Improving the piezoelectric properties of CBN-based ceramic by a Ce ion

CaBi₂Nb₂O₉ (CBN), one of the bismuth-layered structural ferroelectrics, with high Curie temperature (T_C), has great potential in high-temperature applications. In this work, high Curie temperature and piezoelectric constant (d₃₃) are realized in modified CaBi₂Nb₂O₉ ceramics with Ce-substitution. Ce-substitution changes the crystal structures and domain structures of CBN-based ceramics, so as to improve the piezoelectric properties. The optimal performances are obtained with a high d₃₃ value (∼18.0 pC/N) and a T_C value (∼930°C), together with a low tan δ value (∼0.028 at 500°C). Moreover, the thermal stability is also enhanced, where the d₃₃ value maintains 93.9% of its original value after annealing at 900°C for 2 h. Thus, these findings play a meaningful role in devices manufacturing, where the apply temperature is often more than 500°C.     

Chemical vapour infiltration and mechanical properties of carbon open-cell foams

In order to improve their mechanical properties, carbon open-cell foams of two different pore sizes were infiltrated with pyrocarbon by chemical vapour deposition at reduced pressure and using pure propane as precursor. The optimal conditions in terms of deposition rate and uniformity in coating thickness, structure and anisotropy were first investigated. Foam specimens were infiltrated at various stages, with two pyrocarbons of distinct microtextures and their morphology, relative density and geometrical features were evaluated.Compressive crushing tests were conducted to determine the influence of the pore size, the pyrocarbon type and the relative density on the mechanical properties of the pyrocarbon-infiltrated foams. They retain their non-brittle and dissipating behaviour up to relative densities of 0.15. The stiffness, crushing strength and dissipated energy increase significantly with the relative density. The crushing behaviour of the pyrocarbon-foam specimens can be essentially explained using simple structural models and failure mechanisms, according to the Gibson & Ashby’s approach for brittle cellular solids.     

Microstructure of ceramic foams

This paper describes the preparation of ceramic foams by expansion of a ceramic suspension based on a polyurethane system. The microstructure and degree of reticulation of the foamed ceramic were examined and analysed with the help of a simple geometrical model. Like the porous ceramics prepared by the replica processing method, these foamed ceramics possess open cells in a nearly equiaxed shape but the cell size is much finer. The ratio of the window size to the cell size is a useful parameter for characterising the geometry of the foam and is related to the qualitative concept of degree of reticulation. For a face centred cubic array of cells it is related geometrically to the volume fraction of porosity and this relationship is tested using microstructural measurements for a range of ceramic foams.     

Infiltration mechanism of diamond/SiC composites fabricated by Si-vapor vacuum reactive infiltration process

Dense diamond/SiC composites were fabricated by Si vapor vacuum reactive infiltration of carbon-containing diamond porous preform at 1600 °C for 1 h. The microstructural evolution of the composites was investigated. The infiltration mechanisms during reactive infiltration were discussed. The composite consists of diamond, β-SiC and a small amount of Si. Epitaxial growth of nano-sized SiC on diamond and graphite surfaces occurred due to the diffusion-reaction mechanism in the initial stage of infiltration. Growth of micron-sized SiC with no preferential orientation was controlled by solution-precipitation mechanism in the final stage. The infiltration process was determined both by molecular diffusion and capillary effects. Explosive evaporation of molten Si, volume expansion of the solids and heat release during the reaction were the key factors contributing to the rapid densification of diamond/SiC composites. High thermal conductivity (580 W m^?1 K^?1) and low density (3.33 g cm^?3) of the composites were beneficial to thermal management applications.     

Thermal shock resistance of porous ceramic foams with temperature-dependent material properties

The impact of temperature dependence of material properties on thermal shock resistance of porous ceramic foams is studied in this paper. Two cases of thermal shock are carried out: sudden heating and sudden cooling. Finite difference method and weight function method are employed to get the thermal stress field at crack tip. The effects of time dependence and temperature dependence of material properties on thermal shock behavior are analyzed. The thermal shock resistance is acquired based on two different criteria: fracture mechanics criterion and stress criterion. By comparison analysis, results show that taking temperature dependence of the material properties into account is crucial in the assessment of thermal shock resistance of ceramic foams. Cold shock fracture experiments of Al₂O₃ foams with different relative densities are also made, and the obtained results are in coincidence with theoretical results very well.     

