In situ reactive spark plasma sintering of WSi2/MoSi2 composites

MoSi₂ based materials have the potential for use in high temperature structural parts. In this work, WSi₂ reinforced MoSi₂ composites were successfully prepared by mechanical activation followed by in situ reactive spark plasma sintering of Mo, Si, and W elemental powders. Benefiting from the high energy raw materials prepared through ball milling, these mechanically activated reactants started to transform into MoSi₂ at 1000 °C. Full density composites were obtained at a low sintering temperature (1200 °C) within 5 min. The addition of W to the reactants led to a finer microstructure than that obtained using pure MoSi₂, resulting in a significant improvement of mechanical properties. The Vicker's hardness of 20 vol% WSi₂/MoSi₂ was as high as 16.47 GPa.     

Microstructural evolution,mechanical and thermal properties of LaB6 embedded in Si-B-C-N prepared by spark plasma sintering

Si-B-C-N monoliths with 5 wt% LaB₆ additives were prepared by spark plasma sintering at 1250–2000 °C and 50 MPa using a mechanically alloyed mixture of graphite, c-Si, h-BN and LaB₆ powders as the starting materials. Microstructural evolution, mechanical and thermal properties of the as-prepared La/Si-B-C-N monoliths were investigated. The densification of the ceramics starts at 1160° and ends at 1800 °C with the formation of La-containing compounds coupled with SiC and BN(C) phases. La-containing BN(C) grains develop into a lamellar structure at 1900 °C offering improved fracture toughness and decreased Vickers hardness, flexural strength and elastic modulus. The formation of lamellar BN(C) is also responsible for a high thermal expansion coefficient of 4.2×10⁻⁶ /°C.     

Sintering behaviour,microstructural characterisation and thermal expansion properties of Sn substituted ZrMo2O8

The coefficient of thermal expansion (CTE) of ZrMo₂O₈ can be fine-tuned by controlling the amount of tin substitution in zirconium lattice sites. The sintering challenges associated with this material and the optimal sintering conditions were investigated in this study. Powders of tin substituted ZrMo₂O₈ were synthesised by co-precipitation technique. X-ray diffraction studies confirmed the formation of cubic SnMo₂O₈. Sintered pellets were produced from the powders and optimal sintering without decomposition of the phase was achieved at the expense of porosity. The material was found to be thermally stable up to 600 °C using thermogravimetric analysis. Dilatometric analysis of the sintered compacts shows that the CTE of the sample is in the order of 3.9 × 10^?6/K, between 25 °C and 600 °C.     

Mechanical and electrical properties of carbon nanofibers reinforced aluminum nitride composites prepared by plasma activated sintering

High density carbon nanofibers (CNFs) reinforced aluminum nitride (AlN) composites were successfully fabricated by plasma activated sintering (PAS) method. The effects of CNFs on the microstructure, mechanical and electrical properties of the AlN composites were investigated. The experimental results showed that the grain growth of AlN was significantly inhibited by the CNFs. With 2 wt.% CNFs added into the composites, the fracture toughness and flexural strength were increased, respectively to 5.03 MPa m^1/2 and 354 MPa, which were 20.9% and 13.4% higher than those of monolithic AlN. The main toughening mechanisms were CNFs pullout and bridging, and the main reason for the improvements in strength should be the fine-grain-size effect caused by the CNFs. The DC conductivity of the composites was effectively enhanced through the addition of CNFs, and showed a typical percolation behavior with a very low percolation threshold at the CNFs content of about 0.93 wt.% (1.51 vol.%).     

Spark plasma sintering of graphene reinforced zirconium diboride ultra-high temperature ceramic composites

Spark plasma sintering (SPS) of monolithic ZrB₂ ultra-high temperature ceramic and 2–6 vol% graphene nanoplates (GNPs) reinforced ZrB₂ matrix composites is reported. The SPS at 1900 °C with a uni-axial pressure of 70 MPa and soaking time of 15 min resulted in near-full densification in ZrB₂–GNP composites. Systematic investigations on the effect of GNP reinforcement on densification behavior, microstructure, and mechanical properties (microhardness, biaxial flexural strength, and indentation fracture toughness) of the composites are presented. Densification mechanisms, initiated by interfacial reactions, are also proposed based on detailed thermodynamic analysis of possible reactions at the sintering temperature and the analysis of in-process punch displacement profiles. The results show that GNPs can be retained in the ZrB₂ matrix composites even with high SPS temperature of 1900 °C and cause toughening of the composites through a range of toughening mechanisms, including GNP pull-out, crack deflection, and crack bridging.     

