A model SiC-based fibre with a low oxygen content prepared from a polycarbosilane precursor

A model SiC-fibre has been prepared from a polycarbosilane precursor by means of an irradiation oxygen-free curing process. The chemical composition remains unchanged after heat treatments under an inert atmosphere for pyrolysis temperatures of 1600°C. At this temperature, the fibre consists of SiC nanocrystals (mean size 6–10 nm) and free carbon. However, a slow grain growth takes place as the temperature is increased. The fibre retains a high strength at room and high temperatures up to temperatures of 1600 °C when the pyrolysis has been performed under nitrogen. The electrical conductivity was studied as a function of the pyrolysis temperature Tp: For 1100≤Tp≤1200 °C, the conductivity increases by several orders of magnitude due to the reorganization of the free carbon phase at the SiC grain boundaries. Oxidation kinetics of the filaments remain parabolic from 1000–1400 °C. This revised version was published online in November 2006 with corrections to the Cover Date.     

Correlation between microstructure and mechanical behaviour at high temperatures of a SiC fibre with a low oxygen content (Hi–Nicalon)

An oxygen free Si–C fibre has been studied in terms of the chemical, structural and mechanical properties produced as a function of annealing treatments. In spite of its high thermal stability with regard to a Si–C–O fibre the Si–C fibre was subject to moderate SiC grain growth, organization of the free carbon phase and densification within the temperature range 1200–1400°C. The strength reduction at ambient for temperatures ≤1600°C could possibly be due to SiC coarsening or superficial degradation. Bend stress relaxation (BSR) and tensile creep tests show that the as-received fibre undergoes a viscous flow from 1000°C. The thermal dependance of the creep strain rate strongly increases at temperatures ≥1300°C. This feature might be partly explained by the structural evolution of the fibre occurring above this temperature. Heat treated fibres (1400–1600°C) exhibit a much better creep strength, probably due to their improved structural organization. This revised version was published online in November 2006 with corrections to the Cover Date.     

Thermal stability of a PCS-derived SiC fibre with a low oxygen content (Hi-Nicalon)

The oxygen free Si–C fibre (Hi-Nicalon) consists of -SiC nanocrystals (5nm) and stacked carbon layers of 2–3nm in extension, in the form of carbon network along the fibre. This microstructure gives rise to a high density, tensile strength, stiffness and electrical conductivity. With respect to a Si–C–O fibre (Nicalon NL202), the Si–C fibres have a much greater thermal stability owing to the absence of the unstable SiOxCy phase. Despite its high chemical stability, it is nevertheless subject to a slight structural evolution at high temperatures of both SiC and free carbon phases, beginning at pyrolysis temperatures in the range 1200–1400°C and improving with increasing pyrolysis temperature and annealing time. A moderate superficial decomposition is also observed beyond 1400°C, in the form of a carbon enriched layer whose thickness increases as the pyrolysis temperature and annealing time are raised. The strength reduction at ambient for pyrolysis temperatures below 1600°C could be caused by SiC coarsening or superficial degradation. Si–C fibres have a good oxidation resistance up to 1400°C, due to the formation of a protective silica layer.     

Conversion mechanisms of a polycarbosilane precursor into an SiC-based ceramic material

The pyrolysis of a PCS precursor has been studied up to 1600 °C through the analysis of the gas phase and the characterization of the solid residue by thermogravimetric analysis, extended X-ray absorption fine structure, electron spectrocopy for chemical analysis, transmission electron microscopy, X-ray diffraction, Raman and Auger electron spectroscopy microanalyses, as well as electrical conductivity measurements. The pyrolysis mechanism involves three main steps: (1) an organometallic mineral transition (550 < T _p < 800 °C) leading to an amorphous hydrogenated solid built on tetrahedral SiC, Si0₂ and silicon oxycarbide entities, (2) a nucleation of SiC (1000 < T _p < 1200 °C) resulting in SiC nuclei (less than 3 nm in size) surrounded with aromatic carbon layers, and (3) a SiC grain-size coarsening (T _p > 1400 °C) consuming the residual amorphous phases and giving rise simultaneously to a probable evolution of SiO and CO. The formation of free carbon results in a sharp insulator-quasimetal transition with a percolation effect.     