Fabrication and high-temperature properties of Y-TZP ceramic helical springs by a gel-casting process

A rectangular cross-section ceramic helical spring of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) was prepared by the gel-casting process. Both the compressive curves and comprehension rebound curves were tested at room temperature and high temperature. The results showed that springs obeyed Hooke׳s Law at room temperature, as the compression resilience ratio of the samples was nearly 100% under the condition of spring׳s security and no damping loss occurred during the process. Besides, mechanical failures of springs occurred under loads around 128 N with the deformation of 10%. With increasing test temperature the maximum load-carrying capacity of the spring decreased, while the maximum deformation increased. Besides, the load–compression curve showed a yield step when the test temperature was above 800 °C. At elastic stage of spring under high temperature, the compression resilience ratio was also nearly 100%; however, the anelastic effect took place and energy loss increased with the increase in test temperature.     

Improving the structure and properties of an aluminum oxide ceramic

Translated from Steklo i Keramika, No. 7, pp. 19–20, July, 1989.     

Improving the properties of binder jetted ceramics via nanoparticle dispersion infiltration

To improve the dimensional accuracy, surface quality, and mechanical properties of binder jet additive manufactured ceramic parts, in this work, nanozirconia dispersion and binder were used to prepare a jet solution to investigate the effect of nanozirconia content of the jet solution on the properties of zirconia ceramic green and sintered bodies. When the nanozirconia content of the jet solution increased from 0 to 20 wt%, the forming precision of the zirconia ceramic green bodies improved, the surface roughness decreased, and the density increased. After sintering at 1400 °C for 2 h, linear shrinkage along the height, width, and length of the zirconia ceramics decreased by 6.27%, 10.20%, and 5.45%, respectively, while the surface roughness decreased by 42.87%, and the bending strength increased by 145.60%. With increasing nanozirconia content of the jet solution, the spreading and infiltration distance of the jetted solution in the powder layer decreased, the thickness of the deposition layer of nanozirconia on the surface of the zirconia particles in the powder layer increased, and the pore size of the powder layer decreased significantly. This increased the density of the green bodies; thus, improving the sintering properties of the zirconia ceramics.     

Innovative improvement of sintered ceramic electrolytes by salt infiltration

Previously sintered (1500 °C, 4 h) dense pellets of Ce_0.9Gd_0.1O_1.95 (GDC) were covered and heat treated with eutectic mixtures of Na₂CO₃ and Li₂CO₃ (NLC), and their electrical performance was assessed against pure GDC and chemically synthesized GDC + NLC. Microstructural analysis of NLC impregnated samples confirmed slight migration of the molten phase to the interior of the GDC pellets via grain boundaries, resulting in a significant improvement of the grain boundary conductivity, increasing with duration of heat treatment (0.5–2 h) and temperature (600–800 °C range). The observed total conductivity exceeded in almost 20% the corresponding values obtained for standard GDC samples. Cells tested before and after direct current polarization (0.5 V, 500 °C) showed the same electrical performance, discarding the possibility of parallel contributions of salt ions to the total conductivity. Grain boundary engineering using salt infiltration is an effective tool to improve the electrical performance of ceramic electrolytes.     

Fabrication of alumina-based ceramic foams utilizing superplasticity

Model ceramic foams were fabricated by expanding once-sintered dense shells utilizing the superplastic deformation of alumina dispersed with 3 mol% yttria-stabilized zirconia (3YSZ) or magnesia. The grain growth of alumina was suppressed by adding 3YSZ and the resultant grain size and amount of dispersion were closely related to the total porosity. Total porosity of the ceramic foam depended on the grain size and their distributions irrespective of the size of starting powders.     