A study on the interfacial reaction and dielectric properties of Ba0.88(Nd1.40Bi0.42La0.30)Ti4O12/alkali-borosilicate glass composites

This study investigated the effects of sintering parameters and the addition of alkali-borosilicate glass into the Ba_0.88(Nd_1.40Bi_0.42La_0.30)Ti₄O₁₂ B(NBL)T ceramic. The microstructure evolution, ionic exchange phenomenon at phase interfaces and the dielectric properties variation of composites were examined by XRD, EPMA, TEM, RF impedance analyzer and network analyzer, respectively. XRD patterns revealed that interactions between B(NBL)T ceramic and glass during sintering could have caused the change in the preferred orientation as well as the shifting of the crystals’ diffraction angles. EPMA mapping showed that the concentrations of Ba, and Bi decreased along the edge of the B(NBL)T ceramic that is closest to the glass phase, while the opposite trend was seen for Na and Ca. TEM and EDS analyses confirm that an ionic exchange took place during sintering with the glass phase wetting the B(NBL)T ceramic and was responsible for the change in the crystal plane and the variation in lattice parameters. The ionic exchange that occurred between the B(NBL)T ceramic and the glass phase resulted in a decrease in the electrical resistivity of the glass phase, which in turn reduced the dielectric loss.     

Effect of single-walled carbon nanotubes on thermal and electrical properties of silicon nitride processed using spark plasma sintering

Si₃N₄ nanocomposites reinforced with 1-, 2-, and 6-vol% single-walled carbon nanotubes (SWNTs) were processed using spark plasma sintering (SPS) in order to control the thermal and electrical properties of the ceramic. Only 2-vol% SWNTs additions were used to decrease the room temperature thermal conductivity by 62% over the monolith and 6-vol% SWNTs was used to transform the insulating ceramic into a metallic electrical conductor (92 S m⁻¹). We found that densification of the nanocomposites was inhibited with increasing SWNT concentration however, the phase transformation from α- to β-Si₃N₄ was not. After SPS, we found evidence of SWNT survival in addition to sintering induced defects detected by monitoring SWNT peak intensity ratios using Raman spectroscopy. Our results show that SWNTs can be used to effectively increase electrical conductivity and lower thermal conductivity of Si₃N₄ due to electrical transport enhancement and thermal scattering of phonons by SWNTs using SPS.     

Low-temperature sintering and microwave dielectric properties of 3Z2B glass–Al2O3 composites

A potential low temperature co-fired ceramics system based on zinc borate 3ZnO–2B₂O₃ (3Z2B) glass matrix and Al₂O₃ filler was investigated with regard to phase development and microwave dielectric properties as functions of the glass content and sintering temperature. The densification mechanism for 3Z2B–Al₂O₃ composites was reported. The linear shrinkage of 3Z2B glass–Al₂O₃ composites exhibited a typical one-stage densification behavior. XRD patterns showed that a new crystalline phase, ZnAl₂O₄ spinel, formed during densification, indicating that certain chemical reaction took place between the 3Z2B glass matrix and the alumina filler. Meanwhile, several zinc borate phases, including 4ZnO·3B₂O₃, crystallized from the glass matrix. Both of the reaction product phase and crystallization phases played an important role in improving the microwave dielectric properties of composites. The optimal composition sintered at 850–950 °C showed excellent microwave dielectric properties: ?_r = ∼5.0, Q·f₀ = ∼8000 GHz, and τ_f = ∼−32 ppm/°C at ∼7.0 GHz.     

Thermal aging behavior of plasma sprayed LaMgAl11O19 thermal barrier coating

The crystallization behavior of the amorphous phase of the plasma sprayed LaMgAl₁₁O₁₉ (LaMA) coating during thermal aging processes has been investigated. Results indicate that LaMA coating exhibits much similar microstructure and thermal properties such as close coefficient of thermal expansion and specific heat capacity etc. to the sintered LaMA bulk after aging at 1673 K for 20 h. On the other hand, a solid state reaction seems to occur to reform the ideal magnetoplumbite-type LaMA phase coupled with the formations of the La-rich aluminate intermediate phases. When the aging temperature is held between 1273 K and 1473 K, nanosized platelet-like grains as well as sub-grains with high aspect ratios are present. The phase stability has been investigated through the chemical compositions and X-ray diffraction analysis. The recrystallization mechanism of the amorphous LaMA coating has been explored by tracing the microstructure evolutions during thermal aging process.     

