Oxidation of TiC at low temperatures

The isothermal oxidation of TiC powders was carried out at low temperatures of 350–500 °C at oxygen pressures of 3.9, 7.9 and 16 kPa under a static total pressure of 39.5 kPa, achieved by mixing with argon, using an electro-microbalance. The oxidation kinetics are described by the one-dimensional diffusion equation. It was found that oxidation consists of four steps, I (fast step), II (slow step), III (fast step) and IV (slow step), at all the pressures. Two activation energies were obtained in steps II–IV: 125–150 kJ mol^–1 below about 420 °C and 42–71 kJ mol^–1 above that temperature. The low- and high-temperature oxidation mechanisms are discussed in connection with the formation of oxycarbide/titanium suboxides and the crystallization of anatase, followed by the generation of cracks in the grains.     

Oxidation behavior of hot-pressed MoSi2-TiC composite

The oxidation behavior of a hot-pressed MoSi₂-TiC composite was investigated in air over the temperature range of 500 °C–1400 °C. The composite exhibits parabolic oxidation kinetics between 500 °C–800 °C, where the activation energy was calculated to be 32 kJ/mol. However, above 800 °C, it shows two-step parabolic oxidation kinetics, where the first stage (step I) was proposed to be dominated by the oxidation of surface TiC particles. After all the surface TiC particles are oxidized, the oxidation of the composite turns into the second stage (step II) which was controlled by the oxidation of MoSi₂. The activation energy was determined to be 130 kJ/mol and 141 kJ/mol for step I and step II, respectively. A dense cristobalite scale with TiO₂ in lath shapes on the top was observed after oxidation at 1400 °C.     

Au–Pd catalyzed growth of carbon nanofibers mat

Carbon nanofibers (CNFs) mat was synthesized from Au–Pd catalyst at 700 °C for 60 min in 10 kPa of the gas mixture of C₂H₂, H₂ and Ar by thermal chemical vapor deposition (CVD). Fine Au–Pd catalyst as admixture on the growth substrate of SiO₂ was adopted for the growth of CNFs mat. Fine Pd particles without Au particles were detected at the top/inside of as-grown CNFs by TEM-EDS analysis. The Au component within Au–Pd particles with ca. 1.5 nm did not contribute to the CNF growth. On the other hand, the growth scale of individual CNF was ranged from 5 to 150 nm in diameter and less than 100 μm in length, by controlling H₂ or Ar concentration in carbon source gas of C₂H₂/Ar or C₂H₂/H₂/Ar. It seemed that Ar gas accelerated the growth of the CNFs mat and the length of each CNF was increased with increasing the supplying amount of H₂. In contrast, H₂ induced the lower growth density of CNFs mat and the decrease in the diameter of those. The CNFs mainly grow through adsorption and decomposition of acetylene on the surface of Au–Pd catalyst and precipitation of carbon on the surface of those.     

Effect of water vapor on the mechanical behaviors of hot isostatically pressed silicon nitride containing Y2O3

The effect of water vapor on the mechanical behavior of Si₃N₄ ceramics was studied. Strength measurements by flexural dynamic fatigue tests were made at temperatures from 1038°C to 1350°C and at actuator speeds of 8.4 × 10^–2 and 8.4 × 10^–5 mm/s (200 and 0.2 MPa/s). Step stress rupture tests were also performed at 1288°C and 1150°C. Water vapor had a beneficial effect on the flexural strength due to flaw healing, and/or blunting mechanisms. Dynamic fatigue results demonstrated that the beneficial effects of water vapor on the strength increases as temperature increases and/or loading rate decreases. Time-to-failure was always longer in wet air during step stress rupture testing. Creep crack growth by formation and coalescence of cavities ahead of the crack tip generated from the oxidation pits or subsurface pores were the primary mechanism for slow crack growth for NT 164 Si₃N₄.     

A thermoanalytical study of oxidation of TiC by simultaneous TGA-DTA-MS analysis

The oxidation of TiC powders was non-isothermally carried out by heating up to 900 °C at a rate of 5°C min^–1 and at an oxygen pressure of 2–60 kPa, using a simultaneous thermogravimetric analysis-differential thermal analysis-mass spectroscopy (TGA-DTA-MS) apparatus. It was found that oxidation was divided into four stages, I to IV. Stage I, covering the oxidation fraction, , 0–20%, was probably due to formation of oxycarbide (TiC_xO_1–x, 0 < x<0.5) with slight heat and CO₂ evolution. Oxidation in stages II and III at 20–60% was affected by oxygen pressure; the higher pressure giving rise to very sharp exothermic and CO₂ evolution peaks in stage II, which are correlated with the formation of anatase. Oxidation in stage III proceeded gradually with increasing rutile phase. The sample amount also exerted a similar effect on oxidation as the oxygen pressure.     