Development and mechanical characterization of novel ceramic foams fabricated by gel-casting

Porous ceramic materials are of considerable interest for a variety of chemical and industrial applications in extremely harsh conditions, particularly at very high temperatures for long time periods. A combined gel-casting-fugitive phase process employing agar as a natural gelling agent and polyethylene spheres as pore formers was exploited to produce porous ceramic bodies. Alumina and alumina–zirconia powders were used to prepare samples having a porosity of about 65–70–75 vol%. The composite powder was produced by a surface modification route, i.e. by coating a well-dispersed alpha-alumina powder with a zirconium chloride aqueous solution. On thermal treatment, ultra-fine tetragonal zirconia grains were formed on the surface of the alumina particles. SEM observations and image analysis were used to characterize the microstructure of porous samples and uniaxial compressive tests were carried out to measure their mechanical behavior.     

Alternative gelling agents for the gelcasting of ceramic foams

In recent years, a new class of ceramic foams with porosity levels up to 90% was developed as a result of the association of the gelcasting process to the aeration of a suspension containing foaming agents. The gelation of foamed suspensions results from the in situ polymerization of water-soluble monomers. Nearly spherical pores and highly dense struts characterize the structure of these foams, which results in unprecedented properties, such as high permeability, low thermal conductivity, high specific surface area and high mechanical strength. The main drawbacks of this process are the inhibition of the polymerization in the presence of oxygen and the toxicity of the monomers. This work investigates two harmless alternative gelling agents that do not require atmosphere control to set foamed suspensions. The first route consists in the crosslinking of a previously dissolved polyvinyl-alcohol with the addition of an organotitanate. The other approach assessed the use of gelatin as a setting agent. This polymer gels the suspension due to changes in the structure of polypeptidic chains induced by temperature reduction. Gelatin-based systems presented higher storage modulus (G′) than the systems with crosslinked polyvinyl-alcohol chains. This characteristic prevented the formation of cracks during drying of gelatin-based samples, which constituted a limitation of PVAl-based systems. The results point out gelatin as a promising gelling agent to produce ceramic foams without the disadvantages of monomeric systems, such as the toxicity and the necessity of atmosphere control.     

Improving the stability of polylactic acid foams by interfacially adsorbed particles

Polymers such as poly(lactic acid) (PLA), which have poor melt strength, are difficult to foam due to severe cell coalescence during foaming. We show that addition of a few percent of polytetrafluoroethylene (PTFE) particles can stabilize PLA foams against bubble coalescence and collapse. The particles and a chemical blowing agent, were dispersed into the PLA by extrusion, and then foamed by heating. The PTFE‐containing foams remained stable even when the foams were held under molten conditions for extended periods. Foam stability is attributed to an interfacial mechanism: due to their low surface energy, the PTFE particles adsorb on the inner surface of the foam bubbles at a high surface coverage, and endow the bubbles with an interfacial “shell” that prevents coalescence. This mechanism resembles the particle‐stabilization of Pickering emulsions in oil/water systems. Particle adsorption at the interface is a necessary condition for using this approach, and hence this approach is most likely to be successful if the particles have a low surface energy and the polymer has a high surface tension. The approach of using interfacially adsorbed particles can be broadly generalized, and offers the opportunity of foaming various polymers with low melt strength, or for expanding the processing window within which foaming can be conducted. POLYM. ENG. SCI., 56:9–17, 2016. © 2015 Society of Plastics Engineers     

Improving the antifouling properties of ceramic membranes via chemical grafting of organosilanes

For enhanced antifouling surface properties, the alumina membranes were modified through a simple silanization process. Three organosilanes presenting neutral, positive, and negative charges were allowed to graft onto alumina membranes. A small decrease in the pore size and the successful chemical binding of organosilanes were confirmed, respectively. The membrane filtration test using humic acid (HA) was conducted to evaluate the effect of surface charges on fouling resistance. The neutral and negatively charged membranes achieved remarkable flux behaviour due to no charge interaction and electrostatic repulsion force, respectively. Especially, the negatively charged membranes presented the lowest flux decline, the highest flux recovery, and the lowest membrane fouling.     

