Sintering and electromagnetic properties of BaFe12O19 ferrite prepared by co-precipitation method

BaFe₁₂O₁₉ (BaM) was synthesized through the co-precipitation route. Pure phase BaM was formed after calcination of precipitated powder at 900 °C. BaM was sintered at three different temperatures; 1100, 1200, and 1300 °C to study the sintering kinetics by varying the sintering time from 1 to 4 h. Apparent porosity decreased, and bulk density increased with increasing sintering temperature and period. A bulk density of about 4.6 g/cm³ was achieved after sintering at 1300 °C/4 h. The rate-controlling mechanism of BaM densification was the diffusion of oxygen, and the activation energy for the sintering process was 274 kJ/mol. The grain size of BaM increased with rising sintering temperatures. Permittivity increased from about 11 to 17 and the permeability increased from about 10 to 16 with the increase in sintering temperature from 1100 to 1300 °C. Saturation magnetization was also enhanced to about 69 emu/g after sintering at 1300 °C/4 h. Therefore, BaM ferrite synthesized through the co-precipitation route can be effectively used for high-frequency applications after sintering at 1300 °C.

     

The influences of firing temperatures and excess PbO on the crystal structure and microstructure of (Pb<Subscript>0.25</Subscript> Sr<Subscript>0.75</Subscript>)TiO<Subscript>3</Subscript> ceramics

Polycrystalline samples of (Pb_0.25Sr_0.75)TiO₃ (PST75) were prepared by the solid-state reaction method. The effects of firing temperatures and excess PbO on PST75 ceramics were investigated. The PST75 was calcined between 600 and 1000 °C for 3 h and the sintering temperature ranged between 1050 and 1250 °C for 2 h. The optimized calcination and sintering conditions were identified as 950 and 1250 °C, respectively. The lattice parameter c increased, while the lattice parameter a decreased with increased firing temperatures. The average particle size and average grain size increased with increased firing temperatures. After the addition of PbO—excess 0, 1, 3, 5, and 10 wt%—in the PST75 samples, the lattice parameter a decreased. The average particle size and the average grain size increased with the increase of PbO. The porous microstructure slightly decreased with an increasing amount of PbO—up to 3 wt%—then slightly increased with the higher excess PbO. The density was improved by adding 3 wt% of excess PbO. A low dielectric loss was observed from the 3 wt% excess PbO sample.     

Sintering and microstructural studies in the system ZrO2 · TiO2 · CeO2

Tetragonal zirconia polycrystals (TZP) in the system ZrO₂ · TiO₂ · CeO₂ have been prepared from titanium and zirconium alkoxides and cerium nitrate precursors. The change in microstructure with sintering temperature in the range 1300 to 1600° C has been characterized. A fully tetragonal structure with theoretical final density has been achieved after liquid-phase sintering in the range 1350 to 1400° C for 2h. Sintering at temperatures above 1450° C resulted in a loss of stabilizer from the matrix, by the formation of zirconium titanate and to a cerium-, titanium-rich liquid. The loss of stabilizer was such that in the temperature range 1500 to 1600° C, extensive transformation to monoclinic zirconia occurred spontaneously on cooling. The tetragonal zirconia formed after sintering at 1350° C was found to be very stable. Thec/a ratio of the tetragonal phase in this system is higher than in any of the binary TZP systems reported in the literature. The stability of the tetragonal phase is believed to be associated with this highc/a ratio.     

Dielectric Ba(Ti1?xSnx)O3 (x?=?0.13) ceramics, sintered by spark plasma and conventional methods

A two-step sintering approach composed of spark-plasma-sintering (SPS) technique at 1000 °C for 1 min and under a uniaxial pressure of 63 MPa followed by conventional sintering at 1400 °C for 3 h is proposed for synthesis of dense Ba(Ti_0.87Sn_0.13)O₃ ceramics. Starting powders had grain size of about 90 nm and were obtained by co-precipitation. The SPS pellets consist of submicron (300–500 nm) grains. X-ray diffraction analysis of as-prepared Ba(Ti_0.87Sn_0.13)O₃ ceramic shows the occurrence of cubic and tetragonal phase coexistence for the pellets obtained after SPS processing and the presence of only tetragonal phase in the samples after the second (conventional) sintering. Grain uniformity in the final product is high, with average size of ~2 μm. The apparent densities of the sintered pellets at temperature of 1400 °C were ~92% of the theoretical value of Ba(Ti_0.87Sn_0.13)O₃. The ceramics exhibit a high relative dielectric constant of 6,550 and a dielectric loss (tan δ) = 0.078 at Curie temperature of 63 °C and 10 Hz.     

