Permeability and thermal expansion properties of porous LAS ceramic prepared by gel-casting method

The porous lithium aluminosilicate (LAS) ceramics with controllable pore structure were fabricated by gel-casting method. The porosity, pore structure, compression strength, gas permeability, and coefficient of thermal expansion (CTE) of the porous LAS ceramics with different monomer content were investigated. The sample with 5 wt.% monomer content has maximum value of compression (26.62 ± 0.54 MPa). When the monomer content increased to 20 wt.%, the porosity, Darcian gas permeability, and thermal expansion coefficient increased to maximum (63.66 %, 13.3 × 10⁻¹³ m², and 1.1–2.6 × 10⁻⁶ K⁻¹). The non-Darcian gas permeability showed irregular variation (1.35–3.61 m) with the increase of monomer content. A thermal vibration model was induced to investigate the effect of temperature and monomer content on the CTE. The results showed that the CTE increased with the increase of temperature due to the nonlinear thermal vibration of the atoms in lattice and the asymmetry of the force between particles.     

Influence of glass phase on Al2O3 fiber-reinforced Al2O3 composites processed by slip casting

The present work describes the processing of alumina fiber reinforced alumina ceramic preforms consisting of chopped Al₂O₃ fibers (33 wt%) and Al₂O₃ (67 wt%) fine powders by slip casting. The preforms were pre-sintered in air at 1100 °C for 1 h. A lanthanum based glass was infiltrated into these preforms at 1250 °C for 90 min. Linear shrinkage (%) was studied before and after glass infiltration. Pre-sintered and infiltrated specimens were characterized by scanning electron microscopy, energy dispersive X-ray, X-ray diffraction, porosimetry and flexural strength. The alumina preforms showed a narrow pore size distribution with an average pore size of ∼50 nm. It was observed that introducing Al₂O₃ fibers into Al₂O₃ particulate matrix produced warp free preforms with minor shrinkage during pre-sintering and glass infiltration. It was observed that the infiltration process fills up the pores and considerably improves the strength and reliability of alumina preform.     

多孔Al2O3-ZrO2陶瓷的微观结构

以Al₂O₃、ZrO₂陶瓷粉体为溶质,以莰烯为溶剂,以Texaphor963作为添加剂,制备出低粘度高稳定性的陶瓷浆料,采用冷冻注模工艺制备出具有较高强度的陶瓷坯体,采用无压烧结工艺,得到了多孔Al₂O₃-ZrO₂陶瓷制品,并对其微观结构进行了研究。     

Determination of the effects of the pore size distribution and pore connectivity distribution on the pore tortuosity and diffusive transport in model porous networks

An efficient computation method to study flow and transport process of small molecules in porous media using a dual site-bond lattice model, DBSM, is described. The microscopic properties of the porous network take into account the influence of local heterogeneities during the simulations. The numerical experiments demonstrated the combined effect of pore size distribution and connectivity distribution on the mass transport properties and the structural tortuosity. The results indicate that the pore size distribution and percolation phenomena related with pore shielding effects, influence significantly the tortuosity and the effective diffusivity of the porous network. Also, the simulations raise the important role of the connectivity distribution among the various pores in the gas diffusive properties of the poorly connected networks.     

Al2O3/Cu-O composites fabricated by pressureless infiltration of paper-derived Al2O3 porous preforms

Al₂O₃/Cu-O composites were fabricated from the paper-derived alumina matrix infiltrated with a Cu-3.2?wt% O alloy. Paper-derived alumina preforms with an open porosity ranging from ～ 14 to ～ 25?vol% were prepared by sintering of alumina-loaded preceramic papers at 1600?°C for 4?h. Pressureless infiltration at 1320?°C for 4?h of the preforms with Cu–O alloy resulted in the nearly dense materials with good mechanical and electrical properties, e.g. fracture toughness up to 6?MPa?m^0.5, four-point-bending strength up to 342?MPa, Young's modulus up to 281?GPa and electrical conductivity up to 2?MS/m depending on the volume fraction of copper alloy in the composites. The technological capability of this approach was demonstrated using prototypes in various engineering fields fabricated by lamination, corrugating and Laminated Object Manufacturing (LOM) methods.     

