Spark Plasma Sintering and Densification Mechanisms of Conductive Ceramics under Coupled Thermal/Electric Fields

In spark plasma sintering (SPS), thermal and electric fields are applied simultaneously as a material is densified under pressure. The interactions between these two types of physical fields influence the densification behavior during SPS. Moreover, the uniformity and spatial distribution of these fields are also influenced by sample size. In the current investigation, the densification behavior of electrically conductive aluminum‐doped zinc oxide (AZO) ceramics is studied to provide insight into the role played by the thermal and electric fields on densification mechanisms, as a function of sample size. Our results demonstrate that field uniformity and densification behavior depend on sample size, and that ultimately, this behavior can be rationalized in terms of the electrical conductivity characteristics. Our results show that in small samples with a diameter of 20 mm, both thermal and electric fields are spatially uniform, which result in homogeneous microstructure. In large samples with a diameter of 80 mm, however, spatial variations in both thermal and electric fields lead to microstructural inhomogeneities, such as incomplete particle–particle bonding. Furthermore, as the density of the AZO sample increases, the effective electrical conductivity increases due to a decrease in void/pore volume, which changes the densification mechanisms, especially for the larger sample. Thus, for effective sintering of larger samples, a two‐stage sintering sequence is proposed, which relies on the thermal field that evolves once the effective electrical conductivity increases in the sample. We provide experimental confirmation to this suggestion on the basis of results which demonstrate that by extending the hold time from 3 to 30 min, high‐density (99.4%), homogeneous AZO ceramic samples with a diameter of 80 mm can be achieved after sintering at 1200°C.     

Microwave Heated Chemical Vapor Infiltration: Densification Mechanism of SiCf/SiC Composites

Silicon carbide fiber-reinforced silicon carbide matrix composites (SiC_f/SiC) have been produced using microwave heated chemical vapor infiltration. Preferential densification of the composite from the inside out was clearly observed. Although an average relative density of only 55% was achieved in 24 h, representative of an ∼26% increase over the initial fiber vol%, the center of the preform densified to 73% of the theoretical. The densification mechanisms were investigated using X-ray absorptiometry and scanning electron microscopy. The initial inverse temperature profile obtained, which was found to result in the efficient filling of the intratow porosity, although not the intertow porosity, flattened out after approximately 6 h as the densification front moved outward toward the edges. Although not investigated directly, the evidence suggested that this was caused by changes in both the thermal conductivity and microwave absorption characteristics as the samples densified.     

Nanocrystalline Aluminum Nitride: II, Sintering and Properties

Nanocrystalline aluminum nitride powders obtained from gas condensation and in situ nitridation in a forced-flow reactor have been sintered successfully without pressure or additives. The resulting densified pellets showed good thermal conductivity and low oxygen content. A comparison of the improvement in densification with micrometer-size and nanocrystalline yttria additives was undertaken and it was found that the nanocrystalline yttria decreased the sintering temperature significantly. Besides spherical nanoparticles, needle-shaped nanocrystalline aluminum nitride particles that resulted in highly textured compacts when hot pressed could also be produced.     

Energetic characterization of densified residues from Pyrenean oak forest

In Southern Europe there are vast areas of forestry land which are composed of Pyrenean oak (Quercus pyrenaica). However, these areas represent an important source of biomass which can be used for energetic purposes. The maintenance of the Pyrenean oak woodland is carried out by means of the extraction of complete trees, having a final residue made of very different types of branches that can influence on the properties of the pellets. The present work shows the results obtained from the characterization and analysis of different types of pelletized residues of the Pyrenean oak. The results have been classified in order to find out if different pretreatments for the residues, according to their branch size, are necessary. Nevertheless, any significant difference was found among the three classes of samples. On the other hand, during the pelletization of the different types of branches some of their physical characteristics, such as the bulk density, were substantially improved. However, the chemical characteristics of the samples were not significantly altered during this densification process. Furthermore, the pellets showed interesting physical resistance probably due to the high percentage of lignin present in forest biomass which improves their durability.     