The comparative influences of structural ordering, grain size, Li-content, and bulk density on the Li+-conductivity of Li0.29La0.57TiO3

The lattice and total Li⁺-ionic conductivity of Li_0.29La_0.57TiO₃ ceramic (LLTO) sintered at 1200 °C were determined as functions of powder calcination temperature and sintering duration, and these results were correlated with the relative degrees of Li⁺-ordering, Li-content, grain size, and bulk density to assess the relative impact of these parameters on material performance. Under all conditions, LLTO formed with a high degree of tetragonal superstructure to its perovskite related framework, and the lattice conductivity closely followed the relative amounts of the superstructure, as evaluated via determination of the sample ordering parameter from X-ray diffraction data. LLTO powders that were calcined at 900 °C for 1 h and sintered at 1200 °C for 6 h gave lattice conductivity values (~1.14 × 10⁻³ S cm⁻¹) comparable within the highest ranges reported in the literature. This coincided with the lowest degree of tetragonal superstructure formation, and it was also found to be largely independent of the values of Li-content measured on sintered ceramic despite significant Li₂O volatilization at longer sintering times (up to 23 % after 12 h at 1200 °C). Samples of LLTO powder that were calcined at 1100 °C and sintered at 1200 °C for 12 h resulted in the highest total Li-ion conductivity value ~6.30 × 10⁻⁵ S cm⁻¹. The total conductivity of LLTO varied inversely with grain size when the grains were <20 μm but was insensitive to that parameter above that size threshold. The strongest influence on total conductivity was primarily the bulk ceramic density. It was estimated from measured values that as the bulk ceramic density approached the full theoretical value for LLTO the total conductivity could near the lattice conductivity of ~1.2 × 10⁻³ S cm⁻¹.     

Investigation on electrical and microstructural properties of Thick Film Lead-Free resistor series under various firing conditions

The paper presents investigation of four lead free thick film resistor pastes, developed at ITME, denoted R-100, R-1k, R-10k and R-100k with sheet resistivities of 0.1, 1, 10 and 100 kΩ/□, respectively. The resistors were based on RuO₂ as the conductive phase. The aim of the work was to evaluate the influence of firing conditions of the resistive pastes on a sintering process. The pastes were screen printed onto alumina substrate with prefired AgPd lead-free terminations. They were fired at several temperatures from 750 to 950 °C for 10 min at peak temperature, as well as fired at the highest temperature for 6 h, in order to bring the sintering process into the equilibrium. The properties of the resistors, i.e , sheet resistivity and temperature coefficient of resistance (TCR), microstructure changes, glass crystallization upon firing, etc., were examined. Dried and fired resistor samples were evaluated by X-Ray diffraction analysis and by the scanning electron microscopy. The RuO₂ conductive phase maintained the same crystal structure regardless of the firing conditions. No devitrification was observed in lead-free resistors glasses. The lattice constants of RuO₂ were uniform after firing at temperatures over 800 °C. The resistors matched the desired resistivity and the TCR was the least temperature dependent at the firing temperatures around 850 °C.     

Reactive sintering mechanism of Ti + Mo<Subscript>2</Subscript>C and Ti + VC powder compacts

TiC reinforced Ti-matrix composites have been synthesized successfully by reactive sintering of Ti-1.5%Fe-2.25%Mo (wt%) powder compacts with addition of Mo₂C and VC particles. The reactions for the formation of TiC particles start at 600 °C, but the distribution of TiC particles and the densification behavior in the two compacts are significantly influenced by the metal carbides (Mo₂C or VC). The compact with addition of Mo₂C has a relative density of 98% after sintering at 1300 °C for 1.5 h, but TiC particles are agglomerated in the Ti matrix. The compact with addition of VC has a relative density of about 91% after sintering at 1300 °C for 1.5 h, but TiC particles distribute more homogenously in the Ti matrix. Different TiC particle distribution and densification behaviors are attributed to the reaction rates between Ti and metal carbides and the subsequent diffusion process.     