Characterization of submicrometer-sized NiZn ferrite prepared by spark plasma sintering

High-density submicrometer-sized Ni_0.5Zn_0.5Fe₂O₄ ferrite ceramics were prepared by spark plasma sintering in conjunction with sufficient high energy ball milling. They were evaluated by different characterization techniques such as X-ray diffraction, scanning electron microscopy, and dielectric and magnetic measurements. All samples prepared at sintering temperatures ranging from 850 to 925 °C exhibit a single spinel phase and their relative densities and grain sizes range from 90% to 99% and ~100 nm to ~300 nm, respectively. The dielectric constant increases with decreasing grain size until ~250 nm, and then decreases dramatically with further decreasing grain size. The saturation magnetization increases continuously with increasing grain size/density but the magnetic coercivity decreases. The highest dielectric constant and saturation magnetization at room temperature are approximately 1.0×10⁵ and 84.4 emu/g, respectively, while the lowest magnetic coercivity is only around 15 Oe. These outstanding properties may be associated with high density and uniform microstructure created by spark plasma sintering. Therefore, the spark plasma sintering is a promising technique for fabricating high-quality NiZn ferrites with high saturation magnetization and low coercivity.     

Densification and grain growth during solid state sintering of LaPO4

This work is devoted to the kinetic study of densification and grain growth of LaPO₄ ceramics. By sintering at a temperature close to 1500 °C, densification rate can reach up to 98% of the theoretical density and grain growth can be controlled in the range 0.6–4 μm. Isothermal shrinkage measurements carried out by dilatometry revealed that densification occurs by lattice diffusion from the grain boundary to the neck. The activation energy for densification (E_D) is evaluated as 480 ± 4 kJ mol⁻¹. Grain growth is governed by lattice diffusion controlled pore drag and the activation energy (E_G) is found to be 603 ± 2 kJ mol⁻¹. The pore mobility is so low that grain growth only occurs for almost fully dense materials.     

Synthesis of conducting graphene/Si3N4 composites by spark plasma sintering

Graphene/silicon nitride (Si₃N₄) composites with high fraction of few layered graphene are synthesized by an in situ reduction of graphene oxide (GO) during spark plasma sintering (SPS) of the GO/Si₃N₄ composites. The adequate intermixing of the GO layers and the ceramic powders is achieved in alcohol under sonication followed by blade mixing. The reduction of GO occurs together with the composite densification in SPS, thus avoiding the implementation of additional reduction steps. The materials are studied by X-ray photoelectron and micro-Raman spectroscopy, revealing a high level of recovery of graphene-like domains. The SPS graphene/Si₃N₄ composites exhibit relatively large electrical conductivity values caused by the presence of reduced graphene oxide (∼1 S cm⁻¹ for ∼4 vol.%, and ∼7 S cm⁻¹ for 7 vol.% of reduced-GO). This single-step process also prevents the formation of highly curved graphene sheets during the thermal treatment as the sheets are homogeneously embedded in the ceramic matrix. The uniform distribution of the reduced GO sheets in the composites also produces a noticeable grain refinement of the silicon nitride matrix.     

Thermal conductivity of AlN ceramics sintered with CaF2 and YF3

Dense AlN ceramics with a thermal conductivity of 180W/m·K were obtained at the sintering temperature of 1750 °C using CaF₂ and YF₃ as additives. At temperatures below 1650 °C, the shrinkage of AlN ceramics is promoted by liquid (Ca,Y)F₂ and Ca₁₂Al₁₄O₃₂F₂. Liquid CaYAlO₄ mainly improves the densification of the sample when the sintering temperature increases to 1750 °C. The formation of liquid (Ca,Y)F₂ at a relatively low temperature results in homogeneous YF₃ distribution around the AlN particles, which benefits the removal of oxygen impurity in the AlN lattice, and thus a higher thermal conductivity.     

Microstructure and mechanical properties of hot-pressed carbon nanotubes compacted by spark plasma sintering

Bulk carbon nanotube samples were prepared by spark plasma sintering. The as-prepared bulk carbon nanotube material exhibited brittle fracture similar to that of common ceramics. Its fracture toughness was around 4.2 MPa m^1/2 while flexural strength was 50 MPa due to the weak bonding between carbon nanotubes. Obvious carbon nanotube bridging was found during the development of the crack induced by an indenter, which provides a possibility of carbon nanotube tough material.     

Tin oxide-based ceramics of high density obtained by pressureless sintering

Dense tin oxide-based ceramic semiconductors have a high potential as electrodes for aluminum production, glass industry, sputtering targets for transparent conducting thin films, varistors, and thermoelectrics due to their specific electrical properties, high corrosion resistance and ability to withstand high temperatures. The application of these ceramics is still limited because of the necessity to reach density of 99% of TD or greater and high electrical conductivity, and because of the manufacturing difficulties to produce components of different shapes and sizes with low cost. The paper reviews the results of the works conducted towards obtaining tin oxide-based ceramics with density up to 99.5+% of TD through low-temperature pressureless sintering. The selected sintering aids/dopants and firing conditions promoted both liquid phase sintering and electrical conductivity. The uniform microcrystalline structure is obtained.     

