et la faible conductivité thermique de polymère conducteur comme le poly(3-hexylthiophène) (P3HT)
Among the thermoelectric materials, tellurium has a high positive Seebeck coefficient (500 μV/K) at room temperature but with a relative high thermal conductivity (3 W/m.K). Nanostructuring is a solution to improve thermoelectric performances, as the lattice part of the thermal conductivity can be independently lowered. In this work, we synthesized self-standing Te nanostructures by electrodeposition technique, taking advantage of the templating properties of ionic liquids solvents. Single crystalline hexagonal nanostructures were systematically obtained with a preferential orientation along the [001] direction. By varying the conditions of electrodeposition, the size of nanostructures can be tuned. Finally, nanorods of 52 ± 11 nm in diameter and less than 300 nm long were grown by applying a small charge density. Tellurium nanorods were used as precursors of an additional synthesis step. Indeed, by simple redox reaction in Ag(I) or Cu(II) aqueous solution, Te nanostructures have been transformed into single crystalline nanorods Ag2+xTe and Cu1.75Te respectively. In the last part of the work, we tried to incorporate these nanostructures as well as graphene quantum dots in conductive polymer in order to realize flexible hybrid films. The latter should combine the interesting thermoelectric properties of the nanostructures and the low thermal conductivity as poly(3-hexylthiophene) (P3HT)

Construction du diagramme de phase ag-as-te par atd et diffraction rx a temperature ordinaire ou a temperature variable. Mise en evidence de deux systemes quasi-lineaires (ag#2te-as#2te#3 et ag#2te-as) et de trois composes inconnus jusqu'ici (ag#1#1aste#7, agaste#2 et agas#3te#5) dans le systeme ag#2te-as et de trois composes inconnus jusqu'ici (ag#1#1aste#7, agaste#2 et agas#3te#5) dans le systeme ag#2te-as#2te#3. Description de la zone formatrice de verre. Relations entre l'etude de cette zone, l'evolution des temperatures de transition vitreuse et de cristallisation avec le diagramme de phase. Mise en evidence d'une separation de phase sub-liquidus par recuit de produits vitreux

Die vorliegende Dissertation beschäftigt sich mit der Optimierung und der Untersuchung der Materialeigenschaften des thermoelektrischen Materials Ag1-xPb18Sb1+yTe20 (englisch Lead-Antimony-Silver-Telluride: LAST). Bei LAST handelt es sich um Bleitellurid mit geringen Anteilen von Silber und Antimon, welche teilweise gelöst den Gitterplatz von Blei substituieren (Einbau in PbTe-Matrix) bzw. Fremdphasen auf m- und nm-Skala bilden. Seine hohe thermoelektrische Güte wird dabei hauptsächlich der geringen thermischen Gitterleitfähigkeit zugeschrieben, die in ersten Veröffentlichungen mit dem Auftreten nanoskaliger, Silber- und Antimonreicher Einschlüsse und deren Funktion als Phononenstreuer erklärt wurde. Das durch Schmelzsynthese hergestellte Bulkmaterial wurde im Rahmen der Arbeit durch Gefügeabbildung und Elementanalytik untersucht. In Kooperation mit den Projektpartnern sollte daraus eine Korrelation von Struktur- und Funktionseigenschaften abgeleitet, sowie eine reproduzierbare Syntheseroute entwickelt werden. Die elektronenmikroskopische Abbildung der Mikrostruktur erfolgte dabei auf zwei Größenskalen. Auf der µm-Skala wurde die Oberfläche des Bulkmaterials auf Homogenität und Zusammensetzung sowie Anteil des Fremdphasenbestands untersucht. Trotz des sehr komplexen Phasenbestandes aufgrund des quaternären Phasendiagramms und der Vielzahl relevanter Syntheseparameter konnte ein Zusammenhang zwischen Zusammensetzung (Regulierung des Silber- und Antimonanteils bzw. dessen Verhältnis), Temperbedingungen und thermoelektrischen Eigenschaften hergestellt werden. Mithilfe des detektierten Phasenbestandes konnte die Existenz einer Mischungslücke im quasibinären Phasendiagramm 2PbTe-AgSbTe2 nachgewiesen werden. Dabei bilden die Zusammensetzungen zwei der ermittelten Fremdphasen die Phasengrenzen. Die beobachtete spinodale Entmischung erzeugte eine extrem hohe Grenzflächendichte und kann somit ebenfalls einen Beitrag zur Senkung der Wärmeleitfähigkeit liefern. Für die Analyse der Mikrostruktur auf nm-Skala wurden aus der LAST-Matrix mithilfe der fokussierten Ionenstrahltechnik elektronentransparente Schnitte gefertigt. Abhängig von Temperbedingungen und dem Verhältnis von Silber und Antimon wurden auch hier fremdphasige Einschlüsse entdeckt. Dabei konnte ein optimaler Temperbereich von 500 bis 550 °C (bezogen auf einen hohen Gütewert) mit dem Auftreten dieser Einschlüsse korreliert werden. Eine allgemeine, direkte Zuordnung des Vorhandenseins von Nanostrukturen zu guten oder schlechten thermoelektrischen Eigenschaften konnte im Allgemeinen jedoch nicht nachgewiesen werden. Vielmehr wurden deutliche Hinweise gefunden, dass auch die Anordnung von Punktdefekten (Blei-Substitution durch Silber und Antimon) und ggf. Agglomerate aus Punktdefekten in der LAST-Matrix eine Rolle bei der Senkung der Wärmeleitfähigkeit spielen. Im hochaktuellen Entwicklungsgebiet selbstorganisierender Nanostrukturen mit Auswirkungen auf thermoelektrische Eigenschaften wurden substantielle Fortschritte bei der Entwicklung geeigneter, LAST-basierter Thermoelektrika für mittlere Einsatztemperaturen erzielt. Die gewonnenen Erkenntnisse dieser Arbeit zeigen Optionen zur Erzeugung hocheffizienter thermoelektrischer Bauelemente auf, wie unter anderem die bestätigte Stabilität bis zu relativ hohen Einsatztemperaturen (> 500 °C) zeigt.