Deformability enhancement in ultra-fine grained, Ar-contained W compacts by TiC additions up to 1.1%

Nano-sized Ar bubbles give negative influence on the fracture resistance and occurrence of superplasticity in ultra-fine grained (UFG) W–TiC compacts. In order to enhance deformability in UFG, Ar-contained W–TiC compacts, effects of TiC addition on the high-temperature deformation behavior were examined. W–TiC compacts with TiC additions of 0, 0.25, 0.5, 0.8 and 1.1 wt% were fabricated by mechanical alloying in a purified Ar atmosphere and hot isostatic pressing. Tensile tests were conducted at 1673–1973 K (0.45–0.54 T_m, T_m: melting point of W) at initial strain rates from 5 × 10⁻⁵ to 5 × 10⁻³ s⁻¹. It is found that as TiC addition increases, the elongation to fracture significantly increases, e.g., from 3 to 7% for W–0 and 0.25TiC/Ar to above 160% for W–1.1TiC/Ar when tested at 1873 and 1973 K at 5 × 10⁻⁴ s⁻¹. The flow stress takes a peak at 0.25%TiC and decreases to a nearly constant level at 0.5–1.1%TiC. The ranges of the strain rate sensitivity of flow stress, m, and the activation energy for deformation, Q, with TiC additions are 0.17–0.30 and 310–600 kJ/mol, respectively. The observed effects of the TiC additions on the tensile properties are discussed.     

Closed-Space Thermolysis of Hydrous Crystalline Polyantimonic Acid

Unlike the air calcination of Sb₂O₅ · 2H₂O and Sb₂O₅ · 0.2H₂O, which gives only pyrochlore phases up to 840°C, the thermolysis of these hydrates in an autoclave at 400–500°C, under elevated pressures of gaseous products (oxygen and water vapor), yields Sb₆O₁₃ and anhydrous antimony pentoxide with a hexagonal structure (a = 1.342 nm, c= 0.671 nm), which is stable in air up to 710°C.     

High temperature oxidation of hot-pressed aluminium nitride by water vapour

Hot pressed AIN without additives was oxidized et 1100 to 1400°C in dry air, wet air and wet nitrogen gas atmospheres with 1.5 to 20 kPa of water vapour pressure. AIN was oxidized by both air and water vapour, and formed -Al₂O₃ film on the surface above 1150°C. The oxidation kinetics in air were parabolic end were promoted by water vapour. On the other hand, the oxidation kinetics in wet nitrogen were linear below 1250°C and parabolic above 1350°C. The oxidation rate in wet nitrogen was much greater than that in wet air. The rate of oxidation increased with increasing temperature until 1350°C, and then decreased. The parabolic rate constant decreased with increasing temperature and increased linearly with increasing water vapour pressure. The linear rate constant at 1150 to 1250° C increased with increasing the temperature with the apparent activation energy of 250 kJ mol^–1. The relation between the linear rate constant and water vapour pressure was of the Langmuir type.     

Reactivity of layered vanadium pentoxide hydrate with ultrafine metal oxide, TiO2 and ZrO2, particles in an aqueous system

The interactions between vanadium pentoxide hydrate (V₂O₅·nH₂O) sol and colloid solutions of ultra fine titanium dioxide TiO₂ and zirconium dioxide particles ZrO₂ were studied. When mixed with an intrinsic V₂O₅·nnH₂O sol, TiO₂ particles in the mixed sol are sandwiched by V₂O₅·nH₂O layer sheets to form intercalation compounds. An Interlayer distance of V₂O₅·nH₂O was increased by this treatment and the surface area was also increased from 7.9 m² g^–1 for the V₂O₅·nH₂O to ca. 50 m² g^–1. When the TiO₂ sol was contacted with K-type V₂O₅·nH₂O, microporous nature appeared in the sample and the surface area incrased up to ca. 100 m² g^–1. The porous structure was maintained up to 300°C, above which materials were separated into two phases, anhydrous V₂O₅ and anatase type TiO₂. Ultrafine ZrO₂ particles were intercalated stoichiometrically in both intrinsic and K-type V₂O₅·nH₂O giving ZrO₂-V₂O₅·nH₂O for all the mixing ratios from ZrO₂/V₂O₅ = 5 to 20. Physico-chemical properties were almost unvaried and the materials were nonporous. Their surface areas are around 50 m² g^–1 for the former and around 60 m² g^–1 for the latter. The layered structure was maintained up to 300°C above which the sample was crystallized into ZrV₂O₇. The reaction temperature is about 150°C lower than that the heated mixture of ZrO₂ and V₂O₅ powders. The electron microscope observations of the prepared materials showed that the number of the stacked layers was decreased from more than 10 sheets for the sample before intercalation to about 2–4 sheets by exfoliation. This indicates that V₂O₅·nH₂O is exfoliated by ion exchangeably reacting to ultrafine titanium oxide and zirconium oxide particles.     

