Showing papers in "Journal of Materials Science in 1990"

A review of particulate reinforcement theories for polymer composites

Abstract: A concerted survey is presented of the existing theories for predicting the strength and modulus of particulate-filled polymeric composites. The macroscopic behaviour of particulate composites is affected by the size, shape, and the distribution of the inclusions. The interfacial adhesion between the matrix and inclusion is also important. The limitation of theoretical models in describing these parameters and expressing the experimental data on the macroscopic behaviour is demonstrated.

Twinning in ferroelectric and ferroelastic ceramics: stress relief

Abstract: The regular twinning in ceramics and metals below the temperature of a ferroelastic or ferroelectric structural phase transition is a result of energy minimization. Here homogeneous elastic energy is reduced at the expense of twin wall energy. The twin density depends on the gram sizeg; under homogeneous stress the total elastic energy of a grain increases ∝g 3. Any kind of twin wall, however, increases ∝g 2. Below the intersection of these two curves, stress reduction by twinning cannot lower the total energy. Thus there is a critical grain size below which twinning should not occur. Above this limit the width of the twin lamellae increases ∝g 1/2. The shape of the grain then adjusts to the surroundings in two dimensions only. Above another larger critical grain size more complex interfaces with higher surface energy are created, which allow stress relief in the third dimension. A semi-quantitative model is developed with the example of BaTiO3 ceramic, of which the domain patterns are well known. It is representative for many ceramics. The highT c superconductor YBa2Cu3O7−δ also twins according to the same law. For three-dimensional adjustment here a proper interface is missing.

Self-propagating high-temperature (combustion) synthesis (SHS) of powder-compacted materials

Abstract: Self-propagating high-temperature synthesis (SHS) of powder compacts is a novel processing technique currently being developed as a route for the production of engineering ceramics and other advanced materials. The process, which is also referred to as combustion synthesis, provides energy- and cost-saving advantages over the more conventional processing routes for these materials. At the same time, the rapid heating and cooling rates provide a potential for the production of metastable materials with new and, perhaps, unique properties. This paper reviews the research that has been, and is being, undertaken in this exciting new processing route for high-technology materials and examines the underlying theoretical explanations which will, eventually, lead to improved control over processing parameters and product quality.

Low-temperature synthesis and processing of electronic materials in the BaO-TiO2 system

Abstract: The BaO-TiO2 system contains several technologically important electronic materials. BaTiO3, a ceramic with high dielectric constant, is useful for the manufacture of multilayer ceramic capacitors, thermistors and electro-optic components. Titania rich compounds in this system (such as BaTi4O9 and Ba2Ti9O20) are suitable for the manufacture of miniaturized microwave resonators. Conventional processing of these ceramics relies on the solid-state reactions between readily available raw materials (typically TiO2 and BaCO3) and tends to produce coarse, impure, inhomogeneous and multiphase powders. Low temperature, wet chemical routes offer an exciting possibility for the synthesis of high purity, homogeneous, ultrafine and multicomponent powders from which electronic components with tailored and predictable properties could be prepared. A review of new and emerging techniques for the low temperature, wet chemical synthesis of barium titanates is presented. Salient features of several of these processes based on the use of alkoxides, acetates, citrates, chlorides, hydroxides and oxalates of barium and titanium, and combinations thereof, are described. The reaction pathways for the formation of barium titanates are discussed. A comparative summary of the powder characteristics and electrical properties of the barium titanates obtained by different techniques is presented along with a brief discussion of the economic viability of these processes.

Aqueous ferrofluids based on manganese and cobalt ferrites

Abstract: Synthesis of two new aqueous ferrofluids is performed chemically according to Massart's procedure. Manganese and cobalt ferrite magnetic particles are precipitated and treated in order to obtain colloidal sols by creating a charge density on their surface. Such “ionic” ferrofluids can be prepared in an acidic (after a treatment by ferric nitrate) or in an alkaline medium at a concentration as high as 17moll−1, i.e. a volumic fraction of 0.26. This result makes these new compounds of great interest.

