Добірка наукової літератури з теми "Diffusion saturation processes"

The phenomenological theory for describing high-temperature interaction between metal and diluted gaseous medium has been developed. The theory is based on the assumption of duplex contact layer existence in the vicinity of interface (with relative thickness 2 d), where chemical reactions and processes of gas component migration occur. The non-stationary conditions of mass transfer at the interface are described involving effective average parameters. These conditions allow considering a wide spectrum of boundary diffusion phenomena (in a short and prolonged time ranges), in order to describe the kinetics of accumulation of diffusing component close to the interface. The description of the kinetic of gaseous saturation of metal (nitriding and borating) in the diluted medium becomes a partial proof of the suggested models. In order to approach the diffusion phenomena, boundary conditions, which contain, besides the coordinate derivative of concentration function, also the time derivative, were suggested. The derived equations describe the time dependence of change of surface concentration of gaseous component, the kinetics of its accumulation owing to chemical reaction, the specimen mass change owing to both, the diffusive addition dissolution in metal and its chemical interaction. The role of temperature is also discussed.

A mathematical model of amperometric response for a polymer-modified electrode system has been developed. The model is based on nonstationary diffusion equations containing a nonlinear term related to Michaelis–Menten kinetics of the enzymatic reaction. In particular, the interplay between chemical reaction and substrate diffusion is specifically taken into account. The limiting situations of catalytic site unsaturation and site saturation are considered. The analytical solutions for substrate concentration and transient current for both steady and nonsteady-state are obtained using Danckwerts' relation and variable and separable method. An excellent agreement with the previous analytical results are noted. The combined analytical set of solution of steady-state current in all the nearest sites is also described in a case diagram. A general simple analytical approximate solution for steady-state current for all values of α is also given. A two-point Padé approximation is also derived for the nonsteady-state current for all values of saturation parameter α. Limiting case results (α ≪ 1 and α ≫ 1) are compared with Padé approximation results and are found to be in good agreement.

The existing basic nitriding methods do not exploit many of the potential opportunities. To intensify it and increase its efficiency, this paper considers and proposes a new method of low-temperature nitriding, which makes it possible to optimise the classical process and reduce the consumption of ammonia from 2 to 10 times, reduce the nitriding time by 4-6.5 times with an increase in the thickness of the diffusion layer by 2-6 times without reducing the physical and mechanical properties. During the experiment, gas-cyclic and thermogasocyclic nitriding of armco iron was carried out on an experimental setup, which included a system for monitoring and maintaining the temperature in the working volume, a gas supply system, monitoring the flow rate and degree of ammonia dissociation, cleaning and drying gas, as well as two electromagnetic gas valves controlled from the control panel, allowing the processes to be carried out automatically. As a result, a new method of low-temperature nitriding has been developed – under the conditions of a thermo-gas cycle. This method consists in periodic alternation of saturation cycles during flow nitriding and resorption of the nitrided layer with the maximum possible decrease in the saturating capacity of the atmosphere. The proposed new method of thermogasocyclic nitriding is a new, effective hardening technology that allows to reduce the consumption of saturating gas and emissions into the atmosphere by up to 10 times, the nitriding time by 4-6.5 times, and also to increase the thickness of the diffusion layer by 2-6 times without reducing the physical and mechanical properties. A new technological parameter has been established – the duration of half-cycles, which allows simply and effectively regulating the phase composition and structure of the layer in order to obtain the required physical and mechanical properties.

Front propagation for the aggregation-diffusion-reaction equation [Formula: see text] is investigated, where f is a bi-stable reaction-term and D(v) is a diffusion coefficient with changing sign, modeling aggregating-diffusing processes. We provide necessary and sufficient conditions for the existence of traveling wave solutions and classify them according to how or if they attain their equilibria at finite times. We also show that the dynamics can exhibit the phenomena of finite speed of propagation and/or finite speed of saturation.

Innovative approaches in ultrasonic assistant soldering consist consists of increasing the activity of cavitation and accelerating diffusion processes at the interface between the solder and the soldering material. Besides that, it improves the effectiveness of cavitation processes in melts by saturating gas cavities with diamete rs that are smaller than the resonant sizes of cavitation germs. Gas saturation of liquids and melts raises level of cavitation pressure by 20 25%25%, that intensifies US processing of cleaning, soldering and metallization. Modelling diffusion process showed that the US activation increased the concentration of diffusing elements of Zn and Al in the interface depth by 15 20% on average, and the combined activation by the US and electric field increased it by 30 45%. Furthermore, as the energy quantity adsorbed by melt increases, increased amplitude and frequency of US vibrations induces concentration rise. The heat energy was also boosted by combining the activation of the melt–soldered material system with US vibrations energy and high current pulses. This allows for a faster increase in soldering temperature, as well as improved solder wettability.

During the last two decades the macroscopic formulation of moisture transport in wood below the fiber saturation point has motivated many research efforts. From experiments the difference in steady-state and transient transport processes is well known, but could not be explained in a fully physically motivated manner. In the following article, first the microstructure of wood is depicted, followed by a description of the physical background of steady-state and transient transport processes in wood, and thereon based mathematical formulations. For a correct macroscopic description of transient transport processes, three coupled differential equations have to be solved in parallel, which is done using the finite element method. The validation of the whole model by comparison of model predictions with experimentally derived values is currently in progress and will be published in near future.

