Synthesis and Microstructure of Partly-oriented Bismuth Layer Structure Ferroelectrics Ceramics SrBi4Ti4O15

SrBi4Ti4O15 powder was synthesized by conventional solid state synthesis （ CS ） and molten salt synthesis （ MSS ） . MSS method can synthesize plate-like SrBi4Ti4O15 at lower temperature （900℃） than CS method. Plate-like form becomes more distinct when the synthesis temperature increases. This would help cause the grain orientation of the ceramics after sintering. The sintered samples of MSS had grain orientation at （0,0, 10） plane. The degree of （0,0,10） grain orientation F was 62.1% . Hot pressing made （0,0,10） grain orientation more distinct （ F = 85.7% ）. The microstructures of the sintered samples were detected by SEM. Due to the grain orientation the density of samples fabricated by MSS was lower than that of prepared by CS.     

Comparative study of β-Si3N4 powders prepared by SHS sintered by spark plasma sintering and hot pressing

β-Si3N4 powders prepared by self-propagating high-temperature synthesis (SHS) with additions of Y2O3 and Al2O3 were sintered by spark plasma sintering (SPS). The densification, microstructure, and mechanical properties of Si3N4 ceramics prepared using this method were compared with those obtained by hot pressing process. Well densified Si3N4 ceramics with finer and homo- geneous microstructure and better mechanical properties were obtained in the case of the SPS technique at 200°C lower than that of hot pressing. The microhardness is 15.72 GPa, the bending strength is 716.46 MPa, and the fracture toughness is 7.03 MPa?m1/2.     

The Effects of Solid Phase Additives on Sintering Properties of Alumina Bioceramic

1　IntroductionAlumina (Al2 O3 )bioceramicsarewidelyusedinclinicforhardtissuesubstitution ,especiallyforfemurballofartificialhipjoint.Al2 O3 materialhasgoodbiocompati bility ,physiologicinertia ,physicalandchemicalstability ,highhardnessandwearability .Butundernormalcondi tions,pureAl2 O3 isdifficulttobepreparedbecausethesinteringtemperatureishigh (generallyat 170 0℃to2 0 0 0℃ )andtheheat pressure ,atmosphereorvacuumareevenrequired .Inordertoreducethesinteringtemperatureandimprovetheproper…     

Dielectric properties of Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ceramics by impedance spectroscopic method

Various lead-free ceramics have been investigated in search for new high-temperature dielectrics. In particular, Bi₄Ti₃O₁₂ is a type of ferroelectric ceramics, which is supposed to replace leadcontaining ceramics for its outstanding dielectric properties in the near future. Ferroelectric ceramics of Bi₄Ti₃O₁₂ made by conventional mixed oxide route have been studied by impedance spectroscopy in a wide range of temperature. X-ray diffraction patterns show that Bi₄Ti₃O₁₂ ceramics are a single-phase of ferroelectric Bi-layered perovskite structure whether it is calcined at 800 °C or after sintering production. This study focused on the effect of the grain size on the electric properties of BIT ceramics. The BIT ceramics with different grain sizes were prepared at different sintering temperatures. Grain becomes coarser with the sintering temperature increasing by 50 °C, relative permittivity and dielectric loss also change a lot. When sintered at 1 100 °C, r values peak can reach 205.40 at a frequency of 100 kHz, the minimum dielectric losses of four different frequencies make no difference, all close to 0.027. The values of E _a range from 0.52 to 0.68 eV. The dielectric properties of the sample sintered at 1 100 °C are relatively better than those of the other samples by analyzing the relationship of the grain, the internal stresses, the homogeneity and the dielectric properties. SEM can better explain the results of the dielectric spectrum at different sintering temperatures. The results show that Bi₄Ti₃O₁₂ ceramics are a kind of dielectrics. Thus, Bi₄Ti₃O₁₂ can be used in high-temperature capacitors and microwave ceramics.     

Novel phenomenon on valence unvariation of doping ion in Yb:YAG transparent ceramics using MgO additives

Yb:YAG nanopowders were synthesized by the alcohol-water co-precipitation method adding MgO as sintering additives. Appropriate amount of MgO adding can restrict the agglomeration and reduce the particle size of Yb:YAG powders. When the MgO content was 0.04wt%, well-dispersed Yb:YAG powders with ellipsoidal particles of less than 100 nm diameter were obtained. The experimental results showed the valence variation of doping ion Yb³⁺ would not appear when adding MgO as sintering additives, so ceramics showed colorless transparent instead of green due to Yb²⁺ color center using traditional SiO₂ as additives. The transmission of the sintered Yb:YAG ceramics can reach 80.6% even without annealing. Ceramic morphology showed that the grains had uniform-distribution with the size of 10 μm or so, and no impurity and pore existed in the grain boundary and crystalline while using optimal sintering conditions.     

