Showing papers in "Journal of Materials Science in 1993"

Models proposed to explain the electrical conductivity of mixtures made of conductive and insulating materials

Abstract: The electrical conductivity of mixtures of conductive and insulating materials is reviewed In general, the conductivity of such mixtures increases drastically at a certain concentration of the conductive component, the so-called percolation concentration Among the parameters influencing the percolation concentration, the filler distribution, filler shape, filler/matrix interactions and the processing technique are the most important ones On the basis of these parameters, different models have been proposed aimed at the prediction of the conductivity or the percolation concentration It will be shown here that statistical, geometric or thermodynamic models explain the conductivity behaviour of specific mixtures on the basis of insufficient assumptions However, the conductivity seems to be predictable with the help of structure-oriented models

Selective formation of Na-X zeolite from coal fly ash by fusion with sodium hydroxide prior to hydrothermal reaction

Abstract: Hydrothermal treatment of fly ash with alkali gives various types of zeolites such as Na-Pl, Na-A and hydroxysodalite, where the zeolite zone was formed like an egg white, covering the central core of fly ash particles, as evinced in the previous paper. By fusion with sodium hydroxide, most of the fly ash particles were converted into sodium salts such as silicate and aluminate, from which hydrothermal reaction without stirring favourably resulted in the formation of Na-X zeolite. Crystallinity of Na-X zeolite as high as 62% was attained at the optimum condition of NaOH/fly ash = 1.2 and a fusion temperature of 823 K. Fly ash contains 14 wt% mullite (3Al2O3·2SiO2), which was revealed to be a less-active crystalline component for zeolite formation. Aluminium-enriched fly ash gave Na-A in place of Na-X zeolite. Scanning electron microscope images of cubic and octahedral crystals characteristic of Na-A and Na-X zeolite, respectively, obtained from fly ash, are given.

Thermite reactions: their utilization in the synthesis and processing of materials

Abstract: A class of exothermic reactions between, typically, a metal and an oxide, commonly referred to as thermite reactions, is reviewed with emphasis on their utilization in the synthesis and processing of materials. Theoretical and experimental results relating to ignition and combustion (self-propagation) characteristics of these reactions are presented.

The microhardness indentation load/size effect in rutile and cassiterite single crystals

Abstract: The microhardness indentation load/size effect (ISE) on the Knoop microhardness of single crystals of TiO2 and SnO2 has been investigated. Experimental results have been analysed using the classical power law approach and from an effective indentation test load viewpoint. The Hays/Kendall concept of a critical applied test load for the initiation of plastic deformation was considered, but rejected to explain the ISE. A proportional specimen resistance (PSR) model has been proposed that consists of the elastic resistance of the test specimen and frictional effects at the indentor facet/specimen interface during microindentation. The microhardness test load, P, and the resulting indentation size, d, have been found to follow the relationship $$P = a_1 d + a_2 d^2 = a_1 d + (P_c /d_0^2 ) d^2$$ The ISE is a consequence of the indentation-size proportional resistance of the test specimen as described by a 1. a 2 is found to be related to the load-independent indentation hardness. It consists of the critical indentation load, P c, and the characteristic indentation size, d o.

Fibre-matrix adhesion and its relationship to composite mechanical properties

Abstract: Two major areas of enquiry exist in the field of fibre-matrix adhesion in composite materials. One is the fundamental role that fibre-matrix adhesion plays on composite mechanical properties. The other is what is the “best” method used to measure fibre-matrix adhesion in composite materials. Results of an attempt to provide an experimental foundation for both areas are reported here. A well-characterized experimental system consisting of an epoxy matrix and carbon fibres was selected in which only the fibre surface chemistry was altered to produce three different degrees of adhesion. Embedded single-fibre fragmentation tests were conducted to quantify the level of fibre-matrix adhesion. Observation of the events occurring at the fibre breaks led to the documentation of three distinct failure modes coincident with the three levels of adhesion. The lowest level produced a frictional debonding, the intermediate level produced interfacial crack growth and the highest level produced radial matrix fracture. High fibre volume fraction composites made from the same material were tested for on- and off-axis, as well as fracture, properties. Results indicate that composite results can be explained if both differences in adhesion and failure mode are considered. It will be further demonstrated that fibre-matrix adhesion is an “optimum” condition which has to be selected for the stress state that the interface will experience. The embedded single-fibre fragmentation test is both a valuable measurement tool for quantifying fibre-matrix adhesion as well as the one method which provides fundamental information about the failure mode necessary for understanding the role of adhesion on composite mechanical properties.