Rapid Rate Sintering of Yttria Transparent Ceramics

This paper reports on the influence of rapid rate sintering (RRS) on densification and microstructure evolution of yttria transparent ceramics by using vacuum sintering. The presence of temperature gradient has been confirmed during the RRS process. The higher the heating rate (HR), the larger the temperature gradient in the samples would be. By using RRS, e.g., HR = 40°C/min, the samples could be densified very fast to a relative density of 99.6%. However, these samples could not be further densified, due to the presence of difference in densification caused by a heating rate‐induced temperature gradient. By using a two‐step RRS with an intermediate‐temperature thermal treatment, this problem has been successfully addressed. The intermediate‐temperature treatment allowed for the particle neck growth, so that effective thermal conductivity of the compacts was increased greatly. Therefore, the temperature gradient and differentiate densification were effectively prevented. Samples sintered using the two‐step RRS process could be fully densified and excellent in‐line optical transmittance was achieved. It is believed this strategy is applicable to other transparent ceramics, as well as other engineering ceramics.     

Translucent LiSr4(BO3)3 ceramics prepared by spark plasma sintering

Lithium borates are promising materials for thermal neutron detection. However, a strong tendency of borates for glass transformation can lower the detection efficiency of some luminescence centres. Here, we describe the synthesis of well-crystalline translucent borate ceramics. The precursor powder of undoped LiSr₄(BO₃)₃ was prepared using a wet homogenization method and then densified to ceramic pellets at different sintering temperatures using SPS. As the sintering temperature increased, the degree of densification and crystallite connectivity improved, rendering the prepared pellets translucent.     

Bending Creep Test to Measure the Viscosity of Porous Materials during Sintering

A bending creep test is proposed for measuring the change in viscosity of a porous material during densification. Equations, based on simple beam deflection theory, were derived to obtain the viscosity from a series of loading experiments using rectangular samples of different densities. By measuring the deflection in the center of the specimen between the spans, the viscosity of a porous material during densification can be measured. Experiments with porous Y₂O₃-stabilized ZrO₂ beams were used to illustrate the bending creep test. Consistent with theory the viscosity increased from 50 to 400 GPa·s as the sample densified from 87% to 98% density. The rapid increase in viscosity was considered to be a result of both densification and grain growth.     

Viability of using calcined clays, from industrial by-products, as pozzolans of high reactivity

The present work studies the use of clays like high reactivity metakaolin, as pozzolans for concrete. This study adopted two clay types: kaolinite and kaolin by-products from the paper industry. In this second clay, besides the possible technical advantages, the ecological benefit of the use of a by-product must be considered. Initially, the chemical and mineralogical characteristics of the clays were determined. After this, calcination and milling was carried out aiming at obtaining materials with pozzolanic activity. After milling took place in the clays kaolinite and in the by-products, with the best physical characteristics, lab tests were carried out to verify the pozzolanic activity of these materials with Portland cement and with lime. The results show a low performance of the kaolinite and a high performance of the by-products industrial. The low performance of the kaolinite was credited to the lack of homogeneity of the calcination, as the material was obtained after the calcination, and the low efficiency of its milling process.     

Electric current assisted sintering of AlN ceramics: thermal conductivity and transparency

Abstract

Densification, thermal conductivity and transparency of aluminium nitride ceramics densified by electric current assisted sintering are reviewed. Aluminium nitride powders could be densified without any sintering additives by electric current assisted sintering. Sintering additives, Y₂O₃ or CaF₂, and others are effective for densification, increase in thermal conductivity and transparency. As a result of the reaction with the sintering additives and oxide on the surface of the AlN powder, a secondary phase forms between AlN grains, oxygen impurity between aluminium nitride grains decreases and thermal conductivity increases. These mechanisms are effective in the sintering of aluminium nitride ceramics densified by electric current assisted sintering, though sintering time is short, within 5 min. The secondary phase formation does not always give good effect for transparency.     

Novel fire-protecting mortars formulated with magnesium by-products

Several kinds of sprayable mortars are commonly used as passive fire protection of building structures. Several authors have reported the effect of different kinds of aggregates (e.g. vermiculite, fly ashes) in the thermal behaviour of fire-protecting mortars. In this study, the use of magnesium by-products as aggregates in fire-protecting mortars has been evaluated. These by-products were obtained during the calcination process of natural magnesite. Endothermic decompositions of the different aggregates have been determined and analysed by means of thermal techniques. Mortars with different mixtures of these by-products have been prepared. Mechanical properties and temperature behaviour tests have been performed to evaluate the suitability of these substances as aggregates in fire-protecting mortars. During the endothermic decomposition of the studied aggregates the advance of temperature inside the mortar is delayed. Mortar with a mixture of 50% of both magnesium by-products shows a good agreement between mechanical properties and temperature behaviour.     