Fabrication and plasma arc thermal shock resistance of HfB2-based ultra high temperature ceramics

Two hafnium diboride based ceramic matrix composites containing 20% (volume fraction) SiC particle and with or without AlN as sintering additives were fabricated by hot-pressed sintering. The mechanical properties and microstructures of these two composites were tested and the thermal shock resistances were evaluated by plasma arc heater. The results indicate that the composite with AlN as sintering additive has a denser and finer microstructure than composite without sintering additive, and the mechanical properties, thermal shock resistance of the composite with AlN as sintering additive are also higher than those of the composite without AlN. Microstructure analysis on the cross-section of two composites after thermal shock tests indicates that a compact oxidation scale contains HfO₂ and Al₂O₃ liquid phase is found on the surface of composite with AlN, which could fill the voids and cracks of surface and improve the thermal shock resistance of composite.     

Influence of pH on the property of apatite-type lanthanum silicates prepared by sol-gel process

The apatite-type lanthanum silicates with formula La9.33Si6O26 are prepared by sol-gel process. The homogeneity of the sol affected by pH value of the solution is investigated. The viscosity of the sols slightly increases first and then increases abruptly because the predominant reaction mechanism changes from hydrolysis reaction to condensation reaction. In addition, the onset time of the increase for the viscosity shortens from pH 1 to pH 4. The gelation time decreases with increasing pH of the solution. Therefore, the pH of the sols should be less than 4 to form gel. The sol with initial pH 2 shows maximum value of zeta potential and maximum stability. For the sample with initial pH 2, pure apatite-type lanthanum silicates La9.33Si6O26 have been successfully prepared after the dried gel is calcined at 1 000 ℃. In addition, this sample sintered at 1 550 ℃ exhibits the highest ionic conductivity. The activation energies are all less than 0.90 eV.     

Preparation and microstructure of spinel zinc ferrite ZnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> by Co-precipitation method

Spinel zinc ferrites ZnFe₂O₄, prepared by co-precipitation method using the zinc nitrate Zn(NO₃)₂·6H₂O and ferric nitrate Fe(NO₃)₃·2H₂O as the raw materials, were characterized by the thermo gravimetric analysis (TG) and differential scanning calorimeter (DSC), X-ray diffraction (XRD) and scanning electron microscope (SEM). The influence of synthesis conditions, such as Zn/Fe molar ratio, pH value, the sintering temperature and time, on the microstructures was detailedly investigated. The relationships between the microstructures and the synthesis conditions were discussed. The results show that the pure spinel zinc ferrites ZnFe₂O₄ are formed when the Zn/Fe molar ratio is 1.05:2 at pH=8.5 or Zn/Fe molar ratio is 1:2 at Ph=9-10, and the precursors are sintered at 1100 °C for 4 h. Especially no other phases are observed when the Zn/Fe molar ratio is 1:2 at pH=10 and the precursor is sintered above 700 for 4 °C h. The higher sintering temperature and longer sintering time contribute to grain growth.     

Effect of CaO doping on corrosion resistance of Cu/（NiFe2O4-10NiO） cermet inert anode for aluminum electrolysis

The CaO-doped Cu/（NiFe2O4-10NiO） cermet inert anodes were prepared by the cold isostatie pressing-sintering process, and their corrosion resistance to Na3AlF6-K3AlF6-Al203 melt was studied. The results show that the relative density of 5Cu/（NiFe2O4-10NiO） cermet sintered at 1 200 ℃ increases from 82.83% to 97.63% when 2% CaO （mass fraction） is added. During the electrolysis, the relative density of cermet inert anode descends owing to the chemical dissolution of additive CaO at ceramic grain boundary, which accelerates the penetration of electrolyte. Thus, the corrosion resistance to melts of Cu/（NiFe2O4-10NiO） cermet inert anode is reduced. To improve the corrosion resistance of the cermet inert anode, the content of CaO doped should be decreased and the technology of cleaning the ceramic grain boundary should be applied.     

Enhanced Low-field Magnetoresistence in the Absence of Mn Content in Polycrystalline La0.67Ca0.33MnO3

Magnetic and electrical transport properties of the La0.67Ca0.33Mn1-xO3 （x=0-0.16）, which were prepared by the sol-gel method followed by sintering treatment at 1 450, 1 100 and 900 ℃, respectively, were investigated. Experimental results show that, with the increase of x, the resistivity of samples increases and the insulator-metal transition temperature shifts towards lower temperature. Meanwhile, the intrinsic megnetoresistance effect is weakened and the extrinsic magnetoresistance is enhanced. For the samples with x=0.16 and 0.10 sintered at 1 100 ℃ and 900 ℃, respectively, low field magnetoresistance as high as about 50% can be observed. Furthermore, for the samples sintered at 1 100 ℃ and 900 ℃, the grain size is not only controlled by about sintering temperature, but also by the absence of Mn content x.     

