The effects of temperature and liquid crystalline polymer (LCP) on mechanical properties of a blend of polyphenylene sulphide and LCP

The mechanical properties of polyphenylene sulphide (PPS) and liquid crystalline polymer (LCP) blends were investigated over a range of temperatures. The effect of blend composition on the brittle-ductile transition temperature (B-D) was also studied by differential scanning calorimetry and scanning electron microscopy. Blends of various compositions (PPS/LCP; 90/10, 75/25, 50/50 and 25/75) were prepared and injection moulded. The bending test temperature was varied between –40 and 150 °C. The results showed a rapid load drop at the B-D transition region. The B-D transition temperature occurred in unannealed pure PPS, 90/10, 25/75 and 50/50 blends around 75 °C whilst in the annealed sample it was observed around 100 °C. In pure LCP and 25/75, no transition occurred. Partial miscibility of PPS and LCP was confirmed by SEM observations, bending modulus and thermal properties. The use of LCP, as a good reinforcing agent which can improve processability and modulus, is discussed.     

Preparation, thermal expansion, high pressure and high temperature behavior of Al2(WO4)3

The titled compound Al₂(WO₄)₃ was synthesized by a conventional solid state reaction and characterized by powder XRD. It crystallizes in an orthorhombic (Pbcn, No. 60) lattice, with unit cell parameters as 12.582(2), 9.051(1), 9.128(2) Å, and V = 1039.5(3) (Å)³. The compound was found to show negative thermal expansion (NTE) behavior in the temperature range of 25 to 850°C. The average linear NTE coefficient (₁), in this temperature range, was –1.5 × 10^–6 K^–1. The effect of pressure at ambient temperature, was studied by a Bridgman Anvil (BA) apparatus, to reveal that there is no irreversible phase transition up to 8 GPa. The effect of high pressure and high temperature on this compound was studied by a Toroid Anvil (TA) apparatus. This compound has a limited stability under high pressure and temperature, as it undergoes a decomposition to AlWO₄ and WO_3–x with a partial oxygen loss. As an off-shoot of this work, certain new modifications of WO_3–x under pressure and temperature were observed, viz., monoclinic, tetragonal and an orthorhombic modifications at 5 GPa/1400°C, 3 GPa/900°C and 1.8 GPa/1030°C, respectively. The detailed XRD studies of the products are presented here.     

Mechanical properties and fracture morphologies of poly(phenylene sulfide)/nylon66 blends—effect of nylon66 content and testing temperature

Poly(phenylene sulfide) (PPS) was melt blended with Nylon66 and the mechanical properties and corresponding fracture morphologies were investigated. The thermal distortion temperature (HDT) of PPS/Nylon 66 blend showed that the inherent thermal stability of pure PPS can be maintained up to 30 wt% Nylon66, but then it started to decrease linearly thereafter to that of pure Nylon66 based on the rule of mixtures relationship. Tensile tests of PPS/Nylon66 blends at testing temperatures of –30, 25, 75, and 150°C showed that the maximum stress decreased up to 30 wt% Nylon66, and started to increase thereafter. Strain at break showed little change at low nylon content regardless of testing temperature, however, a large strain at break increase could be observed at more than 30 wt% Nylon66 and at 150°C testing temperature. At the same testing temperatures, the impact strength of PPS/Nylon66 blends was investigated, and it was found that an impact strength increase at all testing temperatures could be observed at more than 30 wt% Nylon66.     

Oxidation behaviour of a pressureless sintered AlN-SiC composite

The present study aims to investigate the oxidation behaviour of an AlN-SiC composite, pressureless sintered with the addition of Y₂O₃. Two main aspects are considered: (1) the evaluation of the oxidation kinetics in the temperature range 1300–1450°C for short term tests (30 h) and (2) the degradation of the flexural strength after oxidation at temperatures from 1000 to 1400°C for 100 h, in relationship with the microstructure of the exposed surfaces.The material starts to oxidize notably at temperatures higher than 1300°C. The oxidation kinetics is parabolic in the temperature range 1350–1450°C, the oxidation products are dependent on temperature and exposure time and are mainly constituted by crystalline mullite and alumina.The surface modification induced by long term oxidation does not affect mechanical strength until 1200°C, while after oxidation at 1400°C, the residual strength is about 25% of the starting one. These results are discussed in terms of the microstructure modifications induced by oxidation.     

