Compressive Strength Of Porous Ceramics Research Articles

Lightweight porous silica-alumina ceramics with ultra-low thermal conductivity

Thermal protection has always been an important issue in the energy, environment and aerospace fields. Porous ceramics produced by the particle-stabilized foaming method have become a competitive material for thermal protection because of their low density and low thermal conductivity. However, the study of porous ceramics for composite systems using particle-stabilized foaming method was relatively rare. Here, silica-alumina composite porous ceramics were prepared by particle-stabilized foaming method, which was achieved by tailoring the surface charges of silica and alumina through adjustment of the pH. Porous ceramics exhibited porosity as high as 97.49% and thermal conductivity (25 °C) as low as 0.063 W m−1 K−1. The compressive strength of porous ceramics sintered at 1500 °C with a solid content of 30 wt% could reach 0.765 MPa. Based on the light weight and excellent thermal insulation properties, the composite porous ceramic could be used as a potential thermal insulation material in the spacecraft industry.

Preparation and properties of porous alumina with inter-locked platelets structure

Porous alumina ceramics with alumina platelets was prepared by vapor-solid reaction sintering of AlOF mesophase gas by the reaction of HF and Al2O3. The effect of heating treatment temperatures on porosity, the formation of inter-locked platelets structure and compressive strength of porous alumina ceramics was determined by Archimedes' method, XRD, SEM and compressive tests. The results indicated that after heating at temperatures from 1300 °C to 1600 °C, the porosity of alumina ceramics decreased from 61.6% to 48.4%. Increasing the heating treatment temperature was beneficial to form inter-locked structure between alumina platelets. The maximum compressive strength of porous ceramics with porosity of 48.4% can reach 29.8 MPa heated at 1600 °C; this strength was attributed to the strong bonding between the alumina platelets.

Preparation of Porous Ceramic Building Decoration Materials by Foaming Method and Research on Nanomechanical Properties.

Because of its excellent properties, mullite porous ceramics are widely used in thermal insulation materials, catalyst carriers, gas-liquid filtration, separation materials, etc. At the same time, zirconia not only has the advantages of high melting point, good chemical stability, and high strength but also can significantly improve the strength of ceramics through phase transformation and particle dispersion in the matrix and is widely used in the reinforcement of ceramics. In this paper, using mullite powder as the raw material, Al2O3 and SiO2/ZrSiO4 as the starting material for the mullite self-bonding phase, and AlF3·3H2O, ZrO2, and Y2O3 as additives, the zirconia-reinforced mullite was prepared by the foaming-injection method. The volume density, linear shrinkage rate, microstructure, room temperature, etc. of nanozirconia-reinforced mullite porous ceramics were studied by the amount of the foaming agent, the amount of mullite self-bonding phase powder, the type and amount of additives, etc. Effects of mechanical properties and thermal conductivity were also analyzed. The research results show that zirconia-reinforced mullite porous ceramics were prepared with mullite powder and 6 wt% AlF3·3H2O as raw materials, and ZrO2 and Y2O3 as additives. Adding an appropriate amount of ZrO2 and Y2O3 can significantly improve the mechanical properties of porous ceramics. When ZrO2 is 6 wt% and Y2O3 is 8 wt%, the porosity is 66.4% and the flexural strength and compressive strength of porous ceramics with a large pore size of 168 μm can reach 14.3 MPa and 36.3 MPa, respectively, which are obviously better than the strength of mullite porous ceramics without adding Y2O3 (flexural strength 11.3 MPa, nanocompressive strength 29.4 MPa).

