Sintering condition and PTCR characteristics of porous (Ba,Sr)(Ti,Sb)O3

We fabricated porous (Ba,Sr)(Ti,Sb)O₃ ceramics by adding potato-starch (1–20 wt %) and investigated the effects of sintering temperature (1300–1450 °C) and time (0.5–10 h) on the positive temperature coefficient of resistivity characteristics of the porous ceramics. The room-temperature electrical resistivity of the (Ba,Sr)(Ti,Sb)O₃ ceramics decreased with increasing sintering temperature, while that of the ceramics increased with increasing sintering time. For example, the room-temperature electrical resistivity of the (Ba,Sr)(Ti,Sb)O₃ ceramics for the samples sintered at 1300 °C and 1450 °C for 1 h is 6.8×10³ and 5.7×10² cm, respectively, while that of the ceramics is 6.5×10² and 1.3×10⁷ cm, respectively, for the samples sintered at 1350 °C for 0.5 h and 10 h. In order to investigate the reason for the decrease and increase of room-temperature electrical resistivity of the samples with increasing sintering temperature and time, the average grain size, porosity, donor concentration of grains (N _d), and electrical barrier height of grain boundaries () of the samples are discussed.     

Oxidation characteristics of Ti3AlC2 over the temperature range 500–900 °C

Titanium aluminium carbide (Ti₃AlC₂) displays a unique combination of characteristics of both metals and ceramics coupled with an unusual combination of mechanical, electrical and thermal properties. In this research, the oxidation characteristics of Ti₃AlC₂ over the temperature range 500–900 °C were studied by synchrotron radiation diffraction (SRD) and secondary-ion mass spectrometry (SIMS) experiments which provided elemental and phase compositional depth profiles over this range. Evidence for the outward diffusion of Al during oxidation was shown for the first time by the complementary SIMS results, which suggested the existence of amorphous Al or aluminium oxide at low temperature oxidation. At 500 °C, only anatase-TiO₂ was detected by SRD in addition to the parent Ti₃AlC₂. Transformation of anatase to rutile was observed at 600 °C and was completed by 900 °C. The crystalline phase α-Al₂O₃ was detected at 900 °C but not at lower temperatures.     

Low-fired microwave dielectrics in ZnO-TiO2 ceramics doped with CuO and B2O3

The microwave dielectric properties and the microstructure of low-firing zinc titanate ceramics have been investigated. The microstructure of these ceramics have been studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), and it was found that CuO addition has the important effect of stabilizing the Zn₂TiO₄ phase. When the sintering temperatures are in the range of 870–950 °C, dielectric constant values ( _r ) saturate at 21–25, the Q×f value is 20 000 GHz (at 10 GHz). The temperature coefficient of resonant frequency ( _f ) was 10 ppm °C^–1. Therefore, the 0.5 wt %-CuO and 1.0 wt %-B₂O₃ doped ZnO-TiO₂ ceramics can be used in multi-layer microwave devices requiring low sintering temperature.     

A low-temperature reactive sintering process for the 5Li2O–1Nb2O5–5TiO2 microwave dielectric ceramics

The B₂O₃-doped 5Li₂O–1Nb₂O₅–5TiO₂ composite microwave dielectric ceramics prepared by conventional and low-temperature single-step reactive sintering processes were investigated in the study. Without any calcinations involved, the Nb₂O₅ mixture of Li₂CO₃ and TiO₂ was pressed and sintered directly in the reactive sintering process. More uniform and finer grains could be obtained in the 5Li₂O–1Nb₂O₅–5TiO₂ ceramics by reactive sintering process, which could effectively save energy and manufacturing cost. And relatively good microwave dielectric properties of _r = 41, Q × f = 9885 GHz and τ_f = 43.6 ppm/°C could be obtained for the 1 wt.% B₂O₃-doped ceramics reactively sintered at 900 °C.     

Microwave dielectric and elastic properties of glass-added Ba6−3xR8+2xTi18O54 (x = 2/3)

Microwave dielectric properties of Ba_6−3xSm_8+2xTi₁₈O₅₄ (x = 2/3) [BST] ceramics with the addition of 0–3 wt.% of various glasses have been studied. It has been found that the addition of 0.5 wt.% of the glasses decreases the sintering temperature by about 150 °C. In general, addition of 0.5 wt.% of Zn, Mg and Pb-based glasses deteriorate the quality factor, whereas aluminum and barium borosilicates do not decrease it considerably. The quality factor and dielectric constant decrease with increasing amount of glass. The temperature coefficient of resonant frequency shifts towards positive or negative depending on the composition of the glass. A glass–ceramic composite with a dielectric constant 64, Q × f nearly 8500 GHz and near to zero τ_f could be obtained at a sintering temperature of 1175 °C when 3–4 wt.% Al₂O₃–B₂O₃–SiO₂ glass was added to BST ceramic. The Young's modulus decreases with increasing amount of glass, irrespective of the composition of glass.     