The Deer Horn property is located 150 km south of Smithers in west-central British Columbia and covers 51 km². The deposit is an intrusion-related polymetallic system enriched in Au-Ag-Te-W-Cu with lesser amounts of Bi-Pb-Zn-Mo; the Au and Ag are hosted in telluride minerals. The quartz-sulfide vein system containing the main zones of Au-Ag-Te mineralization and sericite alteration is found in the hanging wall of a local, spatially related thrust fault. The age of the sericite alteration is 56 ± 2 Ma. Biotite K-Ar ages of 57–48 Ma for the nearby Nanika granodiorite intrusive suite indicates that it is likely genetically responsible for the Au-Ag-Te mineralizing event. The telluride minerals are 0.1–525 μm and commonly form whole euhedral to subhedral grains or composite grains of Ag-, Bi-, Pb-, and Au-rich telluride minerals (e.g., hessite, tellurobismuthite, volynskite, altaite, and petzite). Panchromatic cathodoluminescence imaging revealed four generations of quartz. Locally, oscillatory zoning observed in quartz II suggests the participation of hydrothermal fluids. Fine-grained veinlets of quartz III and IV intersect quartz I and II, which is evidence of at least two shearing events; veinlets of calcite intersect all generations of quartz. Three types of fluid inclusions were observed: (1) aqueous liquid and vapour inclusions (L-V); (2) aqueous carbonic inclusions (L-L-V); and (3) carbonic inclusions (V-rich). Fluid inclusions that are thought to be primary or pseudosecondary and related to the telluride mineralization were tested with microthermometry. Homogenization temperatures are 130.0–240.5 °C for L-V inclusions and 268.0–336.4 °C for L-L-V inclusions. Four of eight aqueous carbonic inclusions had solid CO2 melting temperatures from –56.8 to –62.1 °C, indicating the presence of 0.5–13.2% dissolved methane in these inclusions. Sulfur isotope analysis of ³⁴S/³²S using 20 samples of pyrite was conducted. δ³⁴S readings are close to 0 (from –1.6 to 1.6 per mil) and confirm that the sulfur is very likely magmatic/igneous in origin.
Science, Faculty of
Earth, Ocean and Atmospheric Sciences, Department of
Graduate

The 'Golden Quadrilateral' of W. Romania is a rich mineralised magmatic province hosting major porphyry-Cu and epithermal Au-Ag deposits. The mineralisation is associated with extensive magmatism emplaced along a series of NW-SE trending pull-part basins (e.g. Brad-Sacaramb, Rosia Montana). These basins developed during the Miocene, owing to the opposite sense rotation of the ALCAPA and Tisza-Dacia micro-continents, which facilitated extension-related melting of the subduction modified sub-continental lithospheric mantle. The largest of these basins, the Brad-Sacaramb basin contains extensive calc-alkaline, amphibole-rich, porphyritic andesite and dacite volcanics and sub-volcanic intrusions. LA-ICP-MS U-Pb analysis of zircons indicate that these magmatic rocks were emplaced during the Mid-Miocene 9.7 - 13.1 Ma. The rocks display a temporal geochemical evolution from early "normal" island arc magmatism (13.0 - 11.3 Ma) to adakite-like, characterised by high Sr/Y and La/Yb ratios, emplaced after 11.4 Ma. This shift in the geochemistry resulted from the cessation of plagioclase fractional crystalisation in the adakite-like magmas, coupled with the onset of amphibole crystalisation in the lower crustal MASH zone, which depleted the melts in MREEE-HREE. This change in the crystalising assemblage may have related to an increase in the dissolved water content of the melts. High zircon Ce/CE[sup]* (Ce[sup]4+/Ce[sup]3+) and less negative Eu/Eu[sup]* indicate that the high water content of these melts was accompanies by highly oxidising conditions. These conditions may have been important in preventing early sulfide saturation, a process considered to be unfavourable for the formation of economic Au and Cu deposits. K-Ar analysis from hydrothermal illites indicate that lo low to intermediate sulfidation epithermal and porphyry-Cu mineralisation in the Brad-Sacaramb basin occurred between 9.7 and 12.3 Ma. The epithermal deposits typcially developed < 0.5 Myrs after the cessation of magmatism in the immediate vicinity. One of the most prospective deposits in the basin is the Coranda-Hondol deposit (~4.8 Moz at 1.4 g/t Au). Coranda-Hondol is a telluride-rice, intermediate sulfidication Au-Ag deposit, hosted by a series of andesitic stocks (emplaced ~12.6 Ma) and siliciclastic sedimentary formation. The deposit displays a variety of mineralisation styles with pervasive pyrite dissemenations and base-metal sulfide and sulfosalt-telluride -rich veinlets. Microthermonetry of fluid inclusions indicate that the mineralisation was facilitated by a dilute (<10 wt% NaCl) low temperature (~250[degrees]C) fluid. Stable isotopes ([delta][sup]18O[sub]fluid: 6.1 - 9.4 [parts per thousand], [delta]D[sub]fluid: -39.8 - -74.3) support a magmatic source for the fluid which mixed with meteoric waters during the waning stages of the hydrothermal system ([delta][sup]18O[sub]fluid: -0.3 - 1.6). Boiling of the ore fluid during brecciation events triggered the precious metal mineralisation. The Au is predominately hosted within the crystal lattice of textual complex and isotopicalle light ([delta][sup]34S: -19/4 - -6.5 [parts per thousand]) colloform pyrite grains (up to ~450 ppm) developed within these boiling zones.

Dans le domaine de l'enregistrement reversible sur disque optique, la technologie changement de phase concurrence depuis peu la suprematie de l'enregistrement magneto-optique. Dans cette these sont abordes les aspects cles de l'enregistrement a changement de phase. Les alliages a base de tellure entrant dans l'elaboration de ces disques optiques doivent pouvoir passer de facon reversible d'un etat amorphe a un etat cristallin. L'utilisation de trois types de materiaux a ete envisagee. Il s'agit des alliages ternaires gesbte et insbte ainsi que de l'alliage quaternaire aginsbte. Une etude bibliographique des travaux effectues sur ces materiaux a changement de phase est presentee dans cette these. Diverses experimentations (caracterisation structurale, analyse de la cinetique de cristallisation) ont ete effectuees sur quelques compositions. Les comportements optique et thermique des systemes de couches minces utilises en enregistrement optique a changement de phase sont modelises. Les simulations numeriques se revelent etre un outil indispensable et permettent d'optimiser les proprietes du disque optique en ecriture et en lecture. Des prototypes de disques optiques re-inscriptibles a changement de phase sont realises. Ceux elabores a partir d'un alliage de composition ge#2sb#2te#5 donnent les meilleurs resultats. La derniere partie de cette these aborde la faisabilite de disques optiques a plusieurs niveaux d'information disposes sur un meme substrat. Des solutions sont proposees pour l'elaboration d'un disque comportant un niveau d'information pre-enregistre sur lequel est superpose un niveau d'information re-inscriptible. Le cas d'un disque optique a deux niveaux re-inscriptibles a changement de phase est aussi envisage. Ce document constitue une synthese sur l'enregistrement optique a changement de phase et les resultats obtenus permettent d'envisager des applications industrielles. Il ouvre aussi certaines perspectives sur les futures generations de disques optiques.