Sintering temperature-induced electrical properties of (Ba0.90Ca0.10)(Ti0.85Zr0.15)O3 lead-free ceramics

Effects of the sintering temperature on the microstructure and electrical properties of (Ba_0.90Ca_0.10)(Ti_0.85Zr_0.15)O₃ (BCTZ) lead-free piezoelectric ceramics have been studied, where these ceramics were prepared by the conventional oxide-mixed method at varied sintering temperatures from 1300 °C to 1500 °C. These BCTZ ceramics exhibits a phase transition from a rhombohedral phase to the coexistence of rhombohedral and tetragonal phases with an increase of sintering temperature. With an increase of sintering temperature, their relative density and average grain size gradually increase, and electrical properties are improved greatly. These BCTZ ceramics sintered at ～1440 °C have optimum electrical properties: d₃₃ ～ 442 pC/N and k_p ～ 48.9%, making it a promising material for lead-free piezoelectric ceramics.     

In situ high temperature X-ray diffraction study of UO2 nanoparticles

Nanocrystallites of UO₂ with a size of 3–5 nm were studied in situ with high temperature X-ray diffraction (HT-XRD), thermogravimetry (TGA), and differential thermal analysis. The evolution of the crystallite size, the lattice parameter, and the strain were determined from ambient temperature up to 1200 °C. Below 700 °C, a weak effect on the crystallite size occurs and it remains below 10 nm, while a strong expansion of the lattice parameter is measured. The strain decreases with temperature and is completely released at 700 °C. Above this temperature, begins the sintering of the nanocrystallites reaching a size of about 80 nm at 1200 °C. The weight loss curve observed in TGA is assigned to the desorption of water molecules and is correlated with the strain evolution observed by HT-XRD. The linear thermal expansion and the thermal expansion coefficient at 800 °C are 1.3% and 16.9 × 10⁻⁶ °C⁻¹, respectively.     

Spark plasma reaction sintering of ZrO2–mullite composites from plasma spheroidized zircon/alumina powders

Mullite–zirconia ceramic composites are prepared by reaction sintering of plasma spheroidized (PS) zircon–alumina powders in a spark plasma sintering (SPS) system at 1000, 1100, 1200 and 1300 °C with duration of 10 and 30 min. At SPS temperature of 1000 °C, evidence of zircon decomposition is detected, while at 1200 °C, mullite formation dominates the process, resulting in significant increases in microhardness, Young's modulus and fracture toughness values. At SPS temperature of 1300 °C, due to re-crystallization, rapid grain growth, and intergranular micro cracking, there is a slight decrease of microhardness and Young's modulus values. Yet, fracture toughness as high as 11.2±1.1 MPa m^1/2 is obtained by the indentation technique. The results indicate that with optimized sintering parameters, a combination of PS and SPS is effective in preparing high performance mullite/ZrO₂ composites from zircon/alumina mixtures at a relatively low reaction sintering temperature.     

Thermal behavior of SrSO4–SrCO3 and SrSO4–SrCO3–Al2O3 mixtures

The high temperature behavior of SrSO₄, SrCO₃ and Al₂O₃ mixtures was studied. A mixture of 1:1 mole of SrSO₄ and mechanically activated SrCO₃ was mixed and characterized using thermal gravimetric analysis. Some samples were uniaxially pressed and sintered at 1100, 1200 and 1300 °C for 8 h and then analyzed using X-ray diffraction and scanning electron microscopy. Additionally, a mixture of SrSO₄:SrCO₃:Al₂O₃ was uniaxially pressed and sintered at 1500 °C. The decomposition temperature of SrCO₃ was decreased 18° by milling for 180 min. Samples sintered at 1300 °C showed a microstructure free of porosity. X-ray diffraction analysis showed the presence of SrO and SrSO₄ after sintering at 1100, 1200 and 1300 °C. The mixture containing alumina showed the formation of a strontium aluminum oxide sulfate compound in addition to strontium aluminate.     

Mechanical properties and oxidation behaviors of carbon/carbon composites with C–TaC–C multi-interlayer

To improve the mechanical properties and oxidation-resistance properties, a C–TaC–C multi-interlayer structure was introduced in carbon/carbon (C/C) composites by chemical vapor infiltration. Compared with conventional C/C composites, a higher fracture toughness and strength have been achieved by using the C–TaC–C multi-interlayer. In addition, the composites also exhibit a higher preliminary oxidation temperature and a lower mass loss at high temperatures. The oxidation rate of the composites increases with temperature increasing in the range of 700–1300 °C, reaching a maximum value at 1300 °C, then decreases in 1300–1400 °C. A hexagonal structure of Ta₂O₅ phase is obtained when being oxidized at 700–800 °C, and it transforms to an orthorhombic phase at temperatures above 900 °C. The structures of C–TaC–C multi-interlayer are intact without cracks or porosities after being oxidized at 700–800 °C. In 900–1300 °C, the composites are oxidized uniformly with the formation of pores. At temperatures above 1300 °C, there are oxidation and non-oxidation regions with the oxidation process being controlled by diffusion.     

