In-Situ Preparation and thermal shock resistance of mullite-cordierite heat tube material for solar thermal power

In order to improve the thermal shock resistance of solar thermal heat transfer tube material, the mullite-cordierite composite ceramic as solar thermal heat transfer tube material were fabricated by pressureless sintering using α-Al2O3 , Suzhou kaolin, talc, and feldspar as starting materials. The important parameter for solar thermal transfer tube such as water absorption (Wa ), bulk density (Db ), and the mechanical properties were investigated. The phase composition and microstructure of the composite ceramics were analyzed by XRD and SEM. The experimental results show that the B3 sintered at 1 300 ℃ and holding for 3 h has an optimum thermal shock resistance. The bending strength loss rate of B3 is only 2% at 1 100℃ by air quenching-strength test and the sample can endure 30 times thermal shock cycling, and the water absorption, the bulk density and the bending strength are 0.32%, 2.58 g·cm-3 , and 125.59 MPa respectively. The XRD analysis indicated that the phase compositions of the sample were mullite, cordierite, corundum, and spinel. The SEM images illustrate that the cordierite is prismatic grain and the mullite is nano rod, showing a good thermal shock resistance for composite ceramics as potential solar thermal power material.     

Effect of aluminum addition on microstructure and properties of SiO<Subscript>2</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-CaO vitrified bond

Influence of aluminum addition on the structures and properties of SiO₂-B₂O₃-Al₂O₃-CaO vitrified bond at low sintering temperature and high strength was discussed. FTIR and XRD analyses were used to characterize the structures of the basic vitrified bond with different contents of aluminum. The bending strength and the thermal expansion coefficients were also tested. Meanwhile, the microstructures of composite specimens at sintering temperature of 660 °C were observed by scanning electron microscope (SEM). The experimental results showed that the properties of vitrified bond with 1wt% aluminum were improved significantly, where the bending strength, Rockwell hardness, and thermal expansion coefficient of the vitrified bond reached 132 MPa, 63 HRB, and 6.73×10^-6 °C^-1, respectively.     

Synthesis of refractory cordierite from calcined bauxite, talcum and quartz

A cordierite was synthesized from calcined bauxite, talcum, and quartz. The properties and microstructure of the cordierite sintered samples were characterized by Archimedes’ method, thermal dilatometry, X-ray diffraction (XRD), scanning electron microscopy (SEM), and so on. The experimental results showed that calcined bauxite could broaden the range of synthesizing temperature from 1300 °C to 1420 °C and get pure cordierite. The bulk density and linear thermal expansion coeffi cient of the sample synthesized at 1420 °C for 2 h were 1.97 g·cm^?3 and 2.1×10^?6 °C^?1, respectively. The XRD analysis showed that the major crystalline phase was α-cordierite with almost no glassy matters, the SEM images illustrated a small vent hole and the size were 5–100 μm, the well-grown hexagonal and granular cordierite grains had the sizes distributed among 0.1–8 μm, and providing high mechanical strength and lower linear thermal expansion coeffi cient.     

Effect of annealing on microstructure and properties of Si_3N_4-AlN composite ceramics

Aiming at developing novel microwave-transparent ceramics with low dielectric loss,high thermal conductivity and high strength,Si3N4-AlN(30%,mass fraction) composite ceramics with La2O3 as sintering additive were prepared by hot-pressing at 1 800 °C and subsequently annealed at 1 450 °C and 1 850 °C for 2 h and 4 h,respectively.The materials were characterized by XRD and SEM.The effect of annealing process on the phase composition,sintering performance,microstructure,bending strength,dielectric loss and the...     