Combustion synthesis of fine-particle metal aluminates

Abstract: Fine-particle metal aluminates,$MAl_20_4$ where $M = Mg$, $Ca$, $Sr$, $Ba$, $Mn$, $Co$, $Ni$, $Cu$ and $Zn$ as well as $3CaO . Al_2O_3$ $(C_3A)$, $CaAl_{12}O_{19}$ $(CA_6)$ and $MgCeAl_{11}O_{19}$ have been prepared by the combustion of mixtures of the respective metal nitrates (oxidizers) and urea or carbohydrazide (fuels) at 500 or $350^0c$ respectively, over a time of 5min. The solid combustion products were identified by their characteristic X-ray powder diffraction patterns. The fine-particle nature of these metal aluminates was investigated using SEM, TEM, particle size analysis and surface area measurements. The surface areas of the as-prepared metal aluminates using carbohydrazide fuel were higher (45 to 85 $m^2g^{-1}$) compared with urea (1 to 20 $m^2g^{-1}$).

Stable and metastable phase equilibria in the chemical interaction between aluminium and silicon carbide

Abstract: An experimental investigation was carried out on the Al-C-Si ternary system under atmospheric pressure and at temperatures up to 1900 K. From the results obtained, a thermodynamic model based on stable and metastable phase equilibria in the Al-C-Si ternary system was set up in order to provide a general description of the chemical interaction between aluminium and SiC. According to this model, aluminium and SiC are in thermodynamic equilibrium at every temperature lower than 923 K. At 923±3 K, i.e. at 10 K below the melting point of pure aluminium, a quasiperitectic invariant transformation occurs in the Al-C-Si system. In this transformation, solid aluminium reacts with SiC to give Al4C3 and a ternary (Al-C-Si) liquid phase. The carbon content of this liquid phase is very low; its silicon content is 1.5±0.4 at%. From 923 to about 1620 K, aluminium partially reacts with an excess of SiC, leading to a metastable monovariant equilibrium involving SiC, Al4C3 and an aluminium-rich (Al-C-Si) ternary liquid phase, L. The carbon content of this liquid phase, L, remains very low whereas its silicon content increases with temperature from 1.5±0.4 at% at 923 K to 16.5±1 at% at 1620 K. In the temperature range 1670 to 1900 K, two other three-phased monovariant equilibria can be reached by reacting aluminium and SiC. These equilibria involve on the one hand SiC, Al4SiC4 and a liquid phase, L′, and on the other hand, Al4SiC4, Al4C3 and a liquid phase, L″. The former is a stable equilibrium, the latter is a metastable one. At temperatures higher than about 2200 K, the latter metastable equilibrium is replaced by two monovariant stable phase equilibria including the ternary carbide Al8SiC7.

Polymer-filler interactions in rubber reinforcement

Abstract: The reinforcement of elastomers by finely divided fillers, particularly carbon black and silica, is fundamental to the rubber industry. Optimal reinforcement appears to involve both physical and chemical interactions. From a consideration of the effects of particle size as such, it appears that reinforcement, in the sense of tensile enhancement, will occur with any very finely divided filler. Physical factors prevent escape of the polymer from the filler surface (vacuole formation) but allow stress delocalization through interfacial slippage. Occasional stronger bonds may be introduced advantageously to facilitate dispersion, reduce particle/ particle interactions, and optimize practical properties relating to resilience and durability. Several lines of evidence suggest that only a minor amount of strong bonding is necessary or desirable, such that polymer/filler slippage can occur, under stress, over most of the interfacial area.

Strength, deformation, fracture behaviour and ductility of aluminium-lithium alloys.