Promoter action of sulphur on the stabilization of pitch spheres

Elemental sulphur was added into the starting pitch during the preparation of pitch-based spherical activated carbon in order to enhance the stabilization of pitch sphere. Pitch sphere (diameter 0.65–1.0 mm) without adding sulphur needs slow heating rate of 0.5 °C/min, high final temperature of 300 °C and long holding time of 20 h for the successful stabilization in air. While adding elemental sulphur with 2.5–10.0 wt % in total amount into starting pitch decreased the stabilization time significantly, pitch sphere containing 5.0 wt % of sulphur can be stabilized in air very easily at heating rate of 2.0 °C/min up to 270 °C without any holding time, and the successful stabilization time was only 3 h. Pitch molecules reacted with sulphur and some sulphur functional groups, such as C–SH, C–S–C, C=S, O=S=, O=S=O etc., were formed besides the oxygen functional groups under the stabilization condition. All of these sulphur functional groups acted as bridge bonds to make the pitch molecules polymerized so as to high up the softening point of pitch spheres, making the pitch spheres stabilized. Three kinds of sulfocompounds, i.e. H₂S, COS and CS₂ evolved in stabilization process.     

Chemical vapour growth of HfP whiskers and their properties

Whiskers and ribbon-like single crystals of -HfP (hexagonal) have been prepared from HfCl₄+PCl₃+H₂+Ar gas mixtures at 1100–1200 °C using a metal impurity-activated chemical vapour deposition process. The growth conditions, morphology and chemical properties were examined. The 3.5–6.5 mm (average 4 mm) long HfP whiskers were obtained at 1200 °C using Si+Pt or Si+Pd mixed impurities. The HfP whiskers were very stable against oxidation up to 3 h exposure at 1000 °C and for 80 min immersion in concentrated HCl solution at 50 °C.     

Oxidation behavior of TiC-containing Ti<Subscript>3</Subscript>AlC<Subscript>2</Subscript> based material at 500–900 °C in air

The isothermal oxidation behavior of Ti₃AlC₂ based material containing 5 vol% TiC inclusion in air had been investigated at 500–900 °C by means of TGA, XRD, Raman spectroscopy and SEM/EDS. It was demonstrated that, although Ti₃AlC₂ based material exhibited good oxidation resistance at temperatures above 700 °C, anomalous oxidation with higher oxidation rate occurred at lower temperatures of 500 and 600 °C. This interesting phenomenon was due to the formation of microcracks associated with the stress developed within the scales, mainly consisting of anatase, and the volume expansion as Ti₃AlC₂ based material was directly exposed to air at those temperatures. Its oxidation, at temperatures investigated with the exception of 600 °C, generally obeyed a parabolic rate law. The weight gain data for the remaining temperatures were analyzed with an instantaneous parabolic rate constant method by assuming a parabolic rate law. The variations of instantaneous parabolic rate constant with time reflected the complexity of the oxidation behavior of Ti₃AlC₂ based material. These variations were discussed from the viewpoint of the formation of microcracks at 500 °C, and preferred oxidation of TiC inclusion in the initial oxidation and its subsequent depletion at 800 and 900 °C on the basis of X-ray diffraction, Raman spectroscopy, SEM scale morphology observation and composition analysis using EDS. In addition, the deleterious effect of TiC inclusion on the oxidation resistance of Ti₃AlC₂ based material was also investigated and discussed with comparison to monolithic Ti₃AlC₂, which was helpful to understand the discrepancies in reports on the oxidation of Ti₃AlC₂.     

