In-situ synthesis and densification of boron carbide and boron carbide-graphene nanoplatelet composite by reactive spark plasma sintering

Simultaneous synthesis and densification of boron carbide and boron carbide- graphene nano platelets (GNP) were carried out by reactive spark plasma sintering of amorphous boron and graphene nano platelets at temperature ranging from 1200 to 1600?°C, pressure of 50?MPa and heating rate of 50?°C/min and 100?°C/min. X-ray diffraction and Raman spectroscopy confirmed the formation of required phases. Electron microscopic images revealed the formation of sub-micron and nano sized grains of plate like morphology. Sintered product with high relative density of 96%TD was achieved at a temperature of 1600?°C and heating rate of 50?°C/min for B₄C stoichiometric composition and also exhibited maximum hardness of 21.10?GPa.     

Growth of carbon nanotubes with different morphologies by pyrolising acetylene

Abstract

Multiwalled carbon nanotubes (MWCNTs) with different morphologies have been prepared by pyrolysis of a mixture of acetylene-ferrocene over predeposited Co and Ni catalysts at 700°C. A high yield of carbon nanotubes with further purification have been obtained in the optimal conditions. The optimum synthesis parameters included synthesis temperature of 700°C, growth time of 30?min, flowrate of acetylene and hydrogen of 40 and 300?sccm respectively. Multiwall straight, curved, helically, coiled, planar-spiral and V-shaped nanotubes were found with diameters in the range of 10-70?nm and with lengths up to 5?μm. The morphology and structure features of the MWCNTs are characterised using scanning electron microscopy, transmission electron microscopy, energy dispersive spectra, Raman spectroscopy and thermogravimetry analyses.     

Intragranular nanocomposite powders as building blocks for ceramic nanocomposites

A powder-based bottom-up processing scheme is introduced for the production of ceramic nanocomposites. Internal displacement reactions between solid solution powders and metallic reactants proceeding via gaseous intermediates are utilized to generate nanostructured building blocks for the synthesis of ceramic nanocomposites. Subsequent rapid sintering results in ceramic nanocomposites, whose microstructures are inherited from the building blocks. This processing scheme is demonstrated for the production of titanium carbide nanocomposites featuring up to 28 wt.% intragranular tungsten inclusions derived from titanium-tungsten mixed carbide powders. Heat treatment of mixed carbide powders in evacuated ampoules containing titanium sponge and iodine at 1000°C for 24 h resulted in nanocomposite powders featuring tungsten precipitates within titanium carbide grains that were subsequently consolidated via spark plasma sintering at 1300°C for 10 min to produce titanium carbide/metallic tungsten nanocomposites. Transformation of mixed titanium–tungsten carbide powders to titanium carbide/metallic tungsten nanocomposite powders was analyzed via X-ray diffraction. Electron microscopy observations of microstructures pre- and post- sintering showed that the intragranular character of nanocomposite powders can be retained in sintered ceramic nanocomposites. The building block approach demonstrated in this work represents an improved method to make ceramic nanocomposites with majority intragranular character.     

Investigating the effect of SPS‎ parameters on densification ‎and fracture toughness of ZrB2-SiC nanocomposite‎

In this study, the effect of sintering parameters on densification and fracture toughness of spark plasma sintering ZrB₂-SiC nanocomposites was evaluated. For this purpose, ZrB₂-??30?vol% SiC nanocomposites in the conditions of ?1600?°C-4?min, 1700?°C-4?min, 1800?°C-4?min, 1800?°C-8?min, 1800?°C-12?min? were sintered.? Scanning Electron Microscopy (SEM) was used in order to investigate the ?microstructural variations. The bulk density was measured accoring to ASTM C 373–88. Single edge notch beam (SENB) method was used to ?determine the fracture toughness of samples. Microstructural observations showed that ?an increase in sintering temperature led to slight ?increase in SiC grains size but no sensitive variation in ZrB₂. However, increasing the sintering time resulted to increase both ZrB₂ and SiC grain size. Also, it was found, temperature and time ascent always increases the relative density. In addition, it was concluded that optimal temperature and time to reach the highest fracture toughness are 1800?°C and 8?min, respectively. Investigation of SEM images of the Vickers indent and their path propagation showed that the deviation and branching of crack are the most important toughening ?mechanisms in ZrB₂-SiC nanocomposites.?     