Wet chemical etching of silicate glasses in hydrofluoric acid based solutions

Abstract: The etching of silicate glasses in aqueous hydrofluoric acid solutions is applied in many technological fields. In this review most of the aspects of the wet chemical etching process of silicate glasses are discussed. The mechanism of the dissolution reaction is governed by the adsorption of the two reactive species: HF and HF 2 - and the catalytic action of H+ ions, resulting in the breakage of the siloxane bonds in the silicate network. The etch rate is determined by the composition of the etchant as well as by the glass, although the mechanism of dissolution is not influenced. In the second part of this review, diverse applications of etching glass objects in technology are described. Etching of SiO2 and doped SiO2 thin films, studied extensively for integrated circuit technology, is discussed separately.

Comparison of stress concentration versus minimum solid area based mechanical property-porosity relations

Abstract: Stress concentrations due to pore shape are questioned as a fundamental determinant of mechanical property-porosity relations, especially elastic property porosity relations. On the other hand, actual solid load-bearing areas, especially minimum solid areas of porous bodies, clearly are a determinant of mechanical property-porosity effects. The correlation of pore shape-stress concentration effects with elastic properties of ceramics can be explained by the correlation of pore shapes with minimum solid areas.

Dilatational bands in rubber-toughened polymers

Abstract: A theory is advanced to explain the effects of rubber particle cavitation upon the deformation and fracture of rubber-modified plastics. The criteria for cavitation in triaxially-stressed particles are first analysed using an energy-balance approach. It is shown that the volume strain in a rubber particle, its diameter and the shear modulus of the rubber are all important in determining whether void formation occurs. The effects of rubber particle cavitation on shear yielding are then discussed in the light of earlier theories of dilatational band formation in metals. A model proposed by Berg, and later developed by Gurson, is adapted to include the effects of mean stress on yielding and applied to toughened plastics. The model predicts the formation of cavitated shear bands (dilatational bands) at angles to the tensile axis that are determined by the current effective void content of the material. Band angles are calculated on the assumption that all of the rubber particles in a band undergo cavitation and the effective void content is equal to the particle volume fraction. The results are in satisfactory agreement with observations recorded in the literature on toughened plastics. The theory accounts for observed changes in the kinetics of tensile deformation in toughened nylon following cavitation and explains the effects of particle size and rubber modulus on the brittle-tough transition temperature.

Preparation, properties and chemistry of glass- and glass-ceramic-to-metal seals and coatings

Abstract: An overview is given outlining the materials and technologies that have been employed in the preparation of glass- and glass-ceramic-to-metal seals and coatings. Metal/non-metal bonding theories are summarized, and the conditions required for the formation of strong chemical bonding are described and discussed. Particular reference is given to the interfacial chemistry involved for individual glass/metal and glass-ceramic/metal combinations. The major factors responsible for the preparation of high-quality seals and coatings, free from porosity and other undesirable defects, are also outlined and discussed. In addition, a number of applications for seals and coatings are briefly described.

Experimental determinations of the Lorenz number

Abstract: The Lorenz number, which was assumed to be a universal factor, was later found to be deviating from its theoretical Sommerfeld value. Our understanding of the physics of the Weidemann-Franz-Lorenz law is based on a large number of investigations. In view of its importance in the understanding of thermal and electrical conductivity (particularly the electronic contribution), an attempt has been made to review the experimental determination of Lorenz number in metals, semi-metals, alloys and degenerate semiconductors.