Silicon carbide materials obtained from rice husk

Thermal processing of a rice husk sol in air at T≥750°C leads to the phase transformation of amorphous silicon dioxide (SiO₂) into crystobalite (type B samples). Thermal treatment of the same sol at T≈ 1400 °C in a closed graphite crucible leads to the formation of a mixture comprising hexagonal SiC phases and graphite (type A samples). Unannealed type A samples showed high refractory properties, being stable up to 1650°C. Using kaolin binder in silicon carbide articles leads to a decrease in the material refractoriness.     

Structure and photoluminescence of SiC/ZnO nanocomposites prepared by radio frequency alternate sputtering

SiC/ZnO nanocomposites were prepared by radio frequency alternate sputtering followed by annealing in N₂ ambient. Well-crystallized ZnO matrix was obtained after annealed at 750 °C according to X-ray diffractometer patterns. Transmission electron microscopy analyses indicated that the SiC thin layer aggregated to form SiC nanoclusters with the average size of 7.2 nm when the annealing temperature was 600 °C. When the annealing temperatures increased above 900 °C, some of the SiC nanoclusters changed into SiC nanocrystals and surfacial atoms of the SiC nanoparticles were surrounded by a layer of SiO _x (x ≤ 2) according to the Fourier transform infrared spectrums. The SiC/ZnO nanocomposites annealed at 750 °C exhibit strong photoluminescence bands ranging from 250 to 600 nm. UV light originates from the near band edge emission of ZnO and the blue emission peaked at around 465 nm (2.7 eV) may be due to the formation of emission centers caused by the defects in Si–O network, while the green-emission peak at around 550 nm (2.3 eV) may be attributed to the deep level recombination luminescence caused by the vacancies of oxygen and zinc.     

Microwave densification of electrophoretically infiltrated silicon carbide composite

A new method to fabricate SiC composites by microwave heating SiC preforms is described. Preforms were produced by electrophoretically infiltrating SiC fibre (Nicalon) preforms with SiC powder. Samples were heated to 1600°C in a matter of minutes and held at temperature for 5 min to minimize fibre degradation. To achieve densification, heated preforms required the application of a uniform load. Bulk densities increased from ∼ 0.8 gcm-3 for the as-infiltrated preforms to over 1.9 gcm-3 for microwave-heated samples with a small applied load of ∼13 kPa. Microstructural analysis revealed the presence of some pores and cracks in the matrix; however, large areas of dense SiC matrix material are apparent. This revised version was published online in November 2006 with corrections to the Cover Date.     

Continuous SiC-based model monofilaments with a low free carbon content: Part I: From the pyrolysis of a polycarbosilane precursor under an atmosphere of hydrogen

Pyrolysis of a Yajima-type polycarbosilane (PCS) has been performed under an atmosphere of hydrogen on both bulk samples and model monofilaments up to 1000°C, in order to reduce the free carbon content of the resulting ceramics. The organic/inorganic transition occurs within the 400–800°C temperature range, with mainly an evolution of CH₄. At 1000°C, it yields an hydrogenated amorphous ceramic with a C/Si atomic ratio and a free carbon content significantly lower than for its counterpart obtained under inert atmosphere (namely, 1.18 and 9 at% versus 1.72 and 27 at%). Hydrogen is thought to favour the release of the pendent methyl groups of the PCS via demethanation radical reactions. Continuous model filaments were produced via the melt spinning of the PCS, electron beam curing, pyrolysis under hydrogen up to 1000°C, and a final heat treatment under argon up to 1600°C. The ceramic fibres exhibit a C/Si atomic ratio of 1.10, a free carbon content of ≈8 at%, a Young's modulus of 260–300 GPa and a tensile failure stress of 2100 MPa. Their thermal stability is limited to 1400°C due to some oxygen contamination during the process. This revised version was published online in November 2006 with corrections to the Cover Date.     

Re−Ni2B2C superconducting borocarbides: “In situ” growth of thin films, transport and point contact spectroscopy

High quality c-axis oriented films of the intriguing intermetallic superconducting compound YNi₂B₂C have been obtained “in situ” by magnetron sputtering on MgO substrates held at about 800°C. The films showed maximum T_c=15.3 K, †T_c≈0.1 K, room temperature resistivity ρ≈50μΩ·cm, critical current J_c≈10⁵ A/cm² and B_c2≈6 T. Superconducting films were also obtained on Al₂O₃ and LaAlO₃ single-crystal substrates. From the ρ(T) dependence a value of the Debye temperature Θ _D =330±20 K can be deduced. At low temperatures the resistivity follows a quadratic power law possibly indicative of a high value of the electron-phonon interaction parameter λ. In order to clarify the role of λ in these compounds, point contact spectroscopy measurements have been performed on YNi₂B₂C and HoNi₂B₂C bulk samples prepared by inductive melting using a Low Temperature Scanning Tunneling Microscope (LTSTM). In the point contact regime clear evidence of a superconducting gap have been found in both compounds, corresponding to a moderate strong coupling behaviour (2†/KT_c≈3.8).     