Abstract. Thermal, suction and osmotic gradients interact during evaporation from a salty soil. Vapor fluxes become the main water flow mechanism under very dry conditions. A coupled nonisothermal multiphase flow and a reactive transport model of a salty sand soil was developed to study such an intricate system. The model was calibrated with data from an evaporation experiment (volumetric water content, temperature and concentration). The retention curve and relative permeability functions were modified to simulate oven dry conditions. Experimental observations were satisfactorily reproduced, which suggests that the model can be used to assess the underlying processes. Results show that evaporation is controlled by heat, and limited by salinity and liquid and vapor fluxes. Below evaporation front vapor flows downwards controlled by temperature gradient and thus generates a dilution. Vapor diffusion and dilution are strongly influenced by heat boundary conditions. Gas diffusion plays a major role in the magnitude of vapor fluxes.

In the course of reactive diffusion of hydrogen in metal-doped glasses, at some conditions, metallic nanoparticles grow forming quasi-periodic layered structure. We have developed a model defining conditions necessary for the formation of the layered structures. The model indicates relatively narrow range of parameters providing the quasi-periodic growth of the nanoparticles. The layered structure arises at relatively low over-saturation by neutral metal in the diffusion zone, due to the competition of two processes: enrichment of the glass by neutral metal atoms via reducing of metal ions by diffusing hydrogen and depletion of the glass by the metal atoms caused their diffusion to the nanoparticles. The model can be also applied to other situations where reactive diffusion inducing the formation and growth of nanoparticles occurs.

The existing technology for the purification of diffusion juice and its hardware design has not fundamentally changed over the past four decades. The lack of the necessary theoretical developments and experimental data hinders the development of existing and the development of new technological processes. Therefore, the main direction of improving the purification efficiency of juices of sugar beet production is the disclosure of its unused reserves and their implementation in practice. The scientific problem of choosing the rational direction for improving the technology of juice purification, which ensures the production of consumer granulated sugar in the face of changes in a wide range of quality of raw materials, is relevant and has important economic importance, especially in the context of the transition of beet sugar factories to a market economy. One way to solve it is to make fuller use of the adsorption capacity of calcium carbonate particles while increasing the filtration properties of saturation sediments. Therefore, the study investigates the effect of cavitation effects – vapor condensation and hydrodynamic processing of diffusion juice on the processes of purification of diffusion juice, juices of preliminary defecation, first and second saturations. The analysis of the influence of various effects of cavitation processing of juices from the point of view of improving the purification efficiency, the optimal place of the purification process in the technological scheme of production is established.

