Thermodynamic approach to the vaporization and growth phenomena of SiC ceramics. II. The SiC surface under oxidative conditions

Thermodynamic conditions at the SiC surface under oxygen pressure are analyzed from two points of view: (i) the conditions for creation of a first layer of silica and (ii) the conditions for carbon precipitation. The active to passive oxidation transition steady-state is studied using a thermodynamic analysis focused on the chemical potential of silicon and oxygen at the surface of the compound in order to ensure the existence of a clean SiC surface, i.e. a flow balance imposed simultaneously for the Si and C vaporization flows Si/C = 1/1 at the surface. Thermodynamic calculations show that there exists a window in the couple as a function of temperature that corresponds to a bare SiC surface. For such prevailing conditions the SiC erosion flows are calculated as well as the related SiC condensation phenomenon that might explain the SiC transport and vapor phase deposition at high temperature.     

Simultaneously reduced viscosity and enhanced strength of liquid silicone rubber/silica composites by silica surface modification

Silica is the most widely‐used filler to reinforce liquid silicone rubber (LSR), but the high viscosity of LSR/silica suspension significantly limits its processing flexibility. To balance the processibility and reinforcing efficiency of LSR/silica systems, two kinds of enols (propenol and 1‐undecylenyl alcohol) and a saturated alcohol (1‐undecylic alcohol) were employed to modify the silica surface. Various rheological tests were carried out to investigate the processibility as well as filler networking and crosslinking processes of the modified systems. Tensile tests were also adopted to verify the reinforcing effect. It was found that surface modification of silica by 1‐undecylenyl alcohol could significantly reduce the viscosity of its suspension with LSR. Meanwhile, the mechanical strength of LSR could be largely enhanced by six times with 10 wt % modified silica. This work will merit design and production of LSR materials with balanced processibility and mechanical performance. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45544.     

Lightweight porous silica ceramics with ultra-low thermal conductivity and enhanced compressive strength

In recent years, the need for robust thermal protection for reusable spacecraft and vehicles has spurred strong demand for high-performance lightweight thermal insulation materials that exhibit high strength. Herein, we report silica porous ceramics prepared via the direct foaming technique with lightweight, ultra-low thermal conductivity and enhanced compressive strength. Silica particles (particle size: 500 nm and 2 μm) were used as the raw materials. The nano-sized silica particles were easily sintered, thereby improving the compressive strength of the ceramics, whereas the micro-sized silica particles maintained the pore structure integrity without deformation. The addition of nano-silica enhanced the compressive strength by 764% (from 0.039 to 0.337 MPa). In addition, the thermal conductivity of the ceramics was as low as 0.039 W m^?1 K^?1. Owing to these outstanding characteristics, these porous silica ceramics are expected to be employed as thermal insulation material in diverse fields, especially aerospace and space where weight is an important constraint.     

Thermal-shock resistant refractories and ceramics under thermal-shock conditions

Conclusions We propose an approach to evaluating the thermal-shock resistance of solid bodies in a thermal-shock schedule based on the development of elastic waves of compression (expansion) and the final value of the velocity of the thermal waves.A model is put forward and also a calculation equation for the destructive drop in temperature T_p in the thermal-shock schedule.The magnitude characterizing the dissipation of energy during thermal loading equals the ratio of the velocities of the thermal and longitudinal sonic waves in the solid body.During thermal loading of the solid body over the entire external surface it is necessary to take into account the superpositioning of the waves, and the possibility of the sudden increase in the stress on the wave front to a value exceeding the tensile strength of the material, with significantly lower temperature gradients.The calculation equation obtained for determining the destructive temperature limit agrees qualitatively with the known experimental data, and enables us to predict the thermalshock resistance of the traditional refractories, and to determine means of creating new material resistant to thermal shock.This article is published in the nature of a discussionTranslated from Ogneupory, No. 5, pp. 13–15, May, 1988.     

Post-reaction strength of coke under various conditions

The post-reaction strength of coke was studied at different reaction temperatures in CO₂ and mixed gas atmospheres. The influence of solution loss conditions on post-reaction strength of coke, with different levels of reactivity, was studied in this paper.     

