Synthesis of conductive electroceramic material based on antimony doped barium stannate with linear current-voltage characteristic

U okviru ove disertacije ispitivani su uticaji koncentracije dopanta i parametara sinterovanja nastrukturu i svojstva keramičkih uzoraka barijum-stanata dopiranog antimonom, BaSn1-xSbxO3(BSSO, x = 0,00; 0,04; 0,06; 0,08 i 0,10) u cilju dobijanja provodne elektrokeramike sa linearnomstrujno-naponskom (I-U) karakteristikom...The main goal of this doctoral thesis was to investigate the influence of different dopantconcentrations and sintering parameters on the structure and properties of antimony-doped bariumstannate, BaSn1-xSbxO3 (BSSO, x = 0,00; 0,04; 0,06; 0,08 and 0,10) to obtain conductiveelectroceramic samples with linear current-voltage (I-U) characteristics..

The additive influence on sintering of zno-tio2 system according to triad “synthesis-structure-properties”

U okviru ove doktorske disertacije detaljno i sistematski je proučen uticaj aditiva magnezijum oksida na sintezu i sinterovanje sistema ZnO-TiO2. Smeša prahova ZnO, TiO2, MgO (0, 1,25 i 2,50 masenih % MgO), mehanički je aktivirana mlevenjem u visokoenergetskom planetarnom mlinu u toku 15 minuta. Metodama skenirajuće elektronske mikroskopije i rendgenske difrakcije analizirane su strukturne promene u materijalu koje se dešavaju tokom mehaničkog tretmana. Diferencijalnom termijskom analizom određene su karakteristične temperature u intervalu 20-1100°C. Takođe je detaljno ispitan uticaj mehaničke aktivacije na presovanje prahova. Proučavano je sinterovanje u izotermskim uslovima na temperaturama 800, 900, 1000 i 1100°C, kao i reakciono sinterovanje u neizotermskim uslovima u temperaturnim intervalima od sobne do 800, 900, 1000 i 1100°C pomoću dilatometrijske analize. Rendgenostrukturnim proučavanjem uzoraka sinterovanih izotermski utvrđen je fazni sastav dobijenog materijala, a mikrostrukturne karakteristike skenirajućom elektronskom mikroskopijom. Analizom mikrostrukturnih parametara aktiviranih smeša utvrđeno je da se suštinske mikrostrukturne promene odigravaju uglavnom u kristalnoj strukturi ZnO. U toku mehaničkog dejstva dolazi do značajnog smanjenja veličine kristalita cink-oksida, povećanja gustine dislokacija i mikronaprezanja u njegovoj kristalnoj strukturi, dok kod titan-dioksida nije primećena takva pojava. Uticaj aditiva na reakciju u čvrstoj fazi između ZnO i TiO2 praćen je u izotermskim i neizotermskim uslovima. Predložen je mehanizam sinterovanja, utvrđeno je da se sa dodatkom aditiva temperatura početka reakcionog sinterovanja neznatno menja, reakcija formiranja spinela (cink-ortotitanata) se pomera ka višim temperaturama, ali se najveće zgušnjavanje nakon procesa sinterovanja, kako izotermskog tako i neizotermskog, dešava upravo kod smeše sa najvećom koncentracijom dodatog aditiva. Ispitivanjem električnih svojstava utvrđeno je da električna otpornost opada sa frekvencom i temperaturom sinterovanja ali i da raste sa dodatkom aditiva. Na samom kraju, zaključujemo da smo našim istraživanjima predstavljenim u ovoj doktorskoj disertaciji, definisali zavisnosti između sinteze i svojstava, koja direktno vezuje sintezu i strukturu odn. strukturu i svojstva, čime je dat i doprinos prognozi svojstava materijala.In this doctoral dissertation the influence of additive addition on the synthesis and sintering of zinc titanate, Zn2TiO4, was studied. The powder mixtures of ZnO, TiO2 and MgO (0, 1.25 and 2.50 weight % MgO) were mechanically activated by milling using a high-energy ball mill for 15 minutes. Structural changes of the material that occur during mechanical treatment were analyzed using the scanning electron microscopy and X-ray powder diffraction. The characteristics maximums in the temperature range 20-1100°C were determined using differential thermal analysis. Isothermal sintering on temperatures 800, 900, 1000 and 1100°C as well as non-isothermal reaction sintering at the same temperatures were studied. X-ray diffraction was employed to determine a phase composition of sintered samples. Scanning electron microscopy was used in order to determine the phase microstructure evolution. Analysis of microstructural parameters revealed that essential microstructural changes occur mainly in the crystal structure of ZnO. Mechanical activation caused significant reduction of zinc oxide crystallite size, and increase of dislocation density and microstrains in the crystal structure. These phenomena were not observed for titan dioxide. The influence of additive addition on solid state reaction between ZnO and TiO2 was studied in isothermal and non-isothermal conditions. It was established that MgO addition has no influence on sintering temperatures, that moves the temperature of the beginning of reaction to higher temperatures and lags the formation of spinel (zinc-titanate), but the highest densification, after isothermal as well as non-isothermal sintering, is observed within mixture with highest content of mentioned additive. The electrical resistivity data revealed a clear decrease with increasing sintering temperatures as well as slight increase with MgO addition. And finally, we can conclude that observed results presented within this doctoral dissertation gave dependences between synthesis and properties, which directly combine synthesis and structure as well as structure and properties

Dobijanje provodne elektrokeramike na bazi barijum-stanata dopiranog antimonom sa linearnom strujno-naponskom karakteristikom