Preparation of porous SnO2-based ceramics with lattice structure by DLP

Porous tin oxide-based (SnO2-based) ceramics with lattice structures were prepared by digital light processing (DLP) 3D printing technology. A 57 wt% SnO2-based DLP slurry was prepared by rheological experiments. By optimizing the exposure time, the DLP process for preparing SnO2 based ceramics had better molding accuracy. The printing size error could be controlled at about 1.8%. The degreasing and sintering curves were obtained by thermogravimetric-differential scanning calorimetry (TG-DSC) analysis, and the SnO2-based porous ceramics with porosity of 50%–71% were prepared. The SnO2-based ceramics with high densification could be obtained by sintering at 1300 °C for 2 h. The relative density of printed SnO2-based ceramics was about 92%. In this paper, zinc oxide (ZnO) was selected as a sintering additive to promote the densification of SnO2 ceramics. According to the characterization methods of scanning electron microscopy(SEM), X-ray diffraction(XRD), Energy dispersive spectrometer (EDS) and relative density, the optimum doping amount of ZnO was 9 wt%. The influence of pore structure on compressive strength was analyzed by finite element simulation and compressive test. The compressive strength of porous ceramics could be reduced from 1.56 MPa to 0.27 MPa by controlling the porosity of the porous lattice from 50% to 71%. The compressive strength of the solid block was 160 MPa. In this study, the porous SnO2-based ceramics prepared can be used in porous electrodes, lithium-ion batteries, supercapacitors, catalysts, sensors and other fields in the future.

Pore structure and thermal oxidation curing behavior of porous polymer derived ceramics with superhigh porosity fabricated by freeze casting

Porous ceramics with porosity up to 92.5 % have been successfully fabricated by freeze casting of polycarbosilane (PCS) solution. The effect of PCS concentration and thermal oxidation curing on the pore structure and compressive properties was investigated. Curing mechanism and thermodynamics were illuminated through analyzing the molecular structure, curing activation energy, and curing degree. Porous ceramics, mainly composed of SiC and a small amount of SiO2, have dendritic pore structure which well replicates the solidification morphology of camphene solvent. Results of FT-IR and Gaussian computation of PCS electron density show that Si–H and Si–CH3 bonds play a dominant role in thermal oxidation curing reaction. Both curing degree and ceramic yield increase with the increase in curing temperature and time. The curing degree of Si–H bond is close to 52 % and the corresponding ceramic yield is about 83 % when the porous PCS was cured at 200 °C for 90 min. Both polymer concentration and curing time have influences on the compressive strength of porous ceramics.

Preparation of Ca–Si–Al–Mg porous ceramics by Co-operation of Ca&Mg-contained soda residue and altered rock gold tailings

Utilization of two solid wastes of altered rock gold tailings and soda residue as main raw materials, and investigated the effects of soda residue, Al2O3 and MgO contents on bulk density, compressive strength and crystal phase of porous ceramics. The results showed that porous ceramics with the main crystalline phases of diopside can be prepared from high solid waste content (gold tailings + soda residue accounting for 95%) at 1170 °C. The addition of MgO made the final product generate augite phase, which improved the compressive strength of the porous ceramics. The best results of bulk density and compressive strength of the final product are 0.50 g/cm3 and 6.02 MPa respectively.

Preparation of lightweight hollow glass microsphere ceramics by gel casting

Owing to low density, hollow glass microspheres (HGMs) float on common media such as water, ethanol, and tertiary butanol. As a result, HGMs are not suitable to prepare slurries and ceramics by gel, slip, and freeze castings. In this paper, polyacrylamide (PAM) was used as dispersant and thickener agent to prepare homogeneous HGM aqueous slurries with controlled solid loadings and subsequent lightweight HGM porous ceramics were prepared by gel casting. Effects of PAM content on the stability of aqueous slurries as well as effects of solid (i.e., HGM) loading on density, porosity, pore size distribution, and compressive strength of porous ceramics were investigated. Increasing the viscosity of the slurry resulted in HGMs with significantly lower floating rates and more stable HGM aqueous slurries. HGM porous ceramics with densities and compressive strengths of 0.127–0.219 g/cm3 and 0.74–1.71 MPa, respectively, were prepared by gel casting.

Fabrication of porous alumina by directional freeze casting and annealing in TBA–water–sucrose system

The volume change difference between water and tert-butyl alcohol (TBA) hydrate was approximately 10.1% when ceramic slurry with water–TBA was frozen for fabrication of porous ceramics. Residual stress may be produced at the pore walls and led to low compressive strength of the ceramics. In this regard, sucrose was used in the ceramic slurry. Several water–TBA crystals were observed in the sucrose amorphous bodies after directional freezing. Water and TBA in the sucrose amorphous bodies were recrystallized during subsequent annealing. Sucrose was used as a protective agent and annealing additive in the TBA–water–sucrose system to eliminate residual stress and change the pore structure from snowflakes and dendrites into tiny cylinders. Adding sucrose and annealing process exerted minimal effect on the open porosity of porous ceramics but remarkably improved their compressive strength. When TBA and sucrose contents were 30 and 20 wt%, respectively, in the ceramic slurry with 20 vol% alumina content, the compressive strength of porous ceramics was 49.9 MPa after annealing at − 15 °C for 2 h. The value was 3.6-fold higher than that of the ceramic without annealing. Results provide a new method for improving the mechanical properties of porous ceramics prepared by freeze casting.