CaTiO3 linear dielectric ceramics with greatly enhanced dielectric strength and energy storage density

CaTiO₃ is a typical linear dielectric material with high dielectric constant, low dielectric loss, and high resistivity, which is expected as a promising candidate for the high energy storage density applications. In the previous work, an energy density of 1.5 J/cm³ was obtained in CaTiO₃ ceramics, where the dielectric strength was only 435 kV/cm. In fact, the intrinsic dielectric strength of CaTiO₃ is predicted as high as 4.2 MV/cm. Therefore, it should be a challenge issue to enhance the dielectric strength and energy storage density of CaTiO₃ ceramics by optimizing the microstructures. In the present work, dense CaTiO₃ ceramics with fine and uniform microstructures are prepared by spark plasma sintering, and the greatly enhanced dielectric strength (910 kV/cm) and energy storage density (6.9 J/cm³) are obtained. This can be ascribed to the improved resistivity and thermal conductivity, associated with the fine and uniform microstructures. The different post‐breakdown features of CaTiO₃ ceramics prepared by different process well interpret why the enhanced dielectric strength is achieved in the SPS sample. The energy storage density can be further improved to 11.8 J/cm³ by introducing the amorphous alumina thin films as the charge blocking layer, where the dielectric strength is 1188 kV/cm.     

Polyethylene/silica nanorod composites with reduced dielectric constant and enhanced mechanical strength

Reinforcement of materials with low dielectric constant by nanofillers has attained more and more attention. Conventional 1D nanofillers, such as carbon nanotubes, significantly enhance mechanical properties of polymers; however, they do not reduce dielectric constant. In this study, silica nanorods (SNR) were used to prepare polyethylene composites by mechanical blending followed by hot pressing. It was found that the incorporation of SNRs enables enhancement of strength and elastic modulus. Dielectric constant did not increase as predicted when SNRs were incorporated into polyethylene (PE); on contrary, it considerably decreased from ~2.4 to around ~2.2 at SNR loading of 5 wt %. However, when glass fibers were used as fillers, we did not observe such decrease. SEM image and density measurement indicated that the composites were compact with very low porosity. We believe that the incorporation of SNRs alters crystalline and chain arrangement of PE, thus reduces the polarization and dielectric constant. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47143.     

Achieving significantly enhanced piezoelectricity in aurivillius ceramics by improving initial polarization and dielectric breakdown strength

In this work, the neglected role of the dielectric breakdown strength (BDS) in piezoelectricity of bismuth layer-structured ferroelectrics (BLSFs) is revealed, enhanced initial polarization and BDS work together to improve the piezoelectricity of CaBi₄Ti₄O₁₅ (CBT) ceramics via Li/Bi co-substitution. The experimental work, first-principles calculations and finite element simulation are carried out to investigate the effect of Li/Bi co-substitution in depth. The stronger spontaneous polarization (P_s) from the lattice distortion and the more favorable domain switching from the improved grain growth both contribute to an enhanced initial polarization. Besides, the changed grain-scale microstructure improves the hardness and inhibits the local discharge, the higher resistivity related to strong defect dipoles suppresses the heat generation during the poling process, all resulting in an increase of BDS by about 100%, and allowing more domains to align. This work demonstrates an effective strategy to develop BLSFs with enhanced piezoelectricity for high temperature piezoelectric applications.     

Addition of h-BN for enhanced machinability and high mechanical strength of AlN/Mo composites

Ceramic/metal composites, as high-temperature structural materials, have limited applications because of their poor machinability. AlN/Mo/Mo₂B machinable composites were fabricated by adding h-BN to AlN/Mo composites by plasma-activated sintering at various temperatures and at an axial pressure of 30 MPa under argon atmosphere. The sample sintered at 1500 °C exhibited excellent machinability for cemented carbide tools, with a bending strength of approximately 500 MPa and a Vickers hardness value of 9.34 GPa. The effects of sintering temperature on the microstructure, mechanical properties, and machinability were investigated, and the reactions introduced by adding h-BN were studied. These reactions reinforced the composite and newly introduced a Mo₂B phase, which showed better ability to deflect cracks and prevent them from moving deep inside the composite, thus improving its machinability. This research provides a possible solution for improving the machinability of ceramic/metal composites without sacrificing the strength.     