Influence of sintering temperature on piezoelectric properties of (K0.4425Na0.52Li0.0375)(Nb0.8925Sb0.07Ta0.0375)O3 lead-free piezoelectric ceramics

Microstructure characteristics, phase transition, and electrical properties of (K_0.4425Na_0.52Li_0.0375) (Nb_0.8925Sb_0.07Ta_0.0375)O₃ (KNLNST) lead-free piezoelectric ceramics prepared by normal sintering were investigated with an emphasis on the influence of sintering temperature. The microstructure and piezoelectric, ferroelectric, and dielectric properties were investigated, with a special emphasis on the influence of sintering temperature from 1,100 to 1,140 °C. Orthorhombic phases mainly exist in the ceramics sintered at 1,100–1,130 °C, whereas the tetragonal phase becomes dominant when sintering temperature is above 1,130 °C. Because of the existence of MPB-like transitional behavior, the piezoelectric coefficient (d ₃₃), electromechanical coupling coefficient (kp), and dielectric constant (ε) show peak values of 304pC/N, 0.48, and 1,909, respectively, which are obtained in the sample sintered at 1,120 °C, and its Curie temperature (T _C) is about 271 °C.     

The effect of sintering temperature on the development of grain boundary traps in zinc oxide based varistors

A series of zinc oxide based varistors containing 0.5 wt% Bi₂O₃ and 0.5 wt% Mn₂O₃ was prepared by a conventional mixed oxide route and sintered at temperatures between 950° and 1300°C. All samples showed varistor behaviour, although as the sintering temperature was increased from 950°C to 1300°C, the non-linearity coefficient, α, decreased from 22 to 3. Deep level transient spectroscopy of the varistors showed that the main active electron trap migrated to shallower levels within the bandgap as the sintering temperature increased. At the lowest sintering temperature, where α attained the highest values, a second, shallower trap was also activated.     

Effects of addition of BiFeO<Subscript>3</Subscript> on phase transition and dielectric properties of BaTiO<Subscript>3</Subscript> ceramics

Samples of xBiFeO₃–(1 − x)BaTiO₃ (x = 0, 0.02, 0.04, 0.06, 0.07 and 0.08) were synthesized by solid state reaction technique and sintered in air in the temperature range 1,220–1,280 °C for 4 h. X-ray diffraction data showed that 2–8 mol% BiFeO₃ can dissolve into the lattice of BaTiO₃ and form single perovskite phase. The crystal structure changes from tetragonal to cubic phase at room temperature when 8 mol% of BiFeO₃ was added into BaTiO₃. Scanning electron microscope images indicated that the ceramics have compact and uniform microstructures, and the grain size of the ceramics decreases with the increase of BiFeO₃ content. Dielectric constants were measured as functions of temperatures (25–200 °C). With rising addition of BiFeO₃, the Curie temperature decreases. For the sample with x = 0.08, the phase transition occurred below room temperature. The boundary between tetragonal and cubic phase of the BiFeO₃–BaTiO₃ system at room temperature locates at a composition between 7 and 8 mol% of BiFeO₃. The diffusivity parameter γ for compositions x = 0.02 and x = 0.07 is 1.21 and 1.29, respectively. The relaxor-like behaviour is enhanced by the BiFeO₃ addition.     

Anomalous thermal expansion of silver chlorate in the tetragonal and cubic phases

Silver chlorate (AgClO₃) undergoes a phase transition from a tetragonal to a cubic phase at 139° C. The temperature dependence of lattice parameters and the coefficient of thermal expansion in the tetragonal phase between 23° C and 130° C and in the cubic phase between 140° C and 184° C have been investigated. The lattice parameters in the tetragonal phase and the lattice parameter in the cubic phase increase with temperature. In the tetragonal phase the coefficient of expansion ( _c) increases with temperature whereas the coefficient of expansion along a perpendicular direction ( _a) decreases with increasing temperature. In the cubic phase the coefficient of expansion shows a pronounced decrease with increasing temperature. These results have been discussed in the light of the known structure.     