Effect of thermal treatment on microstructure and phase of partially-stabilized zirconia

ZrO2-MgO-Y2O3-CaO-Al2O3 composites with a multiphase microstructure were fabricated by die pressing process and sintered at various thermal programs. The microstructure of this polycrystalline was examined to clarify the role of Al2O3 on grain growth. Environmental scanning electron microscopy (ESEM) micrographs revealed that spinel was beneficial to refine grain. The effect of thermal treatment on the formation of monoclinic phase was discussed. It can be found that, the grain size and the fraction of the monoclinic phase in this partially stabilized zirconia(PSZ) increases after heat treatment at 1 150 ℃. The temperature of heat- treatment, not the time for holding, is the governing factor of the fraction of monoclinic phase.     

Influences of dopants on microstructure and magnetic properties of (Mg<Subscript>0.476</Subscript>Mn<Subscript>0.448</Subscript>Zn<Subscript>0.007</Subscript>)(Fe<Subscript>1.997</Subscript>Ti<Subscript>0.002</Subscript>)O<Subscript>4</Subscript> ferrites

Influences of addition of CaO, CoO and V2O5 on the microstructure and magnetic properties of (Mg0.476Mn0.448Zn0.007)(Fe1.997Ti0.002)O4 ferrites were investigated. The powders of (Mg0.476Mn0.448Zn0.007) (Fe1.997Ti0.002)O4 composition were prepared by using a conventional ceramic powder processing technique. The experimental results showed that the average grain size of the sintered ferrites codoped with 0.03wt% CaO, 0.04wt% CoO and 0.06wt% V2O5 was about 15 μm; the saturation magnetization of ferrites was 68.78 emu/g. The addition of V2O5 in the ferrites can not only increase value of the saturation magnetization, but also decrease the average grain size of (Mg0.476Mn0.448Zn0.007)(Fe1.997Ti0.002)O4 ferrites. Simultaneous incorporation of CoO, CaO and V2O5 dopants into (Mg0.476Mn0.448Zn0.007)(Fe1.997Ti0.002)O4 ferrites can not only improve the saturation magnetization of the materials, but also inhibit abnormal grain growth.     

Microstructure and mechanical properties of TiB<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composites

Effects of Al2O3 and Ni as the additives on the sinterability, microstructure and mechanical properties were systematic studied. The experimental results show that only a relative density about 96.2% of hot-pressing TiB2-30%Al2O3 can be attained due to the plate-like TiB2 particle and its random orientation and excessive Al2O3 grain growth. When sintering temperature is higher than 1 700 ℃, TiB2 grain growth can be found, which obvious improves flexural strength of TiB2 matrix but decreases toughness. It seems that mechanical properties of TiB2-Al2O3 composites are mainly depended on relative density besides grain growth. otherwise, they will be determined by relative density and TiB2 matrix strength together. Anyway, Al2O3 addition can weaken the grain boundary and thus improve the toughness of the materials. A flexural strength of 529 MPa, Vickers hardness of 24.8 GPa and indentation toughness of 4.56 MPa·m1/2 can be achieved inTiB2-30vol% Al2O3.     

Preparation and structure characteristics of nano-Bi2O3 powders with mixed crystal structure

The nano-Bi2O3 powders were prepared by a chemical precipitation method with Bi(NO3)3, HNO3 and NaOH as reactants. The structural characteristics and morphology of nano-Bi2O3 powders were investigated by X-ray diffraction and transmission electron microscopy, respectively. The results show that under the optimum condition that 300g/L Bi(NO3)3 reacts at 90℃ for 2 h, the Bi203 powders with 60 nm on the average and 99.5% in purity are obtained. The prepared nano-Bi2O3 powders contain a mixed crystal structure of monoclinic and triclinic instead of traditional structure of monoclinic α-Bi2O3. And the mixed crystal structure is stable in air. The reason for the appearance of the mixed crystal structure may be that the ionic radius ratio of Bi^3 to O^2- changes easily during the formation of nano-Bi2O3 particles by a chemical precipitation method.     