Young’s modulus evolution at high temperature of SiC refractory castables

The evolution of Young’s modulus versus temperature has been evaluated in SiC-based hydraulically bonded refractories used in waste-to-energy (WTE) plants. Two types of low cement castables (LCC) with 60 and 85 wt% of SiC aggregates have been considered. The study was conducted by the way of a high temperature ultrasonic pulse-echo technique which allowed in situ measurement of Young’s modulus during heat treatment starting from the as-cured state up to 1400 °C in air or in neutral atmosphere (Ar) and during thermal cycles at intermediate temperatures (1000 and 1200 °C). For comparison in order to facilitate interpretation, thermal expansion has also been followed by dilatometry performed in the same conditions. Results are discussed in correlation with phase transformations occurring in the oxide matrix (dehydration at low temperature, crystallization of phases in the CaO–Al₂O₃–SiO₂ system) above 800 °C and damage occurring when cooling. The influence of oxidation of SiC aggregates on elastic properties is also discussed.     

Effect of preparation temperature on the characteristics of Eu-doped borate phosphor particles in the spray pyrolysis

Eu-doped gadolinium borate (GdBO₃:Eu) phosphor particles with fine size and uniform morphology were prepared by spray pyrolysis. The effects of the preparation temperatures on the characteristics of the GdBO₃:Eu phosphor particles prepared by spray pyrolysis were analyzed. The precursor particles obtained at preparation temperatures below 1400 °C had hollow and porous inner structures. On the other hand, the precursor particles obtained at preparation temperature of 1600 °C had dense structure and uniform morphologies. The precursor particles with glass phases were formed at high-preparation temperatures above 1400 °C. The precursor particles prepared at preparation temperature of 1400 °C turned into GdBO₃:Eu phosphor particles with fine size and regular morphology after post-treatment. The GdBO₃:Eu phosphor particles prepared from the precursor particles obtained at a preparation temperature of 1400 °C had the maximum photoluminescence intensity, which was similar to that of the commercial (Y,Gd)BO₃:Eu phosphor particles.     

Dielectric Ba(Ti1?xSnx)O3 (x?=?0.13) ceramics, sintered by spark plasma and conventional methods

A two-step sintering approach composed of spark-plasma-sintering (SPS) technique at 1000 °C for 1 min and under a uniaxial pressure of 63 MPa followed by conventional sintering at 1400 °C for 3 h is proposed for synthesis of dense Ba(Ti_0.87Sn_0.13)O₃ ceramics. Starting powders had grain size of about 90 nm and were obtained by co-precipitation. The SPS pellets consist of submicron (300–500 nm) grains. X-ray diffraction analysis of as-prepared Ba(Ti_0.87Sn_0.13)O₃ ceramic shows the occurrence of cubic and tetragonal phase coexistence for the pellets obtained after SPS processing and the presence of only tetragonal phase in the samples after the second (conventional) sintering. Grain uniformity in the final product is high, with average size of ~2 μm. The apparent densities of the sintered pellets at temperature of 1400 °C were ~92% of the theoretical value of Ba(Ti_0.87Sn_0.13)O₃. The ceramics exhibit a high relative dielectric constant of 6,550 and a dielectric loss (tan δ) = 0.078 at Curie temperature of 63 °C and 10 Hz.     

Thermal effects on fracture of biaxial-oriented poly(ethylene terephthalate) (BOPET) film

The effect of temperature on the fracture behaviour of biaxial-oriented poly(ethylene terephthalate) (BOPET) film was studied using the Essential Work of Fracture (EWF) approach. Fracture tests were performed over the temperature range +25 to +160 °C at the speed of 5 mm/min using double edge notched tension (DENT) specimens. The length of the specimens was either along the machine direction (MD) (0°), transverse direction (TD) (90°) or at 45° to either MD or TD. Ductile tearing of the ligament region was noted over the entire temperature range in all three directions. A linear relationship was found between the specific total work of fracture and the ligament length at all test temperatures. Values of the specific essential work of fracture (w _e) in the MD and TD were similar and smaller than in the 45° direction. Within temperature range 25–140 °C, w _e showed little variation if any with respect to temperature. As expected, the Specific Non-Essential Work of Fracture (βw _p) was temperature dependent. This parameter increased with increasing temperature and reached a maximum around the glass transition temperature of BOPET (T _g ≈ 80 °C). The values of the maxima are respectively 16.15, 20.38 and 17.8 MJm⁻³ for the 0°, 45° and 90°.     

