Synthesis and some properties of Ti3SiC2 by hot pressing of Ti,Si and C powders Part 2 – Mechanical and other properties of Ti3SiC2

Abstract

Some properties of the remarkable Ti₃SiC₂ based ceramic synthesised by hot pressing of elemental Ti, Si, and C powders have been investigated. Its flexural strength by using three point bending tests and fracture toughness by using single edge notched beam tests were measured at room temperature to be in the range 310–427 MPa and about 7·MPa m^1/2, respectively. This material is a relative 'soft' ceramic with a low hardness of 4 GPa. Ti₃SiC₂ is similar to the soft metals and is a damage tolerant material that is able to contain the extent of microdamage. An oxidation test has been performed in the temperature range 1000–1400°C in air for 20 h. The oxidation resistance below 1100°C was good. Two oxidized layers were formed, the outer layer consisting of pure rutile-type TiO₂, and the inner layer a mixture of SiO₂ and TiO₂. The average coefficient of thermal expansion (CTE) of Ti₃SiC₂ was measured to be 9·29 × 10^?6 K^?1 in the temperature range 25–1400°C. The thermal shock resistance of Ti₃SiC₂ was evaluated by quenching the samples from 800°C, 1200°C, and 1400°C, respectively. The retained flexural strength drops dramatically at quenching temperature, but shows a slight increase after quenching from 1400°C compared with quenching from 800°C and 1200°C.     

Effect of synthesized BaTiO3 doping on the dielectric properties of ultra temperature-stable ceramics

The high performance X9R ceramics could be sintered at as low as 1,120?°C by doping 3?mol% synthesized BaTiO₃ (SB) additives into the BaTiO₃-based ceramics, with a dielectric constant greater than 2,200 at 25?°C and dielectric loss lower than 1.7?%. The effects of SB additives on the microstructure and dielectric properties of BaTiO₃-based ceramics were investigated. The dielectric constant of BaTiO₃-based ceramics doped with 3?mol% SB was increased due to the promotion of the densification of ceramics. With SB content up to 4.5?mol%, Ti⁴⁺’s polarization was depressed, which resulted in the decrease of augmented dielectric constant at 25?°C. The partial solid solution was formed between Pb(Ti, Sn)O₃ and BaTiO₃, and the substitutions of Pb at A-sites and Sn at B-sites were existed. The strengthen of Ti–O bonds and higher Curie point of Pb(Ti_0.55Sn_0.45)O₃ was helpful to increased the Curie point of the ceramics effectively. Doped with SB additives, the volume of ferroelectric core was increased, and the sharp peak intensity at Curie point was increased accordingly. Capacitance temperature characteristics was improved attributed to the mutual effects of SB and Pb(Ti_0.55Sn_0.45)O₃. The formation of core–shell structure was sensitive to the sintering temperature, so the dielectric properties of ceramics were highly depended on the sintering temperature.     

Electrophoretic deposition and photoluminescent properties of Eu-doped BaTiO3 thin film from a suspension of monodispersed nanocrystallites

Monodispersed Eu-doped BaTiO₃ nanocrystallites with a pseudo-cubic perovskite phase were successfully prepared using highly concentrated alkoxide solution under hydrothermal conditions at temperatures <100 °C. The obtained nanoparticles had a very narrow particle size distribution and the average value was about 10 nm. By dispersing a piece of BaTiO₃ bulk gel into mixed solvent of 2-methoxyethonal and acetylacetone, we obtained well-dispersed and stable suspensions of Eu-doped BaTiO₃ nanocrystallites with a high transmittance. Using these suspensions, nanostructured thin films with a low porosity and a smooth surface were synthesized on Pt/Ti/SiO₂/Si substrate by electrophoretic deposition. The obtained Eu-doped BaTiO₃ thin films showed strong, visible room temperature photoluminescence (PL) associated with Eu ions. The PL spectra had typical ones for Eu³⁺ band corresponding to ⁵D_o–⁷F_j electronic transitions with a maximum intensity at 613 nm.     

