Showing papers in "Journal of Materials Science in 1988"

Negative Poisson's ratio polymeric and metallic foams

Abstract: Foam materials based on metal and several polymers were transformed so that their cellular architecture became re-entrant, i.e. with inwardly protruding cell ribs. Foams with re-entrant structures exhibited negative Poisson's ratios as well as greater resilience than conventional foams. Foams with negative Poisson's ratios were prepared using different techniques and materials and their mechanical behaviour and structure evaluated.

High-temperature thermal expansion of six metallic elements measured by dilatation method and X-ray diffraction

Abstract: The thermal expansion of silver, gold, copper, nickel, molybdenum and tungsten has been measured by the dilatation method. The thermal expansion values of silver, gold, copper and nickel have also been evaluated from lattice parameter measurements by X-ray diffraction. These six metals were found to exhibit a nearly uniform expansion over the temperature ranges covered. The thermal expansion values obtained by the dilatation method are in good agreement with those determined by X-ray diffraction. The present results appear also to agree well with those reported previously in the literature.

Creep of ceramics

Abstract: This review analyses a wide range of experimental data on the creep of ceramic materials and reveals many similarities with the creep of metals. It is demonstrated that there are two important differences in the creep behaviour of ceramics: (1) there is an enhanced role of diffusion creep, and (2) in the power-law regime, ceramics divide into two categories with stress exponents of ∼ 5 and ∼ 3, respectively. It is concluded that the behaviour with an exponent of ∼ 5 represents fully ductile behaviour as in f cc metals, whereas the behaviour with an exponent of ∼ 3 is due to dislocation climb from Bardeen-Herring sources under conditions where there is either a lack of five independent slip systems or, if five independent slip systems are available, a lack of interpenetration of these systems.

Dip-coating of TiO2 films using a sol derived from Ti(O-i-Pr)4-diethanolamine-H2O-i-PrOH system

Abstract: Diethanolamine (DEA) can suppress the precipitation of oxides from the alcoholic titanium isopropoxide solution in its hydrolysis so that much water can be added to the Ti(O-i-Pr)4-diethanolamine-i-propanol the solution to give a clear solution. Addition of excess water converted the solution to the gel. The conditions for the formation of the clear solution and gel are examined. Uniform transparent TiO2 films can be prepared by dip-coating with the clear solution. The limit of the thickness of uniform films was 200 to 240 nm. Films thicker than 1 μm can be prepared by repeating the coating cycle. These films were well densified. Diethanolamine has some positive effect on the densification of the TiO2 crystals.

Anisotropy of foams

Abstract: Cellular materials are often anisotropic, ie their properties depend on the direction in which they are measured In this paper, we model the mechanical behaviour of anisotropic foams to describe the ratio of foam properties in two orthogonal directions in terms of the mean intercept lengths of the cells Experimental measurements of the mean cell intercept lengths and of the mechanical properties of two polyurethane foams indicate that the model describes anisotropy well

Investigation of combustion of raw and acid-leached rice husk for production of pure amorphous white silica

Abstract: The effects of various acid treatments of rice husk on removal of its metallic ingredients and different combustion temperatures on production of amorphous silica (white ash) were studied. Leaching of husk in dilute HCI (1 N) was proved to be effective in substantially removing most of the metallic ingredients and producing ash completely white in colour. Irrespective of the treatments given, the minimum temperature required for complete combustion within a reasonable period was found to be 500° C. The combustion time varied from 5 h at 500°C to 1.5 h at 700° C. The ash residues obtained from complete combustion of acid-treated husk samples were completely white in colour. On the other hand, under similar conditions, the ash residues obtained from untreated husk remained light brown. The acid treatment of husk did not affect the amorphicity of the silica.