U okviru ove disertacije ispitivani su uticaji koncentracije dopanta i parametara sinterovanja na strukturu i svojstva keramičkih uzoraka barijum-stanata dopiranog antimonom, BaSn1-xSbxO3 (BSSO, x = 0,00; 0,04; 0,06; 0,08 i 0,10) u cilju dobijanja provodne elektrokeramike sa linearnom strujno-naponskom (I-U) karakteristikom. Smeša prahova BaCO3, SnO2 i Sb2O3 mehanički je aktivirana u izopropanolu, kalcinisana na 900 °C i potom sinterovana koristeći tri različite metode: konvencionalno, spark plazma i hladno sinterovanje. Konvencionalno sinterovanje prahova vršeno je na 1200, 1400 i 1600 °C, ali se na nižim temperaturama dobija keramički materijal niskih relativnih gustina. Sinterovanjem na 1600 °C dobijeni su jednofazni uzorci kubne strukture BaSnO3 (BSO). Rezultati skenirajuće elektronske mikroskopije (engl. Scanning Electron Microscopy, SEM) ukazuju na značajno smanjenje veličine zrna nakon dopiranja, ali i na poroznost koja ometa električnu karakterizaciju uzoraka. Transmisiona elektronska mikroskopija visoke rezolucije (engl. High Resolution Transmission Electron Microscopy, HRTEM) ukazuje na prisustvo niskougaonih granica zrna u uzorku BaSn0,92Sb0,08O3. Pouzdana električna merenja izvršena na ovom uzorku ukazuju na njegova poluprovodnička svojstva karakteristična za materijale sa dvostrukom Šotkijevom barijerom (engl. Schottky barrier) na granici zrna. Spark plazma sinterovanje (SPS) vršeno je na temperaturama od 1000, 1100, 1200 i 1250 °C, za vreme od 5, 10, 15 i 30 min, pri pritisku od 60 MPa i sa brzinom zagrevanja od 100 °C min–1. Za dobijanje guste keramike optimalni uslovi sinterovanja su 1100 i 1200 °C, tokom 5 min. Sinterovanjem na 1100 °C dobijeni su jednofazni keramički uzorci kubne strukture BSO. Dopiranje antimonom dovodi do smanjenja veličine zrna, ali i do smanjenja specifične otpornosti u odnosu na nedopirani uzorak. Električna karakterizacija ukazuje na poluprovodnička svojstva svih keramičkih uzoraka, sa najnižom specifičnom otpornošću zabeleženom na uzorku BaSn0,92Sb0,08O3. SPS na 1200 °C dobijaju se keramički uzorci koji osim kubne faze BSO, sadrže i sekundarnu fazu Ba2SnO4, čija se količina u uzorku smanjuje dopiranjem. Na ovoj temperaturi pri visokom pritisku dolazi do reakcije SnO2 iz kalcinisanog praha sa grafitom iz kalupa pri čemu se izdvaja elementarni kalaj i stvaraju uslovi za formiranje sekundarne faze bogate barijumom. Rezultati skenirajuće elektronske mikroskopije emisijom elektrona poljem (engl. Field Emission Scanning Electron Microscopy, FE-SEM) ovih uzoraka potvrđuju da dopiranjem dolazi do smanjenja veličine zrna, ali i smanjenja koncentracije iv dislokacija u keramičkim uzorcima. Svi uzorci osim BaSn0,92Sb0,08O3 ispoljavaju nelinearnu strujno-naponsku karakteristiku, tipičnu za poluprovodnike sa potencijalnom barijerom na granici zrna. Veliki udeo niskougaonih granica u uzorku BaSn0,92Sb0,08O3 dovodi do gubitka potencijalne barijere na granici zrna, što je potvrđeno i impedansnom spektroskopijom, pa ovaj uzorak pokazuje najnižu vrednost specifične otpornosti, konstantnu u temperaturnom opsegu 25–150 °C i linearnu strujno-naponsku karakteristiku. Za razliku od prethodne metode, hladno sinterovanje uzorka BaSn0,92Sb0,08O3 nije dalo zadovoljavajuće rezultate po pitanju faznog sastava, gustine i električnih svojstava. Redukcioni uslovi tipični za spark plazma sinterovanje, uz koncentraciju dopanta od 8 % (mol.) Sb u uzorku BaSnO3 doveli su do formiranja niskougaonih granica zrna, povećanja koncentracije vakancija kiseonika i promene valence Sn, što je dovelo do porasta koncentracije nosilaca naelektrisanja (elektrona) i posledično rezultovalo dobijanjem uzorka sa linearnom I-U karakteristikom i konstantnom vrednosti specifične otpornosti u širem temperaturnom opsegu.The main goal of this doctoral thesis was to investigate the influence of different dopant concentrations and sintering parameters on the structure and properties of antimony-doped barium stannate, BaSn1-xSbxO3 (BSSO, x = 0,00; 0,04; 0,06; 0,08 and 0,10) to obtain conductive electroceramic samples with linear current-voltage (I-U) characteristics. A mixture of BaCO3, SnO2 and Sb2O3 powders was mechanically activated in isopropanol, calcined at 900 °C and then sintered using three different methods: conventional, spark plasma and cold sintering. Conventional sintering at 1200 and 1400 °C resulted in the preparation of low-density ceramic samples. By sintering at 1600 °C, single-phase material with a cubic BaSnO3 (BSO) structure was obtained. The scanning electron microscopy results (SEM) indicate a significant decrease in grain size upon doping but also reveal porosity, which hinders the electrical characterization of the samples. High resolution transmission electron microscopy (HRTEM) indicates the presence of low-angle grain boundaries (LAGBs) in the BaSn0.92Sb0.08O3 sample. Reliable electrical measurements performed on this sample indicate its semiconducting properties characteristic of materials with a double Schottky barrier at the grain boundary. Spark plasma sintering (SPS) was performed at temperatures 1000, 1100, 1200 and 1250 °C, for 5, 10, 15 and 30 min, at a pressure of 60 MPa and with a heating rate of 100 °C min–1. The most optimal conditions for the preparation of dense ceramic materials, in terms of temperatures and sintering time, were 1100 and 1200 °C for 5 min. By sintering at 1100 °C, single-phase ceramic samples with a cubic BSO structure were obtained. In comparison to the undoped sample, doping with Sb leads to a decrease in both grain size and electrical resisitivity. Electrical characterization shows semiconducting properties of all ceramic materials, with the lowest resistivity recorded for the BaSn0.92Sb0.08O3 sample. Apart from the cubic BSO, the spark plasma sintered samples at 1200 °C contained Ba2SnO4 as a secondary phase, whose content decreases upon doping. High sintering temperature and applied pressure created conditions for the reaction of SnO2 from the calcined powder with graphite from the mould, resulting in the evaporation of metallic tin and the formation of the barium-rich secondary phase. The field emission scanning electron microscopy (FE-SEM) results confirm that doping leads to a decrease in grain size and the concentration of dislocations in these ceramic samples. All samples except BaSn0.92Sb0.08O3 exhibit nonlinear current-voltage characteristics, typical for semiconductors with a vi potential barrier at the grain boundary. As a consequence of higher amount of LAGBs present in it, the ceramic BaSn0.92Sb0.08O3, showed linear I-U characteristics in the whole temperature measurement range (25–150 °C) and a significant drop in the electrical resistivity due to the loss of potential barrier at grain boundary. The impedance spectroscopy confirmed these assumptions. Unlike the previous method, cold sintering of the BaSn0.92Sb0.08O3 sample did not give satisfactory results either in phase composition and density or electrical properties. The reducing conditions typical for spark plasma sintering and the dopant concentration of 8 % (mol) Sb in the BaSnO3 sample led to the formation of LAGBs, an increase in the concentration of oxygen vacancies and a change in the valence state of Sn, consequently leading to increase in the charge carriers (electrons) concentration and resulting in the ceramic sample with a linear I-U characteristic and a constant value of electric resistivity in a wide temperature range