Microstructural effects on the phase transitions and the thermal evolution of elastic and piezoelectric properties in highly dense,submicron-structured NaNbO<Subscript>3</Subscript> ceramics

The dielectric, piezoelectric and elastic coefficients, as well as the electromechanical coupling factors, of NaNbO₃ submicron-structured ceramics have been obtained by an automatic iterative method from impedance measurements at resonance. Poled thin discs were measured from room temperature up to the depoling one, close to 300 °C. Dielectric thermal behaviour was determined also for unpoled ceramics up to the highest phase transition temperature. Ceramics were processed by hot-pressing from mechanically activated precursors. Microstructural effects on the properties are discussed. The suppression of the classical maximum in dielectric permittivity in unpoled ceramics at the phase transition at 370 °C was found when a bimodal distribution of grain sizes, with a population of average grain size of 110 nm in between much coarser grains, is observed. The appearance of a phase transition at 150 °C took place when Na vacancies are minimised. The occurrence of a non-centrosymmetric, ferroelectric phase, in the unpoled ceramic from room temperature to ~300 °C, highly polarisable resulting in high ferro–piezoelectric properties was also observed in the ceramic which presents grain size below 160 nm. Maximum values of k _p = 14%, d ₃₁ = −8.7 × 10⁻¹² C N⁻¹ and N _p = 3772 Hz m at room temperature, and k _p = 18%, d ₃₁ = −25.4 × 10⁻¹² C N⁻¹ and N _p = 3722 Hz m at 295 °C were achieved in the best processing conditions of the ceramics.     

Impact of Silver Addition on Room Temperature Magneto-Resistance in La0.7Ba0.3MnO3 (LBMO): Ag x ?( x =0.0,?0.1, 0.2, 0.3, 0.4)

La_0.7Ba_0.3MnO₃ (LBMO): Ag _x (x=0.0, 0.1, 0.2, 0.3, and 0.4) composites are synthesized by a solid-state reaction route, the final sintering temperatures are varied from 1300 °C (LBMO1300Ag) to 1400 °C (LBMO1400Ag), and their physical properties are compared as a function of temperature and Ag content. All samples are crystallized in single phase accompanied by some distortion in main structural phase peaks at higher angles with an increase in silver content. Though the lattice parameters (a,c) decrease, the b increases slightly with an increase in Ag content. The scanning electron micrographs (SEM) showed better grains morphology in terms of size and diffusion of grain boundaries with an increase in Ag content. In both LBMO1300Ag and LBMO1400Ag series, the metal insulator transition (T ^MI) and accompanied paramagnetic-ferromagnetic transition (T _C) temperatures are decreased with an increase in Ag content. The sharpness of MI transition, defined by temperature coefficient of resistance (TCR), is improved for Ag added samples. At a particular content of Ag(0.3), the T ^MI and T _C are tuned to 300 K and maximum magneto-resistance at 7 Tesla applied field (MR^{7 T}) of up to 55% is achieved at this temperature, which is more than double to that as observed for pure samples of the both 1300 and 1400 °C series at same temperature. The MR^{7 T} is further increased to above 60% for LBMOAg(0.4) samples, but is at 270 K. The MR^{7 T} is measured at varying temperatures of 5, 100, 200, 300, and 400 K in varying fields from ±7 Tesla, which exhibits U and V type shapes. Summarily, the addition of Ag in LBMO improves significantly the morphology of the grains and results in better physical properties of the parent manganite system.     

(111)⇒(001) orientational phase transitions and nature of the insulating state in BiSrCaCuO (2212 and 2223) films

The influence of the growth temperature T _s on the structure, optical absorption, and electrical conductivity of BiSrCaCuO films has been studied. It was observed that nonmonotonic changes in the parameters of the films with T _s are caused by (111)⇒(001) phase transitions at T _s ≈550 °C for the 2212 phase and T _s ≈600 °C for the 2223 phase. These phase transitions stimulate the formation of metallic conductivity and are caused by a change in the system of preferential ordering planes of the atoms. Pis’ma Zh. Tekh. Fiz. 24, 13–20 (January 12, 1998)     