碩士
國立臺灣海洋大學
食品科學系
102
Konjac glucomannan (KGM) had good swelling, water vapour transmission, non-cytotoxicity as well as provided wound healing on rat. However, it had not antibacterial property, so wound could not avoid infecting from bacterium. The aim of this study was to prepare KGM film which coupled into antibacterial agent, ‘gold/silver tellurium nanotube (Au/Ag-Te NT)’ for KGM film. We evaluated the Au/Ag-Te NT KGM film for antibacterial properties, cytotoxicity, blood clotting and wound healing. Au/Ag-Te NT produced Reactive oxygen species (ROS). The structure of Au/Ag-Te NT was rod shape composed by Ag/Te and a circle granule Au on the middle area, element analysis revealed that Au formed on the middle area. The X-ray diffraction (XRD) study revealed the major crystal facets for Au/Ag-Te NT at [101], [220] and [221]. The antibacterial test suggested when concentration of Au/Ag-Te NT KGM film higher than 3.40 μg/cm2 showed a good antibacterial property for Staphylococcus aureus and Pseudomonas aeruginosa, and in vitro test revealed that concentration lower than 3.40 μg/cm2 showed a good biocompatibility for L-929 mouse fibroblasts and HaCaT human keratinocyte, while cells attached Au/Ag-Te NT KGM film by pseudopodia. Au/Ag-Te NT KGM film also promoted blood clotting from clotting assay. In addition, Au/Ag-Te NT added did not influence physical properties of KGM film. In vivo test showed that Au/Ag-Te NT KGM film provided a significant higher percentage of wound closure compared with untreated control group and commercial dressing group in the 3rd , 7th and 10th day. Furthermore, histological examination demonstrated significantly advanced granulation tissue, angiogenesis and collagen formation in the 7th, 14th and 21st day. Histologic section revealed the granulation tissue and angiogenesis for Au/Ag-Te NT KGM film at early stage of wound healing in the 3rd and 5th day. These results suggested that Au/Ag-Te NT KGM film had good antibacterial property, biocompatibility, also promoted blood clotting and wound healing. It could be applied as a potential material of wound dressing.

Chalkogensysteme gewinnen aufgrund der halbleitenden Eigenschaften immer mehr an Bedeutung und sind Ziel technischer Anwendungen. Die Aufklärung der Phasengleichgewichte binäreren und ternäreren Chalkogensystemen sind für diesen Forschungszweig von besonderem Interesse. Einen wichtigen Beitrag zur Aufklärung der Phasengleichgewichte liefert die Bestimmung thermodynamischer Daten und die experimentellen Untersuchungen von Zustandsdiagrammen. Reaktivitätsuntersuchungen der intermetallischen und chalkogenhaltigen Verbindungen führen zu neuen Kenntnissen zum Reaktionsverhalten schwerzugängiger Verbindungen. Um eine schnelle und phasenreine Herstellung zu erzielen, kann die mechanische Synthese angewandt werden. Ein anderer Aspekt zur Synthese liegt auf dem Gebiet der Strukturanalyse von Chalkogenidometallaten. Mittels Hydrothermalsynthese werden verschiedene organische Template als Reaktionspartner eingesetzt, um neue Strukturen zu entdecken. Im Rahmen dieser Arbeit wurden Differenz-Thermoanalyse und röntgenographische sowie mikroanalytische Methoden und mikroskopische Gefügeuntersuchungen an den chalkogenhaltigen ternären Systemen Silber-Gallium-Tellur und Zinn-Antimon-Selen durchgeführt.

碩士
國立清華大學
材料科學工程學系
104
Bismuth telluride based compounds have been widely employed in commercial thermoelectric cooling/generation devices owing to their decent thermoelectric properties near room temperature regime. Some trace elements, such as iodine, silver and copper, are commonly added to modulate transport properties of bismuth telluride based compounds. In addition, the figure of merit of thermoelectric materials can be effectively promoted by electrical stressing. In this study, Ag elements were driven into a piece of Bi0.5Sb1.5Te3 sample in contact to a silver foil when applying a high density of electric current at elevated temperature. We demonstrate that Ag concentration profile can be tailored unidirectionally by electrical stressing. The evolution of Ag concentration profile was measured by energy dispersive spectroscopy and modeled based on an electromigration-enhanced diffusion mechanism. It is found that the supersaturated Ag may react with excess Te by forming Ag2Te secondary phase near the grain boundaries. With continuous electrical stressing, Ag2Te will be decomposed by hole wind force. These Ag elements migrate from anode side to cathode side of the Bi0.5Sb1.5Te3 sample under an electric field and a hole wind driving force simultaneously. Due to multiple Ag defect sites, such as interstitials, Sb lattice and grain boundaries, Ag atoms may migrate through different paths and lead to a superposition of different diffusion pockets. A physical mechanism is proposed to explain the evolution of Ag concentration profiles in the electrically stressed Bi0.5Sb1.5Te3 compounds. The effects of electrical stressing and Ag doping on the defect mechanism and thermoelectric transport properties of Bi0.5Sb1.5Te3 compounds are investigated.

碩士
國立清華大學
材料科學工程學系
97
Telluride-based thermoelements can react with Sn-contained solders and form SnTe intermetallic compounds that may deteriorate electrical and mechanical properties of soldered junctions. In addition to the diffusion barrier approach, chemical composition of the solder alloys could be adjusted to suppress or decrease the rate of SnTe formation. In this study the effects of Ag addition (0.1, 1, 3.5, 5 wt.%) in pure Sn on the interfacial reaction between molten solder and Te substrate is explored. It is found that the thickness of SnTe compound is reduced after soldering reaction when more than 1 wt.% Ag is added into Sn solder. For an identical time of 300 minutes, the SnTe thickness is decreased from 570 �慆 for a pure Sn and Te reaction to only 11 �慆 for a Sn-Ag alloy and Te reaction. The suppression of SnTe compound formation is associated with the presence of Ag3Te2 and Ag2Sn3Te5 ternary compounds that are located in between the SnTe compound and the Te substrate. However, the rate at which SnTe initially forms is enhanced as the formation of SnTe originates from two different sources: byproduct of molten solder and Te substrate (Ag3Te2 and SnTe) and excess SnTe when Ag3Te2 further reacts with Sn in the solder (Ag2Sn3Te5 and SnTe). When the thickness of SnTe remains essentially constant, SnTe undergoes a conservative ripening, resulting in surface roughening. Based on top-view scanning electron microscopy micrographs, it is calculated that SnTe particles size increase approximately with the cube root of reaction time.