Crystallization and sinterability of lanthanum-strontium-borate-glass powders

Predmet ove doktorske disertacije je ispitvanje kristalizacionog ponašanja i sinterabilnosti boratnih stakala iz sistema La2O3-SrO-B2O3, o čemu postoji ograničen broj podataka u literaturi. Preliminarna ispitivanja su obuhvatala dobijanje 4 različita sastava ovog sistema, tako što se sadržaj lantana povećavao na račun stroncijuma, dok je sadržaj bora bio konstantan: 5,7La2O3·22,9SrO·71,4B2O3; 9,5La2O3·19,1SrO·71,4B2O3; 14,3La2O3·14,3SrO·71,4B2O3 i 19,1La2O3·9,5SrO·71,4B2O3. Isptivana su fizička i strukturna svojstva kao i kristalizacione karakteristike i sinterabilnost. Kristalizaciono ponašanje stakala praćeno je diferencijalnom termijskom analizom (DTA), rendgenskom difrakcijom praha (XRD), transmisionim i skenirajućim elektronskim mikroskopima (TEM i SEM), dok je sinterabilnost prahova praćena primenom termomikroskopa (TM) i DTA. Rendgenskom difrakcijom praha identifikovane su faze nastale tokom toplotne obrade, odnosno nakon kristalizacije, dok je mehanizam kristalizacije, kao i mikrostruktura stakla nakon toplotne obrade ispitivana SEM-om. Rezultati početnih ispitivanja ukazali su da staklo 14,3La2O3·14,3SrO·71,4B2O3 pokazuje niz specifičnosti u pogledu kristalizacionih karakteristika, kao i sinterabilnosti. Na osnovu rezultata XRD, TEM i SEM analize utvrđen je površinski mehanizam rasta kristalne faze koja je istog sastava kao i staklo (polimorfna kristalizacija). Za ovu fazu u literaturi ne postoje podaci, tako da su na osnovu TEM i XRD analize dobijeni podaci za jediničnu ćeliju. U cilju dobijanja što boljih rezultata sinterovanja pripremljen je prah stakla izuzetno malih veličina čestica. Proces sinterovanja, odnosno, sinter-kristalizacije praćen je korišćenjem DTA, TM i optičkog dilatometra. Ispitivan je uticaj temperature sinterovanja, kao i brzine zagrevanja do izabrane temperature sinterovanja na efikasnost sinterovanja. Stepen denzifikacije, uzoraka je određen merenjem poroznosti nakon toplotne obrade. Za identifikaciju faza koje su nastale u sinterovanim uzorcima korišćena je XRD tehnika. Sinterovani uzorci su analizirani skenirajućim elektronskim mikroskopom, primenom detektora sekundarnih elektrona, SE, i povratno rasejanih elektrona, BSE, da bi se stekao uvid u poroznost i u mikrostrukturu nastale staklo-keramike...The subject of this doctoral thesis is the study of the crystallization behavior and the sinterability of the borate glasses system La2O3-B2O3-SrO. In literature there are limited data from the crystallization characteristics and sinterability of these glasses. Preliminary goal of this study was to investigate 4 different composition of the system, in which the content lanthanum of was increased at the expense of the strontium, while the boron content remained unchanged: 5,7La2O3·22,9SrO·71,4B2O3; 9,5La2O3·19,1SrO·71,4B2O3; 14,3La2O3·14,3SrO·71,4B2O3 and 19,1La2O3·9,5SrO·71,4B2O3. Physical and the structural properties as well as the crystallization characteristics and sinterability of glasses were investigated. Crystallization behavior was studied by the differentially thermal analysis (DTA), X-ray diffraction (XRD), transmission and scanning electron microscopy (TEM and SEM), while the sinterability by the hot stage microscopy (HSM) and DTA. For the identification of the crystalline phases after thermal treatment (after the crystallization) XRD was used. Mechanism of the crystallization and the resulting phase microstructure were observed by SEM. The results of the preliminary investigations revealed that the glass 14,3La2O3·14,3SrO·71,4B2O3 shows the specific crystallization and sinterabilty characteristics. Based on XRD, TEM and SEM results the surface crystallization mechanisam was detected, where the obtained crystalline phase was of the same composition as the initial glass (polymorphic crystallization). In the literature there are no data for this crystalline phase, so XRD and TEM were used for the determination of the unit cell parameters. In order to obtain the best possible sintering results, the glass powder of extremely small particles (nanoparticles) was prepared. The sintering process was followed by the DTA, HSM and the contactless optical dilatometer. The effects of the sintering temperature, as well as the heating rate up to sintering temperatures, on the sintering efficiency were investigated. The quality of sintering, densification of the samples, was evaluated by the measuring the porosity of samples after the thermal treatment. XRD was used to determine the composition of the phase resulting from sintering. The sintered samples were analysed by SEM, equipped with secondary electron (SE) and backscattered electron detectors (BSE), to gain insight in the porosity and the microstructure of the obtained glass-ceramic..