Electronically conductive porous TiN ceramics with enhanced strength by aqueous gel‐casting

In this paper, we report on a study of electronically conductive porous TiN ceramics prepared by aqueous gel‐casting. The effects of solid loading, sintering temperature, and sintering aids on the phase composition, microstructure, and volume fraction of porosity of the prepared porous TiN ceramics are studied. The SEM results show that porosity is uniformly distributed in all of the samples studied. With increasing solid loading and sintering temperature, the volume fraction of porosity decreases slowly. Moreover, the relationship between volume fraction of porosity and mechanical and electrical properties has also been investigated. Our results show that adding Y₂O₃‐TiO₂ as combined sintering aids results in a sharp decrease in the volume fraction of porosity, and the volume fraction range changes from 42%‐60% to 28%‐52%. Moreover, adding sintering aids results in an increase in flexural strength and electrical conductivity with a change in maximum value from 34.6 MPa and 2.3 × 10⁴ S?m^?1 to 101.6 MPa and 5.1 × 10⁴ S?m^?1, respectively.     

Structure and strength of corundum ceramics with additions containing components with low surface tension

The effect of a small amount of additions based on the Al₂O₃ -ZrO₂ eutectic containing microadditions of components with low surface tension on the structure and properties of corundum ceramics is investigated. It is shown that at low surface tension of a melt based on the Al₂O₃ - ZrO₂ eutectic the intergrowth between corundum crystals is stronger, which increases the strength of the corundum materials. The microadditions were Cr₂O₃, TiO₂, and SiO₂. Highest-strength (up to 520 MPa) fine-crystal (3 – 5 µm) materials with low closed porosity (0.5%) were obtained for eutectics with a microaddition of Cr₂O₃. The introduction of a microaddition of TiO₂ caused sharp growth of corundum crystals to 20 – 30 µm, while the strength of the corundum ceramics was 400 MPa. The use of eutectic additions containing small amounts of Cr₂O₃ and SiO₂ gave virtually poreless corundum ceramics after firing in air.Translated from Ogneupory, No. 11, pp. 14 – 18, November, 1994.     

Binary oxide ceramics for enhanced phenols degradation under simulated Solar light

Solvothermal synthesis of ZrO₂–TiO₂ binary oxides and pure counterparts at 150°C in water/isopropanol media is presented. Titanium (IV) isopropoxide and zirconium (IV) propoxide were used as precursors. XRD and TEM techniques were used for structural and morphological characterization of obtained samples. XPS spectra of mixed oxide samples were compared in order to correlate composition of samples with surface properties and presence of defects. Relative positions of defect states within band‐gaps of semiconductors were estimated from UCPL spectra. UV‐Vis DRS spectra revealed that both pure oxides experienced red shift of absorption thresholds compared to reference data. Photocatalytic activities of all samples were probed under simulated Solar light on three model compounds: phenol, 4‐chlorophenol (4‐CP) and 2, 4, 6‐trichlorphenol (TCP). Pure TiO₂ showed the highest photocatalytic activity in the case of phenol. However, pure ZrO₂ and binary oxides showed higher photoactivity in degradation of 4‐CP and TCP. The activity of wide band‐gap semiconductors under simulated Solar light in photodegradation of phenol and its derivatives is most probably the consequence of formation of charge transfer complexes between pollutant molecules and semiconductors surface.     