Дисертація на здобуття наукового ступеня доктора технічних наук за спеціальністю 05.02.08 – технологія машинобудування (13 – механічна інженерія). – Національний технічний університет «Харківський політехнічний інститут», Харків, 2019. У дисертаційній роботі проведено комплекс досліджень, спрямованих на вирішення важливої науково-технічної проблеми в області технології машинобудування: розробка інноваційних та короткотривалих технологій поверхневого зміцнення деталей машин порошковими сумішами керованого складу для забезпечення експлуатаційних властивостей виробів на високому рівні при значному зниженні затрат на їх виготовлення. Наукова новизна отриманих результатів полягає у розробці наукових основ інноваційних та короткотривалих технологій поверхневого зміцнення деталей машин порошковими сумішами керованого складу, що дозволило вирішити актуальну науково-практичну проблему підвищення терміну служби деталей машин та інструменту: – вперше розраховано локальні максимуми поверхневої твердості та глибини дифузійних шарів сплавів і встановлено теоретичні оптимальні умови процесів дифузійного зміцнення, що дозволяє отримати конкретні технологічні параметри проведення хіміко-термічної обробки (ХТО) та забезпечити оптимальні характеристики дифузійних шарів; – вперше створено математичні моделі та номограми існуючих технологій поверхневого зміцнення сталей, що дозволило визначити конкретні умови ХТО (температуру і тривалість), виходячи із заданої глибини дифузійного шару або поверхневої твердості сталей, що суттєво впливає на ефективність реалізації процесів зміцнення; – вперше на основі використання інноваційних технологій і системного аналізу при мінімальних витратах, розроблено загальний методологічний підхід керування технологічними процесами поверхневого зміцнення деталей порошковими сумішами керованого складу при насиченні поверхневих шарів азотом, вуглецем і бором, це дозволило підвищити експлуатаційні властивості виробів при значному скороченні тривалості ХТО; – набули подальшого розвитку розроблені комплексні ХТО, які значно знижують крихкість борованих шарів за рахунок більш плавного зниження твердості від поверхні до серцевини виробів зі сталей, що дозволило підвищити експлуатаційні властивості виробів та термін служби деталей машин та інструменту на відміну від відомих методів ХТО, які підвищують лише поверхневу твердість; – вперше розроблено математичну модель розподілу температури за глибиною дифузійного шару, що дозволило визначити характер залежностей та отримати дані про розподіл температури за глибиною виробу при різних технологічних режимах обробки; – удосконалено технологію борування з паст титанових сплавів за рахунок використання нанодисперсного насичувального середовища, що дозволило скоротити процес борування у 2-3 рази та скоротити технологічний процес виготовлення деталей за рахунок поєднання двох операцій: борування і гартування титанового сплаву; – запропоновано розв’язання крайових задач дифузії методом граничних елементів, що дозволило вперше створити математичну модель розподілу концентрації бору за товщиною зміцненого шару титанового сплаву; − удосконалено технологію інтенсифікації процесів ХТО методами нагрівання струмами високої частоти та за рахунок попередньої лазерної обробки деталей, що дозволило отримати високі експлуатаційні властивості поверхневих шарів при значному скороченні тривалості обробок. Практичне значення роботи полягає у розробці технологій комбінованого зміцнення поверхневих шарів деталей зі сплавів. На основі комплексу проведених теоретичних та експериментальних досліджень, сформульованих принципів, закономірностей і положень отримані наступні практичні результати: 1. Спосіб комбінованої обробки сталевих виробів, що включає попередню лазерну обробку поверхні матеріалу з потужністю лазерного випромінювання -1,0±0,1 кВт, швидкістю пересування лазерного променя - 0,5-1,5 м/хв. з наступним азотуванням. Крім цього, азотування проводять в середовищі меламіну з 3-5% фтористого натрію при температурі 530-560 °C протягом 2-3 годин (патент України №111066). 2. Спосіб дифузійного борування сталевих виробів, що включає попереднє нанесення на поверхню обмазки, в склад якої входить боровмісна речовина, активатор фторид натрію і зв'язуюча речовина розчину клею БФ в ацетоні, і нагрівання струмами високої частоти. При цьому в обмазці як боровмісну речовину використовують поліборид магнію або аморфний бор і додатково введено активатор фторид літію (патент України №116177). 3. Спосіб поверхневого зміцнення сталевих виробів, що включає нанесення на поверхню деталі обмазки, до складу якої' входить боровмісна речовина і активатор, сушіння і нагрівання струмами високої частоти. В обмазці як боровмісну речовину використовують аморфний бор і активатор фторид літію. Нагрівання проводять при температурі 800-1100 °С протягом 1-5 хвилин (патент України №116178). 4. Спосіб отримання твердого покриття на поверхні сталевих виробів, що включає попередню обробку поверхні матеріалу та борування. Проводять попередню лазерну обробку поверхні матеріалу з наступним боруванням в середовищі полібориду магнію, активаторами: фтористий натрій і фтористий літій (патент України №116116). 5. Сплав на основі заліза з ефектом пам'яті форми, що містить: залізо, марганець, кремній, вуглець, хром, нікель, кобальт, мідь, ванадій, ніобій, молібден. При цьому до сплаву додатково введено сірку та фосфор (ваг. %): марганець від 4 до 20; кремній від 1,0 до 4,5; вуглець від 0,1 до 1,0; хром від 10,0 до 25,0; нікель від 1,0 до 10,0; кобальт від 1,0 до 10,0; мідь від 1,0 до 4,0; ванадій від 0,5 до 2,0; ніобій від 0,3 до 1,5; молібден від 0,5 до 2,0; сірка до 0,01; фосфор до 0,045; залізо решта (патент України №116117). 6. Склад для борування сталевих виробів, що містить аморфний бор, тетрафтороборат калію, нітрид бору і доломіт (патент України №117775). 7. Спосіб поверхневого зміцнення титанових сплаві, що включає насичення поверхневих шарів компонентами боровмісного середовища, до складу якого входить боровмісна речовина та активатор, і нагрівання. Насичення поверхневих шарів здійснюють компонентами боровмісного середовища, яке складається з аморфного бору і фториду літію (патент України №117770). 8. Дисперсійно-твердіючий сплав на основі заліза з ефектом пам'яті форми, що містить: залізо, марганець, кремній, вуглець, ванадій, ніобій, вольфрам. Додатково введено алюміній, мідь, нікель, хром, сірку та фосфор (патент України №117757). 9. Розроблені технологічні процеси ХТО були впроваджені для підвищення поверхневої твердості сталевих виробів на ТОВ «АСТИЛ М» (м. Харків), що дозволило підвищити зносостійкість втулки у 1,5 рази після нітроцементації, у 4,3 рази після послідовній нітроцементації та боруванні, у 5 разів після цементації, нітроцементації та боруванні та у 2 рази після боруванні з нагріванням СВЧ у порівняні зі втулкою без поверхневого зміцнення (Акт впровадження від 05.10.2017 р.). 10. Розроблені технологічні процеси комбінованого зміцнення були впроваджені для підвищення поверхневої твердості сталевих виробів на ПАТ «Харківський машинобудівний завод «Світло шахтаря» (м. Харків). Виробничими випробуваннями встановлено, що запропоновані ефективні технологічні процеси комбінованого зміцнення поверхневого шару сталевих виробів дозволили значно прискорити технологічні процеси хіміко-термічної обробки у 2-10 разів, що привело до зменшення витрат на їх проведення за рахунок економії електричної енергії (Акт впровадження від 17.10.2017 р.). 11. Розроблені технологічні процеси були впроваджені на ТОВ «НВЦ ЄТМ» (м. Харків), що дозволило підвищити зносостійкість втулки у 1,5 рази після нітроцементації, у 4,3 рази після послідовній нітроцементації та боруванні (Акт впровадження від 31.10.2017 р.). 12. Прийняті для впровадження в виробництві розроблені номограми, які дозволяють визначити конкретні умови газового азотування (температуру і тривалість) виходячи із заданої глибини азотованого шару або поверхневої твердості виробів зі сталі 38Х2МЮА на ПАТ «Харківський машинобудівний завод «Світло шахтаря» (м. Харків). Встановлено, що запропоновані номограми дозволили значно спростити роботу інженера-технолога, а також номограми дозволили вирішувати зворотну задачу, а саме, оцінити можливу товщину зміцненого шару і поверхневу твердість при одночасному впливі температури і тривалості газового азотування (Акт впровадження від 15.11.2017 р.). 13. Розроблений ефективний технологічний процес нітроцементації у порошковій макродисперсній суміші для підвищення експлуатаційної стійкості зубчастого колеса зі сталі 38Х2МЮА на АТ «Харківський тракторний завод» (м. Харків). Встановлено, що використання макродисперсної суміші прискорило процес хіміко-термічній обробки у 1,5-2 рази при отриманні властивостей поверхневого шару виробу таких, як і після традиційного процесу нітроцементації, що дозволило зменшити витрати на проведення хіміко-термічної обробки у 2 рази (Акт впровадження від 24.01.2018 р.). 14. Розробки, виконані в дисертації, впроваджені в навчальний процес для студентів механіко-технологічного факультету НТУ «ХПІ» спеціальностей 131 «Прикладна механіка» спеціалізації 131-09 «Обладнання та технології ливарного виробництва» та 151 «Автоматизація та комп’ютерно інтегровані технології» спеціалізації 151-07 «Комп’ютеризовані системи управління технологічними процесами» (Акт впровадження від 20.12.2017 р.).