Two-step sintering, phase transformations, electrical and mechanical properties of nanostructured bioceramic materials based on hydroxyapatite

Doktorska disertacija se odnosi na kreiranje gustih nanostrukturnih sinterovanih materijala na bazi hidroksiapatita. Dvostepeno sinterovanje je iskorišćeno za dobijanje bifaznog kalcijum fosfata visoke gustine polazeći od Ca-deficitarnog hidroksiapatita na temperature nižoj od temperature fazne transformacije β→α-trikalcijum fosfat. Kreirana je uniformna mikrostruktura sa prosečnom veličinom zrna od 375 nm, i poboljšane su mehaničke osobine bifaznog kalcijum fosfata. Utvrđeno je i da povećanjem brzine zagrevanja (od 2 do 20 °C/min) poboljšava sinterovanje Ca-deficitarnog hidroksiapatita, što je objašnjeno pomeranjem temperature nastanka β-trikalcijum fosfatne faze ka višim temperaturama. Primenom bržeg zagrevanja došlo je do smanjanja prosečne veličine zrna prilikom neizotermskog (na ~700 nm, uz gubitak frakcije mikrometarskih zrna) i dvostepenog sinterovanja (sa 600 nm na 350 nm) dok je relativna gustina materijala bila iznad 98 %. Metoda dvostepenog sinterovanja je uspešno primenjena i za dobijanje gustog nanostrukturnog hidroksiapatita sa prosečnom veličinom zrna od 75 nm na temperaturama od 900/850 °C. Energija aktivacije sinterovanja određena upotrebom master krive sinterovanja iznosila je 410 kJ/mol. Ukazano je i da efikasnost metode dvostepenog sinterovanja za zaustavljenje rasta zrna prilikom sinterovanja različitih nanoprahova hidroksiapatita do visokih gustina zavisi od mikrostrukture pojedinačnih nanočestica, odnosno uređenosti centralnih i graničnih regiona, kao i kristaliničnosti sintetisanih materijala. Nanoprah hidroksiapatita sa česticama koje imaju visoko-kristalno jezgro i neuređene, amorfne rubove, se znatno bolje sinteruje od slabo-kristalnih nanoprahova bez obzira na graničnu strukturu njihovih čestica. Visokotemperaturska električna provodljivost unutrašnjosti zrna gustog nanostrukturnog hidroksiapatita je povećana za red veličine u odnosu na hidroksiapatit sa mikrometarskom/submikrometarskom prosečnom veličinom zrna, dok se provodljivost granica zrna ne menja znatno. Smanjanje prosečne veličine zrna kod bifaznog kalcijum fosfata i hidroksiapatita visoke gustine je dovelo do poboljšanja tvrdoće i lomne žilavosti.Subject of this PhD thesis is creating of fully dense sintered nanostructured materials based on hydroxyapatite. Two-step sintering method was successfully applied for synthesis of high density biphasic calcium phosphate at sintering temperatures lower than those of β→α tricalcium phosphate phase transformations. Uniform microstructural properties were created, with average grain size of 375 nm and improved mechanical properties. Also, it was found that increase of heating rate (from 2 to 20 °C/min) would enhance densification of Ca-deficient hydroxyapatite. This was explained by kinetic dependence of β-tricalcium phosphate formation. Application of faster heating induced decrease in average grain size for non-isothermal (~700 nm, and disappearing of fraction of large grains) and two-step sintering (from 600 to 350 nm) while relative density was above 98 %. Two-step sintering was also successfully applied for fabrication of fully dense nanostructured hydroxyapatite with average grain size of 75 nm at temperatures of 900/850 °C. Activation energy for sintering process was determined by master sintering curve method, and found to be 410 kJ/mol. It was indicated that efficiency of two-step sintering for densification and suppressing of grain growth of hydroxyapatite nanopowders depends on nanoparticles microstructure, particularly the ordering of its internal and boundary regions, as well as its crystallinity. Hydroxyapatite nanopowder possessing high-crystalline core and non-ordered, amorphous boundary regions expressed enhanced densification when compared with low-crystalline nanopowders, regardless its nanoparitcles’ boundary structure. High-temperature grain interior electrical conductivity of dense nanostructured hydroxyapatite was increased for one order of magnitude when compared to those with average grain size on micrometer/submicrometer level, while grain boundary conductivity did not changed significantly. Decrease of average grain size yielded to improvement of hardness and fracture toughness. of sintered biphasic calcium phosphate and single-phased hydroxyapatite materials

Investigation of the influence of processing parameters on the properties of bioceramic materials based on calcium hydroxyapatite and calcium phosphate obtained by different sintering technique

