Effect of La_2O_3 on Microstructure and Transmittance of Transparent Alumina Ceramics

Optically transparent alumina ceramics were fabricated by conventional process and sintered without pressure in H2 atmosphere. The results indicate that relative densities of alumina specimens increase to theoretical densities （T. D. ） with increasing content of La2O3. With increasing holding time during sintering, much less pores and larger grains were found in the sintered alumina samples. Higher transmittance was achieved in alumina codoped with MgO and La2O3 as compared with that doped with MgO only. The total-transmittance of alumina sample is up to 86% at twavelength range of 300 - 800 nm.

Enhancing microstructural properties of alumina ceramics via binary sintering aids

Alumina ceramics are widely used in many fields such as cutting tools,laser shock materials,roadbed board and refractory.Herein,Al_(2)O_(3)ceramics are prepared by a low-cost pressureless sintering technology,using the binary sintering aids of MgO and SiO_(2).The effects of sintering temperature and the ratio of binary sintering aids on the mechanical properties and microstructure of Al_(2)O_(3)ceramics are investigated.A spinel second phase(MgAl_(2)O_(4))is found out by the analysis of the results of XRD and EDS when MgO and SiO_(2)are introduced in the samples.The optimum properties are found when MgO content is 20 wt.%based on the total sintering aids and the sintering temperature is 1550℃.The bending strength and the bulk density reach a maximum value of 314 MPa and 3.73 g/cm^(3),respectively.The addition of appropriate amount of SiO_(2)makes the formation of liquid phase sintering and the removal of large pores.Meanwhile,a small amount of magnesium oxide doping has an effect on the grain refinement from the microstructure of the sample.Therefore,it is believed that MgO and SiO_(2)are the ideal sintering aids for promoting the densification and property of alumina ceramics.

Thermal Insulation Materials of Alumina Based Porous Ceramics

Significant energy saving effects can be made through the improvement of furnace refractories,especially the thermal insulation refractories. In this study,the preparation and the application of different alumina based porous ceramics were briefly introduced. Alumina based porous ceramics were prepared combined foaming method with gelcasting,sol- gel process or cement curing process. The influences of different preparation methods on the sintering shrinkage, porosity, phase composition, microstructure, compressive strength and thermal conductivity were discussed. Alumina based porous ceramics with relatively high strength and low thermal conductivity could be obtained through the above mentioned methods. Compared with the traditional lining materials,about 40% energy could be saved when they were used as the furnace wall.

Shear bond strengths between alumina-toughened zirconia cores and veneering ceramics and their susceptibility to aging

Objective:To evaluate the shear bond strength(SBS) between alumina-toughened zirconia (ATZ) cores and veneering ceramics,investigate the effect of aging in artificial saliva on SBS and compare it with that of yttria-stabilized tetragonal zirconia polycrystals(Y-TZP).Methods:Bars of ATZ and Y-TZP were layered with veneering ceramics in accordance to the recommendation of the manufacturer.Half of each group(n = 10) was aged at 134℃.(under 2 bar pressure) in an autoclave for 48 h.Subsequently,all specimens were subjected to shear force in a universal testing machine.The interface and fractured surface of the specimens were evaluated using scanning electron microscopy and X-ray energy dispersive spectroscopy.Results:The initial mean SBS values in MPa±SD were 28.9±8.0 for ATZ and 26.2±7.6 for Y-TZP.After aging,the mean SBS values for ATZ and Y-TZP were 22.9±4.9 MPa and 22.8±6.9 MPa,respectively.Neither the differences between the SBS values of the ATZ and Y-TZP groups nor the influence of aging on all groups were statistically significant.Conclusions:The SBS between the ATZ core and the veneering ceramics was not affected by aging.The SBS of ATZ to veneering ceramics was not significantly different compared with that of Y-TZP.

Influence of Alumina Addition on the Optical Property of Zirconia/Alumina Composite Dental Ceramics

The influence of various alumina additions on the optical property of zirconia/alumina composite ceramics was investigated. The relative sintered densities, transmittances, color and the microstructure of the composite ceramics were studied. The experimental results showed that the relative sintered densities and transmittances decreased with alumina addition. The lightness increased obviously but the chroma change was small. Pure zirconia nanopowders sintered densely could obtain the relatively high transmittance, while the transmittance and the lightness of slight addition changed significantly. The zirconia/alumina composite ceramics with alumina addition less than 7.5wt% could achieve the relatively stable and reliable optical properties.

