The Influence of the Synthesis Parameters and Structure on the Electrical Properties of Ba0.77Sr0.23TiO3 Ceramics

Rezultati istraživanja u oblasti elektronske keramike u najvećoj meri direktno zavise od poznavanja procesa konsolidacije (presovanje i sinterovanje) keramičkih prahova, što, posmatrano sa gledišta poimanja prirode, ukazuje na univerzalnost ovoga procesa. Razvoj savremenih keramičkih materijala omogućen je zahvaljujući multidisciplitarnom proučavanju i doprinosu istraživanja u različitim oblastima kao što su nauka o materijalima, fizika, hemija i elektronika. Rezultati naučnog i tehnološkog istraživanja objavljuju se u različitim naučnim publikacijama, što je izraz velikog značaja elektronske keramike u savremenoj tehnici. Prema aktuelnim naučnim istraživanjima koja se odnose na projektovanje i razvoj novih elektronskih keramičkih komponenti (izolatorska, termistorska, elektrooptička, piezoelektrična keramika, memorijski uređaji na bazi feroelektrične keramike itd.), BST keramika kao i jedinjenja sa strukturom perovskita su od posebnog interesa. Kondenzatorska keramika na bazi BST obuhvata: visokofrekventne kondenzatore (promenljivi, nelinearni kondenzatori čija se električna kapacitivnost jako menja sa promenom primenjenog napona), višeslojne kondenzatore sa relativnom dielektričnom konstantom εr (1-15)·103 (zastupljene u tehnologiji debeloslojnih hibridnih kola), čip-kondenzatore sa barijernim slojem kod kojih je εr (20-80)·103, itd. U okviru ove grupe materijala, višeslojni keramički kondenzatori (MLCCs - Multi Layer Ceramic Capacitor) su već duže vreme jedan od najvažnijih i široko korišćenih pasivnih komponenti u elektronskoj industriji. Takođe, BST je široko proučavan i razvijan za potencijalnu primenu u dinamičkim «random access» memorijama i u NVRAM uređajima sa nedestruktivnom modom očitavanja. Tanki filmovi BST pokazuju prednosti u nizu primena, kako u memorijskim elementima u NVRAM i DRAM uređajima (u kompjuterima, mobilnim telefonima, digitalnim kamerama, MP3 plejerima i mnoštvu drugih portabl uređaja), tako i u elektrooptičkim prekidačima, optičkim talasovodima, piroelektričnim detektorima, optičkim modulatorima, itd. Barijum-stroncijum-titanat, Ba1-xSrxTiO3, (BST) je feroelektrični materijal sa tetragonalnom strukturom na sobnoj temperaturi za x<0,3, koji ima perovskitnu strukturu (ABO3). BST je čvrst rastvor sastavljen od titanata, barijum-titanata (BaTiO3) i stroncijum-titanata (SrTiO3). BaTiO3 je feroelektrični materijal sa Kirijevom temperaturom (Tc =393K), dok je SrTiO3 paraelektrični materijal sa neferoelektričnim faznim transformacijama kod koga je Kiri temperatura (Tc =105K). Na sobnoj temperaturi za sistem čvrstog rastvora u feroelektričnoj fazi sadržaj Ba je u opsegu od 0,7 do 1,0; dok je sadržaj Ba u paraelektričnoj fazi manji od 0,7. Kirijeva temperatura BST linearno opada sa povećanjem koncentracije Sr. Zbog toga Kiri temperatura BST a samim tim i njene osobine mogu biti prilagođene širokom opsegu da bi se zadovoljili zahtevi različitih primena. Ova vrsta fleksibilnosti osobina a samim tim i potencijalni opseg primene nije prisutan kod konvencionalne feroelektrične keramike. Postoji nekoliko različitih metoda za sintezu praha Ba1-xSrxTiO3, bilo suvom ili mokrom hemijskom sintezom. Prva grupa metoda zasnovanih na reakcijama u čvrstom stanju, su i najčešće metode za dobijanje BST dok su u drugoj grupi ko-precipitacija, sprej piroliza i neke sol-gel tehnike. Među njima su i hidrotermalne tehnike koje predstavljaju komercijalan način za dobijanje datog materijala. Svaka od ovih metoda ima svoje prednosti i mane. Neke od tih prednosti su dobijanje superfinog praha visoke čistoće i nizak stepen aglomeracije, dok su nedostaci visoka temperatura kalcinacije (od 1000oC do 1200oC), zatim korišćenje veće količine početnog praha, velika veličina zrna zbog čega se ne mogu koristiti za dobijanje materijala sa velikom dielektričnom konstantom. U skorije vreme u cilju sniženja temperature kalcinacije i dobijanja materijala definisane mikrostrukture, sve više se koristi i metoda mehaničke aktivacije. Cilj ovog rada je da pokaže uticaj mehaničke aktivacije i režima sinterovanja na promenu strukture i električna svojstva BST keramike. Na osnovu detaljne analize temperaturske zavisnosti, realne dielektrične propustljivosti i tangensa ugla gubitaka, kao i uticaja mehaničke aktivacije na promenu frekventnih karakteristika biće definisani optimalni tehnološki parametri za dobijanje Ba0,77Sr0,23TiO3 keramike unapređenih električnih karakteristika