Kalcijum-fosfatni biokeramički materiali na bazi kalcijum-hidroksiapatita (HAP) i trikalcijum-fosfata (TCP), zahvaljujući hemijskom sastavu, biokompatibilnosti, bioaktivnosti i osteokonduktivnosti imaju značajnu ulogu pri izradi koštanih implanta. Guste nanostrukturne i kontrolisano porozne forme biokeramičkih materijala na bazi hidroksiapatita i kalcijum-fosfata imaju veliku primenu u maksiofacialnoj, dentalnoj i ortopedskoj praksi. Pažljiva kontrola faznog sastava, veličine zrna, oblika i dimenzija pora, kao i mehaničkih svojstava sinterovanih biokeramičkih materijala jesu ključni faktori za dobijanje implanata optimalnih svojstava. Danas je ogroman broj istraživanja baziran upravo na definisanju optimalnih uslova procesiranja sa ciljem da se dobiju gusti i kontrolisano porozni biokeramički monofazni kalcijum-hidroksiapatitni i bifazni materijali na bazi HAP i TCP, pogodni za ugradnju u ljudski organizam. Cilj ove doktorske disertacije bio je proučavanje procesa formiranja gustih nanostrukturnih biokeramičkih materijala na bazi kalcijum-hidroksiapatita i kalcijum-fosfata mikrotalasnim sinterovanjem, spark plazma sinterovanjem i toplim presovanjem, polazeći od stehiometrijskih i kalcijum deficitarnih nanočestičnih prahova kalcijum-hidroksiapatita dobijenih modifikovanim precipitacionim sintezama. Ispitan je uticaj svojstava polaznih prahova HAP i parametara procesiranja na mikrostukturne i fazne karakteristike, kao i na mehanička svojstva dobijenih gustih sinterovanih biokeramičkih materijala. Definisan je uticaj mikrostrukturnih parametara na žilavost i tvrdoću dobijenih monofaznih i bifaznih biokeramičkih materijala. Nanoindentacijom je utvrđen uticaj veličine zrna i faznog sastava na nanomehaničke karakteristike gustih mikro i nanostrukturnih materijala na bazi HAP i HAP/β-TCP. Svojstva materijala dobijenih mikrotalasnim sinterovanjem, spark plazma sinterovanjem i toplim presovanjem upoređeni su sa svojstvima materijala dobijenih konvencionalnim sinterovanjem.Calcium phosphate bioceramics materials based on hydroxyapatite (HAP) and tricalcium phosphate (TCP), due to their chemical composition, excellent biocompatibility, bioactivity and osteoconduction have received considerable attention as suitable bioceramics for the manufacture of osseous implants. Dense nanostructured and controlled porous forms of hydroxyapatite and calcium phosphate are often used as reparation material in maxillofacial, dental and orthopedic surgery. Careful control of phase composition, grain size and shape and size of pores of sintered bioceramic materials in accordance with good mechanical properties can be key issue for a successful implant. A large number of studies were focused on determining the optimal conditions required to obtain dense or controlled porous HAP and HAP/TCP form suitable for incorporation in living bone. The aim of this PhD thesis was to investigate the processing of dense nanostructured pure hydroxyapatite and biphasic HAP/TCP bioceramics by microwave sintering, spark plasma sintering and hot pressing, starting from stoichiometric and calcium deficient nanosized HAP powders, obtained by two modified chemical precipitation syntheses. The effects of the processing conditions on the microstructure, phase composition and mechanical properties of the obtained materials based on HAP and TCP were investigated. The influence of microstructure and phase composition on the fracture toughness and hardness of sintered HAP and HAP/TCP bioceramics was also defined. The influence of the grain size and phase composition on the nanomechanical behaviour of dense nano and microstructured HAP and biphasic HAP/β-TCP bioceramic materials by nanoindentation was also investigated. The results obtained by microwave sintering, spark plasma sintering and hot pressing were compared with those obtained by conventional sintering methods..

Preparation of zinc oxide varistors with submicronic grain size and ultra-high breakdown field

Cink-oksidni varistori sa izrazito visokim poljem proboja potrebni su u savremenim tehnologijama, gde je poželjno da prenaponska zaštita bude što manjih dimenzija, a istovremeno i električno funkcionalna. U ovoj doktorskoj disertaciji za postizanje visokog polja proboja primenjena su dva smera: dobijanje varistora iz pojedinačno sintetisanih nanodimenzionalnih prekursora (Co/Mn-dopiranog ZnO, Bi2O3 i Sb2O3), kao i metoda uvođenja Bi2O3 i Sb2O3 aditiva u već sintetisani Co/Mn-dopirani ZnO preko rastvora odgovarajućih soli...Zinc oxide varistors with ultra-high breakdown field are necessary for applications in modern technologies, where the overvoltage protection should be simultaneously as small as possible and electrically functional. Preparation of ultra-high breakdown field varistors, that was presented in this work, included two routes: preparation of varistors via separately synthesized nanoprecursors (Co/Mn-doped ZnO, Bi2O3 and Sb2O3), and also by the addition of Bi2O3 and Sb2O3 additives through the solutions of appropriate salts..

Synthesis and characterization of BaTi1-xSnxO3 powders and multilayered ceramic materials