The thesis for the scientific degree of doctor of technical sciences, specialty 05.02.08 – technology of mechanical engineering (13 – mechanical engineering). – National Technical University "Kharkiv Polytechnic Institute", Kharkiv, 2019. In the thesis a set of studies was aimed at solving an important scientific and technical problem in the field of engineering technology: the development of innovative and short-term technologies of machine parts surface hardening with controlled composition powder mixtures to ensure the performance properties of products at a high level with a significant reduction in the cost of their production. The scientific novelty of the results lies in the development of scientific foundations of innovative and short-term technologies of surface hardening of machine parts by powder mixtures of controlled composition, which allowed to solve the actual scientific and practical problem of increasing the service life of machine parts and tools: - for the first time, local maxima of surface hardness and depth of diffusion layers of alloys were calculated and theoretical optimal conditions of diffusion hardening processes were established, which allows obtaining specific technological parameters of the chemical and heat treatment (CHT) process and providing optimal characteristics of diffusion layers; - for the first time created mathematical models and nomograms of existing technologies of surface hardening of steels, which allowed to determine the specific conditions of the CHT process (temperature and duration), based on a given depth of the diffusion layer or surface hardness of steels, which significantly affects the effectiveness of the implementation of strengthening processes; - for the first time through the use of innovative technologies and systems analysis at minimal cost, developed a general methodological approach for control of technological processes of surface hardening of parts by the powder mixtures of controlled composition at saturation of surface layers with nitrogen, carbon and boron, it is possible to improve the performance properties of products with a significant reduction in the CHT duration; – further development of the developed CHT complex, which significantly reduces the fragility of boriding layers due to a more gradual decrease in hardness from the surface to the core products of steels, thus improving the operational properties of the goods and service life of machine parts and tools in contrast to known methods of the CHT, which only increase surface hardness; - for the first time, a mathematical model of temperature distribution over the depth of the diffusion layer was developed, which made it possible to determine the nature of the dependences and obtain data on the temperature distribution over the depth of the product at different processing modes; – improved boriding technology with pastes of titanium alloys through the use of nanodispersed saturating environment, thereby reducing the boriding process to 2-3 times and to shorten the manufacturing process of components by combining two operations: boriding and hardening a titanium alloy; - solutions of boundary-value problems of diffusion by the boundary element method are proposed, which allowed for the first time to create a mathematical model of the distribution of boron concentration over the thickness of the hardened layer of a titanium alloy; - the technology of intensification of processes by CHT of heating by high-frequency currents and by means of preliminary laser processing of details was improved, which allowed to obtain high performance properties of surface layers with a significant reduction in the duration of treatments. The practical value of the work is to develop a technology of combined hardening of the surface layers of alloys making parts. The following practical results are obtained on the basis of a set of theoretical and experimental studies, formulated principles, regularities and the following practical results are obtained: 1. Method of combined processing of steel products, including advanced laser processing of material surface with the laser radiation power of -1.0±0.1 kW, the speed of movement of the laser beam of 0.5–1.5 m/min with subsequent nitriding. In addition, the nitriding is carried out in an environment of melamine with 3 to 5 % of sodium fluoride at a temperature of 530-560 °C for 2–3 hours (the patent of Ukraine No. 111066). 2. Method diffusion boriding steel products, including pre-application to the surface of the coating, which includes boriding substance, the activator sodium fluoride and a binder solution of glue BF in acetone, and heating by high frequency currents. In the coating as boriding substance use polyboride magnesium or amorphous boron, and optionally an activator is introduced lithium fluoride (the patent of Ukraine No. 116177). 3. Method of surface hardening steel parts comprising coating the surface of the part coating, which' is included boriding substance and activator, drying and heating by high frequency currents. In the coating as boriding substance use amorphous boron activator and lithium fluoride. The heating is carried out at a temperature of 800-1100 °C for 1-5 minutes (the patent of Ukraine No. 116178). 4. A method of producing a solid coating on the surface of steel products, including pre-processing the surface of the material and boriding. Carry out a preliminary laser treatment of the surface of the material with subsequent boriding in the environment polyboride magnesium, activators: sodium fluoride and lithium fluoride (the patent of Ukraine No. 116116). 5. The iron-based alloy with shape memory effect, contains: iron, manganese, silicon, carbon, chromium, nickel, cobalt, copper, vanadium, niobium, molybdenum. In this case, the alloy additionally introduced sulfur and phosphorus (weights. %): the manganese from 4 to 20; silicon 1.0 to 4.5; carbon 0.1 to 1.0; chromium, 10.0 to 25.0; nickel 1.0-10.0; cobalt 1.0-10.0; copper 1.0-4.0; vanadium 0.5 to 2.0; niobium from 0.3 to 1.5; molybdenum from 0.5 to 2.0; sulfur up to 0.01; phosphorus up to 0,045; iron-rest (the patent of Ukraine No. 116117). 6. Сomposition for boriding steel products containing amorphous boron, tetrafluoroborate potassium, boron nitride and dolomite (the patent of Ukraine No. 117775). 7. Method of surface hardening of titanium alloys, including a saturation of the surface layers of components boron environment, which includes boriding substance and the activator, and heating. The saturation of the surface layers is performed by components boron environment that consists of amorphous boron and lithium fluoride (the patent of Ukraine No. 117770). 8. The dispersion hardening iron-based alloy with shape memory effect, contains: iron, manganese, silicon, carbon, vanadium, niobium, tungsten. Included aluminum, copper, nickel, chromium, sulfur and phosphorus (the patent of Ukraine No. 117757). 9. Developed technological processes were introduced to improve the surface hardness of steel products at the limited liability company "ASTIL M" (Kharkіv), improving the durability of the sleeve by 1.5 times after nitrocarburizing, 4.3-fold after successive nitrocarburizing and boriding, 5 times after carburizing, nitrocarburizing and boriding and 2 times after boriding with microwave heating compared to a sleeve without surface hardening (the implementation Act from 05.10.2017). 10. Developed technological processes of the combined consolidation was implemented to improve surface hardness of steel products at Public company «Kharkiv machine-building plant "SVET SHAKHTYORA" (Kharkiv). Production tests have proved that the proposed effective technological processes of the combined hardening of surface layers of steel products will significantly accelerate the technological processes of chemical heat treatment in 2-10 times, led to reduced costs for them by saving electrical energy (the implementation Act from 17.10.2017). 11. Developed technological processes have been introduced at the limited liability company "Scientific-production Centre of the European mechanical engineering technology" (Kharkiv), thus improving the durability of the sleeve by 1.5 times after nitrocarburizing, 4.3-fold after successive nitrocarburizing and boriding (the implementation Act from 31.10.2017). 12. Adopted for implementation in production of developed nomograms that allow to define specific conditions of gas nitriding (temperature and duration) based on the desired depth of nitrided layer or the surface hardness of products of steel 38Cr2MoAl at Public company "Kharkiv machine-building plant «SVET SHAKHTYORA" (Kharkiv). Determined that the proposed nomograms greatly simplified the work of the engineer and these nomograms allowed to solve the inverse problem, to estimate the possible thickness of the hardened layer and surface hardness, at the same time of temperature and duration of gas nitriding (the implementation Act from 15.11.2017). 13. Developed an effective technological process of nitrocarburizing in microdisperse powder mixture to improve the operational stability of the toothed wheel of steel 38Cr2MoAl at private joint stock company "Kharkiv Tractor Plant". The use of microdisperse mixture accelerated the process of chemical-heat treatment by 1,5-2 times while getting the properties of the surface layer of the product such as after the nitrocarburizing, which reduced the costs of conducting chemical-thermal treatment by 2 times (the implementation Act from 24.01.2018). 14. The developments made in the thesis introduced in the educational process for students of mechanical engineering faculty of NTU "KhPI" special 131 "Applied mechanics" specialization 131-09 "Equipment and technology of foundry" and 151 "Automation and computer integrated technologies" specialization 151-07 "Computerized control of technological processes" (the implementation Act from 20.12.2017).