Sintering and microstructure development in the system MgO–TiO2

The sintering and microstructure development of magnesia containing 0–10 wt% TiO2 at temperatures in the range 1300–1600°C have been investigated. The addition of TiO2 markedly promoted densification at relatively low temperature, and grain growth. Excess TiO2 over the solid solubility limit of TiO2 (0.3 wt%) reacted with magnesia to form inter- and intra-granular magnesium titanate (Mg2TiO4) above 1300°C. The grain size of MgO increased with increasing TiO2 content, and densification was mainly governed by MgO grain growth. © 1998 Kluwer Academic Publishers     

Polyphenylenesulphide-sealed Ni–Al coatings for protecting steel from corrosion and oxidation in geothermal environments

Polyphenylenesulphide (PPS) polymer was applied as a sealing material to flame-sprayed nickel–aluminium (Ni–Al) coatings to protect the interior surfaces at the ends where the mild carbon steel (MCS) heat-exchanger tubes are jointed to the tubesheet. The aim of applying PPS is to prevent their corrosion, oxidation and abrasive wear, in a low pH, hypersaline brine geothermal environment at 200°C under a hydrothermal pressure of 1.6 MPa. Although the Ni–Al coatings had an excellent thermal conductivity and a good wear-resistance, the inherent open structure of these coatings allowed the hot brine to permeate them easily under such pressure, causing the development of corrosion-induced stress cracks in the MCS. Furthermore, under these conditions, the coatings underwent oxidation with the formulation of Al₂O₃ as the major scale compound and NiO as the minor one. PPS sealant was used to solve these problems. However, one major drawback of PPS was its susceptibility to oxidation reaction with hot brine. This reaction not only incorporated more oxygen into the PPS, generating a sulphide sulphone transformation within PPS, but also it caused the decomposition of PPS, yielding polychloroaryl compound, and sodium sulphate, and also evolving SO₂ gases. The SO₂ gases had a chemical affinity for oxide scales in Ni–Al, forming water-soluble Al₂(SO₄)₃ and NiSO₄ salt reaction products at the PPS/Ni–Al interfaces. Despite the occurrence of such oxidation damage in PPS, an exposure for 14 days showed that there was no development of corrosion-caused cracks at the interfaces between the underlying steel and Ni–Al, nor was a striking oxidation of the sealed coating panels.     

Thermal expansion of the cubic (3C) polytype of SiC

Thermal expansion of the cubic beta or (3C) polytype of SiC was measured from 20 to 1000° C by the X-ray diffraction technique. Over that temperature range, the coefficient of thermal expansion can be expressed as the second order polynominal: ₁₁=3.19×10^–6+ 3.60×10^–9 T–1.68×10^–12 T ² (1/° C). It increases continuously from about 3.2×10^–6/° C at room temperature to 5.1×10^–6/° C at 1000° C, with an average value of 4.45 × 10^–6/° C between room temperature and 1000° C. This trend is compared with other published results and is discussed in terms of structural contributions to the thermal expansion.     

Effect of low temperatures on static and dynamic crack resistance of materials used in lifting-and-transportation devices

The results of investigations of crack resistance under dynamic and static loads are presented for structural materials used in lifting-and-transportation devices (LTD) in an operating temperature range of 203°K to 293°K. A certain correlation was obtained in this temperature interval between the results of dynamic and of static testing of materials used in metal structures of LTD. Practical recommendations are made for utilizing the results of impact tests in predicting the usefulness of the materials and estimating their resistance to brittle fracture.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 26, No. 6, pp. 80–84, November–December, 1990.     

Dielectric spectra of fresh cement paste below freezing point using an insulated electrode

The spectra of complex dielectric constant were measured on a fresh cement paste with a water/cement ratio of 0.4 sandwiched between insulated electrodes in the frequency range 10 kHz–1 MHz and temperature range between 0 °C and — 30 °C. The bulk dielectric constant, 30–20, and conductivity, 6.14×10^–5–0.65×10^–5, in the temperature range –10 to –28 °C were much lower than those at room temperature, owing to the great decrease of ionic mobility caused by freezing the cement paste. The activation energy of 0.31 eV for the ionic conduction in fresh cement paste was obtained from an Arrhenius plot of conductivity at subzero temperature.     

High-temperature interaction of uranium and zirconium with water vapor

We analyze physicochemical processes of interaction between uranium, zirconium, and its alloy (with 1% Nb) with water vapor at temperatures up to 1500°K and study mechanisms and kinetics of these processes. It is shown that, up to 700°K, the kinetics of the process of oxidation of uranium is described by a linear function of time; at temperatures higher than 900°K, this dependence becomes parabolic. Our interpretation of the mechanism of the process of oxidation of uranium takes into account the influence of structural defects and electrochemical properties of uranium oxides formed in the course of this process. The process of oxidation of zirconium and its alloy with 1% Nb by water vapor obeys a cubic law in the temperature range 900–1200°K and a parabolic law in the range 1300–1500°K. The comparative analysis demonstrates that the hydrogen release rate in the process of uranium oxidation is about twice as high as in the process of vapor-zirconium reaction.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 3, pp. 36–43, May–June 1995.     