BaTi1−xSnxO3 (BTS) prahovi (x od 0 do 0.15) sintetisani su reakcijom u čvrstom stanju. Određene su fizičke karakteristike sintetisanih BTS prahova i monomorfnih sinterovanih komponenata: srednja veličina čestica, raspodela veličina čestica, srednja veličina aglomerata, i morfologija kalcinisanih BTS prahova; kristalna simetrija, parametri jedinične ćelije, temperatura faznog prelaza, skupljanje tokom sinterovanja, gustina i mikrostruktura sinterovanih komponenata; kao i njihove električne karakteristike - dielektrična permitivnost, koeficijent termičkog širenja, remanentna polarizacija, koercitivno polje, otpornost zrna (materijala) i otpornost granice zrna. Uočeno je da se sa promenom sadržaja kalaja u BTS uzorcima menjaju i njihove fizičke karakteristike. Povećanje sadržaja kalaja dovodi do smanjenja tetragonalnosti kristalne simetrije i do povećanja zapremine jedinične ćelije. Sa povećanjem sadržaja kalaja smanjuje se procenat skupljanja BTS materijala tokom sinterovanja, povećava se poroznost tj. smanjuje se gustina sinterovanih materijala. Takođe, sa povećanjem sadržaja kalaja u sinterovanim BTS komponentama snižava se temperatura faznog prelaza a raste intenzitet maksimuma dielektrične konstante; smanjuju se vrednosti remanentne polarizacije i koercitivnog polja, osim toga, smanjuje se otpornost granice zrna. Nakon detaljnog ispitivanja karakteristika BTS monomorfnih materijala, pripremljeni su višeslojni BTS materijali u kojima je variran broj slojeva (od 2 do 6), kao i kombinacija sadržaja Sn po slojevima. Nakon sinterovanja u svakom od BTS FGMa formiran je različit gradijent sadržaja kalaja. Ispitano je skupljanje tokom sinterovanja BTS FGMa, njihova mikrostruktura i sadržaj kalaja kroz presek, a određene su i njihove električne karakteristike. BTS FGMi pokazuju veoma složeno ponašanje tokom sinterovanja, koje je uslovljeno difuzijom Sn4+ kroz slojeve FGMa. Difuzija katjona zavisi od broja slojeva i njihove debljine, od početne koncentracije kalaja unutar svakog od slojeva, kao i od brzine zagrevanja tokom neizotermskog sinterovanja. U ispitivanim slučajevima BTS FGMa, iako je postojao gradijent sastava po visini uzoraka, pri čemu se svaki od slojeva skupljao različitom brzinom, nije došlo do pucanja ili raslojavanja kompakata tokom procesa sinterovanja. Mali stepen distorzije uočen je kod onih višeslojnih uzoraka kod kojih je promena u sadržaju kalaja stepenasta (a ne kontinualna). BTS FGMi imaju proširen interval εrmax, tj. imaju relativno veliku vrednost dielektrične konstante u širokom intervalu temperatura. Ovaj efekat je mnogo jasnije izražen kod FGMa sa više slojeva, u kojima se sadržaj kalaja kroz presek postepeno menja. Pokazano je da se kod BTS FGMa vrednost dielektrične konstante i interval temperature faznog prelaza mogu modifikovati/kreirati kombinacijom različitih BTS prahova kao i variranjem broja slojeva. Ipak, treba naglasiti da sa povećanjem broja slojeva dolazi do smanjenja εrmax. To je posledica načina pripreme uzoraka (uniaksijalnog presovanja) i formiranja izolatorskih međuslojeva između BTS slojeva. Tanka izolatorska međupovršina (poroznost) između različitih BTS slojeva menja električne karakteristike FGMa. Što je više takvih međupovršina lošije su dielektrične karakteristike sinterovane monolitne keramike. Uočeno je da jedno- i višeslojni BTS sinterovani materijali imaju bolje dielektrične karakteristike ako se nakon sinterovanja dodatno tretiraju u mikrotalasnom polju u toku 10 min. Pokazano je da se delovanjem intenzivnog ultrazvuka, u trajanju od 3 h, srednja veličina čestica kalcinisanog praha barijum titanata (BT) smanjuje od 1.4 μm do 64 nm. Osim toga, utvrđeno je da se sa smanjenjem srednje veličine čestica poboljšavaju karakteristike sinterovanih BT materijala, tj. povećava se procenat zgušnjavanja materijala tokom sinterovanja, povećava se maksimum dielektrične konstante, i raste otpornost granice zrna. Osim toga, jedno- i višeslojni BTS materijali pripremljeni od kalcinisanih a zatim deaglomerisanih BTS prahova imaju veću dielektričnu konstantu nego materijali pripremljeni od kalcinisanih (aglomerisanih) BTS prahova.BaTi1-xSnxO3 (BTS) powders, with x ranging from 0 to 0.15, were synthesized by solid-state reaction technique. Physical characteristics of synthesized BTS powders and sintered BTS ceramics were examined: average particle size, particle size distribution, average agglomerate size, morphology of synthesized BTS powders; crystal symmetry, unit cell parameters, temperature of phase transition, densification during sintering, density and microstructure of sintered ceramics, also, dielectric permittivity, coefficient of thermal expansion, remanent polarization and coercitive field, grain (bulk) and grain boundary resistivity. It was found that changing of tin content in BTS samples induced changes of their physical characteristics. The increase of tin content in BTS sintered materials led to decreasing of: tetragonality, percent of shrinkage during sintering and density of sintered materials, Curie temperature, remanent polarization, coercitive field, as well as decreasing of grain boundary resistivity. Also, there was increasing of: unit cell volume, porosity, and dielectric permittivity with increasing of tin content in sintered BTS materials. After detailed examination of the sintered BTS materials characteristics, BTS FGMs were prepared with variety of layeres number (from 2 to 6) and tin content within layeres. After sintering, BTS FGMs with different tin concentration gradient were formed. Shrinkages during sintering of BTS FGMs were examined, as well as their microstructure, tin content in cross section, and electrical characteristics. BTS FGMs show very complex behavior during sintering, which is influenced by Sn4+ diffusion through FGM layers. The diffusion of cations depends on the number and thickness of the layers, starting tin content in each layer, and also, on the heating rate during non-isothermal sintering. Even though the content gradient existed in the examined BTS FGMs and each of the layers exhibited different percent of shrinkage, no cracks or delamination were noticed in the samples after the sintering. Small degree of distortion is noticed in multilayered samples with graded (not continual) changes of tin content. Furthermore, it is noticed that multilayer BTS ceramics have a broadened peak of maximum permittivity εrmax, i.e. have a relatively high dielectric constant in a wide temperature range. This effect is clearly expressed in FGMs with more layers, and with continual changes of tin content in cross section. It was found that dielectric properties and phase transition temperature interval of these materials can be modified by combination of different BTS powders as well as layers number. However, it can be emphasized that increasing of the number of the layers decreases εrmax. This effect is a result of insulation layers (porosity) formed between different BTS layers due to FGMs preparation technique (uniaxially pressing). Thin insulation layer between different BTS layers changes electrical characteristics of FGM. Increasing of the insulation layers number makes worse dielectric characteristics of monolithic sintering ceramics. It was noticed that BTS mono- and multilayered ceramics, sintered at 1370 oC, have better dielectric properties if they are additionally treated in microwave oven for 10 min. A high-intensity ultrasound irradiation (ultrasonication) was used to de-agglomerate calcined barium titanate powder. After 3 hours of ultrasonication average particle size was de-agglomerated from 1.4 μm to 64 nm. Besides, with decreasing of the powder's average particle size, characteristics of sintered BT ceramics were improved. Decreasing of average particle size induces higher densification during sintering, increasing of dielectric permittivity, as well as increasing of grain boundary resistance. Besides, mono- and multilayered materials prepared from calcined and de-agglomerated BTS powders possess higher dielectric permittivity than materials prepared from calcined powders

The influence of mechanical activation on the structure and properties of strontium titanate ceramics

