Conventional sintering of LAS–SiC nanocomposites with very low thermal expansion coefficient

Very low thermal expansion materials have been developed in a wide temperature range from ?150 °C to 450 °C. These very low expansion materials are composed of a lithium aluminosilicate matrix with dispersed silicon carbide nanoparticles. The nSiC content in the most stable composite has resulted to be 27 vol.%. Powder processing and sintering temperature control have been found to be of key importance in order to obtain low porosity sintered nanocomposites. Here it is proposed the sintering by pressureless methods (conventional sintering) in order to obtain a LAS–nSiC nanocomposite with improved mechanical properties and excellent expansion behaviour by means of control of the phase reactions and glass formation during sintering.     

Non-contact mechanical property measurements at ultrahigh temperatures

The “Electro-Magnetic Mechanical Apparatus”, a novel non-contact method of mechanical testing of ultrahigh temperature ceramics at high temperatures, was developed where a mechanical stress is applied using Lorenz forces on a sample heated to high temperatures with an electric current. The design of the apparatus and an analysis relating stress to magnetic flux density, electrical current, and specimen dimensions are presented here. Significant creep deformations were observed in ZrB₂–SiC samples deformed under 20 MPa of flexural stress resulting in 0.08% strain after 240 s at 1600 °C and 0.21% strain after 150 s at 1750 °C. A fatigue load of 6 MPa at 60 Hz frequency at 1700 °C in air increased the oxidation rate. This mechanical apparatus has potential application not only for high temperature creep and fatigue experiments but also fracture and elasticity. Though developed for ceramics, this technique can be used to study high temperature mechanical properties of any conducting material.     

Production of nepheline/quartz ceramics from geopolymer mortars

This research has investigated the mechanical properties and microstructure of metakaolin derived geopolymer mortars containing 50% by weight of silica sand, after exposure to temperatures up to 1200 °C. The compressive strength, porosity and microstructure of the geopolymer mortar samples were not significantly affected by temperatures up to 800 °C. Nepheline (NaAlSiO₄) and carnegieite (NaAlSiO₄) form at 900 °C in the geopolymer phase and after exposure to 1000 °C the mortar samples were transformed into polycrystalline nepheline/quartz ceramics with relatively high compressive strength (～275 MPa) and high Vickers hardness (～350 HV). Between 1000 and 1200 °C the samples soften with gas evolution causing the formation of closed porosity that reduced sample density and limited the mechanical properties.     

Catalytic graphene formation in coal tar pitch- derived carbon structure in the presence of SiO2 nanoparticles

A simple and effective way to manufacture graphene from a coal tar pitch (CTP) is demonstrated. Silica (SiO₂) nanoparticles were used to modify the CTP as carbon precursor. A silica nanofiller introduced into the CTP matrix underwent carboreduction during heat treatment to 2000 °C, resulting in the formation of silicon carbide. Surfaces of SiC grains were sites for graphene formation. The influence of SiO₂ on the structure and microstructure of CTP- based carbon matrix, after annealing up to 2800 °C, was studied. Carbon samples were analyzed using X- ray Diffraction (XRD), Transmission Electron Microscopy (TEM) and Raman Spectroscopy. Crystallite sizes (L_a, L_c) and interplanar distance (d₀₀₂) were determined. The presence of SiO₂ in CTP carbon precursor favored the crystallites’ growth in the ‘a′ crystallographic graphite direction, and inhibited their growth on the ‘c′ axis. The crystallites composing of graphene layers, were characterized by an elongated dimension in the ‘a′ axis direction. Above 2000 °C silicon carbide decomposed, followed by the sublimation of silicon from the carbon matrix.     