Electrophoretic deposition and photoluminescent properties of Eu-doped BaTiO3 thin film from a suspension of monodispersed nanocrystallites

Monodispersed Eu-doped BaTiO₃ nanocrystallites with a pseudo-cubic perovskite phase were successfully prepared using highly concentrated alkoxide solution under hydrothermal conditions at temperatures <100 °C. The obtained nanoparticles had a very narrow particle size distribution and the average value was about 10 nm. By dispersing a piece of BaTiO₃ bulk gel into mixed solvent of 2-methoxyethonal and acetylacetone, we obtained well-dispersed and stable suspensions of Eu-doped BaTiO₃ nanocrystallites with a high transmittance. Using these suspensions, nanostructured thin films with a low porosity and a smooth surface were synthesized on Pt/Ti/SiO₂/Si substrate by electrophoretic deposition. The obtained Eu-doped BaTiO₃ thin films showed strong, visible room temperature photoluminescence (PL) associated with Eu ions. The PL spectra had typical ones for Eu³⁺ band corresponding to ⁵D_o–⁷F_j electronic transitions with a maximum intensity at 613 nm.     

Serrated yielding in Nb–V dual phase steel

Abstract

The characteristics of serrated yielding (the Portevin–Le Chatelier effect) in a Nb–V dual phase steel have been studied in the temperature range 85–210°C at strain rates between 1·2 × 10^?5 and 1·2 × 10^?2 s^?1. Serrated yielding was found to initiate only after a critical strain ?_c was reached. The strain between two successive serrations ??_s increases almost linearly with strain, while the stress drop ?σ_c increases with strain up to ?σ_max, then decreases. The exponent β in the mobile dislocation density–plastic strain relationship (ρ_m= ?^β) is 1·09 in the temperature range 85–140°C and 1·34 in the temperature range 140–210°C. The results also indicate that in the same temperature ranges there are two values of activation energy for type A serrations, i.e. 79 and 119 kJ mol^?1 respectively. The results are discussed in terms of substitutional–interstitial solute atom interaction and changes of concentration of interstitial atoms.

MST/934     

Fatigue crack propagation in Ti3 Al based alloys

Abstract

Effects of microstructure, stress ratio, and environment on the fatigue crack growth resistance of Ti–23Al–9Nb–2Mo–1Zr–1·2Si and Ti–23Al–11Nb–0·9Si (at.-%) Ti₃ Al based alloys have been studied at room and elevated temperatures. Only modest effects of microstructure on fatigue crack growth resistance have been obtained at room temperature, and these tend to reduce further at the elevated temperatures of 600 and 700°C both in air and in vacuum. At room temperature the fatigue crack growth resistance of Ti₃ Al based alloys is controlled primarily by the thickness of the retained βphase rather than by its volume fraction and the microstructure with a larger average thickness of retained β laths shows improved fatigue crack growth resistance. However, in some microstructures, the spatial distribution of the β phase can also be deduced to be important. A marked difference on crack growth resistance is obtained for stress ratios of 0·1 and 0·5 both at room temperature and at a temperature of 600°C. The mechanisms of fatigue crack growth in air and vacuum are discussed.     

Effect of the Additive of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> on the Structure Formation and Properties of Composite Materials Based on AlN–SiC

The results of studies on the strength at bending and volumetric electrical resistance of composite materials based on AlN–SiC with additions from 2 to 6 wt % Y₂O₃. It is shown that at increasing the content of Y2O3 in the mixture from 2 to 6 wt % the compaction of the composites intensifies their electrical resistance from (1.4–5.4) × 10⁶ to (1.8–5.94) × 10⁷ Ohm·cm (at 20°C), which at the increasing temperature decreases exponentially and at 800°C for all composites is (5–6) × 10⁴ Ohm·cm. It was determined that materials with the smaller content of Y₂O₃ have somewhat higher value of the ultimate strength during bending, namely, 110 MPa.     