Structures and magnetic properties of nanocomposite CoFe2O4-BaTiO3 fibers by organic gel-thermal decomposition process

The nanocomposite xCoFe₂O₄-(1−x)BaTiO₃ (x=0.2, 0.3, 0.4, 0.5, molar fraction) fibers with fine diameters and high aspect ratios (length to diameter ratios) were prepared by the organic gel-thermal decomposition process from citric acid and metal salts. The structures and morphologies of gel precursors and fibers derived from thermal decomposition of the gel precursors were characterized by Fourier transform infrared spectroscopy, X-ray diffractometry and scanning electron microscopy. The magnetic properties of the nanocomposite fibers were measured by vibrating sample magnetometer. The nanocomposite fibers consisting of ferrite (CoFe₂O₄) and perovskite (BaTiO₃) are formed at the calcination temperature of 900 °C for 2 h. The average grain sizes of CoFe₂O₄ and BaTiO₃ in the nanocomposite fibers increase from 25 to 65 nm with the calcination temperature from 900 to 1 180 °C. The single fiber constructed from these nanograins of CoFe₂O₄ and BaTiO₃ has a necklace-like morphology. The saturation magnetization of the nanocomposite 0.4CoFe₂O₄-0.6BaTiO₃ fibers increases with the increase of CoFe₂O₄ grain size, while the coercivity reaches a maximum value when the average grain size of CoFe₂O₄ is around the critical single-domain size of 45 nm obtained at 1 000 °C. The saturation magnetization and remanence of the nanocomposite xCoFe₂O₄-(1−x)BaTiO₃ (x=0.2, 0.3, 0.4, 0.5) fibers almost exhibit a linear relationship with the molar fraction of CoFe₂O₄ in the nanocomposites.     

Effects of nucleating additives on crystallization and properties of CaO-B2O3-SiO2 glass-ceramics

ZrO₂, TiO₂ and P₂O₅ were doped in CaO-B₂O₃-SiO₂ glass-ceramics as nucleating additives. Effects of different nucleating additives on the phase separation and crystalline behaviors were investigated by using gradient temperature furnace, DTA and XRD. Then, sintering process of the glass-ceramics was investigated by testing sintering shrinkage, dielectric constant and loss. The experimental results shows that the glass-ceramics doped with nucleating additives represents higher crystallization, with ZrO₂ as an exceptional effective dopant to promote the precipitation of wollastonite crystal. Finally, ZrO₂ containing glass-ceramics was chosen to study the influence of sintering temperature and soaking time with the help of X-ray diffraction analysis and density measurement. The glass-ceramics can be well consolidated at 850 °C for 10 min, with low dielectric constant (5.87) and loss (3.21 × 10⁻⁴), which is desirable for LTCC application.     

Nanostructuring and thermoelectric properties of bulk N-type Mg<Subscript>2</Subscript>Si

Preparation and thermoelectric properties of nanostructured n-type Mg₂Si bulk materials were reported. Nanosized Mg₂Si powder was obtained by mechanical milling of the microsized Mg₂Si powder prepared by solid-state reaction. The bulk materials with 30 nm and 5 µm were prepared by spark plasma sintering of the nanosized and microsized Mg₂Si powder, respectively. Both the samples show n-type conduction and the Seebeck coefficient of the sintered samples increase determinately with the grain size decrease from 5 µm to 30 nm. On the other hand, the electrical and thermal conductivity decrease with the decrease of grain size. Accordingly, decreasing their grain size increases their thermoelectric-figure-of-merit. A maximum thermoelectric figure of merit of 0.36 has been obtained for the nanostuctured Mg₂Si sample at 823 K, which is 38% higher than that of microsized Mg₂Si bulk materials and higher than results of other literatures. It could be expected that the properties of the nanocomposites could be further improved by doping optimization.     

The influence of cobalt acetate on sol-gel derived mullite densification behaviour

Monophasic mullite (3Al2O3?2SiO2) samples doped with 0.002 M, 0.02 M, 0.1 M, 0.15 M and 0.2 M of cobalt were prepared by a sol-gel process. Prepared gels were then dried, grinded, pressed into pellets and sintered at 1 000 °C and 1 300 °C for 4 h. Phase formation and densification behavior has been investigated as a function of the cobalt content and sintering temperature. Mullite densification behavior was analyzed .The density of the sintered ceramics was measured using Archimedes method. CoO and Co related compounds were detected by the analysis for G3, G4 and G5 doping levels. The mullite X-ray diffraction suggests predominant incorporation of Co in the glassy phase, whose quantity increased with the doping level.     

Preparation of Bi-system superconductor using sol-gel method

The citrate gel of (Bi, Pb), Sr, Ca and Cu and the (Bi, Pb)-Sr-Ca-Cu-(O, F) superconductor were prepared using citrate as the organic complex agent and 2-ethyl hexanol as the dehydrating agent. The starting material was a high-purity nitrate solution containing the desired ions in the appropriate atomic ratio. The optimum temperature and pH to obtain the homogeneous gel precursor were estimated. IR and DSC were employed to study the thermal decomposition and the calcination of the precursor material. After the calcined powder was sintered, a nearly single-phase (Bi, Pb)₂Sr₂Ca₂Cu₃O _x superconductor with the transition temperature 105 K was synthesized. The effect of the doping of Pb and F was also studied in the paper. Synopsis of the first author Yin Zhoulan, associated professor, born in 1965, received Ph D degree in 1994, professional interest is physical chemistry in metallurgy and materials.