Influence of oxygen content on structure and properties of pressurelessly sintered aluminum-nitride- and zirconium-boride-doped silicon-carbide ceramics

SiC is a widely used material. Understanding how oxygen content affects the SiC structure and properties is crucial. In this paper, heat treatment was used to prepare SiC powder samples with different oxygen contents, which were doped with AlN and ZrB₂ and were densified by pressureless sintering at 2050 °C. The effect of oxygen content on the sintered SiC structure was determined by X-ray diffraction analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, and energy-dispersive spectroscopy. The results indicated that the oxygen content influenced the SiC phase composition, grain boundaries, and densification. Additionally, the interaction between oxygen defects and AlN played an important role in sintering. The nanoindentation, alternating-current impedance, and thermal conductivity of the densified SiC specimens were also evaluated to elucidate the influence of the oxygen content on the densified-SiC functional properties. The results revealed that the oxygen content affected all the measured mechanical, electrical, and thermal properties. Furthermore, surface oxygen impurities suggested that oxygen content had similar critical effects on both the densified SiC structure and properties.     

HIPed TiO2 dense pellets with improved photocatalytic performance

The effects of heating method and temperature on physical, structural and photocatalytic behaviors of TiO₂ pellets prepared by conventional heating and hot isostatic pressing have been evaluated. The pellets of submicron TiO₂ powders were heated to 600, 650, 700, 750 and 1000 °C using both processing methods in order to compare anatase to rutile phase transformation and densification behaviors. Bulk densities and porosities were calculated using the Archimedes method. XRD analysis were performed to calculate anatase/rutile ratios. Microstructures were characterized using SEM. Photocatalytic experiments have been performed under full spectrum irradiation. Degraded methylene blue samples were periodically monitored through UV–vis spectrophotometer to determine degradation kinetics. Anatase to rutile transformation is slightly faster and densification is better for lower temperatures for conventional heating, however HIPing gives better densification above 750 °C as it also retards rutile transformation. Mixed phase structures and HIPed samples showed the best photocatalytic performance which makes this method advantageous.     

Colloidal Processing of Zirconium Diboride Ultra‐High Temperature Ceramics

Colloidal processing of the Ultra‐High Temperature Ceramic (UHTC) zirconium diboride (ZrB₂) to develop near?net‐shaping techniques has been investigated. The use of the colloidal processing technique produces higher particle packing that ultimately enables achieving greater densification at lower temperatures and pressures, even pressureless sintering. ZrB₂ suspension formulations have been optimized in terms of rheological behavior. Suspensions were shaped into green bodies (63% relative density) using slip casting. The densification was carried out at 1900°C, 2000°C, and 2100°C, using both hot pressing at 40 MPa and pressureless sintering. The colloidally processed materials were compared with materials prepared by a conventional dry processing route (cold pressed at 50 MPa) and subjected to the same densification procedures. Sintered densities for samples produced by the colloidal route are higher than produced by the dry route (up to 99.5% relative density by hot pressing), even when pressureless sintering is performed (more than 90% relative density). The promising results are considered as a starting point for the fabrication of complex‐shaped components that can be densified at lower sintering temperatures without pressure.     

Analysis of the Content of Fatty Acid Methyl Esters in Biodiesel by Fourier-Transform Infrared Spectroscopy: Method and Comparison with Gas Chromatography

The increasing importance of sustainability in energy production has led to a global commitment to the use of fuels derived from renewable biological sources, such as biodiesel produced from plant crops or biomass residues, that do not compete with human food for their production. For a biofuel to be considered biodiesel, it must satisfy the specifications described in the UNE 14214, with the UNE-EN 14103 referring to the determination of fatty acid methyl ester content. This standard applies gas chromatography as an analytical technique. Gas chromatography is a widely used technique in the analysis of methyl ester although it has a number of drawbacks such as: long analysis times, a high consumption of high-quality gases and internal standards, does not allow the analysis of different compounds with the same column, etc. From an industrial production point of view, is necessary to know the fatty acid methyl ester content in biodiesel samples quickly. This paper studies the development of an analytical method using Fourier transform infrared spectroscopy (FTIR) as alternative to gas chromatography (GC), since it is a simple, rapid, and precise analytical technique to quantify fatty acid methyl ester content in biofuel samples.     