Influence of B and Ti on hot ductility of high Al and high Al,Nb containing TWIP steels

Abstract

The addition of ～0·002%B and ～0·04%Ti as microalloying additions to improve the poor hot ductility and high risk of cracking on continuous casting of high Al containing twinning induced plasticity (TWIP) steels has been examined. Tensile specimens were either cast in situ or heated to 1250°C before cooling at 60 K min^?1 to test temperatures in the range 700–1100°C and strained to failure at 3×10^?3 s^?1. For tensile specimens reheated to 1250°C, the presence of B with sufficient Ti to combine with all the N improved ductility over the temperature range of 700–950°C, the reduction in area (RA) values being >40%. For the higher strength more complex high Al, TWIP steels having Nb present, there was no improvement in ductility with a similar B and Ti addition, when the average cooling rate after melting to the test temperature was 60 K min^?1. Reducing the cooling rate to 12 K min^?1 resulted in the RA values being close to the minimum required to avoid transverse cracking throughout the temperature range 800–1000°C. Using these additions of B and Ti, transverse cracking was found not to be a problem when continuously casting these high Al containing TWIP steels.     

Effect of Pb(Ti,Sn)O<Subscript>3</Subscript> on the dielectric properties of high dielectric constant X8R BaTiO<Subscript>3</Subscript>-based ceramics

The high dielectric constant X8R dielectric materials could be sintered at 1,240 °C by doping 2.5 mol% Pb(Ti,Sn)O₃ additives into the BaTiO₃ ceramics, with a dielectric constant greater than 3,400 at 25 °C, dielectric loss lower than 2.0% and temperature coefficient of capacitance (TCC) less than ±15% from −55 to 150 °C, which satisfied X8R specification. The effects of Pb(Ti,Sn)O₃ on the microstructure and dielectric properties of BaTiO₃-based ceramics were investigated. Doped with Pb(Ti,Sn)O₃ additives, the partial solid solution was formed between Pb(Ti,Sn)O₃ and BaTiO₃. Due to the high Curie point of Pb(Ti,Sn)O₃, the Curie point of the ceramics was markedly shifted to higher temperature about 150 °C, and the temperature coefficient of capacitance curves was flattened. The increase of the tetragonality (c/a ratio) and the fine microstructure were resulted in the increase of dielectric constant. With Pb(Ti, Sn)O₃ content up to 3 mol%, the depression of Ti⁴⁺’s polarization and the decrease of the tetragonality (c/a ratio) were resulted in the decrease of dielectric constant.     

Hydrothermal preparation of Ba(Ti,Zr)O3 fine powders

Fine powders consisting of aggregated submicron crystallites of Ba(Ti,Zr)O₃ in the complete range of Ti/Zr ratios are prepared at 85–130°C by hydrothermal method, starting from TiO₂ + ZrO₂ · xH₂O mixed gel and Ba(OH)₂ solution. The products obtained below 110°C incorporate considerable amounts of H₂O and OH⁻ within the lattice. As-prepared BaTiO₃ is cubic and converts to tetragonal phase after the heat treatment at 1200°C, accompanied by the loss of residual hydroxyl ions. TEM investgations of the growth features show a transformation of the gel to the crystallite. Ba²⁺ ions entering the gel produce chemical changes within the gel, followed by dehydration, resulting in a cubic perovskite phase irrespective of Ti/Zr. The sintering properties of these powders to fine-grained, high density ceramics and their dielectric properties are presented.     