Effect of particle size distribution of raw powders on pore size distribution and bending strength of Al2O3 microfiltration membrane supports

To lower the sintering temperature of Al₂O₃ microfiltration membrane support, Al₂O₃ powders with particle size distribution of tri-modal are chosen. The results show that the function of fine Al₂O₃ grains depends on their agglomeration state: if fine Al₂O₃ grains distribute discretely, the bending strength of the support increases along with a slight increase in porosity; however, the aggregated fine grains are harmful to both bending strength and pore size distribution of the support. The bridging of medium Al₂O₃ grains between coarse grains contributes to increase the bending strength, but has less effect on porosity. The addition of medium (and/or fine) Al₂O₃ powder has less effect on the pore size distribution of the support if only coarse Al₂O₃ grain forms the support's framework, which suggests a new way to prepare the support with both high bending strength and high porosity at low temperature.     

Infiltration of Al2O3/Y2O3 mix into SiC ceramic preforms

Silicon carbide ceramics are very interesting materials to engineering applications because of their properties. These ceramics are produced by liquid phase sintering (LPS), where elevated temperature and time are necessary, and generally form volatile products that promote defects and damage their mechanical properties. In this work was studied the infiltration process to produce SiC ceramics, using shorter time and temperature than LPS, thereby reducing the undesirable chemical reactions. SiC powder was pressed at 300 MPa and pre-sintered at 1550 °C for 30 min. Unidirectional and spontaneous infiltration of this preform by Al₂O₃/Y₂O₃ liquid was done at 1850 °C for 5, 10, 30 and 60 min. The kinetics of infiltration was studied, and the infiltration equilibrium happened when the liquid infiltrated 12 mm into perform. The microstructures show grains of the SiC surrounded by infiltrated additives. The hardness and fracture toughness are similar to conventional SiC ceramics obtained by LPS.     

Preparation of ultra-active alumina of designed porous structure by successive hydrothermal and thermal treatments of porous anodic Al2O3 films

A porous anodic alumina film was prepared by the anodic oxidation of Al metal sheet in a thermostated and vigorously stirred bath of H₂SO₄ 15% (w/v) at a temperature of 25°C and a current density of 15 mA cm⁻². It had a geometric surface area of 33 cm², a surface density of pores 1.269×10¹¹ cm⁻² and the maximum limiting thickness and porosity achieved at these conditions which are 50.3 μm and 0.42, respectively. This oxide was tried in the catalytic test reaction of the decomposition of HCOOH at temperatures 270–390°C. Then, the oxide was treated hydrothermally in H₂O at 100°C for 5 h and tried in the same test reaction. The procedure of hydrothermal treatment and catalysis experiment was repeated 40 times. In all cases the oxide showed an almost exclusively dehydrative catalytic effect, 98–100%. Both the total activity of the alumina film with the aforementioned constant geometric surface area and its specific activity referred to the unit of oxide mass gave a maximum in the first and a minimum about the fourth hydrothermal treatment; then, they increased strongly with the order of hydrothermal treatment. Despite the decrease of the oxide mass during hydrothermal treatment, the final promotion of the total catalytic activity of oxide was 13.7–10.6 times that of non-treated oxide for temperatures 330–390°C. The corresponding promotion of specific activity was 31.5–24.5 times that of the non-treated oxide. The results of the present study showed that the successive hydrothermal and thermal treatments of porous anodic Al₂O₃ films produce more and more active alumina catalysts. In this way ultra-active alumina catalysts or supports can be prepared.     