Effect of impurities introduced by ball milling on hot pressed boron carbide

Ball milling is commonly used refining process to improve sintering properties of boron carbide. However, this process often leads to the introduction of trace metal impurities into raw materials. It was found in this study that, with prolonged holding time and increased sintering temperature at 2000 °C, these metal impurities led to the recrystallization of boron carbide grains, which significantly affected mechanical properties of sintered boron carbide. Results showed that relative density of sample increased from 98.4%–98.9%, when sintering temperature was raised from 2000 °C to 2050 °C and holding time was extended from 30 min to 60 min. However, under these conditions, flexural strength of boron carbide decreased sharply from 403 MPa to 98 MPa and fracture toughness decreased from 4.4 MPa m^1/2 to 2.3 MPa m^1/2.     

Microstructures and thermal conductivities of reaction sintered SiC ceramics

Abstract

Reaction sintered SiC ceramics were prepared by the silicon melt infiltration method over temperatures of 1450?1550°C. The effects of the carbon and silicon contents of the starting materials as well as the sintering temperature and time on the thermal conductivities and microstructures of the ceramic materials were studied. The thermal conductivities and microstructures of the samples were characterised using thermal conductivity measurements, X-ray diffraction analysis, scanning electron microscopy, energy-dispersive X-ray spectroscopy and mercury injection porosimetry. The results showed that sintering temperature and time as well as the carbon and silicon contents of the green specimens are the main factors affecting the microstructure and porosity of reaction bonded SiC ceramics. Increasing the reaction temperature and time decreased the porosity of the ceramics. This was due to the infiltration of the silicon melt into the ceramic specimens. The thermal conductivity and porosity of the sample sintered at 1550°C for 3 h in an argon atmosphere were 102·5 W m K^?1 and 0·3% respectively.     

Microwave hybrid fast sintering of red clay ceramics

This work aimed to examine the performance of the hybrid sintering of clay ceramic in a microwave furnace, compared to the sintering process in a conventional furnace. The raw materials were subjected to X-ray fluorescence, loss on ignition (LOI), X-ray diffraction, particle size distribution, real specific mass, and thermogravimetric analyses. The red clay ceramic mass was prepared, extruded, pre-sintered in a conventional furnace at 600°C/60 min, and sintered at temperatures between 700 °C and 1100 °C. The sintering conventional (resistive oven) was carried out for 60 min with a heating rate of 10°C/min. In the microwave furnace, the sintering times were 5, 10, and 15 min, with a heating rate of 50°C/min, with a sintering chamber coated with silicon carbide (susceptor). The sintered specimens were characterized according to linear shrinkage, water absorption, apparent porosity, apparent specific mass, X-ray diffraction, Raman spectroscopy analysis, spectroscopy analysis in the ultraviolet and visible regions, microhardness, and scanning electron microscopy. The results showed that microwave sintering promoted an increase in the microhardness and apparent specific mass, and reduction in water absorption and apparent porosity values, due to greater densification in the microstructure. The best results occurred for specimens sintered at 1100°C.     

Formation and reactivity of borides,carbides and silicides. II. Production and sintering of boron carbide

The principal methods of boron carbide production are compared and their thermodynamics are examined. Mechanism of carbide formation and sintering of the products are discussed and investigated further. High-purity stoicheiometric B₄C of submicron size was produced on a semi-technical scale by reduction of diboron trioxide with carbon and magnesium. Rates of sintering were determined from changes in surface areas and average crystallite and aggreagte sizes. Sintering of boron carbide was enhanced by increased temperature and time of calcination. Addition of chromium accelerated sintering at temperatures above 1600° and especially at 1800°. For more extensive sintering, submicron powders from the magnesium reduction process were more suitable than the coarser samples given by the electro-thermal carbon reduction; the latter required ballmilling to provide suitable grain size composition for effective hot pressing.     