Nickel nanoparticles inserted in tBuONa matrix deposited on alumina Part II Thermal treatment and nickel content effects on their stabilty and catalytitic activity

Nanoparticles (1 nm–3 nm) of metallic nickel supported on alumina (4.3% Ni–17.9% Ni w/w) were prepared from a colloidal precursor inserted in an organic matrix. Their structural and stability properties have been studied by X-Ray Diffraction (XRD), Electron Paramagnetic Resonance (EPR) and Thermal Gravimetric Analysis (TGA). Benzene hydrogenation at atmospheric pressure in the temperature range of 75 °C–200 °C was used as a test reaction of their catalytic capability. The thermal stability of the particles depended on the nature of the reactive atmosphere. Thus, a growth in size (up to around 20 nm) is observed under H₂ flow at 350 °C or during benzene hydrogenation but not under air flow at 300 °C. The growth may be due to the coalescence of the metal particles during the heating and decomposition of the stabilizing organic matrix. Under oxidative atmosphere, stable nickel oxide particles, firmly attached to the support, are formed. The catalysts pre-treated under H₂/350 °C were active and stable in benzene hydrogenation. The observed activities depended on the reaction conditions and nickel composition.     

Thermal expansion of isotropic Duralcan metal–matrix composites

The thermal expansion behaviour of Duralcan composites having a matrix of hypoeutectic Al–Si alloy containing SiC reinforcements ranging from 10–40 vol% was investigated. The coefficient of thermal expansion (CTE) of the MMCs was measured between 25 and 350 °C by a high-precision thermomechanical analyser, and compared to the predictions of three theoretical models. At low temperature, the experimental CTEs show substantial deviation from the predictions of the elastic analysis derived by Schapery, while the Kerner model agrees relatively well at high temperature. The overall measured CTE, in the range of 25–350 °C, as a function of the volume fraction of SiC is well predicted using Schapery's lower bound. We interpret these features as being an effect of reinforcement phase geometry and the modified microstructure derived from the Duralcan process and subsequent heat treatments. © 1998 Kluwer Academic Publishers     

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.     

Evaluation of the structural behaviour of annealed nylon 6 fibres from density measurements

Nylon 6 fibres were annealed in the temperatures range 80–185 °C for times from 1–10 h, and the density of the annealed fibres was measured by a system based on the theory of vibrating strings. The fibre diameter was also determined, using the laser forward diffraction technique. Differential thermal analysis (DTA) measurements were used to determine the glass transition temperature and the melting point of nylon 6. Some annealed samples were subjected to X-ray diffraction to clarify the variation of crystallinity with the annealing conditions. The mechanism of structural variations for nylon 6 fibres due to the annealing process is discussed, and structural details for crystalline and non-crystalline phases of a polymer are suggested. The behaviour of the number of oriented chains, number of crystallized nuclei, relative amount of recrystallized material and the shrinkage ratio with annealing time is proposed to explain the thermal structural variations.     

Precipitation behavior of Ag–Pd–In dental alloys

Age-hardening characteristics and precipitation behavior of Ag–25%Pd–3%In–1%Zn–0.5%Ir alloy were investigated in detail by means of hardness testing, X-ray diffraction, electron microscopy and resistivity measurement. The solution treating could be accomplished at 980 °C and the aging in the temperature range from 950 to 850 °C occurred by continuous precipitation. The aging in the temperature range from 850 to 450 °C occurred first, forming GP-zones with a hardness increase and then in overaging stage by forming discontinuous precipitation, which consisted of lamellae of solute (Pd, In, Zn) depleted Ag-rich phase and (Pd,Ag)₃(In,Zn) intermetallic phase. The hardness increased very fast to its peak in 10 min during aging at temperatures between 450 and 550 °C.     