Tensile strength and corrosion resistance properties of porous Al2O3/Ni composites prepared with rice husk pore-forming agent

The mechanical performance and chemical stability of porous alumina materials operating under harsh service conditions are of utmost importance in understanding their operational behavior if they are to stand the test of time. In the present study, the joint effect of nickel (Ni) reinforcement and rice husk (RH) pore-forming agent (PFA) on the tensile strength and the corrosion resistance properties of composite porous alumina ceramics was studied. To exploit the potential of this new porous alumina system, plain and Ni-reinforced porous alumina samples (Al₂O₃-xNi-RH; x?=?2, 4, 6 and 8?wt%) were developed through the powder metallurgy technique. Comprehensive investigation on the tensile strength properties of the developed porous alumina ceramics showed that relative to the plain sample (tensile strength and elastic modulus; 6.1?MPa and 1201?MPa), the presence of highly stable Ni₃Al₂SiO₈ spinelloid promoted the tensile strength enhancement (12.6–6.4?MPa) and the elastic modulus decline (897–627?MPa) of the composite samples. Similarly, corrosion resistance test was performed on the composite porous alumina samples in both 10?wt% NaOH and 20?wt% H₂SO₄ hot aqueous solutions. Overall, the composite samples demonstrated superior chemical stability in NaOH solution as compared with the plain sample. On the other hand, the composites were more prone to attack in H₂SO₄ solution, except for the Al₂O₃-2Ni-10RH composite sample which maintained its superiority over the plain counterpart.     

A novel method for the preparation of porous zirconia ceramics with multimodal pore size distribution

This work presents a new route of processing porous ceramic materials by vacuum induction melting of metallic ternary alloys from the system Ni–Zr–Y. Following internal oxidation results in an interpenetrating network of nickel and oxide phase consisting of yttria and zirconia. After dissolution of the metallic nickel matrix the oxide phase remains as a stable porous ceramic material with a multimodal pore size distribution. The porosity, pore size distribution and specific surface area can be controlled by alloy composition, solidification conditions and oxidation parameters.     

Effect of forming process on the integrity of pore-gradient Al2O3 ceramic foams by gelcasting

Pore-gradient Al₂O₃ foams were produced by gelcasting using the epispastic polystyrene (EPS) sphere template. This approach allows the design of porous ceramics with degree of pore connectivity and height of gradient layers via appropriate selection of the sizes and numbers of spheres. The fabrication processing of open-cell porous ceramics limited by polymeric sponge template, sharp cracks at the strut edges and closed pores can be resolved by this approach. To achieve the optimal manufacturing conditions of maintaining integrity of the network, the effects of solid loads, height of the slurry and the pre-removal of the polymeric foam template on the struts of the ceramic foams were studied. The results revealed that 55 vol.% Al₂O₃ slurries with 0.5 wt.% ammonium polyacrylate kept good fluidity for casting and avoided the inner inordinate shrinkage. Different shrinkage behavior of the top and bottom of the sample was effectively reduced due to approximately same water vapor diffusion areas on the top and bottom. The integrity of dendritic solidification structure maintained perfectly through template pre-removed in dichloromethane compared with direct heating.     

Fabrication and performance of dielectric tape based on CaO-B2O3-SiO2 glass/Al2O3 for LTCC applications

A non-aqueous tape-casting process for fabricating CaO-B₂O₃-SiO₂ glass/Al₂O₃ dielectric tape for LTCC applications was investigated. An isopropanol/ethanol/xylene ternary solvent-based slurry was developed by using castor oil, poly(vinyl butyral), and dibutyl phthalate as dispersant, binder, and plasticizer, respectively. The effects of dispersant concentration, binder content, plasticizer/binder ratio, and solid loading, on the properties of the casting slurry and resultant tape were systematically investigated. The results showed that the optimal values for the dispersant and binder contents, plasticizer/binder ratio, and solid loading were 2.0 wt%, 7.5 wt%, 0.6, and 62 wt%, respectively. The resultant flexible and uniform, 120-μm-thick CaO-B₂O₃-SiO₂ glass/Al₂O₃ tape had a density of 1.90 g/cm^?3, tensile strength of 1.66 MPa, and average surface roughness of 310 nm. Laminated tapes sintered at 875 °C for 15 min exhibited excellent properties: relative density of 97.3%, ε_r of 7.98, tan δ of 1.3 × 10^?3 (10 MHz), flexural strength of 205 MPa, and thermal expansion coefficient of 5.47 ppm/°C. The material demonstrated good chemical compatibility with Ag electrodes, indicating a significant potential in LTCC applications.     