Effect of WC on the Microstructure and Properties of Zirconium Diboride

Tungsten carbide (WC) additions, up to 5 wt%, were added to commercial zirconium diboride (ZrB2) powder. Densification was promoted by the addition of 0.5 wt% carbon. The powders with WC additions were hot-pressed at 2150°C with a pressure of 32 MPa. Bulk densities were measured by the Archimedes method. Scanning electron microscopy was used to determine grain size and morphology, as well as whether any second phases were present. The crystalline phases present were determined by x-ray diffraction. Properties, including hardness, elastic modulus, strength, and thermal conductivity, were measured and will be discussed

Point defects and their effect on dielectric permittivity in strontium titanate ceramics

The origin of dielectric properties of strontium titanate ceramics is investigated using DFT calculations in periodic system. It was determined that the main factors contributing to the increase in dielectric permittivity are: tetragonal distortion of the normally cubic lattice, and charge imbalance induced displacement of titanium center from its central position. Oxygen vacancies were determined to create significantly larger effects than other types of vacancies, like Ti and SrO. The extent of tetragonal distortion was found to be determined by oxygen vacancy distribution, rather than total concentration: relatively symmetrical distribution of oxygen vacancies resulted in smaller tetragonal distortion of the lattice, and, consequently, smaller increase in dielectric permittivity. Charge imbalance naturally destabilizes the cubic lattice, forcing the Ti-atom out of its central position, resulting in tetragonal lattice with increased dielectric permittivity. The process stabilizes the strontium titanate lattice, while increasing the c/a ratio. Therefore, the dielectric permittivity of strontium titanate can be increased by changes to the system that increase tetragonal distortion of the lattice and/or introduce additional negative charge

Influence of Mechanical Activation on the Constituents of the MgO-Al2O3-SiO2-MoO3 System

Cordierite, 2MgO•2Al2O3•5SiO2 (MAS), is high-temperature ceramic material. Cordierite is commonly used material because of outstanding electrical properties, low temperature expansion coefficient and low dielectric constant. In order to accelerate the process of sintering, 5.00 mass% MoO3 has been added to the starting mixtures. The mechanical activation of the starting mixtures was performed in a high energy ball mill during 0-160 minutes. All starting mixtures were sintered at 1100°C, 1200°C and 1300°C for 2h. The particle size analysis (PSA) was employed in order to determine the changes in the particle size of the mechanically treated powders. The phase composition of the starting powders was analyzed by the X-ray diffraction method. Differential thermal (DTA) and thermogravimetric (TG) analysis were used in order to determine characteristic temperatures within the system during heating. Based on the obtained DTA results, it was established that mechanical activation with additive MoO3, has influence on decreasing sintering temperatures for about 150°C