Abstract: The ever increasing need for high strength, improved performance, lightweight and cost-effective materials has resulted in significant improvements and development of new aluminium alloys for structural applications. Lithium additions to aluminium have the potential for providing a class of high strength alloys with exceptional properties suitable for weight-critical applications. In this paper, published studies of composition-processing-microstructure relationships are discussed. Contributions to strength of the solid solution are discussed with reference to the presence of lithium in solid solution, the presence of coherent, ordered precipitates in the matrix and the co-precipitation of binary, ternary and more complex strengthening phases. Microstructural influences on strength are discussed with reference to metallurgical variables. These variables include the intrinsic microstructural features; the presence of dispersoids, the nature and type of matrix strengthening precipitates and the presence of denuded zones adjacent to grain boundaries. The extrinsic and intrinsic micromechanisms governing the deformation characteristics and fracture behaviour are critically examined with specific reference to ageing condition of the alloy, the matrix slip characteristics, and the nature, volume fraction and distribution of strengthening precipitates. The deleterious effects of strain localization and the exacerbating effect of precipitate-free zones are also highlighted. The micromechanics governing the fracture processes are examined and the sequence of events in the fracture process is reviewed in light of the specific role of several concurrent factors involving nature and volume fraction of second-phase particles, deformation mode, and dislocation-microstructure interactions. Past attempts made to improve the tensile ductility and mechanical response of these alloys are also examined so as to provide a better basis for understanding processing-microstructure-deformation interactions.

On short range ordering in the perovskite lead magnesium niobate

Abstract: The structural ordering characteristics of lead magnesium niobate (PMN) and solid solutions (PMN-PT) of PMN with lead titanate have been investigated using transmission electron microscopy (TEM). It is proposed that short range, non-stoichiometric 1∶1 ordering of the Mg and Nb cations on an F-centred superlattice generates space charges which dominate the kinetics of the ordering process and inhibit the development of long-range order. Furthermore, it is demonstrated that by introducing off-valent La3+ ions on to the A-site sublattice, the local charges can be at least partially-compensated and an increase in the extent of structural ordering is consequently observed.

Effective elastic moduli of porous solids

Abstract: The principles of continuum mechanics can be extended to porous solids only if the effective moduli are known. Although the effective bulk modulus has already been determined by approximating the geometry of a porous solid to be a hollow sphere, bounds could only be established for the other moduli. This problem of indeterminacy of the moduli is solved in this study using a particular model from the variation of the effective Poisson's ratio. In addition to this, the results are extended for the hollow sphere to real geometry by introducing a porositydependent factor. These results are compared with experimental data and the agreement is found to be good. As the effective Poisson's ratio cannot be determined accurately using experiments, the derived equation is verified using finite element analysis.

The structure and mechanical properties of sheets prepared from bacterial cellulose Part 2 Improvement of the mechanical properties of sheets and their applicability to diaphragms of electroacoustic transducers

Abstract: A sheet obtained from bacterial cellulose had a remarkably high modulus of elasticity as reported in Part 1 of this series. The Young's modulus of a sheet prepared by squeezing and drying a gel-like pellicle of bacterial cellulose was found to be >15 GPa. In addition, it has been found that treatment of the gel-like pellicles or dried sheets of bacterial cellulose with alkaline and/or oxidative solutions improves the mechanical properties significantly, and the Young's modulus of the resulting sheets approaches 30 GPa. In this paper, improvement of the mechanical properties of bacterial cellulose sheets by the removal of impurities is investigated and the applicability of bacterial cellulose to diaphragms of electroacoustic transducers is discussed

Microstructures and crystallization of electroless Ni-P deposits

Abstract: A study has been made of microstructures and crystallization of the electroless Ni-P deposits containing 11.3 to 23.0 at% P obtained from acidic nickel sulphate baths with sodium hypophosphite as a reducing agent by means of differential scanning calorimetry, X-ray diffractometry and hot stage transmission electron microscopy. The deposits containing low phosphorus content of 11.3 at% could be represented as an fcc Ni-P solid solution of 5 to 10 nm microcrystallites, whereas the deposits containing high phosphorus content were amorphous. The crystallization process of amorphous Ni-P solution involved more than one intermediate phases; precrystallized nickel or off-stoichiometric Ni3(P, Ni) or Ni5(P, Ni)2 phase in which some phosphorus sites are replaced by nickel atoms. The final equilibrium phases were bct Ni3P and fcc nickel crystals regardless of phosphorus content. The amorphous phase containing 20 to 22 at% phosphorus was the most stable among the amorphous Ni-P alloys.