In situ synthesis of three-dimensional nanofiber-knitted ceramic foams via reactive sintering silicon foams

Three-dimensional ceramic nanofiber-assembled materials with large specific surface area and excellent thermal insulation properties are attracting increasing interests for their unique structure and promising applications. In this paper, we propose a facile methodology to fabricate three-dimensional silicon nitride nanofiber-knitted ceramic foams via in situ reactive synthesis from silicon foams. Silicon particle-stabilized foams are fabricated for the first time using long-chain surfactant cetyltrimethyl ammonium bromide as a hydrophobic modifier. First, the fabrication and stability of silicon foams are investigated. Based on the stable silicon foams, silicon nitride-based nanofiber-knitted ceramic foams are synthesized via in situ reactive sintering in nitrogen atmosphere. The novel ceramic foam materials consist of three-dimensional nanofiber-assembled strut wall and nanofiber-spheres in the pores. The diameter of obtained silicon nitride nanofibers ranges from 15 to 100 nm. The unique nanofiber-knitted foams may have potential applications in specific fields, including catalysis, adsorption, separation, and thermal insulation.     

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.     

Effects of heating rate on the structure and properties of SiOC ceramic foams derived from silicone resin

Silicon oxycarbide ceramic foams were fabricated in a single step manufacturing process using in situ foaming of SiOC powders loaded silicone resin. The effects of heating rate on the porosity, compressive strength and microstructure of the ceramic foams were investigated. The porosity (total and open) increased firstly and then decreased with increasing heating rate. It was possible to control the total and open porosity of ceramic foams within a range of 81.9–88.2% and 62.4–72.5% respectively, by adjusting the heating rate from 0.25 °C/min to 3 °C/min while keeping the silicone resin content at 90 vol%. However, the compressive strength decreased with increasing the heating rate progressively, and the average compressive strength of the foams was in the range of 1.0–2.3 MPa. Micrographs indicated that the ceramic foams which cross-linked at a heating rate less than 1 °C/min had a well-defined open-cell and regular pore structure.     

A novel method for preparation of interconnected pore-gradient ceramic foams by gelcasting

A novel approach was introduced to prepare pore-gradient Al₂O₃ ceramic foams with association of gelcasting process and polymer sphere template. This approach allows the design of pore connectivity and gradient height of ceramic foams by the appropriate selection of sphere sizes and numbers. The limitation of ceramic foams fabricated by polymeric sponge process on sponge carrier can be resolved by this approach. Epispastic polystyrene (EPS) spheres with different sizes were employed to array ordered templates. Influence of solid content and dispersant on the viscosity of Al₂O₃ slurry was studied. EPS spheres modified by oxygen plasma to increase the hydrophilicity of surfaces and influence of pre-removal of EPS template on the integrity of networks were investigated. Results showed that 55?vol.% Al₂O₃ slurries with 0.5?wt% dispersant kept good fluidity for casting and permeability in template. Water contact angle of EPS surface decreased from 95.2° to 20°. Some defects in green bodies such as edge-delamination and micropores disappeared after surface modification. The perfect structure and morphology of the ceramic foams were ascribed to the pre-removal of template by solvent. The hierarchical pore structure was fabricated with EPS spheres of 2.1, 2.6 and 3.2?mm diameters. Porosity and compressive strength of the pore-gradient Al₂O₃ ceramic foam were 68.5% and 3.06?MPa at 1,500?°C, respectively.     

Composite ceramic fuel cell fabricated by vacuum plasma spraying

Porous composite cathodes for ceramic fuel cells containing (La_0·8 Sr_0·2)_0·98MnO₃ (LSM) and yttria stabilised zirconia (YSZ) were prepared by vacuum plasma spraying (VPS) on prefabricated planar cermet supported zirconia substrates. Three different types of LSM–YSZ-mixtures with volume-ratio of 50/50 or 70/30% and thickness of 20–50 microns have been compared with conventional LSM-cathodes made by VPS. The electrochemical behaviour was tested between 700 and 900°C using impedance spectroscopy. The composite cathode electrochemical performance is discussed.     

渣油掺炼煤焦油的减压蒸馏过程

为提高资源利用率,考察了掺炼煤焦油对渣油减压蒸馏的影响。结果表明,在渣油中掺炼一定比例的煤焦油可以提高其减压蒸馏的拔出率,为后续加工提供更多的原料。相比较于纯渣油,在550℃对渣油掺炼20%的煤焦油进行蒸馏,可多获得近15个百分点的催化裂化加工原料。扣除煤焦油馏分油后,渣油的拔出率也提高5个百分点。同时,蒸馏残渣的软化点有一定的提高,更适合用做沥青原料。最后探讨了煤焦油对渣油蒸馏过程的强化机理。