Low temperature sintering of Si3N4 ceramics by spark plasma sintering technique

Abstract

Abstract

Low temperature sintering of α‐Si₃N₄ matrix ceramics was developed in the present study using 4?wt‐%MgO together with Al₂O₃ or AlPO₄ as the sintering additives and spark plasma sintering technique. The results suggested that α‐Si₃N₄ ceramics could be densified at low sintering temperature by adjusting both the sintering temperature and sintering additive content. For low temperature sintered α‐Si₃N₄ ceramics, using MgO and Al₂O₃ as the sintering additives, the densification is not complete at a temperature lower than 1600°C, and the mechanical strength is <200?MPa. When MgO and AlPO₄ were used as the sintering additives, the increase in AlPO₄ content not only declines the sintering temperature but also promotes the mechanical property of the sintered Si₃N₄ ceramics. It was the AlPO₄ phosphate binder that played a significant role in low temperature sintering of Si₃N₄ ceramics.     

Utilization of granodiorite in the production of porcelain stoneware tiles

In the present study, the use of granodiorite, as fluxing agent in a body mix for stoneware ceramic tiles production, was assessed. Four batches were formulated using clay from Khaboba, and natural granodiorite from Saint Katherine, Sinai, Egypt. The batches were tailored to completely replace both feldspatic and inert components of stoneware ceramic tiles. Densification was studied according to ISO rules, while sinterability was estimated by optical dilatometry. The dependence of microstructure and mechanical properties of stoneware ceramic tiles on granodiorite content was discussed. Strength measurements showed that increasing granodiorite content the bending strength of the bodies increased. In particular the studied batches can be used for the production of industrial fast firing tiles. The obtained ceramic tiles possess properties similar to commercial ceramic floor and/or wall tiles.     

Microstructure and thermal expansion behavior of diamond/SiC/(Si) composites fabricated by reactive vapor infiltration

Diamond/SiC/(Si) composites were fabricated by Si vapor vacuum reactive infiltration. The coefficient of thermal expansion (CTE) of composites have been measured from 50 to 400 °C. With the diamond content increasing, CTE of composite decreased, simultaneously, the microstructure of the composites changed from core–shell particles embedded in the Si matrix to an interpenetrating network with the matrix. The CTEs of composites versus temperature matched well with those of Si. The Kerner model was modified according to the structural features of the composites, which exhibited more accurate predictions due to considering the core–shell structure of the composites. The thermal expansion behavior of the matrix was constrained by diamond/SiC network during heating.     

Carbon atmosphere effect on Ti3SiC2 based composites made from TiC/Si powders

The effect of carbon activity and CO pressure in the furnace atmosphere is investigated with respect to the phase reactions during heat treatment of TiC/Si powders. Special attention is given to the production and decomposition of Ti₃SiC₂. Samples were heated in graphite and alumina furnaces, connected to a dilatometer which enabled in situ analysis of the phase reactions. The phase compositions of the heat treated samples were determined by X-ray diffraction. The reducing atmosphere of the graphite furnace enhanced the reactivity of the starting powder and enabled phase reactions to take place at a lower temperature than in the alumina furnace. TiSi₂ and SiC phases formed at temperatures below the melting point of Si and were continuously consumed at higher temperatures. Ti₃SiC₂ formed at the melting point of Si regardless of furnace atmosphere. No decomposition of the Ti₃SiC₂ was observed in either furnace.     

Microstructure and properties of WB/Al nuclear shielding composites prepared by spark plasma sintering

In this paper, a novel nuclear shielding material capable of shielding neutrons and gamma rays, WB-reinforced Al (WB/Al) composites, was prepared by spark plasma sintering (SPS) process. The microstructure of the composites was characterized, and the effects of WB content, heat treatment and matrix type on the properties of the composites were discussed. The results demonstrate that the WB particles are uniformly dispersed in the aluminum matrix and formed a good binding interface with the matrix. WAl₁₂ as an interfacial reaction product is identified, and segregation of Si and Mg elements at the reinforcement/matrix interface occurs. The mechanical properties of the WB/Al composites are sensitive to the WB content. The hardness, elastic modulus and bending strength of the composites increase monotonously as the WB volume fraction increasing, up to 234%, 107% and 91.6% higher respectively than those of the monolithic 6061Al. However, the tensile strength reaches a peak point when the volume fraction is 20%. The effects of T6 treatment and matrix type are not pronounced, especially for the composites with high WB content. The thermal neutron and gamma ray shielding properties of the composites both increase with the increase of material thickness and WB content. The WB/Al composites developed in this work show good application prospects in the field of nuclear radiation protection, due to their good mechanical properties and well neutron and gamma-ray shielding performance.