Structural evolutions from polycarbosilane to SiC ceramic

Abstract: The pyrolysis process of a polycarbosilane into a microcrystalline silicon carbide ceramic has been followed up to 1700 ° C mainly by means of solid state29Si and13C nuclear magnetic resonance, transmission electron microscopy and X-ray diffraction analysis. A structural model has been proposed for the amorphous silicon carbide phase that is formed during the pyrolysis process. The ceramic obtained at high temperature is formed by a mixture of β-SiC and α-SiC; however, some difficulties in the identification of the crystalline phases have been pointed out.

Evaluation by indentation of fracture toughness of ceramic materials

Abstract: A transition fracture mode from Palmqvist to median has been observed in a number of ceramic materials A new expression to determine the fracture toughness (KIC) by indentation is presented The KIC values calculated by this formula are independent of the crack profile (median or Palmqvist) and of the applied load This formula has been obtained by modifying the universal curve of Evans and Charles to incorporate Palmqvist and median cracks over a wide range of loads in the case of brittle materials with different mechanical properties (elastic properties: E, v, KIC)

Some observations on the wetting and bonding of nitride ceramics

Abstract: Several series of experiments have been conducted to gain information about the wettability of AlN, BN, Si3N4 and two sialon ceramics by potential braze materials. It was possible to achieve wetting of all five ceramics using aluminium, copper-titanium alloys, and a Ag-28Cu-2Ti alloy. Wetting by aluminium and the Ag-28Cu-2Ti alloy was usually good. Both wetting and non-wetting alloys containing titanium reacted to form TiN and it is argued that the achievement of wettability is associated with a certain degree of hypostoichiometry. While aluminium should also have reacted, no clear evidence was obtained. In supplementary experiments it was found that bonds formed by brazing with aluminium at 1000 °C could have shear strengths as great as 60MPa. Although the experimental work was preliminary in nature, it suggested that good brazing systems could be developed.

Uniaxial compression tests and the validity of indirect tensile strength

Abstract: So-called “indirect” tensile tests have been adopted for various reasons, especially ease of execution and supposed freedom from several notable interferences. The extensive literature reveals much disparity in the conditions for such tests, and no consensus conclusion concerning the optimum protocol or even the interpretation of the results. Briefly, these, and other uniaxial tests in compression, are not well understood, despite long use and much investigation. Trust in centrally initiated tensile failure is misplaced. Experimentally, in a beam-split test, the effect of loading width is shown to be substantial; standard interpretations cannot explain the results. Failure initiated in shear is most probably the mechanism.

Comparison of nacre with other ceramic composites

Abstract: Mother-of-pearl, the highly filled ceramic composite of mollusc shell, is compared with other, less highly filled, artificial ceramics. Stiffness is fairly simply related to volume fraction of ceramic, but no model seems to be adequate to describe this relationship. Strength, stress-intensity factor and the work of fracture are also dependent on the ceramic content but in a much more complex manner. Nacre has the highest value for all these parameters.

Sol-gel synthesis of zirconia barrier coatings

Abstract: A method for applying zirconia barrier coatings using a sol-gel method is described. The coatings of 8 wt % yttria-stabilized zirconia are applied by spin coating a solution containing zirconium alkoxides and yttrium acetate on to stainless steel substrates. Crystallization of the films was observed for thermal treatments in the range 750 to 1050° C. Excellent adhesion at the interface, due to significant coating-substrate interfacial reactions, indicates that this sol-gel route is a feasible method for applying zirconia coatings.