Modified-starch Consolidation of Alumina Ceramics

The alumina ceramics with the homogeneous microstructure and the higher density were fabricated via the modified-starch consolidation process by 1.0 wt% of a modified starch as a consolidator/ binder. The swelling behavior of the modified oxidized tapioca starch was analyzed by optical microscope, and two other corn starches （common corn starch and high amylose corn starch） were also analyzed for comparison. The modified starch used as a binder for the consolidation swelled at about 55 ℃, began to gelatinize at 65 ℃ and then was completely gelatinized at 75 ℃. But the corn starches could not be completely gelatinized even at 80 ℃ for 1 h. The high-strength green bodies （10.6 MPa） with the complex shapes were produced. The green bodies were sintered without any binder burnout procedure at 1 700 ℃ and a relative density of 95.3% was obtained for the sintered bodies, which is similar to that of the sintered sample formed by conventional slip casting. In addition, the effect of temperature on the apparent viscosity of the starch/alumina slurry in the process was investigated, and the corresponding mechanism for the starch consolidation was discussed.

Alumina Ceramics Fabricated by in-situ Consolidation of Pre-gelling Starch

An in-situ consolidation method was developed and optimized to successfully fabricate alumina ceramics using pre-gelling starch. Our results showed that the obtained ceramics have more homogeneous microstructure, higher density, higher flexural strength, and favorable biocompatibility compared to the regular one. During the process, cornstarch granules swelled and deformed but no fracture was observed. After the cornstarch granules bursted, alumina particles were suspended uniformly in the three-dimensional network structure to generate a much smoother surface. Below 0.5 wt% higher cornstarch content increased the flexural strength of prepared ceramics, while above 0.5 wt% the mechanical properties were compromised. Therefore the cornstarch content of 0.5% was the optimal concentration to achieve the highest mechanical strength of the prepared ceramics, with a measured flexural strength of 341 MPa, and a relative density of 96.01%.

Advanced Alumina and Zirconia Ceramics Produced by the Electroconsolidation Method

The possibility to produce advanced alumina and zirconia ceramics by the electroconsolidation method is studied. The technological parameters of Al2O3 and ZrO2 （3 mass% Y2O3 ） production were developed and optimized. Electroconsolidated alumina and zirconia ceramics have higher values of properties in comparison with ordinary sintered samples in air. Advanced proper- ties of electroconsolidated ceramics are defined by homo- geneous, ultradense and fine-crystalline structure that was formed due to the effect to consolidate the materials to high density for a shortest time.

Porous Alumina Ceramics Fabricated by Spark Plasma Sintering

Porous alumina of regular spherical particles was fabricated with the spark plasma sintering （SPS） and then compared to those obtained through conventional hot pressing （HP）. The effects of the parameters of the heating process on porosity were also investigated, Microstructural studies suggest that porous ingots including regular pores be made out of regular spherical alumina particles due to the close sphere packages. A comparative study on the relative necks of the specimens produced by SPS and HP indicates an enhancement of neck growth with SPS. Contrasting the theoretical values to the experimental results over the relative necks indicates that a proper relationship between the relative necks and the porosity can be established by a sintering model.

Fabrication and Properties of Aluminabased Reticulated Porous Ceramics

The primary impregnation slurry was prepared using active alumina(56.25 mass%),kaolin(15 mass%),zirconia(3.75 mass%),deionized water(25 mass%),and extra adding FS(0.2 mass%)and CMC(0.4 mass%).The effects of the active alumina particle size(d50=5.043,2.934,and 1.629μm)on the rheology and the thixotropy of the slurry were researched.It was found that the bimodal activeα-Al2O3(AMA-10)with d50=1.629μm was optimum.The secondary impregnation slurry was prepared using AMA-10,kaolin and zirconia as the main raw materials.Then the alumina-based reticulated porous ceramics were fabricated by the organic foam impregnation method combined with a secondary vacuum impregnation process.The influence of the AMA-10 content on the properties of the ceramics was studied.The residual stress of the specimens was analyzed by finite element analysis.The results show that the smaller alumina particle size and multimodal distribution are beneficial to the thixotropy of the primary impregnation slurry.The secondary vacuum impregnation technique can significantly improve the mechanical properties,the thermal shock resistance and the residual strength of the alumina-based reticulated porous ceramics.With the decrease of alumina content in the secondary impregnation slurry,the residual stress of the external layer of ceramic reinforcement gradually changes from tensile stress to compressive stress,which effectively inhibits the expansion of the surface crack,and remarkably improves the crushing strength retention ratio of alumina reticulated porous ceramics.