碩士
國立臺灣大學
材料科學與工程學研究所
85
Evaporated binary chalcogenide Sb-Te thin films withAg(0 ∼11.4 at%), Cu(0∼10.8 at%) addition were investigated. The crystallization process of these (Sb-Te)-based amorphous films were investigated by isothermally treated at 100℃, 200℃and 300℃, respectively.The structure of these alloy films was confirmed by X-ray diffraction(XRD), Tx and Tm were mea- sured by differential scanning calorimetry(DSC), and the microstructures were observed by transmission electron microscopy(TEM). The reflectivities of these films were measured by spectrophotometer. We found that all of the as-deposited films had an amorphous structure with many naro-crystallines particles, and the crystallization temperature (Tc) of Ag8.3Sb34.9Te56.8 and Cu10.8Sb33.2Te56.0 were more than125℃,172℃, respectively. It indicates that doping Ag or Cu can improve the room temp- erature stability of Sb-Te alloy films. The contrast ratio of as-deposited Ag3.2Sb35.8Te61.0 and Cu4.1Sb37.4Te58.5 are more than 42% and 37% after isothermally treated at 300℃. It indicates that these compositions will be suitable for phase change optical recording materials.

碩士
國立清華大學
材料科學工程學系
101
Thermoelectric materials that can convert thermal energy to electric energy and vice versa have been applied in applications of waste-heat recovery and refrigeration. In most thermoelectric devices, the directions of temperature gradient and electric field are the same. In this research, a longitudinal temperature gradient introduces a lateral electric field, which is called transverse Seebeck effect, has been found in hot-pressed Bi0.4Sb1.6Te3 sample with non-uniform Ag doping. The property of uncoupling heat flow and electric flow directions may provide ideas of developing different device applications. The thermoelectric properties of both Ag-doped and non-doped Bi-Sb-Te samples have been determined experimentally. The transverse Seebeck effect has been found in non-uniform Ag doping sample with asymmetric transverse voltage in hot side and cold side by a self-designed transverse Seebeck measurement apparatus. The experimental and simulation results shows that transverse Seebeck effect should have been referred to different voltage drops between Ag-doped region and non-doped region. Asymmetric transverse voltage may be related to larger charged carrier flow at high temperature side, which has been expressed as an empirical term jemp in simulation. The experimental and simulation results have applied a preliminary model of transverse thermoelectric effect specimen fabrication. The transverse Seebeck coefficient has reached 120 μV/K in simulation.

碩士
國立清華大學
材料科學工程學系
102
Bismuth telluride has been considered as a promising candidate for thin-film thermoelectric (TE) devices due to its superior thermoelectric properties at room temperature regime. Generally, a good thermoelectrics requires a large Seebeck coefficient, a high electrical conductivity and a low thermal conductivity, which can be achieved by optimization of carrier concentration. In this study, both P-type Bi-Sb-Te and N-type Bi-Se-Te thin films were deposited on polyimide substrates by RF magnetron sputtering. A step-like Ag overlayer was evaporated partially on top of the TE films using e-gun deposition. Silver atoms were driven into the TE films by thermal annealing to form a non-uniform doping profile. The research goal is to investigate the effect of Ag doping on thermoelectric transport properties of P- and N-type Bi-Te thin films. The results show that Ag elements have non-uniform distributions in both in-plane and out-of-plane directions of TE thin films. The Ag doping results in 4.7 times increase of carrier concentration in P-type films and 1.75 times increase in N-type films. It suggests that AgSb acceptor-like defects provide large amount of hole carriers in P-type films, while the excessive Te atoms occupy Bi site by forming TeBi donor-like defects counterbalances the contribution of AgBi defects in N-type films. The scanning Seebeck results reveal an asymmetric Seebeck effect in the P-type films. The P-type films with Ag doping achieve a maximum Seebeck coefficient of 285 µV/k, an output current of 5.41 µA and a power density of 10.5 nW/cm2 when heat flows in the direction from high to low doping region of P-type films with a distance between two probes of 5.3 mm. The N-type films with Ag doping do not show obviously asymmetric effect, which is likely attributed to small difference in carrier concentration between doped and non-doped region.

碩士
國立中山大學
材料與光電科學學系研究所
104
Thermoelectricity converts waste heat into electricity directly and reversibly. Ag-Ga-Te ternary system is a promising thermoelectric system, mainly due to the p-type chalcopyrite AgGaTe2. The peak zT(figure-of-merit) value of AgGaTe2 is around 1.2 at the region of 600-800oC. Phase diagram provides materials’ fundamental information, and is important for understanding the phase relations and controlling the microstructures. However, for the Ag-Ga-Te system, only limited information is available. This study is keen to determine the phase relations of Ag-Ga-Te system at 650oC,including: (1) experimentally determine the isothermal section of Ag-Ga-Te system at 650oC by thermally annealing ternary Ag-Ga-Te alloys, (2) experimentally determine the liquidus projection of Ag-Ga-Te system by directly air-cooling or water-quenching ternary Ag-Ga-Te alloys (3) synthesize ternary Ag-Ga-Te alloys with specific composition and measure the thermoelectric properties such as Seebeck coefficient, electrical resistivity and thermal conductivity at elevated temperatures. The homogeneity region of the ternary compound AgGaTe2 at 650oC as well as its primary solidification region are mapped. Moreover, the liquidus projection of Ag-Ga-Te system contains two miscibility gaps that originated from its constituent Ag-Te and Ga-Te systems, and is found to be divided by primary solidification phases Ag2Te, AgGaTe2 and Ag2Ga.