A new tentative phase equilibrium diagram for the ZrO2-CeO2 system in air

Abstract: The phase relationships over a wide range of temperature and compositions in the ZrO2-CeO2 system have been reinvestigated. From DTA results, thermal expansion measurements andKIC determinations it was established that additions of CeO2 to ZrO2 decreases the monoclinic to tetragonal ZrO2 transition temperature, from 990 ° C to 150 ∓ 50 ° C, and an invariant eutectoid point at approximately 15 mol% CeO2 exists. The extent of the different single- and two-phase fields were determined with precise lattice parameter measurements on quenched samples. Evidence for the existence of a binary compound Ce2Zr3O10 (o-phase) was obtained by X-ray diffraction. The o-phase was stable below approximately 800 ° C, above which it decomposes into tetragonal zirconia + fluorite ceria solid solutions. Taking into account the polymorphic tetragonal-cubic transition and the narrowness of the two-phase tetragonal zirconia + fluorite ceria field above 2000 ° C, the existence of a new invariant eutectoid point was assumed, in which the metastable fluorite zirconia solid solution decomposes into tetragonal zirconia + fluorite ceria solid solutions. From the results obtained, the phase diagram also incorporates a eutectic point located at approximately 2300 ° C and 24 mol % CeO2.

Sinterability and electrical conductivity of calcium-doped lanthanum chromites

Abstract: Calcium-doped lanthanum chromites, (La1−xCax) (Cr1−y Cay O3, have been synthesized to investigate effects of calcium doping on sinterability and electrical conductivity. X-ray diffractometric results have revealed that in addition to normal perovskites (La1−xCaxCrO3), chromium-deficient perovskites can exist as a single phase in the composition region 0.1

Elastic constants of MoSi2 and WSi2 single crystals

Abstract: Single crystals of MoSi2 and WSi2 with a body-centred-tetragonal C 1 1b structure were fabricated using a floating-zone method. The elastic wave velocity was measured for samples with various orientations using a simple pulse echo method at room temperature, and six elastic stiffness constantsc ij were calculated. The stiffness constants were a little higher for WSi2 than for MoSi2.c 11 andc 33 of these compounds were approximately equal toc 11 of tungsten and molybdenum, respectively, althoughc ij (i ≠j) was a little higher for these compounds than for molybdenum and tungsten. Young's modulus 1/s 11 was the highest in the direction, and the lowest in the direction. The shear modulus 1/s 66 was high on the {0 0 1} plane and independent of shear direction. It was generally low on the close-packed {1 1 0} plane and largely dependent on shear direction. The elastic constants for the polycrystalline materials were estimated fromc ij ands ij . Poisson's ratiov was 0.15 for MoSi2 and for WSi2, and these values were much lower than for ordinary metals and alloys. The Debye temperature θD was estimated using the elastic-wave velocity of the polycrystalline materials via the elastic constants such as Young's modulus and shear modulus: it was 759 K for MoSi2 and 625 K for WSi2.

Superplasticity in ceramics

Abstract: It is now recognized that superplasticity is a potential deformation process in ceramics. This review summarizes the major characteristics of superplasticity and examines the reports of both transformation and structural superplasticity in ceramic and other non-metallic materials. It is shown that there are both similarities to and differences from metals. Similarities include the variation of strain rate with stress and grain size, but an important difference is the necessity to consider the role of intergranular glassy phases in ceramics. Superplasticity is also important in intermetallic compounds, and in geological materials where there is evidence for superplastic deformation both in laboratory experiments and in natural deformation.

The bonding mechanism of aramid fibres to epoxy matrices: Part 1 A review of the literature

Abstract: Despite considerable attempts to increase aramid-epoxy adhesion, to date, the adhesion levels achieved between aramid fibres and epoxy matrices are less than optimum for some applications. A combination of the aramid fibres' morphology, physical and chemical properties, and the interfacial mechanical stresses is responsible for the lack of success in increasing aramid-epoxy adhesion level. A key to improving the aramid-epoxy adhesion is a basic understanding of the interfacial mechanisms by which fibre and matrix interact. There is a considerable number of publications on aramid fibres and their composites. This paper reviews some of the literature relevant to aramid-epoxy bonding mechanisms