Comparative study of the structure and microstructure of PAN-based nano- and micro-carbon fibers

The work presents results on the manufacture and comparative assessment of the structure and microstructure parameters of polyacrylonitrile polymer (PAN)-based carbon nano- and micro-fibers. Using the same polymer solution, PAN nano- and microfibers were obtained. The PAN nanofibers were obtained by electrospinning, and microfibers were spun using the conventional solution-spinning method. The PAN-based fiber precursors were annealed to 1000 °C, 2000 °C and to 2800 °C. Using X-ray diffraction and Raman spectroscopy, the structural and microstructural parameters of both types of carbon fibers were examined. The morphology of PAN nanofibers and carbon nanofibers (CNF) were studied by SEM. Both types of ex-PAN carbon fibers (nano and micro) have similar the c-axis spacing (d₀₀₂) values and crystallite sizes after heat treatment to 2000 °C presenting turbostratic structure. HR-TEM images of low temperature CNF show uniform microstructure with the misoriented small carbon crystallites along the fiber axis. The ratio of the integrated intensities of the D and G peaks for carbon nanofibers after heat treatment at 2000 °C was distinctly higher in comparison to carbon microfibers (CF). After additional annealing the fibers to 2800 °C a better structural ordering show CNF. The crystallite sizes (L_c, L_a) in CNF were distinctly higher in comparison to the crystallites in CF. CF consist of two carbon components, whereas CNF contain three carbon components varying in structural and microstructural parameters. One of carbon phases in CNF was found to have the interlayer spacing close to graphite, i.e. d₀₀₂=0.335 nm.     

Pitch-based ribbon-shaped carbon-fiber-reinforced one-dimensional carbon/carbon composites with ultrahigh thermal conductivity

Ribbon-shaped carbon fibers have been prepared from mesophase pitch by melt-spinning, oxidative stabilization and further heat treatment. The internal graphitic layers of ribbon-shaped carbon fibers graphitized at 2800 °C show a highly preferred orientation along the longitudinal direction. Parallel stretched and unidirectional arranged ribbon-shaped carbon fibers treated at about 450 °C were sprayed with a mesophase pitch powder grout, and then hot-pressed at 500 °C and subsequently carbonized and graphitized at various temperatures to produce one-dimensional carbon/carbon (C/C) composite blocks. The shape and microstructural orientation of ribbon fibers have been maintained in the process of hot-pressing and subsequent heat treatments and the main planes of the ribbon fibers are orderly accumulated along the hot-pressing direction. Microstructural analyses indicate that the C/C composite blocks have a typical structural anisotropy derived from the unidirectional arrangement of the highly oriented wide ribbon-shaped fibers in the composite block. The thermal conductivities of the C/C composites along the longitudinal direction of ribbon fibers increase with heat-treatment temperatures. The longitudinal thermal conductivity and thermal diffusivity at room temperature of the C/C composite blocks graphitized at 3100 °C are 896 W/m K and 642 mm²/s, respectively.     

Oxidation protection of carbon fibers by coating with alumina and/or titania using atomic layer deposition

Alumina and titania coatings were deposited by atomic layer deposition onto carbon fibers at temperatures of 200 °C or below and reduced pressure. The coatings were smooth, uniform and conformed to the fiber surface. Thermogravimetric analysis (TGA) revealed that the coatings improved the oxidation resistance of the carbon fibers: the oxidation onset temperature of uncoated fibers and fibers coated with 66 nm of alumina was 630 °C. For fibers coated with 20 nm of titania it was 550 °C. Double layer coatings by 50 nm of alumina followed by 13 nm of titania yielded an oxidation onset temperature of 660 °C, while changing the order of the layers, i.e., coating fibers first with 20 nm of titania followed by 30 nm of alumina yielded an oxidation onset temperature of 750 °C. These TGA results were confirmed by a set of additional oxidation experiments conducted at a fixed temperature of 550 °C using a tube furnace in air. In this latter set of additional experiments, the times needed for a complete oxidation of the above mentioned samples were 8 h, 12 h, 10 h, 13 h, and 30 h, respectively.     

Sintering behaviour of silicon carbide matrix composites reinforced with multilayer graphene

The scope of this paper includes preparation and characterisation of dense silicon carbide matrix composites reinforced with multilayer graphene (MLG). Application of graphene as a reinforcement phase should simultaneously improve mechanical properties of SiC matrix composites and act as one of the sintering activators. In the present work the mechanical properties and the microstructure changes of samples sintered with different additions of graphene (0.5, 1, 2, 3, 4 wt%) and boron (0.3, 1 and 2 wt%) were examined. The composites were consolidated at two different temperatures (1800 °C and 1900 °C) using the Spark Plasma Sintering method (SPS). Reference samples with the addition of graphite as a source of carbon (1 and 3 wt%) were also sintered in the same conditions. The abovementioned amounts of graphite are an optimal content which is essential to obtain high density of samples [1], [2], [3], [4], [5], [6], [7], [8], [9]. The influence of MLG on density, mechanical properties and phase structure of the sintered samples were investigated. A high rate of densification for the composites with 0.3 wt% of B and 1 wt% of MLG sintered at 1900 °C was observed. Moreover, these composites showed the highest average of microhardness (2663 HV0.5) and single-phase structure.     