Densification behavior,doping profile and planar waveguide laser performance of the tape casting YAG/Nd:YAG/YAG ceramics

The sintering behavior and doping concentration profile of the planar waveguide YAG/Nd:YAG/YAG ceramics by the tape casting and solid-state reaction method were investigated on the basis of densification trajectory, microstructure evolution, and Nd³⁺ ions diffusion. The porosity of the green body by tape casting and cold isostatic pressing is about 38.6%. And the green bodies were consolidated from 1100 °C to 1800 °C for 0.5–20 h to study the densification and the doping diffusion behaviors. At the temperature higher than 1500 °C, pure YAG phase is formed, followed by the densification and grain growth process. With the increase of temperature, two sintering stages occur, corresponding to remarkable densification and significant grain growth, respectively. The mechanism controlling densification at 1550 °C is grain boundary diffusion. The diffusion of Nd³⁺ ions is more sensitive to temperature than the sintering time, and the minimum temperature required for the obvious diffusion of Nd³⁺ ions is higher than 1700 °C. Finally, planar waveguide YAG/1.5 at.%Nd:YAG/YAG transparent ceramics with in-line transmittance of 84.8% at 1064 nm were obtained by vacuum-sintering at 1780 °C for 30 h. The fluorescence lifetime of ⁴F_3/2 state of Nd³⁺ in the specimen is about 259 μs. The prepared ceramic waveguide was tested in a laser amplifier and the laser pulse was amplificated from 87 mJ to 238 mJ, with the pump energy of 680 mJ.     

Investigation on the calcining condition and sintering temperature of perovskite-type La0.7Ca0.3CrO3 complex oxides by a combustion method

Perovskite-type La_0.7Ca_0.3CrO₃ composite oxides were synthesized by a combustion method. The calcining condition of the synthesized powders and sintering temperature of ceramic specimens were studied. It was found that a pure perovskite structure was direct acquired in the combustion ash and the fine morphology (～100 nm) was observed in the sample calcined at 600 °C. In view of the relative density, microstructure and electrical conducting properties of the sintering ceramics at different temperatures, the preferred sintering temperature was ascertained to be 1300 °C, at which the sample attains a high relative density of 96.8%, showing an electrical conductivity of 53.6 S cm^?1 at 800 °C and much lower activation energy of 0.122 ev. Compared with La_0.7Ca_0.3CrO₃ synthesized by the conventional solid state reaction method, that synthesized by the combustion method exhibits fine morphology and superior sintering activity, effectively lowering the sintering temperature and enhancing electrical conducting properties of the material.     

Thermal stability and thermal expansion behavior of FeCoCrNi2Al high entropy alloy

Present work reports the thermal stability and thermal expansion behavior of dual-phase FeCoCrNi₂Al HEA prepared by Mechanical Activated Synthesis and consolidated by hot pressing. The thermal stability of the phases present in FeCoCrNi₂Al HEA has been extensively studied using in-situ high-temperature X-ray diffraction (HT-XRD) in conjunction with dilatometry and differential scanning calorimetry (DSC). The DSC thermogram shows a single endothermic peak at 1430 °C (1703 K) which belongs to the melting point of the alloy. HT-XRD and dilatometry experiments were carried out from room temperature to 1000 °C (1273 K). HT-XRD study has shown that the room temperature FCC + BCC (face-centred cubic + body-centred cubic) phases remains stable up to 1000 °C (1273 K). Although the amount of BCC phase has increased above 800 °C (1073 K), no additional phase formation was observed in HT-XRD. The coefficient of thermal expansion (CTE) curve shows linear increment up to 1000 °C (1273 K) with a slight change in slope beyond 800 °C (1073 K). Theoretical CTE was computed using the lattice parameter of the FCC phase, obtained from HT-XRD, as a function of temperature and compared with experimental CTE. Third-order polynomial equation was fitted to the experimental CTE data and the constants were evaluated which can be used to predict the coefficient of thermal expansion of the alloy.     

Effect of sintering temperature on the phase composition and microhardness of WC-8 wt % Co cemented carbide

The effect of sintering temperature (800–1600°C) on the phase composition, density, and microhardness of WC-8 wt % Co cemented carbide has been studied using x-ray diffraction, scanning electron microscopy, optical microscopy, and density measurements. The results indicate that, during sintering of the starting powder mixture, containing not only WC and Co but also the lower carbide W₂C and free carbon, W₂C reacts with cobalt metal to form Co₃W. At sintering temperatures from 900 to 1200°C, the reaction intermediate is the ternary carbide phase Co₆W₆C. During sintering at 1300°C, this phase reacts with carbon to form Co₃W₃C. Sintering at 1000°C and higher temperatures is accompanied by the formation of a cubic solid solution of tungsten carbide in cobalt, β-Co(WC). The density and microhardness of the sintered samples have been measured as functions of sintering temperature, and the optimal sintering temperature has been determined.