RESEARCH ON MICROWAVE SINTERING ALUMINA CERAMICS

This paper introduces the characteristic of microwave sintering,introduces and analyses the process of microwave sintering of thealumina ceramics, moreover, compared with the experiments ofconventional sintering method. Based on the research, some resultsare obtained, and experimental basis for microwave sintering ofceramics is put forward.

Effects of alcohol additives on pore structure and morphology of freeze-cast ceramics

The porous alumina ceramics with lamellar structure were fabricated successfully by freeze casting. The viscosities of alumina slurries, pore structures, porosities and mechanical properties of the sintered ceramics were investigated by introducing both types of alcohols as water solidification modifier into the initial slurries, such as ethanol and 1-propanol. With the addition of ethanol or 1-propanol, the viscosities of slurries increased and porosities of sintered ceramics decreased. The compressive strengths of the sintered porous alumina ceramics were improved due to a good connectivity between lamellae with the addition of both types of alcohols. The lowest porosities of 68.52% and 73.72% and highest compressive strengths of 18.2 MPa and 15.0 MPa were obtained by the addition of 30% ethanol in mass fraction and 1-propanol, respectively.

Study on Microstructure of Alumina Based Rare Earth Ceramic Composite

Analysis techniques such as SEM, TEM and EDAX were used to investigate the microstructure of rare earth reinforced Al2O3/(W, Ti)C ceramic composite. Chemical and physical compatibility of the composite was analyzed and in-terfacial microstructure was studied in detail. It is found that both Al2O3 and (W, Ti)C phases are interlaced with each other to form the skeleton structure in the composite. A small amount of pores and glass phases are observed inside the material which will inevitably influence the physical and mechanical property of the composite. Thermal residual stresses resulted from thermal expansion mismatch can then lead to the emergence of dislocations and microcracks. Interfaces and boundaries of different types are found to exist inside the Al2O3/(W, Ti)C rare earth ceramic composite, which is concerned with the addition of rare earth element and the extent of solid solution of ceramic phases.

Numerical Simulations of Tungsten Alloy Rods Penetration into Alumina Ceramic/Armor Steel Composite Target

The Johnson-Holmquist constitutive ceramic model is re-implemented into the LS_ DYNA3D program to simulate the penetration of long rod projectile into ceramic/armor steel composite targets. The damage evolution, fracture propagation, and spall damage in the ceramic/armor targets is represented during the simulation procedure and the accuracy of the penetration depth comparing between the simulating and experimental results is reliable with an error less than 8 %. The relationship between the mass efficiency factor, differential factor and ceramic tiles thickness is given out for the penetration results with/without a cover plate.

Flame Pyrolysis Spraying of Alumina-Yttria Ceramic Coating

Alumina yttria composite oxide coating was obtained on the stainless steel through the pyrolysis of aluminium nitrate and yttrium nitrate by flame spraying in the open atmospheric environment. The isothermal oxidation behaviour of the specimens coated with alumina yttria was investigated in air at the temperature of 1273 K. The results show that coating can promote the selective oxidation of chromium in the alloy and improve the high temperature corrosion resistance of the stainless steel.

Preparation of Alumina Abrasion-Resistant Ceramic Grinding Ball with Spent FCC Equilibrium Catalyst

The chemical composition, structure and thermal stability of the spent FCC equilibrium catalyst from an oil refinery were characterized by XRD, FT-IR, DTA-TG, BET, complete chemical analysis, SEM, and XRF. The spent FCC equilibrium catalyst, clay, barium carbonate, and talc were used as the main raw materials to prepare the alumina abrasion-resistant ceramic balls to be used in the powder grinding mill for manufacture of architecture tiles. The results showed that after proper formulation study, the spent FCC equilibrium catalyst could replace industrial alumina to prepare high performance grinding balls. Meanwhile, the various performance indices of the grinding ball could meet the quality standard for similar products, and additionally, the energy saving effect was achieved in the operation of the grinding section, resulting in a successful comprehensive utilization of solid wastes.