碩士
國立清華大學
電子工程研究所
89
Base on the repeatable、convenient、high storage capacity，and the low cost cause of the mass mass production，the phase change optic disc already become the major high storage capacity media, obviously. And recently, the DVD system was developed very fast，the high storage density was the major trend of the phase change optic disc. The Ge-Sb-Te material system can’t follow this trend, this is also why we choose the Ag-In-Sb-Te material system. But the Ag-In-Sb-Te material system was found the cyclability only have 103 times, evident that this system’s cyclability is lower than former system’s. According to literatures, the reason of cyclability can’t be promoted were very mush, and we aimed at the phase segregation of Te, to study how the improve this problem. We try to used the way whose address in literatures, when sputtering the Ag-In-Sb-Te target mixed the nitrogen gas, promoted the Te and N bond together. The fetter force would confine Te not to segregated self, to achieve our purpose. More advance, we didn’t use the RF power in Sputter chamber to decompose nitrogen, we decompose the nitrogen molecule directly by microwave outside the chamber, attempt to decompose the nitrogen molecule become atomic nitrogen to promote the bonding probability and it’s efficiency. We using the results of DSC analysis to operated by Kissinger equation to prove the method of Microwave enhance N2 atoms increase the activation energy of recroding layer approximately 0.3~0.6 eV, and the Absorbance results of FTIR measure to prove the Te-N bond increase. In cyclability, a four layer structure of the phase chage disc used in the analysis is polycarbonate substrate / ZnS:SiO2 100nm / Ag-In-Sb-Te 200nm / ZnS:SiO2 180nm/ Al-Cr 100nm. The results of the disc dynamic test also observe our Microwave enhance N2 atoms method was 3times than the RF enhance N2 molecule method at cyclability.

碩士
國立清華大學
物理學系
87
Aiming to obtain high density of directly overwritable optical discs, the optical phase change material has been studied. Ag-In-Sb-Te multinary alloy is one of the most investigated materials as phase change recording media. In this work, all films were deposited by R. F. magnetron sputtering. Ag-In-Sb-Te recording film was sandwiched between two ZnS 80 — SiO2 20 at % layers. And the layer structure of the phase change disc used in the experiment is polycarbonate substrate / ZnS:SiO2 115 nm / Ag-In0Sb-Te 20 nm / ZnS:SiO2 15 nm / Al 120 nm which is determined by optical and thermal simulation.The composition of the Ag-In-Sb-Te alloy films was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES), and the crystalline temperature of the thin films was analyzed by differential scanning calorimetry (DSC). Characterization of the crystalline structure had been performed using X-ray diffraction analysis and scanning electron microscopy. The optical properties were determined using optical spectrometer in the wavelength range from 400 to 850 nm. A higher carrier-to-noise ratio was obtained. The results indicates that Ag-In-Sb-Te based optical phase change discs could be a promising candidate for high density rewritable disc system.

博士
國立清華大學
工程與系統科學系
106
Thermoelectric applications have attracted increasing interest recently due to its capability of converting waste heat into electricity without hazardous emissions. Compare with the bulk materials, many researches have been able to experimentally reveal that physical properties were different in low dimensional materials. In nanoscale, the enhancement of the thermoelectric efficiency by the reduction of lattice thermal conductivity (κph) due to phonon blocking. In addition, tuning the position of Femi level to achieve the optimum power factor (σα2) by doping concentration effect is another strategy to improve the zT value. In this thesis, we have synthesized the high quality and large-scale nanomaterials based on the mentioned concept and divided into two systems to investigate the thermoelectric properties. In the first part, the thermoelectric properties of a Bi0.8Sb1.2Te2.9 nanowire (NW) were in-situ studied as it was trimmed from 750 down to 490 and 285 nm in diameter by a focused ion beam. While electrical and thermal conductivities both indubitably decrease with the diameter reduction, the two physical properties clearly exhibit different diameter dependent behaviors. The much lower thermal conductivities were observed as compared with the theoretical prediction of Callaway model. The consequence indicates that in addition to the size effect, extra phonon scattering of defects created by Ga ion irradiation was attributed to the reduction of thermal conductivities. The size dependence of Seebeck coefficient and figure of merit (zT) show the maximum at 750 nm, then decrease linearly with size decrease. The study not only provides the thoroughly understanding of the size and defect effects on the thermoelectric properties but also proposes a possible method to manipulate the thermal conductivity of NWs via ion irradiation. In the second part, A two steps, surfactant-free solution growth process was utilized to synthesize p-type Ag doped SnSe nanocrystals in gram quantities. The formation mechanism of SnSe nanocrystals studied by the high resolution transmission electron microscopy. A clear phase transition near 800 K was discovered in the temperature dependence of thermal conductivity. The thermoelectric properties of SnSe pellets prepared by spark plasma sintering, exhibit a significant increase of zTmax (0.8 at 850 K) in the 3 % Ag doped SnSe. The zTmax value is about 40 % higher than that of the prinstine SnSe. The consequence is mainly attributed to the enhancement of carrier concentration and power factor by Ag doping. Our results demonstrate that this facile chemical method is amenable to fabricate high quality SnSe nanocrystals and might also be applied to other anisotropic crystalline materials.

碩士
國立中央大學
物理研究所
100
Thermal property plays an important role in the characteristic of bulk materials, the measurement is necessary to understand the underlying physics that leads to certain phenomena. However, the accurate measurement and characterization of thermal property of bulk materials can pose many challenges. In order to improve and modify the measurement of thermal property, two investigations of different region have been reported in this thesis. A thermal rectifier is the heat transfer analog to the familiar electrical diode. And thermal rectification is very interest because of its potential to open up new ground for the control of heat transport. In the first study, the thermal rectification in a two–terminal bulk material has been measured. We have prepared pure metal, Au, and polycrystalline insulator, Al2O3, as the two-terminal bulk material which glued with Ag paste and ductile indium wire. Hereby, we made an experimental system (steady state method) which is essentially a measurement of the heat flow through the sample to detect the thermal rectification. The observed thermal conductance of Au-X-Al2O3 (X=Ag, In) in the forward and reverse direction is no rectifying effect with the temperature difference of 10 K between heat baths. We have calculated thermal rectification using thermal conductivity data and found that the calculation is in discordancy with experimental data. In the second study, thermoelectric materials have great potential for use in solid-state power generation and cooling devices. These devices have numerous advantages over conventional methods, but they are not widely used today is because of the poor efficiency. Much of the effort in the thermoelectric field has been directed at improving the efficiency of thermoelectric devices. In order to improve the efficiency of these devices, solid understanding on the properties of the materials from which they are made is required. This usually involves extensive measurement of the thermal and electrical properties of the material. In many cases, the measurement of characteristics of bulk materials is necessary to understand the underlying physics the leads to certain phenomena. Measurement of the properties of bulk material often involves working with specific samples because of synthesis of bulks materials. The measurement of the thermal conductivity of the specific size samples has been a little bit formidable challenge for many years due to the low thermal conductance and the low mechanical strength of the sample. One of the methods developed to address this problem is the steady state method. The steady state method was modified and the results and description of the modified technique are presented in this thesis. Improvements were made to the sample stage and measurement system. The system was evaluated with standards to confirm the validity of the measurements. Investigation of thermoelectric material, the thermal conductivity of CuxBi0.5Sb1.5Te3 (x=0.01) at room temperature is 1.25 W/m-K by steady state method. In addition, the Seebeck coefficient measurement was already established, and the sample stage and measurement are similar to the steady state method. However, our research included an investigation of all properties of thermoelectric materials of CuxBi0.5Sb1.5Te3, and the figure of merit (ZT) has been reported.

