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Dependence of thermal conductivity of nanocrystalline diamond coating on grain size and form is theoretically investigated. Nanodiamond is considered as two-phase material composed of dielectric diamond grains characterizing by three main dimensions and segregated by thin graphite layers with electron or phonon thermal conductivity. Influence of thermal conductance type and thickness of boundary layer on nanodiamond thermal conductivity is analysed. Derived dependences of thermal conductivity on grain dimensions are compared with experimental data. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/35196

U radu je prikazan kratki pregled fizikalnih modela prijenosa topline i električnih grijača. Analiziran je polisilicij, kao materijal za izradu otpornog sloja grijača, te aluminij-nitridna keramika, kao izolacijski materijal za depozicijsku podlogu. Ovi materijali se koriste u mikroelektronici, ali se njihova primjena može proširiti i na ostale grane industrije. Polisilicijski grijač je grijač napravljen u tehnici tankog filma. Grijači napravljeni u tehnici tankog filma danas predstavljaju glavno istraživačko područje na području termičkih elemenata. U ovom radu predstavljen je polisilicijski grijač dobiven depozicijom u LPCVD reaktoru na aluminij-nitridnoj keramici. Polisilicij je poluvodički materijal kojemu se, dodavajući dopande, električne karakteristike mogu mjenjati u širokom opsegu. Budući da struktura deponiranog sloja uvelike ovisi o depozicijskim uvjetima, modeliranje karakteristika polisilicija je vrlo teško, te se uvode određena pojednostavljenja. Na prijenos topline unutar polisilicija uglavnom utječe mehanizam fononskog raspršenja, dok je prijenos struje posljedica djelovanja više mehanizama, kao što su driftno-difuzijski mehanizam, te tuneliranje i termionska emisija. U radu su karakteristike dobivene simulacijom ovih mehanizama u različitim uvjetima. Aluminij-nitridna keramika je relativno nov matrijal, koji ima veliki potencijal za primjenu u termičkim elementima. Ova keramika je električni izolator, vrlo velike toplinske vodljivosti i temperaturne stabilnosti. Temperaturni koeficijent širenja ove keramike je sličan polisilicijskom, pa je ukupno strukturalno naprezanje malo. Aluminij-nitridna keramika pod utjecajem kisika vremenom gubi na čvrstoći i kvaliteti. Prikazan je trodimenzionalni model grijača. Model je simuliran balansiranjem električnih i toplinskih jednadžbi, metodom konačnih elemenata. Prikazani rezultati ukazuju na vrlo dobar prijenos topline od aktivnog grijaćeg člana do grijanog objekta. Moguće je postići veliku gustoću toplinskog toka, preko 200 W/cm2. Short overview of heat transfer physics and electric heaters is given. Polysilicon is analyzed as a material for resistance layer of the heater, and aluminum-nitride ceramic as an insulator material used for deposition substrate. The primary application of this materials is microelectronics, but they also can be used in other industry. Polysilicon heater is thin film heater. Thin film heaters today present the major research field in thermal element area. This work presents polysilicon heater made by LPCVD deposition on aluminum-nitride substrate. Polysilicon is semiconducting material, with very variable electric properties, dopant dependent. Since the structure of deposited layer is very dependent on deposition conditions, polysilicon properties modeling is very difficult, and some simplifications are required. Heat transfer is mainly lead by phonon scattering, while current conduction include several mechanisms, like drift-diffusion mechanism, tunneling, and thermionic emission. The polysilicon properties presented in this work are derived by simulation of this mechanisms in different conditions. Aluminum-nitride ceramic is relatively novel material, with high potential for thermal element application. This ceramic is electric insulator, with very high thermal conductance and temperature stability. Thermal expansion coefficients of this ceramic and polysilicon are similar, and the result structural stress is small. Aluminum-nitride ceramic in oxygen ambient lose on quality by aging. Three-dimensional heater model is given. Model was simulated by heat and electric equations balancing, using finite element method. Derived results show very good heat transfer from heating source to heated object. Very high heat flow density is possible to achieve, over 200 W/cm2.

A new technique of growing large crystalline grain thin films has been developed. This method, that we called Quasi-Rheotaxy, consists in the deposition of thin films on substrates kept at a temperature near their melting point. Several semiconductor thin films which are considered as promising materials for solar photovoltaic conversion such as CdS, CdTe and CuGaInSe2 have been grown by Quasi-Rheotaxy. Low melting point and low vapour pressure metallic elements, such as Sn, Pb, Bi and Tl, have been used as quasi-melted substrates. Stainless steel was used as a base substrate to support the thin layer of the quasi-melted elements which were evaporated in the same vacuum system in which the semiconductor films were prepared. Large crystalline and columnar grains with a size up to 10 μm have been obtained in films grown by Quasi-Rheotaxy. The large crystalline grain thin films, since they are grown on low cost substrates, could be used to build low cost thin film solar cells.