Porous alumina ceramic via gelcasting based on2-hydroxyethyl methacrylate dissolved in tert-butyl alcohol

To obtain porous alumina ceramic with high strength,a novel gelcasting system based on 2-hydroxyethyl methacrylate(HEMA)dissolved in tert-butyl alcohol(TBA)was developed.The polymerization of the HEMA-TBA gelcasting system,the thermal behavior of obtained green body,and the microstructures and mechanical properties of the sintered bodies were investigated by rheometer,TG-DSC,SEM and bending strength testing,respectively.The results show that,(1)10 mg/mL of the initiator(benzoyl peroxide)is the optimal amount for polymerization of this gelscasting system at 25 ℃;(2)The alumina suspension of the HEMA-TBA gelcasting system showing shear-thinning behavior is sufficiently low for gelcasting process;(3)The bending strength of porous alumina ceramic samples,whose porosities range from 42% to 56%,is from(8±0.5)to(91±4.5)MPa.

Low-temperature densification sintering and properties of CaAl_2Si_2O_8 ceramics with MeO·2B_2O_3(Me=Ca,Sr,Ba)

Dense CaAl2Si2O8 ceramics were prepared via a two-step sintering process at temperatures below 1000℃. First, pre-sintered CaAl2Si2O8 powders containing small amounts of other crystal phases were obtained by sintering a mixture of calcium hydroxide and kaolin powders at 950℃ for 6 h. Subsequently, the combination of the pre-sintered ceramic powders with MeO＇2B203 （Me = Ca, Sr, Ba） flux agents enabled the low-temperature densification sintering of the CaAl2Si2O8 ceramics at 950℃. The sintering behavior and phase formation of the CaAl2Si2O8 ceramics were investigated in terms of the addition of the three MeO·2B2O3 flux agents. Furthermore, alumina and quartz were introduced into the three flux agents to investigate the sintering behaviors, phase evolvements, microstructures, and physical properties of the resulting CaA12Si208 ceramics. The results showed that, because of their low-melting characteristics, the MeO·2B2O3 （Me = Ca, Sr, Ba） flux agents facilitated the formation of the CaAl2Si2O8 ceramics with a dense microstructure via liquid-phase sintering. The addition of alumina and quartz to the flux agents also strongly affected the microstructures, phase formation, and physical properties of the CaA12Si208 ceramics.

TLP bonding of alumina ceramic and 5A05 aluminum alloy using Ag-Cu-Ti interlayer

In order to join alumina ceramic to 5A05 aluminum alloy and obtain the excellent airtightness of joints whose maximum service temperature is 623 K, transient liquid phase （TLP） bonding technique was ,investigated using Ag-Cu-Ti alloy as interlayer. The wetting experimental results confirm that Ti can react with alumina ceramic at 833 K by adding 2 wt.% Ti in Sn. But during bonding alumina ceramic and 5A05 aluminum alloy with Ag-Cu-Ti interlayer at 833 K, Ti preferentially reacts with Al and there is no reaction layer on alumina ceramic/Ag-Cu-Ti interface, which finally results in a poorly airtight joint.

Sea Urchin Spines as a Model-System for Permeable,Light-Weight Ceramics with Graceful Failure Behavior.Part Ⅱ.Mechanical Behavior of Sea Urchin Spine Inspired Porous Aluminum Oxide Ceramics under Compression

Sea urchin spines were chosen as a model system for biomimetic ceramics obtained using starch-blended slip casting. Porous alumina ceramics with cap-shaped layers with different alternating porosities were found to have superior fracture behavior under bulk compression compared to ceramics with uniform porosity.They fail in a cascading manner,absorbing high amounts of energy during extended compression paths.The porosity variation in an otherwise single phase material mimicks the architectural microstructure design of sea urchin spines of Heterocentrotus mammillatus,which are promising model materials for impact protection.