博士
國立清華大學
化學工程學系
101
Thermoelectric material has been recognized as a promising candidate in the application of sustainable energy. The quaternary Pb-Ag-Sb-Te system has attracted much attention because it contains high-zT AgPb18SbTe20 (zT>2 at 800K), PbTe and AgSbTe2. The phase stability, microstructural evolution and resultant electrical/thermal transport properties of quaternary Pb-Ag-Sb-Te and the constituent systems (Ag-Sb-Te, Pb-Sb-Te and Ag-Pb-Te) are investigated. The liquidus projections of both ternary Ag-Pb-Te and Pb-Sb-Te systems are constructed. In particular, the Ag-Pb-Te ternary-eutectic alloy, with composition of Ag-4.3at%Pb-62.6at%Te, forms a partially aligned nano-sized lamellar microstructure, which comprises both the PbTe and Ag5Te3 phases, and an additional dotted PbTe of 200-600nm. This particular ternary-eutectic alloy is unidirectionally solidified using the Bridgman method, resulted in a nanostructured composite with an extremely low thermal conductivity(κ) of 0.3 W/mK and a zT peak of 0.41 at 400K. The phase diagrams of ternary Ag-Sb-Te system are constructed as well, including the 400℃ and 250℃ isothermal sections and the liquidus projection. The ternary AgSbTe2 is stabilized at 400℃ but not at 250℃, with homogeneity region of 49.0-53.0at%Te and 28.0-30.0at%Sb. The nano-scaled microstrucutre and crystal structures of the non-stoimeteric AgSbTe2 are analyzed by the transmission electron microscope (TEM). In particular, an ordered array of nano-wire microstructure, comprising a 200nm Ag2Te and a matrix of AgSbTe2+δ-Sb2Te, was resulted from a Class I reaction: L=AgSbTe2+Ag2Te+δ-Sb2Te with liquid composition of Ag-40at%Sb-36.0at%Te at 496.5℃. To understand and guide production of uniform bulk samples of this composite, the liquidus projection of quaternary Pb-Ag-Sb-Te system at 36.0at%Te isoplethal section is constructed experimentally using quenched samples. High-resolution transmission electron microscopy (TEM) confirms that these three phases are simultaneously present at the nanometer scale. Furthermore, unidirectional solidification experiments of the ternary eutectic alloy using the Bridgman method are carried out to examine the alloy's solidification behaviors. Pb-alloyed AgSbTe2 (PbxAg20Sb30-xTe50, x=3,4,5 and 6) are also unidirectionally solidified using the modified Bridgman method. The as-solidified 5at%Pb and 6at%Pb alloys, which exhibit high phase purity of AgSbTe2, contain grain-boundary inhomogeneity and nano-precipitates of δ-Sb2Te, leading to an extremely low thermal conductivity (κ) of 0.3-0.4 W/mK. A peak zT of 0.7-0.8 is found in as-solidified 5at% specimen at 425K. However, after annealing at 673K, the zT peak of 5at%Pb(annealed) decreases to 0.4, presumably due to increase in grain size and decrease in inhomogeneity.

Die vorliegende Dissertation beschäftigt sich mit der Optimierung und der Untersuchung der Materialeigenschaften des thermoelektrischen Materials Ag1-xPb18Sb1+yTe20 (englisch Lead-Antimony-Silver-Telluride: LAST). Bei LAST handelt es sich um Bleitellurid mit geringen Anteilen von Silber und Antimon, welche teilweise gelöst den Gitterplatz von Blei substituieren (Einbau in PbTe-Matrix) bzw. Fremdphasen auf m- und nm-Skala bilden. Seine hohe thermoelektrische Güte wird dabei hauptsächlich der geringen thermischen Gitterleitfähigkeit zugeschrieben, die in ersten Veröffentlichungen mit dem Auftreten nanoskaliger, Silber- und Antimonreicher Einschlüsse und deren Funktion als Phononenstreuer erklärt wurde. Das durch Schmelzsynthese hergestellte Bulkmaterial wurde im Rahmen der Arbeit durch Gefügeabbildung und Elementanalytik untersucht. In Kooperation mit den Projektpartnern sollte daraus eine Korrelation von Struktur- und Funktionseigenschaften abgeleitet, sowie eine reproduzierbare Syntheseroute entwickelt werden. Die elektronenmikroskopische Abbildung der Mikrostruktur erfolgte dabei auf zwei Größenskalen. Auf der µm-Skala wurde die Oberfläche des Bulkmaterials auf Homogenität und Zusammensetzung sowie Anteil des Fremdphasenbestands untersucht. Trotz des sehr komplexen Phasenbestandes aufgrund des quaternären Phasendiagramms und der Vielzahl relevanter Syntheseparameter konnte ein Zusammenhang zwischen Zusammensetzung (Regulierung des Silber- und Antimonanteils bzw. dessen Verhältnis), Temperbedingungen und thermoelektrischen Eigenschaften hergestellt werden. Mithilfe des detektierten Phasenbestandes konnte die Existenz einer Mischungslücke im quasibinären Phasendiagramm 2PbTe-AgSbTe2 nachgewiesen werden. Dabei bilden die Zusammensetzungen zwei der ermittelten Fremdphasen die Phasengrenzen. Die beobachtete spinodale Entmischung erzeugte eine extrem hohe Grenzflächendichte und kann somit ebenfalls einen Beitrag zur Senkung der Wärmeleitfähigkeit liefern. Für die Analyse der Mikrostruktur auf nm-Skala wurden aus der LAST-Matrix mithilfe der fokussierten Ionenstrahltechnik elektronentransparente Schnitte gefertigt. Abhängig von Temperbedingungen und dem Verhältnis von Silber und Antimon wurden auch hier fremdphasige Einschlüsse entdeckt. Dabei konnte ein optimaler Temperbereich von 500 bis 550 °C (bezogen auf einen hohen Gütewert) mit dem Auftreten dieser Einschlüsse korreliert werden. Eine allgemeine, direkte Zuordnung des Vorhandenseins von Nanostrukturen zu guten oder schlechten thermoelektrischen Eigenschaften konnte im Allgemeinen jedoch nicht nachgewiesen werden. Vielmehr wurden deutliche Hinweise gefunden, dass auch die Anordnung von Punktdefekten (Blei-Substitution durch Silber und Antimon) und ggf. Agglomerate aus Punktdefekten in der LAST-Matrix eine Rolle bei der Senkung der Wärmeleitfähigkeit spielen. Im hochaktuellen Entwicklungsgebiet selbstorganisierender Nanostrukturen mit Auswirkungen auf thermoelektrische Eigenschaften wurden substantielle Fortschritte bei der Entwicklung geeigneter, LAST-basierter Thermoelektrika für mittlere Einsatztemperaturen erzielt. Die gewonnenen Erkenntnisse dieser Arbeit zeigen Optionen zur Erzeugung hocheffizienter thermoelektrischer Bauelemente auf, wie unter anderem die bestätigte Stabilität bis zu relativ hohen Einsatztemperaturen (> 500 °C) zeigt.