The effect of molecular weight on slow crack growth in linear polyethylene homopolymers

Abstract: The rate of initiation and growth of cracks in linear high-density polyethylene with different molecular weights was observed in single-edge-notched tensile specimens under plane strain condition as a function of applied stress, notch depth and temperature. The initial rates of crack initiation all have the form of Cσm a0nexp (−Q/RT) or AKpexp (−Q/RT) where σ = stress, a0 = notch depth and K= stress intensity factor. For the different molecular weights, m, n, P and Q are almost the same where m=5, n=2, P=4.7 and Q=115 kJ mol−1, but the constants C and A varied as (¯Mw−¯Mc)−1 where ¯Mc is a limiting molecular weight for sudden fracture. A molecular model based on tie-molecules has been used to explain the dependence on ¯Mw. The effect of ¯Mw on the fast-fracture strength at low temperature and the relationship to tie-molecules have also been investigated. Quantitative relationships between the concentration of tie-molecules and the fracture behaviour have been obtained.

Fibre-matrix bond strength studies of glass, ceramic, and metal matrix composites

Abstract: An indentation test technique for compressively loading the ends of individual fibers to produce debonding has been applied to metal, glass, and glass-ceramic matrix composites; bond strength values at debond initiation are calculated using a finite-element model. Results are correlated with composite longitudinal and interlaminar shear behavior for carbon and Nicalon fiber-reinforced glasses and glass-ceramics including the effects of matrix modifications, processing conditions, and high-temperature oxidation embrittlement. The data indicate that significant bonding to improve off-axis and shear properties can be tolerated before the longitudinal behavior becomes brittle. Residual stress and other mechanical bonding effects are important, but improved analyses and multiaxial interfacial failure criteria are needed to adequately interpret bond strength data in terms of composite performance.

The effect of surface constraints on the ordering of block copolymer domains

Abstract: The effect of surfaces on the ordering of block copolymer microdomains has been studied by TEM observation of thin-film droplets. Poly(styrene butadiene) block copolymers exhibiting spherical, cylindrical and lamellar microdomain morphologies were formed by solvent evaporation on a carbon substrate, annealed aboveT g, and stained with OsO4 prior to TEM observation. Samples with lamellar morphologies exhibited orientations of the lamellae in which the domains were either perpendicular to the substrate surface, or parallel to the substrate surface in which a layered structure resulted. Specimens in which cylinders were formed revealed sheets in which the cylinder axes lie parallel to the substrate, as well as parallel to each other. Stacking of such layers was observed, with exceptional registration between layers such that the cylinders in one layer were located over the inter-domain regions of the cylinders in adjacent layers. Specimens with spherical microdomain morphologies revealed a layer structure that consisted of hexagonally packed arrays of the spherical micelles. The stacking of layers of discrete thickness of all three types of microdomain morphology to produce the observed geometries indicates that the exterior of bulk samples may also exhibit such surface steps that reflect the ordering of the microdomains within the sample. Regardless of the microdomain type, all samples revealed a peripheral region in the thinnest area of the droplets in which the formation of the normal microdomain morphology (i.e. that of spheres, cylinders or lamellae) was inhibited. The uniform thickness of this region provides evidence that preferential segregation of the block component with the lowest surface energy takes place regardless of the sample composition.

Development of a new continuous Si−Ti−C−O fibre using an organometallic polymer precursor

Abstract: A polytitanocarbosilane, which is useful as the precursor polymer for ceramic fibre, was synthesized using polydimethylsilane, polyborodiphenylsiloxane and titanium tetraisopropoxide. The polytitanocarbosilane was melt-spun and using the continuous heat-treatment process from the polymer fibre to ceramic fibre, flexible Si-Ti-C-O fibre was produced. The density, tensile strength and Young's modulus of this amorphous ceramic fibre were found to be 2.35 g cm−3, 3.0±0.2 and 220±10 GPa, respectively. The Si-Ti-C-O fibre retained its high tensile strength to higher temperatures (about 1200° C). The specific resistance of this ceramic fibre covered a wide range of 107 to 10−1ωcm. This ceramic fibre is considered to be useful as reinforcement fibre for composites.