Synthesis of Mg-α-sialon from the mixture of silicon, aluminum and magnesia powders in a flowing nitrogen atmosphere

Synthesis of Mg-α-Sialon has been investigated by the mixture of silicon, aluminum and magnesia powders in a flowing nitrogen atmosphere in the range of 1300–1600 °C, when Mg-α-Sialon is designed with a chemical formulation of Mg _x Si_12−3x Al_3x O _x N_16−x in present work. The results showed that Mg-α-sialon initially occurred at 1400 °C and basically increased with elevated temperatures. For the samples of x = 0.6, 0.8 and 1.0 the products mainly consisted of Mg-α-Sialon with small amounts of Si, AlN and 21R AlN-polytypoid phases at 1600° C. However, in final products of x = 1.2, 1.4 and 1.6 only a little of Mg-α-Sialon formed and a great amount of Si remained in these samples at all the fired temperatures. Fortunately, the content of Mg-α-Sialon in these samples were obviously increased by adding a small amount of α-Si₃N₄ as seeds before nitridation.     

Spark-plasma-sintering temperature dependence of structural and piezoelectric properties of BNT–BT<Subscript>0.08</Subscript> nanostructured ceramics

(Bi_0.5Na_0.5)TiO₃ doped with 8 mol% BaTiO₃ powder prepared by sol–gel was compacted and sintered by spark-plasma-sintering method. The influence of spark-plasma-sintering temperature on the densification and piezoelectric properties of these ceramics was studied. Starting from BNT–BT_0.08 powder gel with a microstructure consisting of particles with average size of 50 nm, ceramics with grain size of 60–90 nm and density of about 98.9–99.6% of the theoretical density were obtained by spark-plasma-sintering at 800–900 °C. Increasing the sintering temperature by SPS from 800 to 900 °C lead to the increase of d ₃₃, k _p, ε₃₃^T and, decrease of Q _m. Typical d ₃₃ and k _p values of BNT–BT_0.08 ceramics sintered by spark-plasma-sintering at 900 °C were 8 and 0.029, respectively.     

Density and Volume Fraction of Supercritical CO<Subscript>2</Subscript> in Pores of Native and Oxidized Aerogels

The density and volume fraction of an adsorbed phase of carbon dioxide (CO₂) in aerogels was investigated using a formalism based on independent measurements of neutron transmission and small-angle neutron scattering from fluid-saturated absorbers (Rother et al. J. Phys. Chem. C 111, 15736 (2007)). The range of excess fluid pressures (0 <  P <  8 MPa) and temperatures (T = 35°C and 80°C) corresponded to the supercritical regime above the critical temperature T _C = 31.1°C and critical density ρ _C = 0.468 g · cm⁻³ of the bulk fluid. The results demonstrate that a porous aerogel matrix works to create an adsorbed phase with liquid-like fluid densities reaching ~1.1 g · cm⁻³ and ~0.8 g · cm⁻³ at T = 35°C and 80°C, respectively. Thus, despite the fact that the density and volume fraction of the adsorbed fluid both decrease with temperature, the dense adsorbed phase is still present in the aerogel at temperatures far exceeding the T _C. Heat treatment (“oxidation”) of the aerogel at 500°C for 2 h, which removes a significant fraction of the alkyl groups from the aerogel surface, has little effect on the adsorption properties. The observed reduction of the density and volume fraction of the adsorbed CO₂ with temperature and its minor dependence on the surface modification are consistent with predictions of the pore-filling model.     

Silicide–carbide composites obtained from alloyed melt infiltration

The development of a C_f/(Mo, Ti)Si₂–SiC composite using melt infiltration technique was investigated. C/C preforms and also C_f-felts were infiltrated with an alloyed melt of Si, Ti and MoSi₂. The amount of each element was selected so that the melting point of the alloy was lower than 1600 °C. It was then possible to prevent the melt from reacting heavily with the carbon fibers and preserve their reinforcing effect in case of the C/C preforms. After infiltration no residual silicon could be detected in the matrix of the infiltrated C/C composites. The infiltrated C/C samples reached a maximum bending strength of 210 MPa at room temperature. At 1600 °C there is even an increase in their bending strength to 250 MPa. Infiltrated felts showed monolithic and brittle characteristics. Their bending strength at room temperature was not higher than 150 MPa. Because of softening of the residual silicon, the strength of the infiltrated felts was reduced at high temperatures. The felt samples which were infiltrated with an alloyed melt showed higher mechanical strength than pure silicon infiltrated felts both at room temperature and at 1600 °C.