Synthesis and characterization of bismuth and tellurium single crystals doped with selenium, zirconium and arsenic

Istraživanja u okviru ove doktorske disertacije obuhvatila su sintezu i karakterizaciju monokrista bizmuta i telura dopiranih selenom, cirkonijumom i arsenom dobijenih Čohralski i Bridžman postupkom. Jedinjenja na bazi bizmut telurida predstavljaju vrlo važne materijale za proizvodnju termoelektričnih hladnjaka i uređaja za proizvodnju električne energije. Za karakterizaciju monokristala korišćeno je više savremenih, standardizovanih, visoko rezolutivnih metoda i analitičkih tehnika. Rezultati prve serije eksperimenata pokazuju sintezu i eksperimentalna ispitivanja poluprovodnika n tipa Bi2Te2.88Se0.12 dobijenog metodom Čohralskog. Praćenje strukturnih promena uzoraka vršeno je elektronskim mikroskopom. Elementarni sastav i empirijska formula jedinjenja određeni su pomoću energetsko disperzivne spektrometrije (EDS). Ponašanje uzorka prilikom zagrevanja ispitano je korišćenjem diferencijalne termijske analize (DTA). Fizičke i hemijske osobine materijala su praćene u funkciji povećanja temperature termogravimetrijskom analizom (TGA). Rendgenskom difrakcijom (XRD) dobijeni su podaci položaja difrakcionih maksimuma 2θ (º), vrednosti međupljosnih rastojanja dhkl (Å) za sve refleksije, kao i odgovarajući relativni intenziteti I/Imax koji su dati grafički i tabelarno. Na osnovu dobijenih vrednosti I/Imax i dhkl i upoređivanjem sa literaturnim podacima i JCPDS i ICSD standardima, identifikovano je da je dobijen monokristal i proračunati su parametri jediničnih ćelija poluprovodnika. Vrednosti parametara rešetke dobijene difrakcionom analizom praha Bi2Te2.88Se0.12 veoma su slične konstatama rešetke Bi2Te3, što ukazuje na to da je mali deo telura zamenjen selenom. Vrednosti Vikersove mikrotvrdoće su u opsegu između HV 187 i HV 39.02 i smanjuju se sa povećanjem opterećenja. Pokazalo se da vrlo složen proces infracrvene termografije može biti primenjen za karakterizaciju termoelektričnih elemenata i modula. Rezultati završne serije eksperimenata pokazuju sintezu monokristalnih ingota, BiSbTeSe dopiranog cirkonijumom i BiTeSe dopiranog arsenom, sintetisanih Bridžman metodom. Za utvrđivanje hemijskog sastava proučavanih uzoraka, kao i provere i potvrđivanje homogenosti uzoraka korišćena je analiza energetsko disperzivne spektrometrije (EDS). Merenja rentgenske difrakcije (XRD) pokazala su da dobijeni kristalni ingoti predstavljaju monokristal i potvrđuju jedinjenje Bi2Te3 tipa. Određena je orjentacija (00l) monokristala dopiranog cirkonijumom. Tačka topljenja monokristala dopiranog cirkonijumom određena je dilatometrijskom analizom...Researches in this doctoral dissertation included synthesis and characterization of bismuth telluride single crystals doped with selenium, zirconium and arsenic obtained using Czochralski and Bridgman method. Compounds based on bismuth telluride are very important materials for thermoelectric refrigerators and devices for electricity production. For the characterization of monocrystals, many modern, standardized, high resolution methods and analytical techniques were used. The results of the first series of experiments show synthesis and experimental investigation of the n type Bi2Te2.88Se0.12 semiconductor obtained by the Czochralski method. Monitoring of structure changes of the samples was carried out by electron microscope. The elemental composition and empirical formula of the compound were obtained by energy dispersive spectrometry (EDS) analysis. The behavior of sample upon heating was studied using differential thermal analysis (DTA) technique. Changes in physical and chemical properties of materials were measured as a function of increasing temperature by thermogravimetric analysis (TGA). Data of diffraction peaks 2θ (º) positions, values of interplaned hkl (Å) for all reflections, as well as corresponding relative intensities I/Imax, which are given graphically and in table form, were obtained by X-ray diffraction. Based on the obtained values of I/Imax and dhkl and comparing with the literature data and with JCPDS and ICSD standards, it has been identified that a single crystal has been obtained and the parameters of the semiconductor unit cells were calculated. The lattice parameters values obtained by X-ray diffraction analyses of Bi2Te2.88Se0.12 powder are very similar to Bi2Te3 lattice constants, indicating that a small portion of tellurium was replaced with selenium. The Vickers microhardness values are in range between HV 187 and HV 39.02 and decrease with load increasing. It has been shown that very complex process of infrared thermography can be applied for characterization of thermoelectric elements and modules. The results of the final series of experiments show the synthesis of monocrystal ingots, BiSbTeSe doped with zirconium and BiTeSe doped with arsenic, synthesized by the Bridgman method. An analysis of energy dispersive spectrometry (EDS) was used to determine the chemical composition of the samples studied, as well as checking and confirming the homogeneity of the samples. Measurements of X-ray diffraction (XRD) showed that the resulting crystalline ingots represent a single crystal and confirm the compound of Bi2Te3 type. The orientation (00l) of monocrystal doped with zirconium was determined. The melting point of monocrystal doped with zirconium was determined by dilatometric analysis. Mobility, concentration, resistivity/conductivity, of majority of charge bearers and Hall coefficient of single crystals, of BiSbTeSe doped with Zr and BiTeSe doped with arsenic, were determined using a Hall Effect measurement system based on the Van der Pauw method. From different parts of BiSbTeSe single crystal ingot doped with zirconium, samples were cut and cleaved normally and parallel to the crystallization direction..