Mechanical properties of thermally cycled nylon bonded Nd-Fe-B permanent magnets

The effect of thermal cycling on the stress-strain behavior of polyamide (nylon) and polyphenylene-sulfide (PPS) based injection molded Nd-Fe-B magnets was investigated after test specimens were cycled between –40 and 150°C for 50, 500, or 5000 repetitions. It was found that PPS based magnets exhibit higher ultimate strengths, higher modulus and lower toughness than nylon based magnets. Furthermore, formulations containing platelet morphology particles exhibited higher strengths and modulus than those containing spherical morphology particles, with increases in particle volume fraction leading to a decrease in strength. Differences in strength, modulus, and toughness were attributed to the degree of bonding between the matrix and the magnet powder in the various formulations, the degree of crosslinking, along with the effects of powder morphology. Additionally, it was found that while the stiffness of these materials increased with thermal cycling, their toughness decreased significantly, by as much as 99%. The extent of these effects was found to be dependent on the polymer matrix, powder morphology, and volume fraction of powder in the magnet. Finally, it was found that the PPS composites showed less relative change due to thermal cycling than the Nylon composites.     

Solid-state deformation of polytetrafluoroethylene powder

Polytetrafluoroethylene (PTFE) powder of a high molecular weight (~ 10⁷) was drawn by solid-state extrusion in the temperature range 100–340°C, which covers the glass transition temperature (125°C) and the ambient melting point (334°C). Draw was attainable only above 100°C. The maximum achievable extrusion draw ratio (EDR_max) was almost constant, ~ 10, from 100–280°C, yet increased rapidly with further increasing temperature, reaching a maximum of 60 at 330–340°C. At yet higher temperatures, the drawability was lost due to melting. The structure and properties of drawn products were found to be complexely affected by extrusion temperature and EDR. For extrusion at 330–340°C, near the melting point, an effective and high draw was achieved. The crystalline chain orientation function, crystallite sizes, both along and perpendicular to the chain axis, differential scanning calorimetry heat of fusion, and flexural modulus increased with EDR and approached a maximum at EDR of 30–40, depending on the extrusion temperatures. Above a specific EDR, the efficiency of draw decreased due to the formation of flaws. The highly oriented PTFE consisted of microfibrils of a significantly large lateral dimension (~ 45 nm) compared to those (6–20 nm) generally found in oriented polymers. The modulus of a drawn PTFE was sensitive to the test temperatures, reflecting the reversible crystal/crystal transitions at ~ 19 and 30°C. The optimization of the extrusion conditions resulted in the maximum achieved flexural modulus at 24°C of 20 GPa at an EDR 40 for extrusion at 340°C.     

Superplastic properties of a TiAl based alloy with a duplex microstructure

The superplastic properties of a engineering TiAl based alloy with a duplex microstructure were investigated with respect to the effect of testing temperatures ranging from 950°C to 1075°C and strain rates ranging from 8 × 10^–5 s^–1 to 2 × 10^–3 s^–1. A maximum elongation of 467% was achieved at 1050°C and at a strain rate of 8 × 10^–5 s^–1. The apparent activation energy was calculated to be 345 kJ/mol. Also, the dependence of the strain rate sensitivity values on strain during superplastic deformation was examined through the jump strain rate tests, and microstructural analysis was performed after superplastic deformation. It is concluded that superplasticity of the alloy at relatively low temperature and relatively high strain rate results from dynamic recrystallization, and grain boundary sliding and associated accommodation mechanism is related to superplasticity at higher temperature and lower strain rate.     

Kinetics of direct nitridation of pelletized silicon grains in a fluidized bed: experiment, mechanism and modelling

A fluidized-bed nitridation of pelletized silicon grains having a wide size distribution was carried out in the temperature range 1200–1300°C under conditions free of external heat and mass transfer effects. N2(30%–90%)–H2(5%–50%)–Ar (balance) mixtures were used as the nitriding gas at atmospheric pressure. Both the yield of -Si3N4 and the final overall conversion of silicon are affected by temperature and nitrogen gas concentration in a nitriding atmosphere, but hydrogen gas has a minor effect on either of these. After accounting for some of the structural changes that occur during nitridation, a simple model was derived. The model has shown that the pseudo-asymptotic exponential conversion trend in the second nitridation stage could be explained by various reaction mechanisms, adjusted for properties of the size distribution of silicon grains and the experimentally observed spalling of the product scale from the silicon surface. In the investigated range of experimental conditions, nitridation could be considered as having an apparent activation energy of Eapp340 kJ mol-1. © 1998 Chapman & Hall     

Thermophysical properties of ceramic matrix composites produced by polymer pyrolysis

A unidirectional composite and a series of bidirectionally reinforced composites were fabricated using carbon fibre reinforcement in a silicon carbide matrix, which was produced by the pyrolysis of a polymer precursor. The thermal expansion over the temperature range 20–1000 °C has been measured and the thermal diffusivity measured over the temperature range 200–1200 °C. Thermal diffusivity data was converted to conductivity data using measured density and literature specific heat data. Metallographic examination has been carried out on the composites and the results are discussed in terms of the observed microstructural features.