Conduction mechanism and dielectric properties of BiFeO3–BaTiO3 solid solutions

The first measurements are reported for the frequency-dependent conductivity of (1?x)BiFeO₃–xBaTiO₃ (x = 0.10, 0.15 and 0.30) solid solutions in the frequency range of 10⁰–10⁶ Hz and in the temperature range of 50–300 °C. Powder X-ray diffraction confirms the formation of solid solutions. The dielectric properties were seen to improve with increasing BaTiO₃ (BT) content. The conductivity (AC and DC) measurements reveal an inverse variation of the frequency exponent ‘s’ with temperature, high density of states and thermally activated process. The calculated density of states was found to be N(E_f) = 80.2 × 10³² eV^?1 cm^?1 at 1 kHz and 50 °C for BiFeO₃–10 % BaTiO₃ (BFO–10 % BT) solid solution. The impedance spectroscopy analysis confirms the presence of grain and grain boundary affecting the conductivity. Our results provide the first unambiguous evidence of conduction in crystallite BFO–BT solid solutions through correlated-barrier-hopping model.     

Preparation and dielectric properties of BaTiO3-based X8R ceramics co-doped with BIT and CBS glass

The effects of Bi₄Ti₃O₁₂ (BIT) on phase purity and dielectric properties of BaTiO₃ (BT) ceramics have been investigated. Results show that BT samples doped with 1–3 mol% BIT adopt a single phase. However, secondary phase Bi₂Ti₂O₇ is observed when BIT content exceeds 3 mol%. Tetragonality and the Curie temperature (T _C) firstly increase and then decreases with an increase in BIT content. The 3 mol% BIT-doped BT ceramic sintered at 1,250 °C exhibits good dielectric properties of ε_r = 2,692, tan δ = 0.0152, ρ_v = 5.8 × 10¹² Ω cm, and the variation of dielectric constant as compared with that at room temperature is about ?20 % at ?55 °C and less than 11 % at 150 °C. It is found that the addition of calcium borosilicate glass (CBS) in BT-BIT ceramics can effectively lower the sintering temperature from 1,250 to 1,050 °C and further enhance the capacitance temperature stability. The permittivity decreases with an increase in CBS content from 1 to 10 wt%. Secondary phase BaBi₄Ti₄O₁₅ exists in the CBS doped BT-3BIT systems. All of CBS doped samples satisfy the X8R specification. Typically, the sample with 3 wt% CBS has ε_r = 1,789, tan δ = 0.0115, ρ_v = 9.67 × 10¹² Ω cm. The variation of permittivity as compared with that at room temperature is about ?12 % at ?55 °C and less than ± 11 % at 150 °C. The as-prepared materials have great potential as EIA X8R-type multilayer ceramic capacitors.     

Influence of titanium and nitrogen on hot ductility of C–Mn–Nb–Al steels

Abstract

The influence of small additions of titanium on the hot ductility of C–Mn–Nb–Al steels has been examined. Titanium and nitrogen levels varied in the ranges 0·014–0·045 and 0·004–0·011 wt-%, respectively, so that a wide range of Ti/N ratios could be studied. The tensile specimens were cast and cooled at average cooling rates of 25, 100, and 200 K min^-1 to test temperatures in the range 1100–800°C and strained to failure at a strain rate of 2 × 10^-3 s^-1. It was found that ductility in the titanium containing niobium steels improved with a decrease in the cooling rate, an increase in the size of the titanium containing precipitates, and a decrease in the volume fraction of precipitates. Coarser particles could be obtained by increasing the Ti/N ratio above the stoichiometric ratio for TiN and by testing at higher temperatures. However, ductility was generally poor for these titanium containing steels and it was equally poor when niobium was either present or absent. For steels with ～0·005 wt-%N ductility was very poor at the stoichiometric Ti/N ratio of 3·4 : 1. Ductility was better at the higher Ti/N ratios but only two of the titanium containing niobium steels gave better ductility than the titanium free niobium containing steels and then only at temperatures below about 950–900°C. One of these steels had the lowest titanium addition (0·014 wt-%), thus limiting the volume fraction of fine Ti containing particles and the other had the highest Ti/N ratio of 8 : 1. However, even for these two steels ductility was worse than for the titanium free steels in the higher temperature range. The commercial implications of these results are discussed.     