MoSi2–Al2O3 electroconductive ceramic composites

Al₂O₃–MoSi₂ composites with MoSi₂ volume fractions between 16 and 40% were fabricated from commercial ceramic Al₂O₃ and intermetallic MoSi₂ powders by granulation, cold isostatic pressing and vacuum-sintering. The addition of MoSi₂ had only a slight influence on the densification of the composites, with sintered densities of 98% for samples with 16 vol.% MoSi₂ and 94% for samples with 40 vol.% MoSi₂. Composites with MoSi₂ contents of 20 vol.% and higher were electroconductive due to the formation of a three-dimensional percolating network of the conductive MoSi₂ phase.     

Microstructure refinement approaches of melt-grown Al2O3/YAG/ZrO2 eutectic bulk

Microstructure developments of melt-grown Al₂O₃/YAG/ZrO₂ ceramic bulks were investigated by controlling composition, cooling rate, heterogeneous nucleation sites and melt superheating treatment. The solidification microstructure of sample with hypereutectic composition (ZrO₂ 20 mol%) is finer than that with hypoeutectic or eutectic ones. With increasing the cooling rate, microstructure of melt-grown samples develops from colony to dendrite and finally to cell. The microscopy and the components of samples vary with the melt superheating temperature and the type of heterogeneous nucleation sites. The microstructure evolutions of melt-grown Al₂O₃/YAG/ZrO₂ eutectic relate to the melt undercooling level and the solid–liquid interfaces stability.     

Mechanical properties of Al2O3 + ZrO2 + nano-SiCp ceramic composites

The mechanical properties of Al₂O₃ matrix composites reinforced by ZrO₂(2 mol% Y₂O₃) and nanometre scale SiC dispersions have been investigated. It is shown that the Al₂O₃ matrix is simultaneously strengthened and toughened by both ZrO₂(2 mol% Y₂O₃) and nano-SiC particles. The maximum flexural strength and fracture toughness of the composites are 945 MPa and 7.3 MPam^1/2, respectively. The reinforcing effect of both t-m phase transformation of ZrO₂ (2 mol% Y₂O₃) and nano-SiC particles appears to be synergetic.     

HF/HCl acid resistance mechanisms of alumina ceramics in the Al2O3-MgO-CaO-SiO2-Y2O3 system

Silicon compounds in raw materials are the main reason for the low HF/HCl acid resistance of alumina ceramics. Y₂O₃ can improve the acid resistance of alumina ceramics. This work aimed to reveal the mechanisms of the effects of Y₂O₃ on the form of Si and the durability of the ceramic. An experiment on a high-temperature reaction between Y₃Al₅O₁₂ and a polycrystalline alumina ceramic was designed. The effect of corrosion time on the acid solubility of the alumina ceramic was investigated. The results show that Si can dissolve in Y₃Al₅O₁₂ to generate solid solutions, impeding the generation of Si-containing compounds with bad acid resistance, and decreasing the content of amorphous Si. The acid solubility of the ceramic was only 0.95%, even when the corrosion time was extended to 60 times the industry standard. This revelation of the acid resistance mechanisms can provide a new idea for designing corrosion-resistant ceramics.     