太阳能热发电用堇青石-莫来石复相陶瓷的制备及抗热震性

以合成莫来石和合成堇青石为原料,采用常压烧结制备了太阳能热发电用堇青石-莫来石复相陶瓷。研究了质量比、烧结温度等对样品的吸水率、气孔率、体积密度、抗折强度及抗热震性等的影响。结果发现,最优配方是1 440℃烧结的A3样品,堇青石和莫来石质量比为7∶3,气孔率为0.30%,吸水率为0.12%,体积密度为2.487g/cm3,抗折强度达68.49MPa,抗热震循环(室温～1 100℃)30次无裂纹。通过XRD和SEM分析发现样品由低温堇青石、高温堇青石和莫来石组成。该复相陶瓷可作为潜在的太阳能输热管道材料。     

Preparation and properties of 4.25Cu-0.75Ni／NiFe2O4 cermet

4.25Cu-0.75Ni/NiFe₂O₄ cermets were prepared by doping NiFe₂O₄ ceramic matrix with the mixed powders of Cu and Ni or Cu-Ni alloy powder as the electrical conducting metallic elements. The effects of technological parameters, such as the adding modes of metallic elements, the ball milling time, the sintering time and the sintering temperature, on the relative density and resistivity of the cermets were studied. The results show that the resistivity of 4.25Cu-0.75Ni/NiFe₂O₄ cermets decreases with increasing temperature, and has a turning point at 590 °C, which is similar to that of NiFe₂O₄ ceramic. The sintering temperature and adding modes of metallic elements have a great influence on the properties of 4.25Cu-0.75Ni/NiFe₂O₄ cermets. When the sintering temperature increases from 1200 °C to 1300 °C, the relative density increases from 89.86% to 95.33%, and the resistivity at 960 °C decreases from 0.11 Ω · cm to 0.03 Ω · cm, respectively. When the metallic elements are added with the mixed powders of Cu and Ni, the cermets of finely and uniformly dispersed metallic phase, high density and electric conductivity are obtained. The relative density and resistivity at 960 °C are 90.23% and 0.04 Ω · cm respectively for the cermet samples sintered at 1200 °C for 2 h, which are both better than those of the cermets prepared under the same technique conditions but with the metallic elements added as 85Cu-15Ni alloy powders. Foundation item: Project (G1999064903) supported by the National Key Fundamental Research and Development Program of China; project(2001AA335013) supported by the National High Technology Research and Development Program of China; project (50204014) supported by the National Natural Science Foundation of China     

Effect of sintering temperature on aging resistance and mechanical properties of 3Y-TZP dental ceramic

In order to investigate the effect of sintering temperature on aging properties and mechanical properties of 3Y-TZP dental ceramic in simulated oral environment,3Y-TZP nanopowder compacts were pressurelessly sintered at 1 350 ℃,1 400 ℃,1 450 ℃,1 500 ℃,respectively,then were treated by soaking in artificial saliva(65 ℃,pH=7)for two months.The treated specimens sintered at 1 350 ℃ showed there was no phase transformation but whose strength and toughnesswere significantly improved(P<0.05),while those sintered at 1 400 ℃-1 500 ℃ revealed a small amount of phase transformation and insignifi cant mechanical reinforcement(P>0.05).No micro-cracks were detected but increment in lattice volume was found in all specimens.Lowering sintering temperature favors aging resistance and mechanical reinforcement of 3Y-TZP in a simulated oral environment.     

Microstructure and mechanical properties of 8YSZ ceramics by liquid-phase sintering with CuO-TiO2 addition

The 8% (mass fraction) yttrium-partially-stabilized zirconia (8YSZ) ceramic was fabricated via liquid phase sintering at 1 200–1 400 °C by adding different mass ratios of CuO-16.7%TiO₂ (molar fraction) as sintering aid. Relative density, microstructure, Vickers hardness and bending strength as a function of sintering temperature and additive content were investigated. The experiment results show that liquid phase sintering at low temperature can be realized through adding CuO-16.7% TiO₂ to 8YSZ. The Vickers hardness and bending strength of samples with sintering aid are generally much higher than those of samples without sintering aid for all sintering temperatures, and increase with the increase of sintering temperature. When the addition content of CuO-16.7% TiO₂ is beyond 0.5%, the relative density, Vickers hardness and bending strength decrease with the increase of the mass ratio of sintering aid. Low additions of sintering aid are beneficial to aiding densification; high additions of sintering aid are detrimental to the sintered properties mainly due to greater amounts of pores generated by the volatilization of oxygen with the eutectic reaction between copper oxide and titanium dioxide. It is found that the fine grain size and high relative density are two main reasons of the high bending strength and Vickers hardness of the materials.     