A study of the structural, electrical and optical properties of copper tellurium oxide glasses

Abstract: The results of an investigation of some properties of CuO-TeO2 glasses are reported. X-ray diffraction, differential scanning calorimetry and optical absorption measurements are discussed with respect to the compositions. D.c. conductivity is measured in the temperature range 293 to 453 K and discussed in terms of small polaron theory.

Diamond synthesis from methane-hydrogen-water mixed gas using a microwave plasma

Abstract: Fine particles of diamond or diamond-like carbon were synthesized from methane-hydrogen-water mixed gas using a microwave plasma. The growth rate was several times as fast as that of methane-hydrogen mixed gas, which is the conventional method to prepare diamond from the gas phase. Promotion of the methane decomposition reaction and the removal reaction of byproduct graphite by OH radicals produced in the plasma were credited with enhancing the rate.

Electrical properties and microstructure in the system ceria-alkaline earth oxide

Abstract: The ionic conduction of oxygen in the ceria-alkaline earth oxide system was investigated as a function of temperature, partial pressure of oxygen and oxide composition, together with its crystal structure, density, and microstructure. Undoped ceria and its solid solution with alkaline earth oxides have a cubic fluorite structure. The ionic conductivity of ceria is greatly enhanced by additions of calcia and strontia, even when they are added in excess of the solubility limit. The conductivities of ceria-calcia and ceria-strontia were much higher than those of calcia-and yttria-stabilized zirconia. Up to the limit of calcia and strontia, the ionic transference number was nearly unity in the temperature range between 600 and 900°C. With an increase in calcia and strontia content, the ionic conductivity was little affected by the presence of a second phase of CaO and SrCeO3.

Polymer optical fibres

Abstract: The background to the development of polymer optical fibres (POFs) and the methods used for their manufacture are described. The optical properties of polymers are then discussed in the context of fibres, taking the most common POF materials, polymethylmethacrylate and polystyrene, as examples. The poor market penetration of POFs, due to inadequate performance in terms of attenuation, bandwidth and temperature resistance, is then explained. The article concludes with a discussion of techniques which show great promise for the development of a new generation of high-performance POFs which will satisfy market demands.

Synthetic and biological composites formed byin situ precipitation

Abstract: Filler particles are frequently put into polymers to improve the stiffness or strength There are a number of other applications such as magnetic coatings and piezoelectric transducers where second phase particles are introduced to give the composite special properties Many biological materials, including bone, are also composite with particles reinforcing a polymer matrix In the preparation of the synthetic materials the filler is normally blended into the polymer In the biological materials the particles are grownin situ within the polymeric matrix, and under the control of the matrix Here we review the growth of particles when constrained by a polymeric matrix, both in natural and synthetic materials We discuss the principles governing such processes and the potential for the production of highly structured composite materials by this route

Sintering of hydroxylapatite-zirconia composite materials

Abstract: Sintering of hydroxylapatite-zirconia (doped with 3 mol% Y2O3) composite powder compacts was studied. Hydroxylapatite powder was prepared from Ca(OH)2 and H3PO4, and zirconia powder was prepared from ZrOCl2 · 8H2O and YCl3. The sinterability of hydroxylapatite-zirconia composite powder compacts depends strongly on differential shrinkage between the powder components of the composite. Smaller differential shrinkage results in better sinterability. By increasing the calcination temperature of zirconia powder and/or decreasing that of hydroxylapatite powder improves the sinterability of the composite powder compacts. The phase distribution and total amounts of crystal phases depend on the sintered density of compacts. Hydroxylapatite and cubic zirconia are the major phases of compacts with high sintered densities, whereas α- and β-tricalcium phosphate and CaZrO3 are the major phases of compacts with low sintered densities.