La technique originale de spectroscopie par saturation d'absorption, dite holeburning spectral, est mise en oeuvre pour étudier l'élargissement homogène de transitions optiques à 1,55 μm dans deux types de nanostructures. Pour les boîtes quantiques GaN/AlN, notre expérience constitue la première mesure directe de la largeur homogène de la transition intrabande s − pz. Des études en puissance démontrent le rôle prédominant des processus Auger dans la relaxation de population des niveaux. Le profil spectral d'absorption homogène s'avère gaussien. La forte augmentation de la largeur homogène entre 5K et 30K suggère des mécanismes de décohérence autres que le couplage aux phonons acoustiques, comme la diffusion spectrale. Dans le cas des nanotubes de carbone, notre dispositif expérimental permet d'étudier finement l'évolution du spectre d'absorption homogène de la transition électronique fondamentale sur une large gamme de puissance et pour des températures allant de 5K à 300 K. Les études en puissance mettent en évidence quantitativement la contribution prédominante de l'élargissement collisionnel et la contribution marginale de la réduction de force d'oscillateur au signal nonlinéaire. Deux processus d'interaction à deux excitons sont analysés : l'annihilation exciton-exciton (EEA) et la diffusion exciton-exciton (EES), et nous révélons la nature hybride Wannier-Frenkel particulière des excitons dans les nanotubes de carbone. Finalement, nous étudions le déphasage assisté par phonons et nous mettons en évidence les caractéristiques du couplage exciton-phonon, liées au caractère unidimensionnel.