Effect of nano-ZrO2 on microstructure and thermal shock behaviour of Al2O3/SiC composite ceramics used in solar thermal power

The Al2O3-ZrO2(3Y)-SiC composite ceramics used in solar thermal power were prepared by micrometric Al2O3,nano-ZrO2 and SiC powders under the condition of pressureless sintering.The bulk density and bending strength of samples with 10vol% nano-ZrO2 sintered at 1480℃ were 3.222 g/cm3 and 160.4MPa,respectively.The bending strength of samples after 7 times thermal shock tests (quenching from 1000℃ to 25℃ in air medium) is 132.0MPa,loss rate of bending strength is only 17%.The effect of nano-ZrO2 content on the microstructure and performance of Al2O3-ZrO2(3Y)-SiC composite ceramic was investigated.The experimental results show that the bending strength of samples with above 10vol% nano-ZrO2 content has decreased,because the volume expansion resulting from t-ZrO2 to m-ZrO2 phase transformation is excessive;Adding proper nano-ZrO2 would be contributed to improve the thermal shock resistance of the composite ceramics.The Al2O3-ZrO2(3Y)-SiC composite ceramic has promising potential application in solar thermal power.     

Low-temperature sintering and microwave dielectric properties of Li2MgTi3O8 ceramics doped with BaCu（B2O5）

The influences of BaCu(B₂O₅) (BCB) addition on sintering, microstructure and microwave dielectric properties of Li₂MgTi₃O₈ ceramics were investigated using X-ray diffractometry, scanning electron microscopy and microwave dielectric measurements. The experimental results show that a small amount of BaCu(B₂O₅) addition can effectively reduce the sintering temperature to 900 °C, and induce only a limited degradation of the microwave dielectric properties. Typically, the best microwave dielectric properties of ɛ _r=24.5, Q×f =24 622 GHz, τ _f=4.2×10⁻⁶ °C⁻¹ are obtained for 1.0% BCB-doped Li₂MgTi₃O₈ ceramics sintered at 900 °C for 3 h. The BCB-doped Li₂MgTi₃O₈ ceramics can be compatible with Ag electrode, which may be a strong candidate for low temperature co-fired ceramics applications.     

Effect of ZnO-B<Subscript>2</Subscript>O<Subscript>3</Subscript> glass addition on the microwave dielectric properties of BaO-Nd<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiO<Subscript>2</Subscript>-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> system

The effect of ZnO-B₂O₃(ZB) glass addition on the sintering behavior, microstructures and microwave dielectric properties of BaO-Nd₂O₃-TiO₂-Bi₂O₃ (BNTB) system was investigated with the aid of X-ray diffraction, scanning electron microscopy and capacitance meter. It is found that the ZB glass addition, acting as a sintering aid, can effectively lower the sintering temperature of BNTB system to 850 °C. The dielectric constant of BNTB-ZB ceramics increases with the increase of soaking time and the value of dielectric loss decreased with increasing soak time. The optical dielectric properties at 1 GHz of ɛ=74, tan δ=4×10⁻⁴, and TCC=25 ppm/°c were obtained for the BNTB system doped with 25 wt% ZB glass sintered at 850 °C for 2 h, representing that the BNTB-ZB ceramics could be promising for multilayer low temperature co-fired ceramics applications.     