Cilj ove doktorske disertacije je bio da se analizira i sagleda uticaj mehaničke aktivacije na strukturu i svojstva nedopiranog i dopiranog SrTiO3 praha, kao i uticaj na strukturu i svojstva keramike dobijene sinterovanjem ispresaka pomenutih prahova. U slučaju sinterovanih nedopiranih SrTiO3 uzoraka dodatno su razmatrane promene u brzini densifikacije i kinetici sinterovanja. Uticaj mehaničke aktivacije i dopanta na električna svojstva stroncijum-titanatne keramike je analiziran merenjem frekventne zavisnosti relativne dielektrične permitivnosti i tangensa ugla dielektričnih gubitaka na sobnoj temperaturi. Magnetnim merenjima, kod dopiranih SrTiO3 uzoraka dobijenih dodavanjem mangan(IV)-oksida, praćen je uticaj mehaničke aktivacije, koncentracije dopanta i jačine magnetnog polja na vrednost specifične magnetizacije. Mehanička aktivacija SrTiO3 praha je vršena u visoko-energetskom planetarnom mlinu sa kuglama, pri čemu je vreme aktivacije nedopiranog praha iznosilo: 5, 10, 30, 60, 90 i 120 minuta, dok je efekat dopiranja razmatran za aktivaciju od 10, 30 i 120 minuta. U cilju dobijanja SrTiO3:Mn sistema, tipa Sr1-xMnxTiO3 (SMnT) ili SrTi1-xMnxO3 (STMn), u polazni SrTiO3 prah je dodat mangan(IV)-oksid (MnO2), pri čemu su izabrane vrednosti za x bile: 0,03, 0,06 i 0,12. Raspodela veličina čestica je ukazala da je mehanička aktivacija dovela do usitnjavanja čestica polaznog praha, uz istovremenu pojavu šire raspodele veličina čestica pri dužim vremenima aktivacije. Skenirajućom i transmisionom elektronskom mikroskopijom (SEM i TEM) analizirana je mikrostruktura i morfologija polaznog i mehanički aktiviranih prahova. BET metodom je utvrđeno da se najviša vrednost specifične površine kod nedopiranih prahova postiže pri aktivaciji u trajanju od 60 minuta, dok pri daljoj aktivaciji specifična površina ostaje približno konstantne vrednosti, usled povećane sekundarne aglomerizacije. Pokazano je da je gustina ispreska kod praha aktiviranog tokom 10-30 minuta najveća. Zapaženo je sniženje intenziteta XRD pikova sa porastom vremena aktivacije, kao i širenje difrakcionih linija, usled smanjenja veličine kristalita i porasta mikronaprezanja. Primenjena mehanička aktivacija je omogućila formiranje kubnog nanokristalnog SrTiO3 praha, pri čemu veličina kristalita opada i do ~20 nm sa uvećanjem vremena aktivacije. Uočena promena u vrednosti parametra kristalne rešetke a je posredno ukazala i na moguće promene u koncentraciji kiseonikovih vakancija. Analizom Ramanovih spektara je praćen uticaj mehaničke aktivacije na promene u fononskom spektru prahova, sa posebnim akcentom na polarne TO mode, čiji oblik i intenzitet bitno zavise od defekata kao što su kiseonikove vakancije. Analiza optičkih svojstava kristalnih materijala pomoću UV-Vis spektroskopije je ukazala na sniženje energije zabranjene zone sa uvećanjem vremena aktivacije. Primenom dilatometrije je ispitivan uticaj mehaničke aktivacije na početni stadijum sinterovanja dvostrano presovanih SrTiO3 prahova. Uočeno je značajno smanjenje temperature početka skupljanja ispreska, kao i uticaj na brzinu skupljanja i na konačne gustine sinterovanih uzoraka. Primena Dornove metode je ukazala na značajno smanjenje efektivne energije aktivacije transporta mase u početnom stadijumu sinterovanja, sa porastom vremena mehaničke aktivacije polaznog praha. Za uzorke dobijene sinterovanjem do 1300 oC, uz izotermsko zadržavanje na maksimalnoj temperaturi u trajanju od 2h, je izvršena korelacija između zapaženih strukturnih promena i promena u dielektričnim svojstvima, nastalim usled mehaničke aktivacije. Posebna pažnja je posvećena uticaju promena: gustine, poroznosti, veličine zrna i kristalita, kao i uticaju promena parametra kristalne rešetke, na dielektrična svojstva. Promene vrednosti relativne dielektrične permitivnosti su razmatrane i sa stanovišta promena u oblasti granice zrna, posebno imajući u vidu rezultate koji su ukazali na promenu koncentracije kiseonikovih vakancija...The aim of this doctoral dissertation is to analyze and evaluate the effect of mechanical activation on the structure and properties of undoped and doped SrTiO3 powders, as well as the effect on the structure and properties of the ceramics obtained by sintering these powders. For the sintered undoped SrTiO3 samples, changes in the densification rate were also analyzed. The effect of mechanical activation and dopant on the electrical properties of strontium titanate ceramics was analyzed by measuring the frequency-dependent relative dielectric permittivity and the loss tangent at room temperature. Magnetic measurements performed on the doped SrTiO3 samples obtained by the addition of manganese(IV) oxide were used to monitor the influence of mechanical activation, dopant concentration and magnetic field strength on magnetization. The mechanical activation of the SrTiO3 powder was performed in a high-energy planetary ball mill; the activation times of the undoped powder were: 5, 10, 30, 60, 90, and 120 minutes, while the doping effect was analyzed for the activation periods of 10, 30, and 120 minutes. In order to obtain SrTiO3:Mn systems belonging to Sr1-xMnxTiO3 (SMnT) or SrTi1-xMnxO3 (STMn) compound types, manganese(IV) oxide (MnO2) was added to the starting SrTiO3 powder, where the values selected for x were: 0.03, 0.06 and 0.12. The particle size distribution indicated that mechanical activation resulted in the comminution of the starting powder particles, accompanied with a wider particle size distribution at longer activation times. The microstructural and morphological analysis of the starting and mechanically activated powders was performed using scanning and transmission electron microscopy (SEM and TEM). The BET method revealed that 60-minute activation resulted in the highest specific surface area in undoped powders, while further activation led to a relatively constant specific surface due to increased secondary agglomeration. The density of the powder activated for 10-30 minutes was the highest. The intensity of XRD peaks decreased with longer activation times and diffraction lines broadened due to decreased crystallite sizes and increased microstrain. Mechanical activation led to the formation of cubic nanocrystalline SrTiO3 powder, with the minimum crystallite size of 20 nm. The observed change in the value of the crystal lattice parameter indirectly indicated possible changes in the concentration of oxygen vacancies. The analysis of Raman spectra revealed the effect of mechanical activation on the phonon spectrum of powders, with particular emphasis on polar TO modes which shape and intensity generally depend on defects such as oxygen vacancies. The analysis of the optical properties of crystalline materials by UV-Vis spectroscopy indicated a correlation between decreased band gap energy and increased activation times. The effect of mechanical activation on the initial stage of sintering of bilaterally pressed SrTiO3 powders was investigated by dilatometry. A significant decrease in the temperature of the onset of densification was observed, as well as the effect of mechanical activation on the densification rate and on the final density of the sintered samples. The application of the Dorn method indicated a significant decrease in the effective activation energy of mass transport in the initial sintering stage, with a longer time of the mechanical activation of the starting powder. For the samples obtained by sintering up to 1300 °C, with isothermal retention at the maximum temperature for 2 h, a correlation between the structural changes and the changes in dielectric properties resulting from mechanical activation was observed. Particular attention was paid to the effect of changes: density, porosity, grain and crystallite size, as well as the effect of crystal lattice parameter changes on the dielectric properties. The changed values of the relative dielectric permittivity were analyzed in the context of the changes in the grainboundary region, with special respect to the results indicating changes in the oxygen vacancy concentration