A simple and efficient way to prepare porous mullite matrix ceramics via directly sintering SiO2-Al2O3 microspheres

A novel route was proposed to fabricate porous mullite matrix ceramics by directly sintering SiO2-Al2O3 microspheres (SAMs) in air, which was simple and efficient compared with other methods. The effects of sintering temperature on the phase composition, microstructure, porosity and compressive strength of porous ceramics were studied. It is found that the mullite phase generates when sintered at above 1450°C. SEM results shows that the sintering temperature has a significant influence on the formation of pores morphology. As the sintering temperature increases from 1200°C to 1650°C, the original small holes inside and between the SAMs develop into macropores, whose pore size increases to eight times more than that of SAMs at a maximum, and the distribution of pores is relatively uniform. Porous mullite matrix ceramics with the porosity of 81.37% and compressive strength of 6.25±0.91MPa have been obtained through this method at the sintering temperature of 1550°C.

Fabrication of Porous Al<sub>2</sub>O<sub>3</sub> Ceramics by Freeze Drying Technique and Annealing Treatment

Porous Al2O3 ceramics were fabricated by directional freezing and low pressure drying with sucrose solution as the cryogenic medium. The pore structure of the porous ceramics was changed by annealing in the environment of higher than the glass transition temperature of sucrose solution after directional freezing because of changing the size and distribution of crystalline solid. The effects of the annealing time on the pore structure, open porosity and mechanical property of porous ceramics were investigated. The results showed that the pore size of porous ceramics increased substantially with the increase of annealing time. The open porosity of porous ceramics changed slightly with the increase of annealing time, while the compressive strength of porous ceramics showed a trend of decrease. The pore size range of porous Al2O3 ceramics fabricated is from 6.0μm to 110.2μm, the range of porosity was 40.35%-64.58%, the compressive strength range of porous Al2O3 ceramics was from 25.9MPa-126.6MPa. The porous Al2O3 ceramics with different pore structure can be obtained by changing the annealing time.

Preparation and properties of reticulated porous γ-Y2Si2O7 ceramics with high porosity and relatively high strength

Dense γ-Y2Si2O7 ceramics are promising silicates with very low thermal conductivity, good damage tolerance and hot corrosion resistance. However, processing of porous γ-Y2Si2O7 was not reported in early literatures. In this paper, reticulated porous γ-Y2Si2O7 ceramics with high porosity (~85%) and relatively high compressive strength (~1.28MPa) are successfully fabricated by the polymeric sponge impregnation method. The main compositions of the slurry are Y2SiO5 and silica sol. Samples are sintered at the temperature of 1550°C to obtain porous γ-Y2Si2O7 ceramics by reaction bonding; and the samples are recoated 3 times to enhance the compressive strength of porous ceramics. Scanning electron microscopy and X-ray tomography investigations show that the pore size distribution of the as-prepared porous γ-Y2Si2O7 ceramic ranges from 200 to 700μm; meanwhile, the sample possesses fully open and highly interconnected pores. Influences of pretreatments of sponge on the compressive strengths of samples are also investigated. This work reports a new porous ceramic material (γ-Y2Si2O7) with high permeability, high strength and controllable macrostructure.

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A low temperature processing route for fabricating porous SiC ceramics by carbothermal reduction has been demonstrated. Effects of expandable microsphere content, sintering temperature, filler content, and carbon source on microstructure, porosity, compressive strength, cell size, and cell density were investigated in the processing of porous silicon carbide ceramics using expandable microspheres as a pore former. A higher microsphere content led to a higher porosity and a higher cell density. A higher sintering temperature resulted in a decreased porosity because of an enhanced densification. The addition of inert filler increased the porosity, but decreased the cell density. The compressive strength of the porous ceramics decreased with increasing the porosity. Typical compressive strength of porous SiC ceramics with ~70% porosity was ~13㎫.