Characterization of high strength mortars with nano alumina at elevated temperatures

In this study, the effect of elevated temperatures on chemical composition, microstructure and mechanical properties of high strength mortars with nano alumina was investigated. Mortars with 1, 2 and 3% nano alumina as cement replacement were prepared and then exposed to 100 °C, 200 °C, 300 °C, 400 °C, 600 °C, 800 °C and 1000 °C. XRD, DSC and SEM tests were carried out to identify chemical composition and microstructure changes in the cement matrix after being exposed to elevated temperatures. Residual compressive strength, relative elastic modulus and gas permeability coefficient of samples were also obtained. A brittleness index was defined to monitor changes in brittleness of samples after being exposed to elevated temperatures. Nano alumina enhanced compressive strength of samples up to 16% and improved residual compressive strength. An increase in the relative elastic modulus, higher energy absorption and lower permeability were also observed when 1% nano alumina was added.     

Structural,textural, surface chemistry and sensing properties of mesoporous Pr,Zn modified SnO2–TiO2 powder composites

Mesoporous Zn and Pr modified SnO₂-TiO₂ mixed powders (Sn:Ti:Zn:Pr contents 60:20:15:5) have been prepared by a modified sol–gel method involving Tripropylamine (TPA) as chelating agent, TritonX100 as template and Polyvinylpyrrolidone as dispersant and stabilizer, respectively. The obtained gels have been dried at different temperatures and calcined in air at 600 and 800 °C, respectively. Phase identification of the synthesized samples and their evolution with the calcination temperature has been performed by X-ray diffraction. N₂ adsorption/desorption isotherms were found to be characteristic for mesoporous materials, showing relatively low values for the specific surface area (15–32 m² g⁻¹) and nanometric sized pores. In case of the sample calcined at 800 °C, a bimodal pore size distribution can be observed, with maxima at 20 and 60 nm. SEM results demonstrate a porous nanocrystalline morphology stable up to 800 °C. The surface chemistry investigated by XPS reveals the presence of the elements on the surface as well as the oxidation states for the detected elements. At 800 °C a diffusion process of Sn from surface to the subsurface/bulk region accompanied by a segregation of Ti and Zn to the surface is noticed, while Pr content is unchanged. The sensing properties of the prepared powders for CO detection have been tested in the range of 250–2000 ppm and working temperatures of 227–477 °C.     

The resistance to oxidation of an HfB2–SiC composite

The oxidation resistance of an hot-pressed HfB₂–SiC composite was studied through non-isothermal and isothermal treatments at temperatures up to 1600 °C in air. The most severe oxidation conditions consisted of repeated heating-cooling cycles at 1600 °C for up to 80 min of exposure. A thermogravimetric test for over 20 h at 1450 °C provided evidence that, at this temperature, the oxidation kinetics fits a paralinear law until 10 h, when a partial rupture of external oxide scale occurs (i.e. a break-away reaction). Afterwards, the weight gain data fit a linear law. The main secondary phases formed in the composite during hot-pressing, namely BN, Hf(C,N) and a Si-based compound, although in limited amounts, influenced the oxidation resistance at temperatures below 1350 °C. At temperatures higher than about 1400 °C, the presence of SiC particles markedly improved the oxidation resistance due to the formation of a protective borosilicate glassy coating on the exposed surfaces.     