Characterization of B4C-SiC ceramic composites prepared by ultra-high pressure sintering

Additive-free boron carbide (B₄C) – silicon carbide (SiC) ceramic composites with different B₄C and β-SiC powders ratio were densified using the high-pressure “anvil-type with hollows” apparatus at 1500 °C under a pressure of 4 GPa for 60 s in air. The effect of starting powders ratio on the composites sintering behavior, relative density, microstructural development, and thermomechanical properties was studied. The sintered samples hardness was found to be in the range from 24 to 31 GPa. The thermal conductivity measurements, conducted in the temperature range from room temperature to 1000 °C, showed that the thermal diffusivity of sintered samples was between 6 and 9.5 mm²/s whereas the thermal conductivity was in the range from 16 to 28 W/(m K). The results of this study show that the high-pressure sintering can be a very effective low-temperature densification method for the obtainment of additive-free B₄C - β-SiC ceramic composites.     

Production of titanium-containing metal-ceramic composites based on boron carbide in the nanocrystalline state

ABSTRACT

The results of the study of the production technology, phase composition, structure and physico-mechanical properties of metal-ceramic materials based on boron carbide and their components are presented. Boron carbide was obtained by direct synthesis from chemical elements using amorphous boron and carbon black. By mechanical dispersion, solid reagents were converted into an ultrafine state. Using a chemical method, nanoscale (70–80?nm) boron carbide was synthesised from suspension solutions of amorphous boron and liquid hydrocarbons. Boron carbide-based metal-ceramic composite powder B₄C–(Co–Ni–Ti) was obtained by mechanical dispersion of the constituent components. Based on results of studying of the temperature-dependence of wetting angle of boron carbide with Co–Ni–Ti metallic alloy, the compacting modes of metal-ceramic composite powders by plasma-spark sintering and hot pressing have been developed. The influence of the component content of the binder metal (alloy) on some physico-mechanical properties (linear expansion coefficient, hardness, and bending strength) of hardmetal-ceramic materials based on boron carbide was studied. It was found that the optimum content of the metal component in the composite is ～ 25?wt-%. In the temperature range 300–600°C, the materials obtained are characterised by stable dimensional factors, since in this temperature range the thermal conductivity coefficient does not depend much on temperature. At room temperature, their bending strength is about 1?GPa. A new method of chemical synthesis of nanocrystalline ceramic compositions of boron carbide and titanium diboride using suspension solutions for the preparation of powders and their spark plasma sintering was also developed to obtain a compacted material of composition B₄C+30?wt-%TiB₂, which has a high hardness of 95 HRA (with maximum microhardness 45.6?GPa) and sufficient strength (with a bending strength of 834?MPa).     

Effect of chromium additive on sintering and oxidation behavior of HfB2-SiC ceramics

The effect of chromium admixture on the processes in the HfB₂-SiC ceramic powder system during its pressureless sintering at 1600?°C was studied. It was shown that an increase in chromium content from 0% to 15.5% in the HfB₂-SiC ceramic powder mixture leads to a continuous increase in its relative density up to 90%. A transient liquid phase Cr-Si-C-B is formed at 1600?°C, and it promotes intense sintering of HfB₂ and SiC powders. The oxidation resistance of HfB₂-SiC-Cr ceramics was studied in static air at 1000–1500?°C. It was shown that the oxidation resistance is greatly improved due to a decrease in the porosity of the sintered ceramic system because of chromium additive. The presence of chromium oxide in the formed surface glassy layer can also lead to the increase in the oxidation resistance. These results suggest that chromium can be considered as a promising sintering additive for HfB₂-SiC and similar systems.     