High-temperature oxidation of silicon nitride-based ceramics by water vapour

Hot-pressed Si₃N₄, sintered Si₃N₄ and three kinds of sialon with different compositions were oxidized in dry air and wet nitrogen gas atmospheres at 1100 to 1350° C and 1.5 to 20 kPa water vapour pressure. All samples were oxidized by both dry air and water vapour at high temperature, and formed oxide films consisting of SiO₂, Y₂Si₂O₇ and Y4A1209. The oxidation rate was in the order sialon > sintered Si₃N₄ > hot-pressed Si3N4. The oxidation rate of sialon increased with increasing Y₂O₃ content, and oxidation kinetics obeyed the usual parabolic law. The oxidation rates in dry air and wet nitrogen were almost the same: the rate in wet nitrogen was unaffected by water vapour pressure above 1.5 kPa. The activation energy was about 800 kJ mol^–1.     

Sintering properties of submicron TiC powders from carbon coated titania precursor

The sintering behavior of submicron titanium carbide (TiC) synthesized from carbon coated titania (TiO₂) precursor was investigated in TiC-Ni system. The densification was examined as functions of initial carbon content (30.95–34 wt.%) and Ni content (3–20 wt.%). The sintered density of TiC-Ni was markedly decreased with increased carbon content in the precursor. The amount of Ni had a relatively small influence on the densification of submicron TiC-Ni cermet compared with TiC (commercially available HCS)-Ni cermets. The results show that submicron TiC with only 3 wt.% Ni can be sintered to densities above 95% TD in flowing Ar+10H₂ at 1500°C and below. The improvements in densification result from the capillary force increase since it is inversely dependent on the particle size. With decreased Ni content, the Vickers hardness increased and the fracture toughness decreased, as expected. However, the sufficient densification cannot be achieved for commercial HCS TiC powder sintered with Ni (<10 wt.%) under the same conditions. Therefore, both the Vickers hardness and fracture toughness decreased as the Ni content decreased. This was due to the increase of porosity in the sintered samples containing commercial TiC powder.     

Synthesis of TiC Whiskers through Carbothermal Reduction of TiO2

TiC whiskers were produced through carbothermal reduction of TiO₂ in the presence of potassium (K₂CO₃) and nickel (NiCl₂). The effect of potassium, nickel, and heating rate on the formation of whiskers was studied. Potassium was found to be an essential constituent for whisker formation. Nickel acts as a catalyst for TiC whisker formation only in the presence of potassium. The yield of whiskers was maximum at 1000–1200°C. At higher temperatures, formation of particulates of TiC was the dominant process. An increase in K₂CO₃ concentration during fast heating and decrease in K₂CO₃ concentration during slow heating was found to be beneficial in increasing the formation of TiC whiskers. A vapor–liquid–solid growth mechanism of whisker formation was explained.     

9Cr-1Mo钢在含水蒸汽气氛中的氧化行为

研究了9Cr-1Mo(ASME T91)钢在(Ar 10%H_2O)气氛中600℃、650℃和700℃下的高温氧化行为.在恒温氧化10 h过程中,在600℃氧化遵从抛物线规律;而在650℃和700℃氧化则遵从分阶段抛物线规律,且后一阶段的速度常数高于前一阶段的;随着温度的升高,氧化速度明显增大;氧化激活能为157.2 kJ/mol.T91钢氧化时表面形成了多层结构的氧化膜,从外到内依次为Fe_2O_3、Fe_3O_4和(Fe,Cr)_3O_4;钢基体也发生了内氧化,内氧化物为FeO和Cr_2O_3.只有在700℃氧化时没有外层Fe_2O_3形成和内氧化发生.9Cr-1Mo钢氧化后冷却至室温后,氧化膜发生开裂或局部剥落.最外层Fe_2O_3与次外层Fe_3O_4间出现很大的裂隙,Fe_2O_3层容易剥落.同时探讨了9Cr-1Mo钢在含水蒸汽气氛中的氧化机制.     