A model describing the suspension diffusion process of gas molecules in liquid media is presented in this paper. This process is not yet solved by a satisfactory model for micro-scale applications at this time. The new model allows the simulation of diffusion processes in continuous media considering the molecular mass flux in a suspension/carrier phase mixture. Modelling the diffusion of gas suspensions in liquid media the saturation mass ratio is reached near the liquid/gas surface very quickly. The increase of gas concentration in the liquid domain depends on the elapsed time and the physical properties of gas and liquid media. The molecular gas velocity is described by a Maxwell probability density function. Based on this spectral method macroscopic physical values are modelled to describe time-dependent global concentration changes. Modelling the gas species diffusion the molecular convection is considered. Modelling the mass flux of the molecular gas suspension characteristic time scales are developed describing the completion level of the saturation progress based on non-dimensional formulations of the molecular convection equation. The present model is implemented in a CFD code and validated by a family of parametric simulation results depending on the saturation mass ratio of the suspended gas phase. This simulation result array shows the dependency of saturation time and saturation mass ratio of the suspended gas molecules. Based on this relation macroscopic diffusion processes in micromixers and microchannels are described with this model and without an extra solution of molecule trajectories or spectral fields of molecule velocity.

Thermal and water management is critical to fuel cell performance. It has been shown that gas diffusion layer (GDL) can impose the mass transport limit; for example, it can block the reactant transport to active layer when flooding occurs at high current density conditions. Micro porous layer (MPL) in conjunction with backing layer (BL) has been used as a GDL material and was shown to be effective for water management. To study the transport processes in GDL and MPL modified GDL, an analytical solution is derived current study for calculation of two-phase, multicomponent transport in GDL. Two models were considered, the unsaturated flow model (UFM) and the separate flow model (SFM). Comparison of the calculated saturation level and oxygen mass fraction shows that UFM calculation can underestimate, as well as overestimate the saturation and oxygen concentration. The SFM was used to study the effects due to GDL property variations. The calculation shows that increase in liquid water transport in an MPL modified GDL is due to the abrupt change of liquid water flow rate when a step change in porosity or permeability is imposed. The calculation further shows that particle size of around 1 μm would be a good choice for MPL as it results in higher oxygen concentration at active layer and lower saturation in GDL.

The processes occurring at saturation of the surface with various elannotation The paper deals with issues related to the kinetics of saturation of high-density powder materials obtained by hot stamping (GS). The effect of surface treatment, temperature and time of saturation of the surface of highdensity powder materials with nitrogen and carbon is investigated. The graphs of the influence of these factors are given. The structures of the diffusion layer and the effect of the surface features of high-density powder materials on saturation with nitrogen and carbon are described. Comparative studies on the saturation of the surface layer at the SC of high-density powder materials and rolled steels have been carried out. It is established that the thickness of the diffusion layer on the iron powder is greater than that on low-carbon electrical steel and the magnitude of the difference slightly decreases with increasing temperature.

Diffusion layer saturation analysis (DLSA) is introduced in order to further investigate water transport within the cell. The analysis relies on two separate experimental processes. First, an ex-situ investigation of the relative humidity of the gas streams and their resulting pressure drop is performed. Next, multiple variables of the cathode and anode gas streams are manipulated in-situ to create an evaporative driving force to remove water out of the porous layers. Multiple gas stream settings are investigated as well as the temperature set point of the cell. The ex-situ pressure drop data is used to infer how much water is being removed from the system and to begin to estimate an overall water balance. Multiple cathode and anode GDL configurations are tested in order to further investigate initial saturation conditions of the GDLs. By coupling the voltage results to the calculated water removed from the system, it can be seen which configurations had low initial voltage due to GDL oversaturation. The potential for DLSA both as a diagnostic tool and an investigative technique into multiphase flow in the porous layer is demonstrated.

Water and heat are produced in the cathode catalyst layer of a polymer electrolyte fuel cell (PEFC) due to the oxygen-reduction reaction. Efficient water removal from the gas diffusion layer (GDL) to the flow channel is critical to achieve high and stable PEFC performance. Water transport and removal strongly depend on local temperature because the saturation concentration of water vapor rises rapidly with temperature, particularly in the temperature range of practical interest to PEFC applications. Detailed investigations of two-phase flow in the GDL have been reported in the literature, but not on the rate of phase change – either from liquid to vapor as in the case of evaporation or from vapor to liquid as in the case of condensation. In the present work, a two-phase, non-isothermal numerical model is used to elucidate the phase-change rate inside the cathode GDL of a PEFC. Results computed from our model enable a basic understanding of the phase-change processes occurring in a PEFC.