Preparation and electrochemical properties of SrCe 0·4 Zr 0·4 Yb 0·2 O 2·9 electrolyte

The perovskite Yb-doped strontium cerate–zirconate material, SrCe_0·4Zr_0·4Yb_0·2O_2·9, was prepared by solid-state reaction and the structure was characterized by X-ray diffraction. The calcination process of the powder was investigated by thermogravimetric/differential thermal analysis (TG–DTA). The high temperature conductivities were measured by d.c. four-probe technique in the temperature range from 500 to 950°C in wet hydrogen and effect of temperature on conductivity was investigated. The conductivity increased with the elevation of temperature from 500 to 950°C. The highest conductivity of 4·4 × 10^???2 S· cm^???1 was observed for SrCe_0·4Zr_0·4Yb_0·2O_2·9 at 950°C. The current–voltage (I–V) and current–power (I–P) characteristics of the single cell (H₂, Pt/SrCe_0·4Zr_0·4Yb_0·2O_2·9/Pt, O₂) at temperature range from 600 to 850°C were tested. With the temperature increasing from 600 to 850°C, the open circuit voltage (OCV) decreased from 1·164 to 1·073 V and the ionic transfer number decreased from 0·996 to 0·946. At 850°C, the maximum power density of 25·2 mW· cm^???2 was observed.     

Effect of spark plasma sintering temperature on the phase equilibria and dielectric properties of BaTi2O5 ceramics

The effect of sintering temperature (ranging from 1055 to 1200 °C) on the phase ingredient and dielectric property of the nominal BaTi₂O₅ ceramics (starting with the Ba/Ti of 1:2) fabricated by a spark plasma sintering method were systematically studied. At the first stage, BaTi₂O₅ component was enhanced in the sintering temperature range of 1055–1120 °C; it turned out to be the dominant phase. For these BaTi₂O₅ phase dominated ceramics, the Curie temperature T _c rised on increasing the sintering temperature and saturated around 440 °C with the maximum dielectric constant of 500. Further increasing the sintering temperature, the decomposition of the obtained BaTi₂O₅ into BaTiO₃ extensively happened; the ceramics turned to be the BaTi₂O₅ and BaTiO₃ coexisting state. These ceramics can be characterized by two dielectric anomalies. One at ~420 °C stood for the phase transition of BaTi₂O₅ while the other at ~150 °C stood for the transition of BaTiO₃, which is exceptionally high as the normal BaTiO₃ ceramics. Further increasing the sintering temperature (until 1200 °C) would dramatically enhance the BaTiO₃ phase; the ceramics showed T _c at 130 °C with the maximum dielectric constant of 1800.     

Nanostructured barium titanate thin films from nanoparticles obtained by an emulsion precipitation method

Spherical non-agglomerated BaTiO₃ precursor particles of 3–5 nm size were prepared by an emulsion precipitation method that consisted of the complexation of Ba- and Ti-precursors in separate water-in-decane emulsions, followed by mixing and controlled precipitation upon reactive decomposition of tetramethyl ammonium hydroxide. Stable clear yellow dispersions with a final particle yield of ∼60% were obtained. In situ high temperature X-ray diffraction (XRD) measurements indicated that the initially amorphous particles crystallised into single-phase tetragonal BaTiO₃ at 600 °C. Thin BaTiO₃ films of 25–350 nm thickness were prepared from the precursor dispersions by spin coating or controlled deposition on Si wafers. The average crystallite size after annealing at 800 °C was approximately 26 nm, but some larger grains of 100–150 nm size were also observed. Fresnoite (Ba₂TiSi₂O₈) was likely to be formed near the interface between the oxide layer of the Si substrate and the BaTiO₃ film.     