Effects of binary additives B2O3–Y2O3 on the microstructure and thermal conductivity of aluminum nitride ceramics

Aluminum nitride (AIN) ceramics, with binary additives B₂O₃-Y₂O₃, were sintered at temperatures from 1700 to 1850 °C. The microstructure and sintering characteristics were studied by XRD, HREM, SEM and TEM/EDS, which showed that Y₂O₃ gave different yttrium aluminates through the reaction with Al₂O₃ under different conditions. With the increase of sintering temperature, the yttrium-to-aluminum atomic ratio Y/Al decreased in the secondary phases of the sintered bodies. It was discovered that B₂O₃ could dissolve in the yttrium aluminates, forming some ordered structure with a superlattice. After sintering at 1850 °C for 4 h, a specimen with a fine microstructure and a thermal conductivity of 190 Wm^–1K^–1 was obtained.     

Synthesis and characterization of low density calcia stabilized zirconia ceramic for high temperature furnace application

Five weight percent calcia stabilized zirconia (CSZ) low density ceramic material was synthesized from zirconia and calcia powder by solid state reaction. Powder specimens were sintered at various temperatures for different time durations in air and argon atmosphere. Physico-chemical properties such as thermal stability, purity, crystalline phases identification, particle size, particle size distribution, specific surface area and porosity of CSZ were measured by thermogravimetry and differential thermal analysis (TG/DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Taller (BET) and particle size analyzer (PSA). Total relative shrinkage and coefficient of thermal expansion were estimated by dilatometry. The density and Vicker's microhardness were measured in order to evaluate the material performance. A combination of cubic zirconia and calcium zirconium oxide (Ca_0.15Zr_0.85O_1.85) phases were found when specimen sintered at 1500 °C in air while under argon atmosphere at 1750 °C and 1850 °C, the ZrO and Ca_0.15Zr_0.85O_1.85 phases were established. At higher sintering temperature (1950 °C) for 4 h in argon medium three phases ZrO, CaZrO₃ and Ca_0.15Zr_0.85O_1.85 were observed. It was found that with the increase of sintering time these phases convert into a single cubic phase Ca_0.15Zr_0.85O_1.85. Pellets sintered at different temperatures and duration were also studied by scanning electron microscopy showing that the grain structure becoming fine with increase of temperature and duration.     

Thermophysical properties of porous SiC ceramics fabricated by pressureless sintering

Highly porous SiC with approximately 30–41% porosity was fabricated by pressureless sintering without sintering additives at temperatures in the range 1700–2000 °C. Thermal diffusivities, specific heats, thermal conductivities and thermal resistivities of sintered samples are reported for temperatures from room temperature to 1000 °C. The thermal diffusivities and thermal conductivities of all samples decreased significantly with increasing temperature over this range, whereas specific heats and thermal resistivities increased. At any given temperature, the greater the porosity of the SiC, the lower the thermal conductivity.     

Kinetics and sintering mechanisms of hydro-thermally obtained hydroxyapatite

In this paper, the kinetics and sintering mechanism of hydro-thermally synthesized hydroxyapatite in the temperature range 900–1100 °C are presented. It was found that at 900 °C, the sintering process is very slow, whereas at 1000, 1050, and 1100 °C is much faster, caused by activated sintering with liquid phase/phase of solution on spots with concentrated material impurity and the formation of reaction/polyeutectic mixtures dominates. The activation energies of sintering were also determined. At all the investigated sintering temperatures, an approximate value of 90 kJ mol⁻¹ was found.     

Influence of ZnO additions to 0.8(Mg0.95Co0.05)TiO3–0.2Ca0.6La0.8/3TiO3 ceramics on sintering behavior and microwave dielectric properties

The effects of ZnO addition on the microstructures and microwave dielectric properties of 0.8(Mg_0.95Co_0.05)TiO3–0.2Ca_0.6La_0.8/3TiO₃ ceramics were investigated. ZnO was selected as liquid phase sintering aids to lower the sintering temperature of 0.8(Mg_0.95Co_0.05)TiO3–0.2Ca_0.6La_0.8/3TiO₃ ceramics. With ZnO additives, the densification temperature of 0.8(Mg_0.95Co_0.05)TiO₃–0.2Ca_0.6La_0.8/3TiO₃ can be effectively reduced from 1450 to 1200–1325 °C. The crystalline phase exhibited no phase difference at low addition levels (0.25–2 wt.%). It is found that low-level doping of ZnO (0.25–2 wt.%) can significantly improve the density and dielectric properties of 0.8(Mg_0.95Co_0.05)TiO₃–0.2Ca_0.6La_0.8/3TiO₃ ceramics. The quality factors Q × f were strongly dependent upon the amount of additives. Q × f values of 36 000 and 13 000 GHz could be obtained at 1200–1325 °C with 1 and 2 wt.% ZnO additives, respectively. During all additives ranges, the relative dielectric constants were significantly different and ranged from 23.1 to 27.96. The temperature coefficient varies from 14.1–24.3 ppm/°C.     