Polycrystalline material generally exhibits degradation in its mechanical properties and shows more tendency for intergranular fracture due to segregation and diffusion of hydrogen on the grain boundaries (GBs). Understanding the parameters affecting the diffusion and binding of hydrogen within GBs will allow enhancing the mechanical properties of the commercial engineering materials and developing interface dominant materials. In practice during forming processes, the coincidence site lattice (CSL) GBs are experiencing deviations from their ideal configurations. Consequently, this will change the atomic structural integrity by superposition of sub-boundary dislocation networks on the ideal CSL interfaces. For this study, the ideal ∑ 3 111 [110] GB structure and its angular deviations in BCC iron within the range of Brandon criterion will be studied comprehensively using molecular statics (MS) simulations. The clean GB energy will be quantified, followed by the GB and free surface segregation energies calculations for hydrogen atoms. Rice-Wang model will be used to assess the embrittlement impact variation over the deviation angles. The results showed that the ideal GB structure is having the greatest resistance to embrittlement prior GB hydrogen saturation, while the 3° deviated GB is showing the highest susceptibility to embrittlement. Upon saturation, the 5° deviated GB appears to have the highest resistance instead due to the lowest stability of hydrogen atoms observed in the free surfaces of its simulation cell. Molecular dynamics (MD) simulations are then applied to calculate hydrogen diffusivity within the ideal and deviated GB structure. It is shown that hydrogen diffusivity decreases significantly in the deviated GB models. In addition, the 5° deviated GB is representing the local minimum for diffusivity results suggesting the existence of the highest atomic disorder and excessive secondary dislocation accommodation within this interface.

In the paper, a two-phase, one-dimensional steady model of a proton exchange membrane fuel cell is developed for the porous cathode electrode. The interacted phenomena of cathode flooding and mass-transport limiting are investigated. In the model, the catalyst layer is treated as a separate computational domain with finite thickness and structure. Furthermore, through the Tafel kinetics, the mass transport processes of oxygen and liquid water are coupled. The transport of liquid water across the porous electrode is driven by the capillary force based on the Darcy’s law. However, the transport of gas is driven by the concentration gradient based on the Fick’s law. From the simulated results, it is found that the thin catalyst layer is very detriment for better understanding of the concurrent phenomena inside the electrode of a fuel cell, particularly, the flooding phenomena. More importantly, the saturation-level jump at the gas diffusion layer and catalyst layer interface is attained in order to satisfy the continuity of the capillary pressure on both sides of the GDL and CL interface. Meanwhile, the results show that the flooding phenomenon within the CL is more serious than that in the GDL, which has a significant influence on the mass transport of oxygen. Finally, the effect of some important parameters, such as the boundary value of saturation, absolute permeability, the cathode surface overpotential, on the saturation distribution inside the electrode is also obtained.

The objective of this work is to investigate the dynamic interactions between the vadose and the phreatic zones during breaking solitary wave runup and drawdown over a fine sand beach. Extreme wave runup and drawdown in the nearshore region can lead to soil failure in the form of severe erosion, liquefaction, or slope instability. However, the physics of the nearshore region is difficult to simulate numerically due to the greatly varying time scales between the four governing processes: loading and unloading caused by wave runup and drawdown, propagation of the saturation front, pore pressure diffusion, and soil consolidation. Such processes are also difficult to simulate experimentally via model-scale wave tank studies due to the inability to satisfy all the similarity requirements for both the wave and the porous media in a 1g environment. Hence, the goal of this work is to perform a 1D study using a multiphase model to describe the transient responses of the species saturation, pore fluid pressure, effective stresses, and skeleton deformation. Results are shown for three simulations: (1) full-scale simulation, (2) 1:20 laboratory-scale simulation without scaling of the porous media, and (3) 1:20 laboratory-scale with consistent scaling of the soil permeability. The results suggest that the scaling of porous media between the pore fluids and soil skeleton has a significant influence on the transient response of both the vadose and the phreatic zones.

This paper is concerned with the modeling of complex thermal-fluid phenomena that arises during the process of sterilization of canned food. In the sterilization process, food contained with a typical cylindrical can is subjected to a sudden change in temperature as the can is immersed in a steam bath at saturation temperatures of the order of 120 degrees C. Because of this transient heating event, complex transient buoyant motion is initiated in the can and therefore the heating of the food is as a result of diffusion and buoyant or natural convection processes. A Finite Volume method was used to investigate the unsteady two dimensional (axisymmetric) natural convection of a non-Newtonian liquid food during sterilization. The non-Newtonian liquid food was modeled using carboxy-methyl cellulose. Because its apparent viscosity depends on both temperature and non-linear velocity gradients, the resulting mathematical formulation is highly non-linear, thereby requiring a highly coupled implementation of the Finite Volume Method. Transient two dimensional velocity and temperature fields were obtained for different heating conditions and can aspect ratios. It was found that lower can aspect ratios and lower index n generally lead to shorter sterilization times.