Evaluation of the sinterability,densification behaviour and microhardness of spark plasma sintered multiwall carbon nanotubes reinforced Ti6Al4V nanocomposites

Structural and industrial demands for lightweight engineering materials with exclusive properties have been rising in recent decades for automobile and aerospace applications. This has encouraged various innovations in materials engineering communities to synthesis advanced engineering materials using improved fabrication technique such as spark plasma sintering (SPS). In this study, titanium-based nanocomposites were synthesized by reinforcing Ti6Al4V reinforced with (0.5, 1.0 and 1.5 wt%) multiwall carbon nanotubes (MWCNT) powders. The starting powders were blended by shift-speed ball milling. Thereafter, SPS technique was used to consolidate the admixed powders by employing the following sintering parameters; sintering rate, 100 °C/min, compressive pressure, 50 MPa, holding time, 10 min and sintering temperatures of 900–1100 °C. The influence of MWCNT additions on the sinterability, densification behaviours and microhardness of the sintered nanocomposites were investigated. The results revealed that the densification of the sintered nanocomposites was in the range of 97.51–99.61% which decreased with an increase in concentration of the MWCNT. Meanwhile, the densification and microhardness improved tremendously with an increase in sintering temperatures.     

A new method for solid-state diffusion of boron atoms into powders of a multicomponent carbide

Multicomponent boron-containing carbide (ie, Zr-Ti-C-B) composites show good ablation resistance. The present work is the first report to introduce the powder fabrication of Zr-Ti-C-B using a new method for solid-state diffusion of boron atoms. First, the nonstoichiometric carbide (ie, Zr_0.8Ti_0.2C_0.8) with carbon vacancies was fabricated by free-pressureless spark plasma sintering. Different boron sources such as B₂O₃, B, and B₄C were used to react with the nonstoichiometric carbide. The Zr_0.81Ti_0.19C_0.86B_0.14 can be finally generated through the solid-state diffusion of boron atoms using the B₂O₃ boron source at 1300°C followed by carbon thermal reduction using the phenolic resins at 1600°C.     

Synthesis of nanocrystalline boron carbide by sucrose precursor method-optimization of process conditions

Nanocrystalline boron carbide powder was synthesized by a precursor method using B₂O₃ as the source of boron and sucrose as the source of carbon. Precursor was prepared at different temperatures ranging from 300 to 800 °C. The optimum temperature for the precursor preparation was found to be 600 °C. All the precursors were heat treated at different temperatures from 1000 to 1600 °C for different duration of heating, ranging from 5 to 240 min under vacuum. The products thus obtained after heat treatment were characterized using X-ray diffraction. The boron carbide obtained was nanocrystalline and the average X-ray crystallite size was found to be ~ 33 nm. Boron, total carbon and free carbon contents also were determined. The free carbon content was found to be less than 3% for samples heated at 1600 °C for 10 min. Effect of heat treatment temperature on the morphology of the synthesized product was studied using scanning electron microscope.     

Effect of zirconia content on mechanical and thermal properties of mullite–zirconia composite

Abstract

Mullite–zirconia composites were prepared by adding various zirconia contents in the mullite ranging from 0 to 30 wt-% and sintering at 1400–1600°C for 2 h. The phase composition examined by X-ray diffraction showed that mullite was the major phase combined with developed t-ZrO₂ and m-ZrO₂ phase as a function of zirconia content, especially at 1600°C, wherein m-ZrO₂ predominated. Density increased when the zirconia content and sintering temperature were increased ranging from 2·2 to 3·53 g cm^?3. The morphology of mullite grain showed elongated grains, whereas dispersed zirconia showed equiaxed and intergranular grains. Flexural strength was continuously improved by adding zirconia during the sintering temperature ranging from 1400 to 1500°C, whereas flexural strength was initially improved up to 5 wt-% of zirconia addition and deteriorated with more than 5 wt-% of zirconia content during sintering between 1550 and 1600°C. The maximum strength, 190 MPa, was obtained when sintering mullite with 30 wt-% of zirconia content at 1500°C. The degradation of strength at high sintering temperature may be a result from more occurrence of m-ZrO₂ phase. Thermal expansion of sintered specimens indicated linear change and hysteresis loop change. The hysteresis loop obtained with increased zirconia content resulted in the t–m phase transformation. Martensitic start temperature M_s was determined to be 530°C for 15 wt-% zirconia sintered at 1500°C, implying that the t–m phase transformation occurred.     