Anodic polarization and corrosion of cemented carbides with cobalt and nickel binders

Abstract: The polarization of pure cobalt, nickel and tungsten, and hardmetals based on WC/Co and WC/Ni, in a solution of 001 M H2SO4 + 099 M Na2SO4 (pH 255), has been investigated using potentiodynamic and metallographic techniques Pure cobalt dissolved rapidly, nickel had an excellent passivity, while tungsten had a high degree of passivity over the whole potential range up to 2O V (SCE) Extensive dissolution of WC was observed metallographically at potentials in the range 06 to 14V (SCE) The polarization of WC/Co hardmetals reflected mainly the behaviour of the binder phase modified by the presence of tungsten in the binder and passivating film The effects of the carbon in the binder are discussed in relation to the tungsten in the binder Tungsten carbide grain size, and the presence of TaC and TiC, had no noticeable effects Hardmetals with nickel binders had a superior performance compared with cobalt binders Chloride in the solution had a relatively small effect

Physical and mechanical properties of YBa2Cu3O7-δ superconductors

Abstract: The physical and mechanical properties of YBa2Cu3O7−° superconductors are examined. These properties are related to powder preparation method, powder characteristics, sintering behaviour and sintered microstructure. The sintering atmosphere and sintering schedules affect the final microstructure very strongly and determine, in conjunction with starting powder characteristics, the sintered density. The mechanical properties such as Young's modulus, bend strength and critical stress intensity factor (fracture toughness) are measured and related to microstructure as determined by electron microscopy. Control of microstructure by careful powder selection and sintering schedule is seen as key to optimizing the physical and mechanical properties of the material. Finally attention is drawn to fabrication techniques and how these must be optimized in order to realize the mechanical properties which are necessary if these are to be useful as engineering materials. Comparisons between fabrication techniques show that uniaxial powder pressing suffers from limitations in terms of specimen complexity and densification whereas the favoured route, termed viscous processing, gives a more homogeneous microstructure, higher strength and allows near theoretical density to be achieved.

Bibliography Resource structure properties of natural cellulosic fibres — an annotated bibliography

Abstract: We present a review of the work on the structure and properties of some natural lignocellulosic fibres with a classified list of references on resource, structure, physico-mechanical properties and on the uses of these fibres. This list of references includes papers published in scientific journals and in the proceedings of conferences.

Electrical conductivity in non-stoichiometric titanium dioxide at elevated temperatures

Abstract: The electrical conductivity of polycrystalline titanium dioxide prepared by a liquid mix technique was measured for the oxygen partial pressure range of 10° to 10−19 atm and temperature range of 850 to 1050° C. The data were found to be proportional to the −1/6 power of oxygen partial pressure for the oxygen pressure range 10−19 to 10−15 atm, and proportional to $$P_{{\text{O}}_2 }^{ - 1/4}$$ for the oxygen pressure range >10−15 atm. The region of linearity where the electrical conductivity varied as the −1/4 power of $$P_{{\text{O}}_2 }$$ increased as the temperature was decreased. There was evidence of p-type behaviour for $$P_{{\text{O}}_2 } > 10^{ - 2}$$ atm in the temperature range 950 to 850° C, although the measured data were insufficient to assign a pressure dependence. Electrical conductivity minima in the log σ against log $$P_{{\text{O}}_2 }$$ plot moved to lower $$P_{{\text{O}}_2 }$$ as the temperature was decreased in the range 950 to 850° C. The measured oxygen pressure dependence of electrical conductivity in the lowest $$P_{{\text{O}}_2 }$$ region supports the oxygen vacancy defect model. The observed data are consistent with the presence of very small amounts of acceptor impurities. A binding energy of ∼ 0.67 eV between the acceptor impurity and its compensating oxygen vacancy was also determined.