碩士
國立成功大學
材料科學及工程學系碩博士班
92
This study aims to prepare Ag-In-Sb-Te amorphous thin films with various Ag contents by sputtering and investigates the effect of Ag content on the phase stability and optical properties. 　　In order to evaluate the phase stability of amorphous films, differential Scanning Calorimeter（DSC） is applied to examine the exothermic behavior of crystallization under different heating rates. Also, the Kissinger method is used to calculate the activation energy for crystallization. The results show that an increase in Ag content causes a higher activation energy, and thus makes the crystallization process more difficult. This implies that increasing Ag content can improve the phase stability of Ag-In-Sb-Te amorphous films. 　　The results of phase analysis by X-ray diffraction show that after the crystallizing heat treatment held at different temperatures (160 oC、180 oC、200oC、250 oC、300 oC), the microstructure of the Ag-In-Sb-Te films is a mixture of Sb and small amount of AgInTe2. Notably, AgInTe2 can not be detected in the 6 Ag and 13Ag specimens suffering the heat treatment at 200oC. 　　Optical properties of the films with amorphous and crystallized states are also investigated. The results indicate the reflectivity of the amorphous of 6Ag and 13Ag films are similar but 8Ag specimen is lower others, however, those of the crystallized films decreases in turn from 6Ag, 8Ag and 13 Ag. Therefore, reflectivity contrast of the 8Ag sample is greater than that of the 6Ag specimen, which is in turn greater than that of the 13Ag specimen. 　　From the aforementioned properties, this study proposes that the Ag-In-Sb-Te thin film with the Ag content of 8 at%, which possesses appropriate phase stability and superior optical performance, is recommended for application as the recording material for CD-RW disc.

Chalcogenide glasses are a class of covalent amorphous semiconductors with interesting properties. The presence of short-range order and the pinned Fermi level are the two important properties that make them suitable for many applications. With flash memory technology reaching the scaling limit as per Moore’s law, alternate materials and techniques are being researched at for realizing next generation non-volatile memories. Two such possibilities that are being looked at are Phase Change Memory (PCM) and Programmable Metallization Cell (PMC) both of which make use of chalcogenide materials. This thesis starts with a survey of the work done so far in realizing PCMs in reality. For chalcogenides to be used as a main memory or as a replacement to FLASH technology, the electrical switching parameters like switching voltage, programming current, ON state and OFF state resistances, switching time and optical parameters like band gap are to be considered. A survey on the work done in this regard has revealed that various parameters such as chemical composition of the PC material, nature of additives used to enhance the performance of PCM, topological thresholds (Rigidity Percolation Threshold and Chemical Threshold), device geometry, thickness of the active volume, etc., influence the electrical switching parameters. This has motivated to further investigate the material and experimental parameters that affect switching and also to explore the possibility of multi level switching. In this thesis work, the feasibility of using two chalcogenide systems namely Si15Te85-xGex and Ge15Te85-xAgx in the form of amorphous thin films for PCM application is explored. In the process, electrical switching experiments have been carried out on thin films belonging to these systems and the results obtained are found to exhibit some interesting anomalies. Further experiments and analysis have been carried out to understand these anomalies. Finally, the dynamics of electrical switching has been investigated and presented for amorphous Si15Te85-xGex thin films. From these studies, it is also seen that multi state switching/multiple resistance levels of the material can be achieved by current controlled switching, the mechanisms of which have been further probed using XRD analysis and AFM studies. In addition, investigations have been carried out on the electrical switching behavior of amorphous Ge15Te85-xAgx thin film devices and optical band gap studies on amorphous Ge15Te85-xAgx thin films. Chapter one of the thesis, gives a brief introduction to the limitations in existing memory technology and the alternative memory technologies that are being researched, based on which it can be inferred that PCM is a promising candidate for the next generation non volatile memory. This chapter also discusses the principle of using PCM to store data, realization of PCM using chalcogenides, the material properties to be considered in designing PCM, the trade offs in the process of design and the current trends in PCM technology. Chapter two provides a brief review of the electrical switching phenomenon observed in various bulk chalcogenide glasses, as electrical switching is the underlying principle behind the working of a PCM. In the process of designing a memory, many parameters like read/write operation speed, data retentivity and life, etc., have to be optimized for which a thorough understanding on the dependence of electrical switching mechanism on various material parameters is essential. In this chapter, the dependence of electrical switching on parameters like network topological thresholds and electrical and thermal properties of the material is discussed. Doping is an efficient way of controlling the electrical parameters of chalcogenides. The nature of dopant also influences switching parameters and this also is briefly discussed. Chapter three provides a brief introduction to the different experimental techniques used for the thesis work such as bulk chalcogenide glass preparation, preparation of thin amorphous films, measurement of film thickness, confirmation of amorphous nature of the films using X-Ray Diffraction (XRD), electrical switching experiments using a custom made setup, crystallization study using XRD and Atomic Force Microscopy (AFM) and optical band gap studies using UV-Vis spectrometer. Vt is an important parameter in the design of a PCM. Chapter four discusses the dependence of Switching voltage, Vt, on input energy. It is already established that the Vt is influenced by the composition of the base glass, nature of dopants, thickness of films and by the ambient temperature. Based on the results of electrical switching experiments in Si15Te74Ge11 amorphous thin films a comprehensive analysis has been done to understand the kinetics of electrical switching. Chapter five discusses a current controlled crystallization technique that can be used to realize multi-bit storage with a single layer of chalcogenide material. In case of PCM, data is stored as structural information; the memory cell in the amorphous state is read as data ‘0’ and the memory cell in crystalline state is read as data ‘1’. This is accomplished through the process of electrical switching. In order to increase the memory density or storage density, multi-bit storage is being probed at by having multiple layers of chalcogenide material. However, with this technique, the problems of inter-diffusion between different layers cannot be ruled out. In this thesis work, a current controlled crystallization technique has been used to achieve multiple stable resistance states in Si15Te75Ge10 thin films. Chapter six discusses the mechanism behind multi state switching exhibited by certain compositions of Si15Te85-xGex thin films. Crystallization studies on certain Si15Te85-xGex films have been carried out using XRD and AFM to understand the phenomenon of multiple states. The results of these experiments and analysis are presented in this chapter. Chapter seven discusses the results of electrical switching experiments and optical band gap studies on amorphous Ge15Te85-xAgx thin films. Chapter eight gives the conclusion and scope for future work.