Effect of temperature on hydrothermal synthesis of analcime and viséite

The zeolite minerals analcime (Na₁₆(Al₁₆Si₃₂O₉₆) · 16H₂O) and viséite (Na₂Ca₁₀(Al₂₀Si₆P₁₀O₆₀(OH)₃₆) · 16H₂O) were synthesized by hydrothermal treatment of artificial glasses of the respective same composition in a temperature range between 80 and 630°C at 100 MPa H₂O pressure. The crystal symmetry of both zeolites varied systematically with temperature under the given experimental conditions, starting from orthorhombic symmetry at low synthesis temperatures, tetragonal at medium leading to cubic symmetry at highest zeolite synthesis temperatures. The crystal sizes varied between 500 nm and 100 μm. In case of analcime water free tectosilicates instead of zeolites were formed at T≥500°C while viséite was formed up to 630°C. The investigations showed the direct correlation between chemical composition of the starting materials and the formed zeolite phase under the given p–T conditions.     

Sintering and properties of SrBi2(Ta1−x V x )2O9 ceramics

The effects of vanadium doping on the sintering, microstructure, dielectric properties, and ferroelectric properties of SrBi₂(Ta_1–x V _x )₂O₉ ceramics were investigated. The densification and grain-growth processes of the vanadium doped ceramics were shifted to a lower temperature range. For the ceramics with relative density 90%, the dielectric constant is 120–125 and 100–130 for the undoped and doped ceramics, respectively, and the dielectric loss tangent is below 1%. As compared with the undoped ceramics, the ferroelectric properties can be significantly improved by doping with an appropriate amount of vanadium and sintering at 1000°C. The variations of dielectric and ferroelectric properties are influenced by the incorporation of vanadium into crystal lattice and several microstructural factors.     

Sintering and characterization of fully dense aluminium nitride ceramics

Aluminium nitride ceramics with no sintering additives could be densified to close to theoretical density (99.6% theoretical) by pressureless sintering of tape-cast green sheets at 1900 °C for 8 h. The thermal conductivity and bending strength of the specimens were 114 Wm^–1 K^–1 and 240 MPa, respectively. The effect of Y₂O₃ additive on sinterability, thermal conductivity and microstructure of aluminium nitride ceramics was investigated. Thermal conductivity increased with increasing amount of Y₂O₃ additive, sintering temperature and holding time at the sintering temperature. Samples with a thermal conductivity up to 258 Wm^–1 K^–1 were fabricated by elimination of the grain-boundary phase.     

The effect of Sb2O5 additions on the dielectric properties of Ag(Nb0.8Ta0.2)O3 ceramics

The sintering behavior and dielectric properties for perovskite Ag(Nb_0.8Ta_0.2)O₃ ceramic with Sb₂O₅ doping was explored. A small amount of Sb₂O₅ (2.5 wt.%) led to high densification at temperatures < 1060 °C. The dielectric constant increased and the temperature coefficient decreased with increasing concentration of Sb₂O₅, and the dielectric constant reached 673, combined with a low temperature coefficient of 147 ppm/°C, and dielectric loss of 0.0044 (at 1 MHz) for the sample with 3.5 wt.% Sb₂O₅ sintered at 1080 °C.     

Effect of CuO on the sintering and cryogenic microwave characteristics of (Zr0.8Sn0.2)TiO4 ceramics

The effect of CuO on the sintering temperature, microstructure and microwave dielectric properties of (Zr_0.8Sn_0.2)TiO₄ (ZST) modified with 1 wt% of ZnO has been investigated. Microwave dielectric properties of ZST ceramics are measured from cryogenic to room temperatures (15–290 K). Crystallite sizes of sintered ZST ceramics as derived from XRD are in the 30–50 nm range. The addition of CuO effectively reduced the sintering temperature to 1300 °C, possibly due to liquid-phase effects. Addition of CuO did not cause any secondary phases up to 1.5 wt% of CuO. The dielectric constant (ε_r) and temperature coefficient of resonant frequency (τ_f) of ZST ceramics do not significantly vary with temperature, whereas the unloaded quality factor (Q_u) changes noticeably. It is found that the Q_u factor of the sample without CuO decreased with increase in temperature, whereas the samples with addition of CuO up to 1.0 wt% showed less dependence on temperature. The Q_u factor of CuO-free ZST is 15,000 and that of ZST with 0.5 wt% of CuO is 11,800 at 15 K. The Q_u factor while measured at room temperature ranged between 2900 and 7000. Efforts were made to understand whether the increase in Q_u factor at both cryogenic and room temperatures is the result of intrinsic or extrinsic factors.     