Adhesion Effects of Intermediate Layers on the Densification of Ceramic Powders

In the ceramic technology the first step to produce sintered bodies is the manufacturing of powders which then are densified. The adhesion mechanisms between the single particles and the agglomerates produced from them determine the densification process. Starting from theoretical considerations adhesion mechanisms, such as solid bridge formation, adhesive bonding and glide-promoting effects, are discussed in principle. Subsequently, the effects of surface-active substances on the densification behaviour of clay-ceramics and oxide-ceramic bodies are discussed. Further, the evaluation of the action of additives to the powder mixtures on the microstructure of the compacts, such as porsity and texture, leads to a compaction equation which describes the transition from the powder pile to a densified green body.     

Asynchronous densification of zirconia ceramics formed by stereolithographic additive manufacturing

Stereolithography has been proven as a feasible approach to make crack-free ceramic macrostructure with customized designs, but the microstructure, especially pore structure remains to be tailored more precisely for better performance, where the sintering protocol and related densification characteristics could play a vital role as the slurry preparation and debinding protocol do. Herein we report a phenomenon named “asynchronous densification”, that is, the surface region of zirconia ceramics formed by stereolithographic additive manufacturing would be densified prior to the bulk at 1200°C during the conventional pressureless sintering in air. The cause of this asynchronism is unclear but supposed to be correlated with low packing density, high sintering activity, poor thermal conduction of ceramics and impurities. Early densification of the surface may have negative effects towards ceramic components with more homogeneous microstructure, suppressed pore coalescence and limited grain growth, and therefore needs to be better controlled through optimization in sintering protocol.     

Thermal and dynamic mechanical behavior of poly(lactic acid) (PLA)-based electrospun scaffolds for tissue engineering

Electrospun scaffolds can find numerous applications, including biomedical; for example, tissue engineering. Poly-L-lactic acid is considered suitable for these applications, but its low-thermal stability and its poor mechanical properties limit this polymer use. The aim of this work is to obtain a modulation of the final scaffolds characteristics such as fibers dimension, wettability, elasticity, and resistance to rupture through the choice of the polymers to be electrospun. Different electrospun scaffolds containing gelatin, Poly-DL-lactic acid, different percentages of cellulose nanocrystals and an elastin peptide have been produced. Thermal stability, physical structure, and its mechanical behavior have been studied. Results suggest that the electrospun scaffolds show better thermal and mechanical properties than bulk materials; that is, the scaffolds with the best hydrophilic and thermomechanical properties are the samples containing 3% (wt/wt) of CNCs and elastin peptide.     

生物质固体成型燃料研究现状及发展前景

概述了国内外生物质固体成型燃料技术及设备的研究现状。讨论了生物质固体成型燃料的成型机理和影响因素,主要概括了几种成型工艺及设备的优缺点。最后指出我国生物质固体成型燃料存在的问题和今后的发展趋势。     

Densification of complex shape ceramics parts by SPS

Spark Plasma Sintering (SPS) equipments, with higher productivity and hybrid heating modes, allow reducing thermal gradients in large samples and broaden their application potential. Furthermore, strong shape limitations remain: only simple shapes can be obtained because of the use of uniaxial pressure.The aim of this paper is to propose a SPS densification method for complex shape ceramic or powder metallurgy parts, without any modification of the tools and equipment. The samples to be sintered are placed in a powder bed in the classical die used in SPS. Different powder beds have been tested.A numerical model was developed in order to allow a rapid simulation of the force field around a part surrounded by a powder bed in a SPS tool. The optimization of the operating conditions was carried out, first, on small simple-shape samples, then, on complex geometry parts which were densified to demonstrate the feasibility of the proposed sintering process.     

Mathematical modelling of the combustion of a single wood particle

A mathematical model describing the thermal degradation of densified biomass particles is presented here. The model uses a novel discretisation scheme and combines intra-particle combustion processes with extra-particle transport processes, thereby including thermal and diffusional control mechanisms. The influence of structural changes on the physical–thermal properties of wood in its different stages is studied together with shrinkage of the particle during its degradation. The model is used to compare the predicted data with data on the mass loss dynamics and internal temperature of several particles from previous works and relevant literature, with good agreement.