Microstructure and microanalysis of hardened ordinary Portland cement pastes

Abstract: Hardened ordinary Portland cement pastes of various ages have been examined by analytical transmission electron microscopy (TEM) and electron microprobe analysis (EMPA). The stability of the various hydrate phases in the electron microscope is discussed. Although all are subject to damage in varying degrees, even the least stable phase, AFt, can be recognized in relict form in the TEM. The basic framework of the microstructure and the differentiation into inner and outer hydration product are well-established at 24 h hydration. Although the dominant inner product formed within the boundaries of the original anhydrous grains is C-S-H gel, particles of AFt, AFm, Ca(OH)2, a magnesium-rich phase and an iron-rich phase are occasionally observed within the inner product. The Ca∶Si ratio of the C-S-H gel determined by TEM shows significant variation from one region to another in a given paste. There is no relationship between the average Ca∶Si ratio of the C-S-H and the maturity of the paste, although young pastes appear to show a bimodal distribution. Microanalysis by EMPA gives Ca∶Si ratios in substantial agreement with those found by TEM but it is essentially impossible to obtain by EMPA analyses of outer product C-S-H without admixture of other phases, particularly sulphoaluminate phases. Despite the presence of small amounts of embedded phases as revealed by TEM, single-phase inner product C-S-H can be analysed by EMPA. The compositions of AFt and AFm phases have been obtained by TEM and the results do not require the substitution of silicon in the formulae.

Documentation of damping capacity of metallic, ceramic and metal-matrix composite materials

Abstract: High-damping materials allow undesirable mechanical vibration and wave propagation to be passively suppressed This proves valuable in the control of noise and the enhancement of vehicle and instrument stability Accordingly, metallurgists are continually working toward the development of high-damping metals (hidamets) and high-damping metal-matrix composites (MMCs) MMCs become particularly attractive in weight-critical applications when the matrix and reinforcement phases are combined to provide high-damping and low-density characteristics In selecting the constituents for an MMC, one would like to have damping capacity data for several prospective component materials Based upon data which have been published in the scientific literature, a concise documentation is given of the damping capacity of materials by three categories: (a) metals and alloys, (b) ceramic materials, and (c) MMCs

The hot compaction of high modulus melt-spun polyethylene fibres

Abstract: The production of solid section highly oriented polyethylene by compaction of melt-spun polyethylene fibres is described. Differential scanning calorimetry, X-ray diffraction and electron microscopy have been used to determine the structure of the compacted polymer. The essential feature of the process is shown to be selective surface melting of the fibres to form a polyethylene/polyethylene composite of very high integrity, yet maintaining a very high proportion of the strength and stiffness of the fibres.

Splat-quench solidification: estimating the maximum spreading of a droplet impacting a solid surface

Abstract: This study investigates the mechanisms that contribute to determining the maximum spreading of a liquid droplet impacting a solid surface in connection with splat-quench solidification. This paper defines two domains, the viscous dissipation domain and the surface tension domain, which are characterized by the Weber and the Reynolds numbers, and that are discriminated by the principal mechanism responsible for arresting the splat spreading. This paper illustrates the importance of correctly determining the equilibrium contact angle (a surface tension characteristic that quantifies the wetting of the substrate) for predicting the maximum spreading of the splat. Conditions under which solidification of the splat would or would not be expected to contribute to terminating the spreading of the splat are considered. However, our a priori assumption is that the effect of solidification on the spreading of a droplet, superheated at impact, is secondary compared to the effects of viscous dissipation and surface tension.

Synthesis of cast metal matrix particulate composites

Abstract: The present review begins by briefly tracing history in the early days of development of cast metal-matrix composites and also outlines different casting routes for their synthesis. The problems faced by the quality of cast products and their relation to the process variables and characteristics of a given process, constitute the main theme of the review. The development of microstructure has been discussed in view of nucleation behaviour anticipated on the basis of estimated interface energies. The solidification around dispersoids and in regions away from it has been highlighted. Porosity in cast composites (its origin and control in cast components by suitable mould design) has engaged attention because of damage to mechanical properties due to porosity. The chemical reactions at the interface between dispersoid and matrix during processing of certain important systems of composites, have been described and the means of controlling these reactions have been indicated. The review concludes by drawing attention to the potential for application of cast composites in different industrial components and underlines the necessity of research in certain related fields so that industrial application of cast metal-matrix composites will soon become a reality.