Effects of functionalized MWNTs with GMA on the properties of PMMA nanocomposites

The acid modification of multiwall carbon nanotubes (MWNTs) was performed by an HNO₃/H₂SO₄ solution. The glycidyl methacrylate (GMA) undergoing an opening‐ring was grafted onto the surface of acid‐modified MWNTs. The surface properties of MWNTs were investigated by Fourier transform infrared spectrometer (FTIR), Raman spectra, transmission electron microscopy (TEM), X‐ray diffraction, and thermogravimeric analysis. Then the MWNTs/ poly(methyl methacrylate) (PMMA) nanocomposites were prepared by in situ polymerization. The tribological and dielectric properties of nanocomposites were studied. As a result, GMA was grafted on the surface of MWNTs. The tribological and dielectric properties of MWNTs/ PMMA nanocomposites were improved as the content of the surface‐modified MWNT increased. The marked improvement in tribological and dielectric properties were attributed to the good dispersion of MWNTs that were bonded with C?C on the surface that participated in the polymerization of MMA. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

In situ synthesis and thermal shock resistance of mullite used for thermal storage ceramics in solar thermal power generation

Abstract

Mullite ceramic, as one of high performance thermal storage ceramics for solar thermal power generation systems, was in situ fabricated via semidry pressing and pressureless sintering in the air. Andalusite (57–68 wt-%) and calcined bauxite (24–29 wt-%) were used as the raw materials, with kaolin and a tiny of boric acid being added to promote the densification and improve the mechanical properties. The best physical properties and thermal shock resistance were obtained on an optimum A3 sample sintered at 1600°C for 3 h, i.e. a bending strength of 120·44 MPa and 30 cycles thermal shock cycling without cracking (wind cooling from 1000°C to room temperature) with a loss of bending strength of 8·7%.     

纯B4C和掺碳B4C的烧结机制

研究了中位粒径为0．42μm的纯B4C和掺碳B4C的烧结致密化过程。根据烧结温度和保温时间对线收缩率的影响。得出了它们的烧结动力学方程;由特征指数n值对比研究了它们的烧结致密机制。纯B4C的烧结致密机制为体扩散和晶界扩散,而掺碳B4C的烧结机制主要为晶界扩散,因此,掺碳对B4C起到了活化烧结的作用,在2160℃烧结45min,掺碳B4C烧结后相对密度大于90％,掺入的碳除了固溶于B4C晶格中之外,其它均以游离石墨形式存在,不形成新相。掺碳还导致B4C晶粒尺寸大大减小。     

Rapid microwave sintering of alumina ceramics with an addition of carbon nanotubes

Compacted alumina samples with an addition of 1.0 and 2.5 wt % carbon nanotubes (CNT) have been microwave sintered at heating rates 50 and 100°С/min to a maximum temperature of 1550–1600°С with zero hold time. The densification kinetics has been studied using optical dilatometry. No noticeable influence of CNT on the development of thermal instability during rapid microwave sintering of alumina has been detected. The relative densities of the samples containing 1.0 and 2.5 wt % CNT sintered at a maximum temperature of 1550 °C and zero hold time were 93.8 and 87.5%, respectively. Microwave sintering with a repeated development of thermal instability has resulted in an increase in the final density to 95.0%.     

Diametral strength testing of hydroxyapatites doped with yttrium and fluoride

Abstract

Abstract

This study aimed to investigate the diametral strength testing of hydroxyapatite (HA) doped with Y and fluoride with different compositions. Hydroxyapatites were synthesised by precipitation method and sintered at 900, 1100 and 1300°C for 1?h. High amounts of doping caused a decrease in relative densities of HAs. Higher sintering temperatures helped in increasing the relative densities. No second phases were observed by X-ray diffraction spectra of 2·5?mol.-%Y and 2·5?mol.-%F doped HA after the sintering at all temperatures. Trace amounts of β-tricalcium phosphate was found in 7·5?mol.-%Y and 2·5?mol.-%F doped HA sintered at 1100 and 1300°C. Diametral strength of doped HAs mostly enhanced with the addition of Y³⁺ and F^?. 2·5YFHA sintered at 1300°C had the highest diametral strength of 11·6?MPa with a relative density of 94·3% of theoretical density.