Synthesis and characterization of ceramic composite materials based on silicon-carbide and cordierite

Predmet ove doktorske disertacije je sinteza i karakterizacija kompozitnih keramičkih materijala na bazi silicijum-karbida i kordijerita. Silicijum-karbid, SiC, je kovalentno jedinjenje male gustine, velike tvrdoće, odlične termičke stabilnosti, dobre toplotne provodljivosti, dobre otpornosti prema termošoku. Ovakva svojstva omogućavaju njegovu brojnu primenu u raznim oblastima. Zbog kovalentne prirode veze, dobijanje guste SiC keramike je nemoguće bez dodatka aditiva. Dosadašnja ispitivanja su pokazala da se dodatkom smese oksida, kao aditiva, dobija gusta SiC keramika na temperaturama ispod 2100°C. Kordijerit, 2MgO 2Al2O3 5SiO2, poseduje odličnu termičku stabilnost, nizak koeficijent toplotnog širenja i dobru otpornost prema termošoku. U ovom radu kordijerit je dobijen iz dva izvora: iz spinela, MgAl2O4, i iz Mg-izmenjenog zeolita LTA a zatim je uz dodatak silicijum-karbida korišćen za sintezu kompozitnih materijala. Kompozitni materijali dobijeni su mehaničkim mešanjem SiC sa 30 i 50 mas.% kordijerita, dobijeno je četiri vrste kompozita. Posebna pažnja posvećena je proučavanju procesa sinterovanja u cilju dobijanja keramike sa poboljšanim fizičko-hemijskim svojstvima variranjem parametara sastav-temperaturapritisak. Praćen je uticaj sadržaja dodatog kordijerita na termostabilnost SiC/kordijeritnog kompozitnog materijala pošto poznavanje otpornosti vatrostalnih materijala prema termošoku određuje njihovu primenu. Dodatak grafita omogućio je formiranje pora tokom sinterovanja. Sadržaj dodatog grafita značajno utiče na poroznost i čvrstoću kompozitnog materijala pa je proučavan uticaj poroznosti na termostabilnost kompozitne keramike. U okviru istraživanja korišćene su nedestruktivnih metode karakterizacije: ultrazvučne metode za određivanje dinamičkog Jungovog modula elastičnosti i čvstoće tokom ispitivanja termošoka, kao i primena alata za analizu slike, Image Pro Plus program, radi određivanja stepena oštećenja uzoraka pre i u toku ispitivanja termostabilnosti. Uspostavljene su zavisnosti i predloženi modeli između izmerenih brzina prostiranja ultrazvučnih talasa, Jungovog modula elastičnosti, kao i stepena oštećenja uzoraka sa brojem ciklusa termošoka. Ustanovljeno je veoma dobro slaganje između rezultata predloženih modela posmatranih veličina sa brojem ciklusa termošoka. Ovakav pristup je doveo do razvoja modela za predviđanje smanjenja čvrstoće kompozitnih materijala usled termošoka na osnovu promena brzine prostiranja ultrazvučnih talasa kroz kompozitne materijale, kao i stepena oštećenja površine kompozitnih materijala. Takođe, ovaj pristup je iskorišćen radi razvijanja modela koji određuju kritični broj ciklusa, koje kompozitni materijali izdrže a da ne dođe do razaranja 50 % čeone površine usled izlaganja termošoku, na osnovu promena posmatranih parametara (brzina prostiranja ultrazvuka kroz materijal i stepen oštećenja površine)...The aims of this dissertation are synthesis and characterization of silicon-carbide and cordierite based ceramic composite materials. Silicon-carbide, SiC, is low density covalent compound with high hardness value, excellent thermal stability, high thermal conductivity and good thermal shock resistance. These properties of SiC allow numerous applications in different fields. Covalent bond of SiC precludes formation of high density SiC ceramics without additive. Previous investigations show that the combination of oxide mixture and SiC results in dense SiC ceramics at temperatures up to 2100°C. Cordierite, 2MgO 2Al2O3 5SiO2, has excellent thermal stability, low coefficient of thermal conductivity and good thermal shock resistance. In this dissertation cordierite was synthesized by using two sources: spinel, MgAl2O4, and Mg-exchanged zeolite LTA and then, the mechanical commixtures with SiC powder were prepared with the aim of obtaining four types of composite materials with 30 and 50 wt% of cordierite. Particular attention in this thesis was focused on sintering process. Processing parameters such are composition, temperature, and pressure were changed with the aim of synthesizing ceramics with good physical-chemical properties. Variation of cordierite content in composite materials was applied due to examine his impact on thermal stability of SiC-cordierite composite materials. Graphite, as pore former during sintering, was added to change porosity of SiC-cordierite composite materials. Since addition of graphite has strong influence on porosity and hardness, the influence of graphite additions, 10, 20, and 30 wt%, on thermal stability of SiC-cordierite composite materials was investigated. Nondestructive methods of characterization were applied: ultrasonic pulse velocity technique for determination of dynamic Young modulus of elasticity and strength during the thermal shock investigation as well as tools for image analyses Image pro Plus program for determination of degradation level before and during the thermal shock experiment. Correlations and models between measured ultrasonic velocity, Young modulus of elasticity and degree of degradation were established. Good agreement between the results of suggested models of analyzed parameters with number of thermal shock cycles were obtained. This approach leads to the models for prediction of strength degradation level of composite materials during the thermal shock based on changes of ultrasonic velocity through the composite materials and level of surface degradation. Based on this analysis, model for prediction of critical number of cycles, the number of cycles that composite materials exposed to thermal shock can withstand without surface degradation less than 50 %, was developed..