Raman scattering characterization of gel-derived titania glass

Abstract: The preparation by the sol-gel method of bulk vitreous TiO2 is reported and its transformation both by controlled thermal treatment and by laser-induced heating in the anatase and rutile crystalline forms is studied by Raman spectroscopy. The results are compared with those existing for amorphous TiO2 thin films. The dependence of the Raman frequencies and linewidths on the incident laser power is presented and the Raman spectrum of the brookite (a third naturally occurring form of TiO2) is also reported as a comparison.

Characterization of the pyrolytic conversion of polysilsesquioxanes to silicon oxycarbides

Abstract: A series of silsesquioxane copolymers synthesized by hydrolysis and condensation of phenyl- and methyltrimethoxysilanes have been studied as preceramic polymers. The pyrolytic conversion to ceramics was characterized by thermogravimetric analysis, 29Si and 13C nuclear magnetic resonance and Raman spectroscopy. The pyrolysed materials were further characterized by differential thermal analysis. X-ray diffractometry and transmission electron microscopy. The ratio of phenyl to methyl groups in the copolymer was found to control polymer structure and rheology, as well as ceramic composition and char yield. On pyrolysis to 1000 °C under inert conditions, silicon oxycarbides were formed, along with glassy carbon. On heating from 1200 °C to 1400 °C, the oxycarbide structure diminished, and the materials were comprised primarily of amorphous silica, amorphous Si-C, some small crystallite SiC and graphitic carbon. The carbon content increased, and char yield decreased, with increasing concentration of phenyl groups in the copolymer. The presence of free carbon appears to inhibit the crystallization of silica. Significant carbothermal reduction was observed only above 1500 °C. Oxidation studies of the pyrolysed materials indicated the presence of at least two forms of carbon.

Fracture toughness and fracture mechanisms of PBT/PC/IM blend

Abstract: Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques were employed in the morphology and fracture mechanisms studies on a commercial polybutylene terephthalate/polycarbonate/impact modifier (PBT/PC/IM) blend. The fracture mechanisms involved at different temperatures under both impact and static loading were revealed. It was found that massive plastic deformation of the matrix material occurred after rubber particle cavitation; and it was this plastic deformation that was responsible for the drastic enhancement in fracture toughness although the widespread cavitation did absorb a considerable amount of energy as well. The major source of toughness was the same for both impact and static fracture tests, but the toughening processes became effective at a much lower temperature under static than impact conditions. The sequence of toughening events was also observed using TEM.

Effects of porosity on electric fatigue behaviour in PLZT and PZT ferroelectric ceramics

Abstract: Electric fatigue, namely the decay of the polarization and the consequent elastic strain with increased number of switching cycles under high a.c. field, severely limits the applications of ferroelectric and piezoelectric materials in high-strain electro-mechanical actuators and in thin films used in non-volatile memory devices. Electric fatigue tests have been conducted on lead zirconate titanate (PZT) and lanthanum-doped lead zirconate titanate (PLZT) ferroelectric ceramics. It was found that electric fatigue can be initiated by various factors, the porosity being one of them. Electric fatigue occurred in low-density(93%–97%) PLZT 7/65/35 ceramics after 104 switching cycles, while the high-density (>99%) PLZT specimens of the same composition did not fatigue after 109 switching cycles. It was also observed that for PZT ceramics, fatigue proceeded much more slowly in the samples with higher density (∼98%) than those with lower densities (92%–96%). A tentative explanation for the origin of the fatigue mechanism associated with porosity is proposed.

Formation of submicrocrystalline structure in the titanium alloy VT8 and its influence on mechanical properties

Abstract: It is shown that the formation of microstructure with grain size up to 0.06 μm may occur during the course of plastic deformation of the Ti-6Al-3.2Mo (α+β)-alloy with the initial coarse-grained lamellar structure. The formation of submicrocrystalline structure results from the development of dynamic recrystallization concurrent with the process of spheroidization. The temperature of superplastic deformation significantly decreases while strength characteristics at room temperature sharply increase in the alloy with such a microstructure.