Sintering and Electronic Conducting Properties of Lao.7Ca0.3CrO3 Perovskite Synthesized by a Glycine-nitrate Process

La0.7 Ca0.3 CrO3 powder consisting of superfine and uniform particles （ 100-200 nm ） were synthesized using a glycine-nitrate process （ GNP ）. The sintering and electronic conducting properties of La0.7 Ca0.3 CrO3 were invetigated in the sintering temperature range of 1 200-1 450 ℃. The desired morphology of the powder significantly improved its sinterability. La0.7 Ca0.3 CrO3 ceramics sintered at 1 250-1 450 ℃ show high relative densities above 95 % . The ceramics sintered at 1 250-1 400 ℃ have very similar electronic conduct- ing properties, providing electronic conductivities around 55 Ω^-1 cm^-1 at a measuring temperature of 800 ℃ . Further increasing the sintering temperature to 1450 ℃ led to an apparent degradation of electronic conducting properties. This research demonstrates the advantage of the GNP in producing La0.7 Ca0.3 CrO3 with respect to the enhanced sintering properties and superior electronic conducting properties.     

Spark plasma sintered AlN-BN composites and its thermal conductivity

A series of samples of hexagonal boron nitride-aluminum nitride ceramic composites with different amounts of CaF2 as sintering aid were prepared by spark plasma sintered at 1700-1850 ℃ for 5 min.The addition of CaF2 effectively lowered the sintering temperature and promoted the densification of AlN-BN composites.With the increase of sintering temperature,the density increased,and the contiguity of AlN grains enhanced in AlN-BN composites.Thermal conductivity of AlN-BN composites increased with the increase in CaF2 content and sintering temperature,and there is a maximum value of 78.6 W·m^-1·K^-1 when the sample with 3wt% CaF2 sintered at 1800 ℃.     

Shape memory effect during preparation of alumia ceramic tubes

Pure alumina ceramic tube and 95 alumina ceramic(the ceramic with 95.84% alumina) tube were prepared by using self-prepared alumina micrometer powder without agglomeration as raw material. The ceramic green was shaped by isostatic pressing and sintered at different temperature from 800 to 1 600 ℃ for 2 h. The 95 ceramic tube sintered at 1 550 ℃ for 2 h had mean particle size of 4 μm, bend strength of 437 MPa and volume density of 3.714 g/cm3. Shape memory effect during sintering was observed. XRD results showed that no phase transition occurred during shape memory process, which indicated that shape memory effect was not caused by phase transition. Several probable causes of the alumina ceramic shape memory effect were discussed in this paper.     

Relationship between Liquid Phase Content and the Orientation of PMNT Ceramics by TGG Method

The effects of glass frit on the sintering and electric properties of PMN-PT textured ceramics were investigated. The glass frits, including PbO, Bi2O3 and ZnO, were selected since liquid phase sintering lowered the PMN-PT sintering temperature. The piezoelectric properties of PMN-PT ceramics with glass frit addition are strongly dependent on the densification. The addition of glass frits into PMN-PT matrix reduced the sintering temperature to 1 100 ℃ instead of 1 150 ℃ for samples without glass. The piezoelectric coefficients (d33) of PMN-PT textured ceramics achieved 568 pc/N with 1 wt% excess PbO.     

Effect of MnO<Subscript>2</Subscript> on properties of SiC-mullite composite ceramics for solar sensible thermal storage

For improving the properties of SiC-mullite composite ceramics used for solar sensible thermal storage, MnO₂ was introduced as sintering additive when preparing. The composite ceramics were synthesized by using SiC, andalusite, α-Al₂O₃ as the starting materials with non-contact graphite-buried sintering method. Phase composition and microstructure of the composites were investigated by XRD and SEM, and the effect of MnO₂ on the properties of SiC composites was studied. Results indicated that samples SM1 with 0.2 wt% MnO₂ addition achieved the optimum properties: bending strength of 70.96 MPa, heat capacity of 1.02 J·(g·K)^-1, thermal conductivity of 9.05 W·(m·K)^-1. Proper addition of MnO₂ was found to weaken the volume effect of the composites and improve the thermal shock resistance with an increased rate of 27.84% for bending strength after 30 cycles of thermal shock (air cooling from 1 100 °C to RT).     