Oxidation of silicon carbide in a wet atmosphere

The effects of water vapour on oxidation of pressureless-sintered silicon carbide containing alumina as a densifying aid were studied in a wet air flow with 10, 20, 30 and 40vol% H₂O at 1300° C for 100h. The oxidation kinetics were determined in a wet air flow with 20 vol % H₂O and in a dry air flow at 1300° C for times up to 360 h. The weight gain on oxidation showed an increasing tendency with increasing water vapour content. Water vapour in the atmosphere strongly influenced oxidation and accelerated the reaction. Oxidation in a wet atmosphere proceeded in a diffusion-controlled manner, in the same process as that for the dry atmosphere. No remarkable differences were noticed in the microstructure of the oxide layer and the surface roughness between the samples oxidized under the four wet conditions, but the surface roughness increased with increasing oxidation time. Water vapour evidently accelerated the devitrification of amorphous silica and promoted oxidation. Oxidation in a wet atmosphere (10 to 40 vol % H₂O for 100 h and 20 vol % H₂O up to 360 h) had a slight degrading effect on the flexural strength. The microstructure or surface roughness of the oxide layer formed during oxidation presumably had very little effect on the room-temperature strength.     

Synthesis of Fe-substituted Al-mordenites by hydrothermal method

Al-mordenite and Fe-mordenite were synthesized with the tetraethylammonium as atemplate by hydrothermal method at 150°C in the Na₂O–Al₂O₃–SiO₂–H₂O and Na₂O–Fe₂O₃–SiO₂–H₂O system, respectively. Synthesis of several Al-mordenites substituted with Fe, Al/Fe ratio = 75/25, 50/50, 25/75, were also attempted in the Na₂O–Al₂O₃–Fe₂O₃–SiO₂–H₂O system under the same conditions by the hydrothermal method. The continuous solid solution of Fe in Al-mordenite was successfully obtained in mordenites with various Al-Fe molar ratios. Al-mordenite crystal was tablet-like with approximately 20–30 m in diameter and 5–10 m in thickness. Fiber-like Fe-mordenite grew up to 20–30 m in length and 5 m in diameter. The morphology of Fe-substituted Al-mordenite was cubic-like with 5–10 m in size. The size of Fe-substituted Al-mordenite decreased with the increase of Fe-content.     

Oxidation behaviour and effects of oxidation on the strength of SiC-whisker reinforced alumina

The oxidation behaviour and effect of oxidation on the strength of a SiC-whisker-reinforced-alumina composite material (Al₂O₃-SiC_w) were investigated. The oxidation mechanism of the composite material was determined by thermogravimetric analysis (TGA) and compositional analysis. Changes in the fracture strength and surface morphology were also determined and related to the oxidation mechanism. Weight changes of samples exposed to flowing Ar with various levels of oxygen partial pressure, P _O2 at 1400 °C were monitored continuously with a microbalance. Changes in strength were measured after exposure to flowing Ar with different P _O2 at 1400 °C for various periods of time. The P _O2-range employed in this experiment was from 5×10^–7 to 1×10^–3 MPa. In contrast to the oxidation behaviour of monolithic SiC materials, weight gains were detected for the whole P _O2-range investigated. However, despite the weight gains in the low P _O2-region (P _O2 < 1 × 10^-5 MPa), significant reductions in strength were observed which were apparently due to the loss of SiC whiskers from the sample surface via the formation of volatile SiO. This SiO gas reacted with the Al₂O₃ matrix to form a non-protective aluminosilicate glass on the surface, resulting in a linear weight gain with time. In the high P _O2-region, typical parabolic weight gains were observed as a result of the formation of an aluminosilicate glass on the surface by a reaction between SiO₂, formed by the oxidation of SiC whiskers, and the matrix alumina. The observed increases in strength of the specimens with exposure are believed to be due to blunting of existing surface flaws with a product oxide.Now with the Department of Inorganic Materials Engineering, Seoul National University, Seoul, 151-742 Korea.     

Oxidation kinetics of SiC deposited from CH3SiCl3/H2 under CVI conditions

Oxidation tests were performed on SiC deposits prepared from CH₃SiCl₃/H₂ under chemical vapour infiltration conditions, at temperatures ranging from 900–1500 °C under a flow of pure oxygen at 100 kPa (passive oxidation regime). The kinetics of growth of the silica layer were established from thickness measurements performed by spectroreflectometry. They obey classical parabolic laws from which rate constants are calculated. Within 1000–1400 °C, the oxidation process is thermally activated with an apparent activation energy of 128 kJ mol^–1. Above 1400 °C and below 1000 °C, an increase in the activation energy is observed which is thought to be related to a change in the mechanism of the oxygen transport across the silica layer forT>1400 °C and tentatively to stress effects forT<1000 °C. The kinetics data are compared to those measured on silicon single crystals (used as a standard) and to other reported data on SiC.