Structure of mesophase pitch-based carbon fibres

Abstract: The structure of five samples of commercially available carbon fibres with ultra-high modulus produced from mesophase pitch was studied by the complementary techniques of high resolution electronmicroscopy, X-ray diffraction and transverse magnetoresistance effect. The fibres with high strength and elongation to failure were found to be composed of turbostratic carbon structure, which was different from the three-dimensional graphite structure in ultra-high modulus carbon fibres. Transmission electron microscope examination revealed that the mesophase pitch-based fibres with high strength have a basic structure unit with folded sheets arranged nearly parallel to the fibre axis similar to those of high modulus carbon fibres produced from PAN. The present fold structure was suggested to contribute consequently to the lower graphitizability of the fibres and to the strong effects on the fibre strength. By controlling the microstructure, it is expected that the crystallographic as well as the mechanical properties could be improved significantly even from the same kind of precursor materials such as mesophase pitch.

Coordination state of Nb5+ ions in silicate and gallate glasses as studied by Raman spectroscopy

Abstract: Raman spectra of K2O-Nb2O5-SiO2 glasses are measured in order to compare the coordination state of Nb5+ ions in gallate glasses with that in silicate glasses. It is found that less-distorted NbO6 octahedra with no non-bridging oxygens as well as NbO6 octahedra with non-bridging oxygens and/or with much distortion are present in the K2O-Nb2O5-SiO2 glasses. The Raman band in the 800 to 900 cm−1 region is attributed to the NbO6 octahedra with non-bridging oxygens and/or with much distortion. The broad bands in the 600 to 800 cm−1 region are attributed to less-distorted NbO6 octahedra with no non-bridging oxygens. An increase in the molar ratio Nb2O5/K2O leads to an increase in the oxygens shared by more than two polyhedra and/or a decrease in non-bridging oxygens for the NbO6 octahedra which possess non-bridging oxygens, or to an increase of distortion for much-distorted NbO6 octahedra. At the same time, an increase in the molar ratio Nb2O5/K2O increases the less-distorted NbO6 octahedra with no non-bridging oxygens. In short, GaO4 tetrahedra and NbO6 octahedra compete to attract alkali ions in gallate glasses but such competition is not found in silicate glasses.

The infrared and Raman spectra of ZrO2-SiO2 glasses prepared by a sol-gel process

Abstract: Infrared and Raman spectra were measured and interpreted on the basis of structure for various ZrO2-SiO2 glasses that were prepared by a sol-gel process involving zirconium nitrate. Interpretation of the spectra indicates the presence of both Zr-O-Si and Si-O-H linkages depending upon heat treatment conditions, and the evidence of zirconium atoms with eight fold coordination for glasses with high ZrO2 contents. The crystalline products formed during glass devitrification were monitored by vibrational spectra. The formation and detection of tetragonal ZrO2, zircon, monoclinic ZrO2 and α-cristobalite depended upon the conditions of devitrification.

A study on the microstructure and phase transformation of electroless nickel deposits

Abstract: Electroless Ni-7.4 to 10% P deposits obtained from acidic nickel sulphate baths with sodium hypophosphite as a reducing agent were analysed by transmission electron microscopy, X-ray diffraction and thermal analysis. The deposits could be represented better by a microcrystalline structure composed of 4 to 5 nm fcc Ni-P solid-solution grains rather than an amorphous structure. The deposits also had the (111) texture, which persisted in nickel grains even after phase separation of nickel and Ni3P by heating in the case of the low nickel content, whereas the texture approached the random orientation with increasing phosphorus content. The phase transformation temperature was independent of the phosphorus content.

The Curie point temperature of Ba(Ti1?x Zr x )O3 solid solutions

Abstract: The Curie point temperature of solid solutions made by firing mixtures of BaTiO3 and BaZrO3 powders was found to depend markedly on sintering conditionS. However, when the ceramic was made from a homogeneous powder, wherein each particle of powder had the same chemical composition, the Curie point temperature was only slightly dependent on the firing schedule. Using chemically homogeneous powders, Ba(Ti1−xZrx)O3 withx=0.05, 0.10 and 0.20, the Curie point temperature shift was found to be a non-linear function of x.