Chalcogenide glasses are a class of covalent amorphous semiconductors with interesting properties. The presence of short-range order and the pinned Fermi level are the two important properties that make them suitable for many applications. With flash memory technology reaching the scaling limit as per Moore’s law, alternate materials and techniques are being researched at for realizing next generation non-volatile memories. Two such possibilities that are being looked at are Phase Change Memory (PCM) and Programmable Metallization Cell (PMC) both of which make use of chalcogenide materials. This thesis starts with a survey of the work done so far in realizing PCMs in reality. For chalcogenides to be used as a main memory or as a replacement to FLASH technology, the electrical switching parameters like switching voltage, programming current, ON state and OFF state resistances, switching time and optical parameters like band gap are to be considered. A survey on the work done in this regard has revealed that various parameters such as chemical composition of the PC material, nature of additives used to enhance the performance of PCM, topological thresholds (Rigidity Percolation Threshold and Chemical Threshold), device geometry, thickness of the active volume, etc., influence the electrical switching parameters. This has motivated to further investigate the material and experimental parameters that affect switching and also to explore the possibility of multi level switching. In this thesis work, the feasibility of using two chalcogenide systems namely Si15Te85-xGex and Ge15Te85-xAgx in the form of amorphous thin films for PCM application is explored. In the process, electrical switching experiments have been carried out on thin films belonging to these systems and the results obtained are found to exhibit some interesting anomalies. Further experiments and analysis have been carried out to understand these anomalies. Finally, the dynamics of electrical switching has been investigated and presented for amorphous Si15Te85-xGex thin films. From these studies, it is also seen that multi state switching/multiple resistance levels of the material can be achieved by current controlled switching, the mechanisms of which have been further probed using XRD analysis and AFM studies. In addition, investigations have been carried out on the electrical switching behavior of amorphous Ge15Te85-xAgx thin film devices and optical band gap studies on amorphous Ge15Te85-xAgx thin films. Chapter one of the thesis, gives a brief introduction to the limitations in existing memory technology and the alternative memory technologies that are being researched, based on which it can be inferred that PCM is a promising candidate for the next generation non volatile memory. This chapter also discusses the principle of using PCM to store data, realization of PCM using chalcogenides, the material properties to be considered in designing PCM, the trade offs in the process of design and the current trends in PCM technology. Chapter two provides a brief review of the electrical switching phenomenon observed in various bulk chalcogenide glasses, as electrical switching is the underlying principle behind the working of a PCM. In the process of designing a memory, many parameters like read/write operation speed, data retentivity and life, etc., have to be optimized for which a thorough understanding on the dependence of electrical switching mechanism on various material parameters is essential. In this chapter, the dependence of electrical switching on parameters like network topological thresholds and electrical and thermal properties of the material is discussed. Doping is an efficient way of controlling the electrical parameters of chalcogenides. The nature of dopant also influences switching parameters and this also is briefly discussed. Chapter three provides a brief introduction to the different experimental techniques used for the thesis work such as bulk chalcogenide glass preparation, preparation of thin amorphous films, measurement of film thickness, confirmation of amorphous nature of the films using X-Ray Diffraction (XRD), electrical switching experiments using a custom made setup, crystallization study using XRD and Atomic Force Microscopy (AFM) and optical band gap studies using UV-Vis spectrometer. Vt is an important parameter in the design of a PCM. Chapter four discusses the dependence of Switching voltage, Vt, on input energy. It is already established that the Vt is influenced by the composition of the base glass, nature of dopants, thickness of films and by the ambient temperature. Based on the results of electrical switching experiments in Si15Te74Ge11 amorphous thin films a comprehensive analysis has been done to understand the kinetics of electrical switching. Chapter five discusses a current controlled crystallization technique that can be used to realize multi-bit storage with a single layer of chalcogenide material. In case of PCM, data is stored as structural information; the memory cell in the amorphous state is read as data ‘0’ and the memory cell in crystalline state is read as data ‘1’. This is accomplished through the process of electrical switching. In order to increase the memory density or storage density, multi-bit storage is being probed at by having multiple layers of chalcogenide material. However, with this technique, the problems of inter-diffusion between different layers cannot be ruled out. In this thesis work, a current controlled crystallization technique has been used to achieve multiple stable resistance states in Si15Te75Ge10 thin films. Chapter six discusses the mechanism behind multi state switching exhibited by certain compositions of Si15Te85-xGex thin films. Crystallization studies on certain Si15Te85-xGex films have been carried out using XRD and AFM to understand the phenomenon of multiple states. The results of these experiments and analysis are presented in this chapter. Chapter seven discusses the results of electrical switching experiments and optical band gap studies on amorphous Ge15Te85-xAgx thin films. Chapter eight gives the conclusion and scope for future work.