Direct observation of ternary solid state transformation in Bi–Cd–In alloy system

Abstract

Cast or solution treated specimens of a Bi–9·0Cd–26·7In (wt-%) alloy were observed to form a fine, three phase microstructure on aging at room temperature, replacing a single phase formed at a higher temperature. The three phases resulting from this solid state reaction were found to grow with a lamellar morphology into the high temperature phase, with a growth rate of 0·5–1·0 μm h^-1 at room temperature. The equilibrium temperature for the transformation was found to be ～25°C. Using a Hitachi S-4500 field emission SEM, the phase transformation was followed in progress at magnifications of 3000 and 10 000 times. It was noted that a volume change was associated with the transformation. It was concluded that the transformation is of the ternary eutectoid type.     

Effects of deposition temperature on phase purity, orientation, microstructure and property of cobalt substituted bismuth ferrite thin films

Multiferroic BiFe_0.95Co_0.05O₃ thin films were fabricated on Pt/Ti/SiO₂/Si substrates at various temperatures by pulsed laser deposition. It was found the deposition temperature had great effects on phase purity, orientation, microstructure and multiferroic properties of these films. The optimized deposition temperature was close to 600?°C. Polarization–electric field (P–E) and magnetization–magnetic field (M–H) hysteresis loops at room temperature were observed simultaneously in the films fabricated at 600?°C. The remnant polarization, coercive electric field (P _r , E _c ) and the remnant magnetization, coercive magnetic field (M _r , H _c ) of the films deposited at 600?°C were (0.95?μC/cm², 31?kV/cm) and (0.59?emu/cm³, 130 Oe), respectively. These results might have implications for further investigations on high quality BiFe_0.95Co_0.05O₃ multiferroic films.     

Stability and transport properties of microcrystalline Si1−xGex films

The crystallization evolution of boron and phosphorus doped amorphous Si_1−xGe_x films (5×10¹⁷–5×10²⁰ cm⁻³), deposited on SiO₂/Si(001) substrates by molecular beam in high vacuum at room temperature, were studied by XRD, TEM and SEM. The amorphous Si_1−xGe_x films were fully crystallized at ∼600°C. Up to 800°C no morphology changes were observed. Between 800 and 950°C, voids and hillocks were gradually developed in the films, which consequently collapsed. The Hall concentration and mobility were characterized in the Si_1−xGe_x films, annealed between 600 and 800°C. The mobility and conductivity of p-Si_0.5Ge_0.5 films at room temperature were found to be relative high: 60 cm²/V s and 2000 (Ω cm)⁻¹, respectively.     

Preparation of indium-tin oxide (ITO) aciculae by a novel concentration-precipitation and post-calcination method

Indium-tin oxide (ITO) aciculae were prepared by adding tin into indium hydroxide aciculae, which were synthesized by a concentration-precipitation method, and subsequent calcining. X-ray powder diffraction (XRD) indicated that indium hydroxide aciculae were partially crystallized and ITO aciculae were a well-crystallized solid solution, and both of them had a cubic structure. Using scanning electron microscope (SEM), it was found that the cross-sectional diameters of most of ITO aciculae were in the range of 2 to 9 μm, and the aspect ratios of about 95% of aciculae were more than 6. Energy dispersion spectrometer (EDS) and phenylfluorone spectrophotometry analysis were used to measure Sn content of ITO aciculae, and it was revealed that the Sn content of the surface layer was higher than that of the bulk. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) showed that the intensive dehydration of In(OH)₃ took place in the temperature rage of 260–280 °C and the formation of ITO solid solution started at temperature higher than 280 °C. According to the results of XRD, TGA–DTA and N content analysis, indium-containing nitrates or nitrites maybe existed in indium hydroxide aciculae. The specific resistance of the pellet formed by pressing ITO aciculae at a pressure of 10 MPa was measured by a four-probe method at room temperature, and it was as low as 1.2×10⁻² Ω cm.