experimental methods for quantifying effects of water content and other dynamic environmental factors on bacterial growth in partially-saturated soils. Towards this end we reviewed critically the relevant scientific literature and performed theoretical and experimental studies of bacterial growth and activity in modeled, idealized and real unsaturated soils. The natural wetting-drying cycles common to agricultural soils affect water content and liquid organization resulting in fragmentation of aquatic habitats and limit hydraulic connections. Consequently, substrate diffusion pathways to soil microbial communities become limiting and reduce nutrient fluxes, microbial growth, and mobility. Key elements that govern the extent and manifestation of such ubiquitous interactions include characteristics of diffusion pathways and pore space, the timing, duration, and extent of environmental perturbations, the nature of microbiological adjustments (short-term and longterm), and spatial distribution and properties of EPS clusters (microcolonies). Of these key elements we have chosen to focus on a manageable subset namely on modeling microbial growth and coexistence on simple rough surfaces, and experiments on bacterial growth in variably saturated sand samples and columns. Our extensive review paper providing a definitive “snap-shot” of present scientific understanding of microbial behavior in unsaturated soils revealed a lack of modeling tools that are essential for enhanced predictability of microbial processes in soils. We therefore embarked on two pronged approach of development of simple microbial growth models based on diffusion-reaction principles to incorporate key controls for microbial activity in soils such as diffusion coefficients and temporal variations in soil water content (and related substrate diffusion rates), and development of new methodologies in support of experiments on microbial growth in simple and observable porous media under controlled water status conditions. Experimental efforts led to a series of microbial growth experiments in granular media under variable saturation and ambient conditions, and introduction of atomic force microscopy (AFM) and confocal scanning laser microscopy (CSLM) to study cell size, morphology and multi-cell arrangement at a high resolution from growth experiments in various porous media. The modeling efforts elucidated important links between unsaturated conditions and microbial coexistence which is believed to support the unparallel diversity found in soils. We examined the role of spatial and temporal variation in hydration conditions (such as exist in agricultural soils) on local growth rates and on interactions between two competing microbial species. Interestingly, the complexity of soil spaces and aquatic niches are necessary for supporting a rich microbial diversity and the wide array of microbial functions in unsaturated soils. This project supported collaboration between soil physicists and soil microbiologist that is absolutely essential for making progress in both disciplines. It provided a few basic tools (models, parameterization) for guiding future experiments and for gathering key information necessary for prediction of biological processes in agricultural soils. The project sparked a series of ongoing studies (at DTU and EPFL and in the ARO) into effects of soil hydration dynamics on microbial survival strategy under short term and prolonged desiccation (important for general scientific and agricultural applications).

Time Domain Reflectometry (TDR) and other in-situ and remote sensing dielectric methods for determining the soil water content had become standard in both research and practice in the last two decades. Limitations of existing dielectric methods in some soils, and introduction of new agricultural measurement devices or approaches based on soil dielectric properties mandate improved understanding of the relationship between the measured effective permittivity (dielectric constant) and the soil water content. Mounting evidence indicates that consideration must be given not only to the volume fractions of soil constituents, as most mixing models assume, but also to soil attributes and ambient temperature in order to reduce errors in interpreting measured effective permittivities. The major objective of the present research project was to investigate the effects of the soil geometrical attributes and interfacial processes (bound water) on the effective permittivity of the soil, and to develop a theoretical frame for improved, soil-specific effective permittivity- water content calibration curves, which are based on easily attainable soil properties. After initializing the experimental investigation of the effective permittivity - water content relationship, we realized that the first step for water content determination by the Time Domain Reflectometry (TDR) method, namely, the TDR measurement of the soil effective permittivity still requires standardization and improvement, and we also made more efforts than originally planned towards this objective. The findings of the BARD project, related to these two consequential steps involved in TDR measurement of the soil water content, are expected to improve the accuracy of soil water content determination by existing in-situ and remote sensing dielectric methods and to help evaluate new water content sensors based on soil electrical properties. A more precise water content determination is expected to result in reduced irrigation levels, a matter which is beneficial first to American and Israeli farmers, and also to hydrologists and environmentalists dealing with production and assessment of contamination hazards of this progressively more precious natural resource. The improved understanding of the way the soil geometrical attributes affect its effective permittivity is expected to contribute to our understanding and predicting capability of other, related soil transport properties such as electrical and thermal conductivity, and diffusion coefficients of solutes and gas molecules. In addition, to the originally planned research activities we also investigated other related problems and made many contributions of short and longer terms benefits. These efforts include: Developing a method and a special TDR probe for using TDR systems to determine also the soil's matric potential; Developing a methodology for utilizing the thermodielectric effect, namely, the variation of the soil's effective permittivity with temperature, to evaluate its specific surface area; Developing a simple method for characterizing particle shape by measuring the repose angle of a granular material avalanching in water; Measurements and characterization of the pore scale, saturation degree - dependent anisotropy factor for electrical and hydraulic conductivities; Studying the dielectric properties of cereal grains towards improved determination of their water content. A reliable evaluation of the soil textural attributes (e.g. the specific surface area mentioned above) and its water content is essential for intensive irrigation and fertilization processes and within extensive precision agriculture management. The findings of the present research project are expected to improve the determination of cereal grain water content by on-line dielectric methods. A precise evaluation of grain water content is essential for pricing and evaluation of drying-before-storage requirements, issues involving energy savings and commercial aspects of major economic importance to the American agriculture. The results and methodologies developed within the above mentioned side studies are expected to be beneficial to also other industrial and environmental practices requiring the water content determination and characterization of granular materials.