Effect of Al2O3 on mechanical properties of Ti3SiC2/Al2O3 composite

The effect of Al₂O₃ on mechanical properties of Ti₃SiC₂/Al₂O₃ composite fabricated by SPS was studied systematically. The results show that the hardness of the Ti₃SiC₂/Al₂O₃ composite can reach 10.28 GPa, 50% higher than that of pure Ti₃SiC₂. However, slight decrease in the other mechanical properties was observed with Al₂O₃ addition higher than 5–10 vol.%, which is believed to be due to the agglomeration of Al₂O₃ in the composite.     

Characterization and tribological investigation of sol-gel Al2O3 and doped Al2O3 films

Thin films of Al₂O₃ and doped Al₂O₃ were prepared on a glass substrate by dip coating process from specially formulated ethanol sols. The morphologies of the unworn and worn surfaces of the films were observed with atomic force microscope (AFM) and scanning electron microscope (SEM). The chemical compositions of the obtained films were characterized by means of X-ray photoelectron spectroscopy (XPS). The tribological properties of obtained thin films sliding against Si₃N₄ ball were evaluated and compared with glass slide on a one-way reciprocating friction tester. XPS results confirm that the target films were obtained successfully. The doped elements distribute in the film evenly and exist in different kinds of forms, such as oxide and silicate. AFM results show that the addition of the doped elements changes the structure of the Al₂O₃ films, i.e., a rougher and smoother surface is obtained. The wear mechanisms of the films are discussed based on SEM observation of the worn surface morphologies. As the results, the doped films exhibit better tribological properties due to the improved toughness. Sever brittle fracture is avoided in the doped films. The wear of glass is characteristic of brittle fracture and severe abrasion. The wear of Al₂O₃ is characteristic of brittle fracture and delamination. And the wear of doped Al₂O₃ is characteristic of micro-fracture, deformation and slight abrasive wear. The introduction of ZnO is recommended to improve the tribological property of Al₂O₃ film.     

Dynamics of internal diffusion during the hydrogenation of 1,5,9-cyclododecatriene on Pd/Al2O3

Dynamic and pseudo-steady state diffusion–reaction models were simulated for the three-phase consecutive hydrogenation of 1,5,9-cyclododecatriene on a shell Pd/Al₂O₃ catalyst in order to examine the time evolution of concentration profiles inside the catalyst pellet: a model accounting only for the dynamics of the active layer and a model taking also into account the inert part of the catalyst were compared.

In the conditions of the semibatch experiments (T=433 K, p_H2=1.2 MPa), all the models lead to the same bulk concentration–time curves, but the hydrocarbon concentration profiles in the pores are dependent on the model. The influence of the diffusion in the inert part of the catalyst on the bulk concentrations becomes nonnegligible only when the external liquid volume (out of the catalyst) is reduced.

The transient evolution of the concentration profiles in the pores show that hydrogen concentration reaches its steady state within a few seconds, while the evolution of the organic concentration profiles is slower.

Furthermore, the reaction rate has been found to be only affected by the hydrogen diffusion. The diffusion of organics can control the reaction rate only for low values of organic concentration and higher pressure in hydrogen.     

Determination of the porosity and pore size distribution of SiC ceramic foams by nuclear methodologies

Abstract

Abstract

Two nuclear methodologies were used for porosity and pore size distribution determination of SiC ceramic foams. Thirty samples were analysed, six of each one of the following pore densities: 30, 45, 60, 80 and 100 pores per inch (ppi). The two nuclear techniques employed were X-ray microtomography and gamma ray transmission. For the microtomography technique, the spatial resolution of the images was 32?μm. For the gamma ray transmission methodology, a NaI (Tl) scintillation detector and Am-241 radioactive source were used. The gamma transmission technique was precise for porosity determination in relationship to the nominal values supplied for the sample manufacturer. The 30 and 45?ppi samples analysed by the microtomography technique present average porosities equivalent to the nominal porosity, and the other samples present an average of 4·6% smaller values. The 30 and 45 ppi sample two-dimensional images show voids inside the structural solid material of the ceramic.