Reaction mechanism and microstructure evolution of reaction sintered <Emphasis Type="Italic">h</Emphasis>-BN

Hexagonal boron nitride ceramic (h-BN) based on the nitridation of B powders was obtained by reaction sintering method. The effects of sintering temperature on the mechanical properties and microstructure of the resultant products were investigated and the reaction mechanism was discussed. Results showed that the reaction between B and N₂ occurred vigorously at temperatures ranging from 1 000 °C to 1 300 °C, which resulted in the generation of t-BN. When the temperature exceeded 1 450 °C, transformation from t-BN to h-BN began to occur. As the sintering temperature increased, the spherical particles of t-BN gradually transformed into fine sheet particles of h-BN. These particles subsequently displayed a compact arrangement to achieve a more uniform microstructure, thereby increasing the strength.     

Effect of ZrO<Subscript>2</Subscript> dopant on the sintering behavior and performance of mcmb-derived carbon laminations

Liquid ball-milling dispersant method was used to prepare the ZrO2-doped carbon laminations from mesocarbon microbeads(MCMBs). After sintering at 1 300 ℃ in nitrogen atmosphere, the effect of ZrO2 concentration on sintering behavior, electric conductivity as well as bending strength of the carbon laminations was investigated in detail. The results showed that the volumetric shrinkage rate of the carbon laminations decreased from 38.2% to 30.9% when the ZrO2 concentration in raw materials varied from 0 to 16...     

Influence of soaking time on nonlinear electrical behavior and dielectric properties of TiO2-based varistor ceramics

The influence of soaking time on the nonlinear electrical behavior and dielectric properties of TiO₂-based varistor ceramics was investigated. Based on single sintering process, six disk samples of (Sr, Bi, Si, Ta)-doped TiO₂-based varistor ceramics were fabricated by sintering at 1 250 °C for 0.5–5.0 h. The samples were characterized by X-ray diffraction, voltage-current characteristics, energy spectra, metallographs, breakdown voltages, and apparent dielectric constant. It is found that the breakdown electrical field intensity at a current density of 10 mA/cm² decreases from 5.5 to 4.1 V/mm first and then increases to 7.0 V/mm, the nonlinear coefficient increases from 2.39 to 2.62 first and then decreases to 2.42, and the apparent dielectric constant increases from 98 200 to 115 049 first and then decreases to 73 865 with the soaking time increasing from 0.5 to 5.0 h. These indicate that the optimal soaking time is 2.0–3.0 h considering both nonlinear electrical behavior and dielectric properties.     

Microstructures and mechanical,electrical, high-temperature properties of Cu/Ti<Subscript>2</Subscript>AlC FGM fabricated by hot-pressing

The microstructure and the electrical, thermal, friction, and mechanical properties of Cu/Ti₂AlC fabricated by hot-pressing at 900 °C for 1 h were investigated in the present work. Microstructural observations have shown that the plate-like Ti₂AlC grains distribute irregularly in the network of Cu grains, and well-structured, crack-free bonds between the layers. With the increase in the content of Ti₂AlC from layer A to layer D, the electrical resistivity increases from 1.381×10^-7 Ω·m to 1.918 ×10^-7 Ω·m, the hardness increases from about 980.27 MPa to about 2196.01 MPa, and the friction coefficient from above 0.20 reduces to about 0.15. Oxidation rate increases with the increases of temperature. Exfoliation was obviously observed on the surface of oxidation layer A. The surface of layer D was still intact and the spalling and other defects were not found. The mass decreases in the acid solution, and increases in the alkaline solution. The largest corrosion rate is found in 6.5% HNO₃ or 4% NaOH solution.