Preparation of carbon fibers from linear low density polyethylene

Carbon fibers were produced from linear low density polyethylene (LLDPE) instead of commonly used precursors, such as viscose rayon, mesophase pitch and polyacrylonitrile (PAN). Cross-linked fibers were produced at various temperatures, times and stress conditions during a sulfuric acid treatment using LLDPE fibers obtained from dry-wet spinning. The effects of cross-linking were analyzed using a range of characterization techniques, such as differential scanning calorimetry, color change, fourier transform infrared spectroscopy, elemental analysis, density, scanning electron microscopy, and single filament mechanical properties. The carbonization process of cross-linked fibers was carried out at 950 °C for 5 min in a nitrogen atmosphere. The carbon fibers with the best mechanical properties were obtained from the cross-linked fiber with the highest tensile modulus. In particular, the carbon fibers with the best mechanical properties (tensile strength and tensile modulus of 1.65 GPa and 110 GPa, respectively), similar to commercial-grade carbon fiber, were obtained from the cross-linked fiber that had undergone a carbonization process with a stress of 0.25 MPa after an acid treatment for 150 min at 140 °C and a stress of 0.26 MPa.     

Effect of PyC interphase thickness on mechanical behaviors of SiBC matrix modified C/SiC composites fabricated by reactive melt infiltration

A dense carbon fiber reinforced silicon carbide matrix composites modified by SiBC matrix (C/SiC-SiBC) was prepared by a joint process of chemical vapor infiltration, slurry infiltration and liquid silicon infiltration. The effects of pyrolytic carbon (PyC) interphase thickness on mechanical properties and oxidation behaviors of C/SiC-SiBC composites were evaluated. The results showed that C/SiC-SiBC composites with an optimal PyC interphase thickness of 450 nm exhibited flexural strength of 412 MPa and fracture toughness of 24 MPa m^1/2, which obtained 235% and 300% improvement compared with the one with 50 nm-thick PyC interphase. The enhanced mechanical properties of C/SiC-SiBC composites with the increase of interphase thickness was due to the weakened interfacial bonding strength and the decrease of matrix micro-crack amount associated with the reduction of thermal residual stress. With the decrease in matrix porosity and micro-crack density, C/SiC-SiBC composites with 450 nm-thick interphase exhibited excellent oxidation resistance. The residual flexural strength after oxidized at 800, 1000 and 1200 °C in air for 10 h was 490, 500 and 480 MPa, which increased by 206%, 130% and 108% compared with those of C/SiC composites.     

A study of the oxidation behavior of CrN and CrZrN ceramic thin films prepared in a magnetron sputtering system

CrN and CrZrN ceramic thin films were produced by a planar type reactive sputtering system on glass and stainless steel substrates. We investigated oxidation resistance of CrN and CrZrN ceramic thin films with different Zr contents. The structure of the films at different thermal-annealing temperatures was investigated by X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). The mechanical properties of the films at different thermal-annealing temperatures were measured by nano-indentation. The results of this study showed that the addition of few amount of Zr (0.4 at%), can improve thermal stability of CrZrN ceramic thin film and increase the oxidation temperature of the film from 600 °C to 800 °C. The relatively good oxidation resistance (800 °C) and high hardness of the film with the lowest Zr content, indicates that this film is a good candidate for high temperature applications.     

Structure and conductivity of NiO/YSZ composite prepared via modified glycine-nitrate process at varying sintering temperatures

Nickel oxide and Yttria-stabilized zirconia (NiO/YSZ) composite is one of the most promising mixed conducting electrode materials in both solid oxide electrolysis cell and solid oxide fuel cell applications. In this study, 50 wt% NiO and 50 wt% YSZ composite was synthesized via a modified glycine-nitrate combustion process (GNP) and the effect of sintering temperatures (1100 °C, 1300 °C and 1500 °C) on its microstructure and electrical properties were investigated. TG/DTA and in-situ high temperature XRD revealed the thermal property behavior and the structural changes of the as-combusted precursor material. For all the samples sintered at different temperatures, room temperature XRD patterns revealed a distinct cubic phases of both YSZ and NiO while SEM images showed a porous microstructure. The total conductivities at 700 °C are 9.87 × 10⁻³, 5.26 × 10⁻³, 4.02 × 10⁻³ S/cm for the 1100, 1300, and 1500 °C with activation energies of 0.1722, 0.3555, and 0.3768 eV, respectively. Conductivity measurements of the different sintered samples revealed that the total conductivities as well as the activation energies are greatly affected by different sintering temperatures.     