Dependence of ceramic fracture properties on porosity

Abstract: A connected-grain model developed earlier to study the modulus of elasticity as a power-law of density was extended to study the dependence of the flexural strength of polycrystalline ceramics on porosity. Relations were derived for specific surface fracture energy, fracture toughness and flexural strength as power laws of (1 −p), wherep is porosity. Model validity was confirmed with data on α-alumina, UO2, Si3N4, and the YBa2Cu3O7−δ superconductor.

Solid-solution hardening

Abstract: Recent advances in the understanding of solid-solution hardening (SSH) of crystalline materials, as well as some basic early papers are briefly reviewed. This survey shows that models of SSH based on the concept of a frictional drag on dislocations migrating through fields of point-like obstacles, whether randomly dispersed or clustered, do not encompass the principal features of SSH, e.g. the temperature dependence of the yield stress, the stress and temperature dependences of the activation volume, and the phenomenon of stress equivalence. However, a model based on the nucleation of slip, involving the breakaway of dislocation segments from several pinning points, formulated in closed form, is shown to account satisfactorily for the principal observations.

Tensile properties and inhomogeneous deformation of ferrite-martensite dual-phase steels

Abstract: The tensile properties and inhomogeneous deformation of coarse ferrite-martensite dual-phase steels containing 17–50% martensite were analysed. The stress of dual-phase steels at equal strain increased with increasing volume fraction of martensite, f, but the rate of increase was reduced after f=0.3. The strain hardening rate was dependent on f at small strains (ɛ ⩽ 0.03), however, it became independent of f at larger strains. It was found that the deformation of the dual-phase steels divided into three different stages when f was less than about 0.3. The concurrent in situ stress-strain states of ferrite, martensite and their composite, and the stress ratios and strain ratios between ferrite and martensite were evaluated by means of a new stress and strain partition theory. The martensite phase deformed plastically after the uniform strain for f 0.25. The theoretical analyses for inhomogeneous deformation implied that the volume-fraction dependence of the stress and the characteristics of the strain-hardening rate were influenced by the plastic deformation of martensite. Further, the in situ stress-strain curves of ferrite and martensite and the internal stresses at respective phases were calculated from the partitioned stresses and strains.

Microbuckling instability in elastomeric cellular solids

Abstract: Compressive properties of elastic cellular solids are studied via experiments upon foam and upon single-cell models. Open-cell foam exhibits a monotonic stress-strain relation with a plateau region; deformation is localized in transverse bands. Single-cell models exhibit a force-deformation relation which is not monotonic. In view of recent concepts of the continuum theory of elasticity, the banding instability of the foam in compression is considered to be a consequence of the non-monotonic relation between force and deformation of the single cell.

Si-C-N ceramics with a high microstructural stability elaborated from the pyrolysis of new polycarbosilazane precursors

Abstract: Novel polycarbosilazanes (PCSZs) were prepared by stepwise synthesis and thermal crosslinking of polysilasilazane (PSSZ) copolymers. Their pyrolysis under inert gas, producing Si-C-N ceramics, was investigated up to 1600 °C by analyses performed on the solids (elemental analysis; EPMA; TGA, density; 1H, 13C and 29Si solid state NMR, i.r. XRD, electrical conductivity) and analyses of the evolved gases (gas chromatography and mass spectrometry). From 250 to 450 °C, a first strong weight loss was observed, which was due to the formation and elimination of low-boiling-point oligomers. The weight loss closely depends on the cross-linking degree of the ceramic precursor resulting from the PSSZ/PCSZ conversion. Then, the organic/inorganic transition took place between 500 and 800 °C, proceeding via evolution of gases (mainly H2 and CH4) and yielding a hydrogenated silicon carbonitride. This residue remained stable up to 1250 °C although it progressively lost its residual hydrogen as the temperature was raised. Then, crystallization occurred between 1250 and 1400 °C, yielding β-SiC crystals surrounded by free-carbon cage-like structures. Finally, above 1400 °C, the remaining amorphous Si-C-N matrix underwent a decomposition process accompanied by nitrogen evolution and a second substantial weight loss. At 1600 °C, the pyrolytic residue was a mixture of β-SiC and free carbon. So, the amorphous silicon carbonitride resulting from the pyrolysis of PCSZ precursors was found to be appreciably more thermally stable than the previously reported Si-C-O ceramic obtained by pyrolysis of polycarbosilane precursors.