Formation of mullite and other alumina-based ceramics via hydrolytic polycondensation of alkoxides and resultant ultra- and microstructural effects

Abstract: This manuscript discusses the formation of alumina-based ceramic materials from alkoxide starting materials. Particular attention is given to Al2O3-SiO2 systems, especially mullite. When the oxide network of these materials is formed via chemical polymerization of alkoxides, the ultrastructure of the resultant condensates is fundamentally affected by the nature and kinetics of the condensation reactions. Crystallization of mullite from an amorphous Al6Si2O13 at ∼ 980°C appears to be related to the intimacy and homogeneity of the aluminium-silicon bonding in the precursor networks. Network inhomogeneities due to the different hydrolysis rates of mixed alkoxides diminish by reduction of the chemical encounter rates with water during the hydrolytic polycondensation. Mullite produced from homogeneous and stoichiometric condensate shows a large spontaneous densification at ∼ 980° C. The microstructure of this material shows no indication of a liquid phase at the grain boundaries.

Thin films of molybdenum and tungsten disulphides by metal organic chemical vapour deposition

Abstract: Thin films of MoS2 and WS2 have been prepared on various substrates (glass, quartz, LiF, MgO, mica, molybdenum, gold, platinum, aluminium, copper, steel, graphite, MoSe2) by metal-organic chemical vapour deposition using sulphur or hydrogen sulphide and the hexacarbonyls of the transition metals as volatile components. The deposition technique is described here. The thin layers have been examined by X-ray powder diffraction and scanning electron microscopy. Time-resolved microwave conductivity measurements were used to detect the photoactivity of these materials.

Electrical conductivity of polyethylene-carbon-fibre composites mixed with carbon black

Abstract: The study of electrical conductivity of high-density polyethylene-carbon-fibre composites mixed with different concentrations of carbon black is reported. The influence of the mixing procedure of the additives and material preparation is examined with regard to the conductivity values. The use of these two filler types in polyethylene composites combines the conducting features of both. Thus, while fibres provide charge transport over large distances (several millimetres), carbon black particles improve the interfibre contacts. Results are discussed with reference to simple electrical models. It is shown that for composites in which the segregated carbon black-polyethylene component lies above the percolation threshold the electrical interfibre contacts are activated through carbon black particle bridges, leading to a conductivity rise. This effect is more relevant in the case of shorter fibres. Processing of the material involving fibre orientation, such as in injection-moulding, decreases drastically the conductivity level reached.

Characterization of B2O3-SiO2 glasses prepared via sol-gel

Abstract: B2O3-SiO2 glasses were prepared by the sol-gel method from boron and silicon alkoxides. The gels were densified by several heat-treatments at temperatures above 800° C. The gel-glass transition was studied with the data obtained from differential thermal analysis, dilatometry and thermal evolution of density. The structural features of the glassy materials prepared were analysed by means of infrared (IR) and near-infrared (NIR) spectroscopy. The IR spectra indicated the existence of mixed Si-O-B bonds, preferential located at the outside of the material particles. The NIR spectra, recorded from gels heat-treated at low temperatures, demonstrated the existence of a great number of OH− groups varying in nature, as well as their evolution and their positional changes on the silica particle surface as a function of the heat-treatment.

Raman study of alumina gels

Abstract: The sol-gel transition of an alumina sol is followed by Raman spectroscopy. Other spectroscopy techniques are also used to obtain additional information. The sol is made of particles of pseudo-boehmite which give rise to a Raman line at 360 cm−1. The treatment of the sol by ethyl-ether extraction of 2-butanol confirms the presence of this band. Its assignment is made after a study of the spectra obtained during gelation on wet and dried gels. Raman spectroscopy does not show the presence of aluminium polycations in the sol. However, 27Al nuclear magnetic resonance shows clearly the peak of the tetrahedrally coordinated aluminium atom located at the centre of these polycations. The presence of polycations contributes to explaining the slope change observed in the curve giving the Raman line intensities as a function of the aluminium content of the sample. X-ray diffraction and infrared spectroscopy enable us to correlate the Raman spectra of the gels heat-treated at various temperatures with respective structures.