The Influence of Mechanical Activation on Sintering Process of BaCO3-SrCO3-TiO2 System

In this article the influence of mechanical activation on sintering process of barium-strontiumtitanate ceramics has been investigated. Both non-activated and mixtures treated in planetary ball mill for 5, 10, 20, 40, 80 and 120 minutes were sintered at 1100-1400 °C for 2 hours in air atmosphere. The influence of mechanical activation on phase composition and crystal structure has been analyzed by XRD, while the effect of activation and sintering process on microstructure was investigated by scanning electron microscopy. It has been established that temperature of 1100 °C was to low to induce final sintering stage for the system. Electrical measurements have been conducted for the densest ceramics sintered at 1400 °C for 2 hours

Influence of synthesis parameters and thermal treatment on functional properties of Fe3O4-BaTiO3 multiferroics obtained by mechanical activation

Mechanical activation of a mixture of polycrystalline powders Fe3O4 (50% wt.) and BaTiO3 (50% wt.) was performed in a planetary ball-mill, with different milling times (3 h, 6 h and 12 h). Average crystallite size determined by XRD analysis ranges from 12 to 30 nm, depending on the milling time. The activated powders were pressed into disc-shaped samples, 8 mm in diameter and 1.5 mm thick, applying the pressure of 500 MPa,. Thermoelectric measurements conducted in the temperature range from room temperature to 350 °C revealed that the electrical resistivity of the sample depends on temperature and activation time. At room temperature, the maximum value of specific electrical resistivity (ρ0 = 1 MΩm) was observed for the sample obtained by pressing the powder activated for 6 h. Magnetic properties of pressed powder samples were studied using a modified Faraday method. At room temperature, the pressed powder activated for 3 h exhibited the maximum value of magnetization, M0= 0.86 Am2/kg. Multiple heating of the pressed samples, for 10 min, was performed in a magnetic field of 20 KA/m. After cooling, the highest magnetization values were observed for the samples previously heated at 380 °C, while the maximum one (M’= 1.04 Am2/kg) corresponds to the sample activated for 3 h

Correlation between isothermal expansion and functional properties change of the Fe81B13Si4C2 amorphous alloy

The structural changes effect on functional properties of ribbon shaped samples of the Fe81B13Si4C2 amorphous alloy during annealing process was investigated in this paper. Differential scanning calorimetry method has shown that this alloy crystallizes in one stage, in temperature range from room temperature up to 700°C. Structural relaxation process was investigated by sensitive dilatation method in nonisothermal and isothermal conditions. It has been shown that structural relaxation process occurs in two stages by measuring thermal expansion at constant temperatures of t1=420°C, t2 = 440°C and t3 = 460°C. The first stage is characterized by linear logarithmic dependence of thermal expansion upon time at constant temperature. The second stage of structural relaxation process is characterized by linear dependence of isothermal expansion upon the square root of process time. These results imply that the first stage of structural relaxation process is a rapid kinetic process, while the second stage of structural relaxation process is a slow diffusion process. The rate constants k11 = 2,27⋅10- 3 s-1, k12 = 2,79⋅10-3 s-1, k13 = 3,6⋅10-3 s-1, k21 = 0,67⋅10-4 s-1, k22 = 3,72⋅10-4 s-1, k23 = 21,53⋅10-4 s-1 and activation energies E1 = 48,64 kJ/mol and E2 = 366, 23 kJ/mol were determined for both stages of structural relaxation process. The distinct correlation between structural relaxation process and magnetic susceptibility relative change was determined by thermomagnetic measurements. It has been shown that magnetic susceptibility can be increased by up to 80%, by convenient annealings after structural relaxation process, at magnetic field intensity of 8 kA/m

Influence of mechanical activation on mechanical properties of PVDF-nanoparticle composites