The engulfment of foreign particles by a freezing interface

Abstract: The interactions of second-phase particles, liquid droplets or gas bubbles with a solidification front form the basis of various materials synthesis and purification processes and the design of microstructures in cast metal-matrix composites, as well as frost heaving and biological cell interactions. The physical mechanisms of this interaction phenomenon are based upon surface thermodynamic factors, solidification parameters, and fluid dynamic effects such as fluid drag and buoyancy. An overview is presented of the role of various factors which determine the nature as well as the kinetics of foreign particle-solidification front interactions, and the current status and limitations of the various theoretical models of the phenomenon.

Sol-gel processing of titanium-containing thin coatings

Abstract: Using tetrabutylorthotitanate as a precursor, titanate coatings with and without modifiers such as SnO2 and Fe2O3 have been successfully obtained. The hydrolysis and polycondensation mechanism is discussed in relation to the sol-gel transition. The gel-formation region was obtained as well as stable-layer formation and phase-separation regions in the Ti(OBun)4-C2H5OH-H2O system. Coatings deposited on glass substrates exhibited a strong tendency to aggregate and with increasing temperature to crystallize in anatase or rutile phases (500 and 1000 °C, respectively). Structural characteristics of the gel products were investigated by SEM, X-ray and electron diffraction measurements, as well as by small-angle X-ray scattering. It was established that after thermal treatment at 500 °C for 10 min, spherical aggregates were formed in the coatings, having an average radius of approximately 5.1 nm.

Effect of aluminium addition on the combustion reaction of titanium and carbon to form TiC

Abstract: Aluminium was incorporated into the reactant mixture with a molar ratio of Ti/C of 1.0 to study the effect of its addition on the combustion reaction between titanium and carbon to form TiC. Thermal analysis of the reactant mixture and component analysis of the reaction product suggest that the combustion reaction of the Ti-C-Al system proceeds in such a way that aluminium initially reacts with titanium to form titanium aluminide compounds of TiAl3, Ti2Al, and TiAl with heat evolution, and then the reaction between titanium and carbon and the decomposition of titanium aluminide to titanium and aluminium is subsequently followed. As the amount of aluminium incorporated was increased over the range of 0 to 40 wt%, the grain size of TiC decreased from approximately 15 μm to 0.4 μm. It was also observed that most of the aluminium in the TiC-Al composite was distributed on the surface of the spherical TiC grain.

Temperature dependence of the elastic moduli, dilational and shear internal frictions and acoustic wave velocity for alumina, (Y)TZP and β′-sialon ceramics

Abstract: Young's, shear and bulk moduli, Poisson's ratio and Lame parameters, longitudinal and transverse internal friction values and acoustic wave velocity anisotropy factors for three kinds of polycrystalline compounds, α-alumina, yttria-stabilized tetragonal zirconia polycrystal, (Y)TZP, and β′-sialon, (Si,Al)3(N, O)4, were simultaneously measured over a temperature range 295–1773 K, by an ultrasonic pulse sing-around method. These elastic moduli and Lame parameters decreases and Poisson's ratio increases with increasing temperature, suggesting activation of a shear mode in the high-temperature region. The high-temperature shear internal friction for (Y)TZP and sialon were more sensitive to relief of strain and softening of glassy phase at grain boundaries, respectively, compared with the dilational friction.

A model for lapping of glass

Abstract: Lapping of glass and other brittle materials is an important economic activity. Nevertheless, it has not received much scientific attention, despite the fact that it is also related to problems of wear (three-body abrasion). Therefore, lapping of glass has been analysed in terms of the concept of lateral fracture, by studying the influence of material parameters, namely Young's modulus, hardness and fracture toughness, on material removal and surface roughness for two different sets of experimental conditions. The concept was found to be well applicable, and was therefore used to develop a model of three-body abrasion by material removal via rolling and indenting abrasives. The model gives a good description of the two experiments. It allows an average normal force per abrasive to be determined from Preston's coefficient and the characteristics of the workpiece and abrasive.