Characterization of precipitates in a 7.9Cr–1.65Mo–1.25Si–1.2V steel during tempering

In this paper, the precipitates formed during the tempering after quenching from temperature 1150 °C for 7.90Cr–1.65Mo–1.25Si–1.2V steels are investigated using an analytical transmission electron microscope (A-TEM).The study of this tempering is carried out in isothermal and anisothermal conditions, by comparing the results given by dilatometry and hot hardness.Tempering is performed in the range of 300–700 °C. Coarse primary carbides retained after heat treatment are V-rich MC and Cr–Mo-rich M₇C₃ types. In turn, it gives a significant influence on the precipitation of fine secondary carbides, that is, secondary hardening during tempering. The major secondary carbides are Cr–Mo–V-rich M′C (and/or) Cr–Mo-rich M₂C type. The peak hardness is observed in the tempering range of 450–500 °C. In the end, we observe between 600 and 700 °C, that this impoverished changes the phase. At these high temperatures of tempering, we observe that there is a carbide formation of the types M₆C developing at the expense of the fine M₇C₃ carbides previously formed.     

The effects of zinc bath temperature on the coating growth behavior of reactive steel

The purpose of this work is to identify the influence of zinc bath temperature on the morphology and the thickness of reactive steel (Fe–0.1 wt.%Si alloy) coatings. The Fe–0.1 wt.%Si samples were galvanized for 3 min at temperatures in the range of 450–530 °C in steps of 10 °C. The coatings were characterized by using scanning electron microscopy/energy dispersive X-rays analysis. It was found that the coating thickness reaches the maximum at 470 °C and the minimum at 500 °C, respectively. When the reactive steel is galvanized at temperatures in the range of 450–490 °C, the coatings have a loose ζ layer on the top of a compact δ layer. With the increase of the galvanizing temperature, the ζ layer becomes looser. When the temperature is at 500 °C, the ζ phase disappears. With the increase of temperature, the coatings change to be a diffuse-Δ layer (δ+ liquid zinc).     

Thermal expansion of silicon carbide materials

The influence of porosity and sintering additives on the thermal expansion of silicon carbide ceramics with additions of Al₂O₃, B(B₄C), and BeO is investigated in the temperature range 50–700°C.Notation thermal expansion coefficient - b constant in the exponent - n porosity - T temperature Daghestan State University, Makhach-Kala. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 66, No. 6, pp. 739–741, June, 1994.     

Microwave dielectric properties and sintering behavior of nano-scaled (α + θ)-Al2O3 ceramics

The dielectric properties at microwave frequencies and the microstructures of nano (α + θ)-Al₂O₃ ceramics were investigated. Using the high-purity nano (α + θ)-Al₂O₃ powders can effectively increase the value of the quality factor and lower the sintering temperature of the ceramic samples. Grain growth can be limited with θ-phase Al₂O₃ addition and high-density alumina ceramics can be obtained with smaller grain size comparing to pure α-Al₂O₃. Relative density of sintered samples can be as high as 99.49% at 1400 °C for 8 h. The unloaded quality factors Q × f are strongly dependent on the sintering time. Further improvement of the Q × f value can be achieved by extending the sintering time to 8 h. A dielectric constant (_r) of 10, a high Q × f value of 634,000 GHz (measured at 14 GHz) and a temperature coefficient of resonant frequency (τ_f) of −39.88 ppm/°C were obtained for specimen sintered at 1400 °C for 8 h. Sintered ceramic samples were also characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM).     

Silver cofirable Bi1.5Zn0.92Nb1.5O6.92 microwave ceramics containing CuO-based dopants

Bi₂O₃–ZnO–Nb₂O₅ system has emerged as a good low sintering (1050 °C) microwave material because it exhibits high dielectric constant and low temperature coefficient of resonance frequency (τ_f). We have lowered the sintering temperature of Bi_1.5Zn_0.92Nb_1.5O_6.92 (BZN) below 900 °C by using 3 wt.% of CuO-based dopants, such as 0.21BaCO₃–0.79CuO (BC) and 0.81MoO₃–0.19CuO (MC). The doped BZN exhibits high microwave dielectric constant at 2.3 GHz (k  120). The interfacial behavior between BZN and silver was investigated by using X-ray diffractometer, scanning electronic microscope, and electronic probe microanalyzer. The extent of silver migration of MC and BC dopants is reduced at least by one order of magnitude as compared with V₂O₅ dopant when the samples was prepared at 900 °C for 4 h. Thus, CuO-based dopants can replace V₂O₅ to lower the sintering temperature of BZN and to be cofired with silver.