Formation of metal composites by displacement under bubbling conditions

The present paper is aimed at studying the kinetics of copper powder deposition process onto iron—carbon alloy substrates by a ‘cementation’ type of reaction. The variables included copper sulphate concentration, reaction time, temperature, pH, rate of helium gas bubbling and the surface area of the test specimen. The properties of the composite powders were also investigated as well as the surface topography of the deposited copper powders. The properties embodied fluidity, bulk density, specific surface area, porosity and packability. The composite material — after compaction and sintering — was evaluated by both tensile and ‘Brinell’ hardness tests.It was concluded that copper deposition by cementation is controlled by the transport of the reacting species from the bulk of the solution to the test specimen. This was explained in terms of Fick's law for the rate of mass transfer by diffusion in the boundary liquid film surrounding the solid.     

Behavior of strontium titanate ceramics in reducing conditions suggesting enhanced conductivity along grain contacts

In reducing conditions the resistance of dense strontium titanate ceramic samples decreases markedly with decreasing average grain size. This trend is interpreted on assuming enhanced conductivity along grain contacts in parallel with a RC bulk term. The temperature dependence is consistent with this model and indicates that the bulk may still control the overall behavior at sufficiently high temperatures (typically for T>700 °C). The resistivity of fine grained porous samples is intermediate between the results obtained for fine grained dense samples and dense samples with large average grain size. An extended model is thus proposed to describe the behavior of porous samples by assuming a series association of isolated channels connected by the interior of adjacent grains.     

Properties of Thermoresponsive N-maleyl gelatin-co-P(N-isopropylacrylamide) hydrogel with ultrahigh mechanical strength and self-recovery

Thermo-responsive polymer hydrogels with superior strength and toughness are potential candidate materials in biomedical field, such as drug delivery system and tissue engineering. By introducing maleylgelatin (MAGEL) into the conventional PNIPAAm hydrogel, a series of composite P(NIPAAm-co-MAGEL) hydrogels with combined features were fabricated. Thermo-responsive behaviors, equilibrium swelling ratio (ESR), compression strength, tensile strength (TS) and elongation at-break (E), cyclic compression tests, and thermal stability properties of hydrogels with different amount of MAGEL were investigated. Experimental data indicated that the amount of MAGEL could modulate the mechanical property of the composite hydrogel. With the increase of the MAGEL contents from 0 to 50%, the composite hydrogel with relatively high water content possessed good compressive strength, tensile strength and stretrability. Only when the weight ratio of MAGEL/NIPAAm was 30:70, did the P(NIPAAm-co-MAGEL) have a homogeneous distribution and stable 3D networks which played a significant role in the properties of the hydrogels. Cyclic compression tests results indicated that P(NIPAAm-co-MAGEL) hydrogel had an excellent thermo-reversible ability. This research would expand the scope of the PNIPAAm hydrogel applications.     

Effect of pores on dielectric breakdown strength of alumina ceramics via surface and volume effects

To investigate the effect of pores on the dielectric breakdown (DB) of alumina ceramics under AC voltage, we implant pores at different positions (surface, interior, and the whole body) of the ceramic samples by adding the polyvinyl alcohol solid particles at different positions in the green bodies. Meanwhile, two electrode setups are used to test the dielectric breakdown strength (DBS), in order to analyze the effect of pore distribution on the DB, and the morphology of the DB channels is characterized. The results reveal both the pores on the surface and in the bulk reduce the DBS, and the pore distribution affects the shape and position of the DB channel. Besides, pre-breakdown in a low-DBS layer is found when the voltage is not high enough to produce DB. It confirms that the effect of pores on the DB is a combination of the volume effect and the surface effect.     

不同介质中聚四氟乙烯复合材料的摩擦磨损性能

分别在碱液、水、油和干摩擦条件下考察了碳纤维和玻璃纤维填充聚四氟乙烯复合材料的摩擦磨损性能。利用SEM观察了不同介质中磨损面和对摩面的形貌,并探讨了其磨损机理。结果表明,不同介质中摩擦系数的大小关系是μ干>μ水或油>μ碱,磨损率是W水>W干>W碱或油。水、碱和油都不同程度地阻止了转移膜的形成。碱液和油具有很好的冷却与润滑作用,摩擦系数低,磨损小;然而水分子降低了填料和基体的界面粘接强度,造成犁削和磨粒磨损加重。