The influence of mechanically activated fillers (ZnO, BaTiO3 and SrTiO3 ultra-fine powders) on mechanical properties of poly(vinylidene) fluoride (PVDF) and oxide nanoparticle composite was investigated using molecular simulations. Mechanical activation leads to the creation of new surfaces and the comminution of the initial powder particles, which affects the crystallization of PVDF matrix. In addition, prolonged mechanical activation leads to agglomeration of nanoparticles into “soft” and “hard” agglomerates of different sizes. All of this has a significant effect on mechanical properties of PVDF-nanoparticle composites. Microstructural changes due to mechanical activation in ZnO, BaTiO3 and SrTiO3 powders were investigated using SEM and XRD, while dependence of mechanical properties on nanoparticle size was investigated using molecular simulation. These show that smaller nanoparticles significantly enhance the mechanical properties of PVDF-nanoparticle composite and allow use of mechanical activation as a means of reducing the amount of nanoparticle filler in the composite, while achieving the same of superior mechanical properties.Conference poster: [https://hdl.handle.net/21.15107/rcub_dais_861

Influence of mechanical activation on mechanical properties of PVDF-nanoparticle composites

Mechanical activation leads to the creation of new surfaces and decrease in particle size of the initial powder particles, which affects the crystallization of PVDF matrix. In addition, prolonged mechanical activation leads to agglomeration of nanoparticles agglomerates of different sizes. Microstructural changes due to mechanical activation in ZnO, BaTiO3, SrTiO3 and TiO2 powders were investigated using electron microscopy (TEM and SEM), x-ray diffraction (XRD) and particle size analysis (PSA). These were correlated with molecular simulations of mechanical properties of PVDF-nanocluster composites with different oxide nanocluster sizes (1, 1.5 and 2 nm) Each system was simulated using a periodic system of metal oxide nanocrystal inside β-PVDF matrix. β-PVDF was chosen for its superior properties over α- and γ- phases. Geometry of each system was optimized prior to the calculation of mechanical properties using GULP v4.3 software package. Molecular simulations show that, in general, mechanical properties (Young modulus as an indication of elasticity, Shear modulus as an indication of hardness) for all systems improve with decrease in nanocluster size. This suggests that mechanical activation of fillers in PVDF-metal oxide nanoparticle composites should have a positive effect on the mechanical properties of composite systems. This suggests mechanical activation as an easy an inexpensive method of improving mechanical properties of polymer-nanocrystal composite materials.Conference abstract: [https://hdl.handle.net/21.15107/rcub_dais_882

Synthesis of BaFe12O19-BaTiO3 multiferroics by mechanical activation

A mixture of polycrystalline powders of Fe (70 % wt.) and BaTiO3 (30 % wt.) was ball-milled in a planetary mill under air atmosphere, for different time intervals: 60, 120, 180, 240, 300 and 360 min. During the mechanical activation, the powder was exposed to oxygen from the air, resulting in formation of iron oxides: FeO and then Fe2O3 and Fe3O4. XRD and SEM analyses of the activated powders revealed that the weight fraction of the iron oxides in the mixture and microcrystal size depend on the activation time. For the powders activated for different time intervals, average crystallite size (Dhkl), dislocation density (ρn) and average microcrystal size of BaTiO3 and Fe were determined. In order to investigate the influence of thermally induced structural changes on magnetic properties, the change of magnetic properties of the pressed activated powders during multiple heating in a magnetic field of 10KA/m was measured. Maximum magnetization of the samples was reached after heating at 620 K. Pressed powder samples were sintered at temperatures of 1100 oC and 1200 °C for 2h giving the different phase diagrams. The samples sintered at 1100 oC include BaTiO3, BaFe12O19 and BaFeO2,67 as the dominant components. The samples sintered at 1200 °C containing only two components, BaTiO3 and BaFe12O19, exhibited pronounced ferromagnetic and ferroelectric propertie