Interaction of 18Cr-10Ni stainless steel with liquid aluminium

Abstract: The dissolution of an 18Cr-10Ni stainless steel in liquid aluminium at 700 to 850 ° C was found by the rotating disc technique to be non-selective and diffusion controlled Experimentally determined values of the parameters characterizing the dissolution run are presented In the case of saturated aluminium melts two intermetallic layers were found to form between the steel and the melt material at 700 °C The compact layer adjacent to the steel surface is probably a solid solution based upon the Fe2Al5 compound Its thickness,x, tends with increasing time to the limiting valuex max = 10μm The porous layer adjacent to the melt material is probably a solid solution based upon the FeAl3 compound After a certain period of non-linear growth its thickness,Y, increases with time,t, according to the linear law:Y = 1 × 10−8 t + 6 × 10−6 m The time dependence of the total thickness of both layers is well described in terms of the “paralinear” kinetics In the case of undersaturated aluminium melts the formation of a single-phase intermetallic layer, 3 to 11 μm thick, was observed at 700 ° C for 100 to 600 sec The steel-to-aluminium transition joints with good mechanical properties were made by interaction of a solid steel material with liquid aluminium

Glasses from aerogels

Abstract: Silica glasses are obtained by the densification of aerogels. The transformation of the material into a glass is followed by differential thermal analyis, thermo-gravimetric analysis, dilatometry and by the evolution of the structural, textural and mechanical properties of the material. The organic species and the hydroxyl groups are removed by oxidation and chlorination heat treatments in such a way as to avoid bloating and crystallization phenomena during sintering. Densification is obtained by heat treatment at a low temperature (1100 to 1300 ° C). The densified aerogel shows physical properties identical to those of molten silica. Moreover, this material is very pure and its water content is low. The same process can be extrapolated to multicomponent glasses and composite materials.

The influence of aluminium content to the stacking fault energy in Fe-Mn-Al-C alloy system

Abstract: Four Fe-30Mn-0.9C-XAl alloys are employed to investigate the influence of aluminium content to the stacking fault energy in Fe-Mn-Al-C alloy system. The range of aluminium content is zero to 8.47 wt%. Based on the thermodynamic model, the stacking fault energy can be obtained through calculation. Increasing the aluminium content will make the stacking fault energy of Fe-30Mn-0.9C based alloys increase at 300 K.

Splat-quench solidification of freely falling liquid-metal drops by impact on a planar substrate

Abstract: Results are presented of a study of the splat-quench solidification of small, freely falling liquid drops of the alloy Nitronic 40W, which were allowed to impact on a solid, planar, horizontal substrate. The principal variable was the substrate material, with substrates of copper, alumina and fused quartz being used. The shapes of the solidified splats were correlated with a simplified model for the energetics of the splatting process and with the thermal conductivity of the substrate. The measured results are qualitatively in agreement with theoretical predictions, and suggestions are offered for a more comprehensive model of splat-quench solidification. A relationship between sessile droplet diameter and parent wire diameter is also presented and discussed.

Powder preparation and compaction behaviour of fine-grained Y-TZP

Abstract: Two wet chemical preparation methods are described for yttria-doped tetragonal zirconia powders. Both methods yield powders with an extremely small crystallite size (8 nm) and a narrow size distribution. The agglomerate and aggregate structure of these powders have been investigated by several techniques. Gel precipitation from an alkoxide solution in water (ldquoalkoxiderdquo synthesis) results in a ceramic powder with irregular-shaped weak and porous agglomerates, which are built up from dense aggregates with a size of 18 nm. Gel precipitates formed from a metal-chloride solution in ammonia (ldquochloriderdquo synthesis) do not contain aggregates. Both types of agglomerate are fractured during isostatic compaction. Hydrolysis and washing under (strong) basic conditions probably decrease the degree of aggregation. The aggregate morphology and structure are key parameters in the microstructure development during sintering of a ceramic. Several characteristics of these powders are compared with those of a commerical one (Toyo Soda TZ3Y).

Mechanically driven syntheses of carbides and silcides

Abstract: Most metal carbides or slicides may be synthesized at room temperature, by ball milling mixtures of elemental powders for some tens of hours with a vibratory mill. Both stable and metastable compounds containing a high density of defects can be obtained. In general, phases stable at low temperatures are synthesized. This observation allows us to confirm recent estimations for the maximum temperature (∼600 K) attained in these powders during mechanical alloying. Exceptions are found for some MSi2 suicides with M = titanium, iron or molybdenum for which both low- and high-temperature phases are formed.