Fabrication of large diameter SiC monofilaments by polymer route

We demonstrate the possibility to fabricate SiC monofilaments with large diameters of 100 μm by a polymer route using a dry-spinning process. The properties of the spinning solution and the parameters of the spinning process were optimized to achieve a circular cross section of the spun filaments despite their large diameter. The evolution of the diameter and the mechanical properties of the filaments with pyrolysis temperature were studied. Filament shrinkage started above 400 °C. A radial shrinkage of about 25% was measured for pyrolysis temperatures of 1200 °C. The mechanical properties significantly start to increase at pyrolysis temperatures above 600 °C. At a diameter of 100 μm the filaments show a tensile strength of 620 MPa and a tensile modulus of 138 GPa after pyrolysis at 1200 °C. A decrease in the filament diameter leads to an improvement of the mechanical properties. We demonstrate the fabrication of these SiC monofilaments on spools.     

Influence of the carbonization temperature on the mechanical properties of thermoplastic polymer derived C/C-SiC composites

Carbon/Carbon (C/C) composites derived from the thermoplastic polymer polyetherimide (PEI) were pyrolized up to 1000 °C, subsequently carbonized in inert atmosphere up to 2200 °C and afterwards infiltrated with liquid silicon. The investigation of fibers and matrix with Raman microspectroscopy revealed, that an increased carbonization temperature leads to an increased carbon order as well as an incipient stress-induced graphitization of the carbon matrix close to the fiber surfaces at 2200 °C. The derived C/C-SiC samples show a maximum flexural strength of 180 MPa with C/C composites treated at 2000 °C and monotonically increasing Young’s moduli ranging from 49 GPa with C/C preforms treated at 1600 °C up to 59 GPa after carbonization at 2200 °C. The carbon fiber strength was evaluated with a single fiber tensile test, which showed a monotonically increased Young’s modulus and a decrease of the strength after carbonization at 2200 °C.     

Preparation and high-temperature performance of HfC-based nanocomposites derived from precursor with Hf-(O,N) bonds

A novel precursor was synthesized by reacting hafnium chloride with dicyandiamide and dimethylformamide. The precursor was characterized via FT-IR and NMR, as well as TG. Subsequently, the precursor was annealed in Ar over a range of temperatures from 1000 °C to 2000 °C, and the microstructural evolution of the ceramics was investigated by XRD, XPS, and TEM. The results show that the carbothermal reduction of the precursor starts at 1150 °C and the ceramic yields at 1500 °C reach 44.6 wt%. The obtained powders exhibit a uniform distribution and are composed of N-doped HfC and graphite. The N-doped structure postponed the oxidation of the HfC(N) ceramics. The HfC(N) ceramics were first oxidized to yield HfO₂, carbon, and nitrogen, and then the carbon was oxidized with the evolution of CO₂. The presented synthesis method is believed to be applicable to the preparation of other high-performance ceramics.     

Processing and properties of sol gel derived alumina–carbon nano tube composites

High purity alumina–carbon nano tube (CNT) composites were prepared by an aqueous sol–gel processing route. CNTs were dispersed in alumina sol containing appropriate amount of MgO precursor. Aqueous slurry of alumina was seeded into the sol followed by gelation, drying and calcination at 1000 °C for 1 h. The calcined powder consisting of alumina-coated CNTs and alumina was milled, sieved, dried, pressed and pressureless sintered at 1400–1600 °C for 1 h in nitrogen atmosphere. Sintered samples were further isostatically hot pressed at 1300 °C and the properties were compared with the pressureless sintered samples. Phase formation was followed by XRD study, CNT retention was confirmed by Raman studies and the samples were further characterized for mechanical and microstructural properties.     

Pressureless sintering of single-phase tantalum carbide and niobium carbide

This work presents the results of studies on the preparation of single-phase polycrystalline tantalum carbide and niobium carbide. It has been found that it is possible to obtain polycrystals with high density in the pressureless sintering process at temperatures up to 2000 °C and therefore relatively low temperatures such as for the compounds with one of the highest melting points; TaC – 3985 °C and NbC – 3600 °C. Only carbon as a sintering additive was used. The main role of carbon is to reduce of oxide contamination. It has been shown that the determination of the amount of carbon required to reduce oxide contamination is only possible through the experimental method.