Showing papers in "Journal of Materials Research in 1988"

Mechanical deflection of cantilever microbeams: A new technique for testing the mechanical properties of thin films

Abstract: The mechanical deflection of cantilever microbeams is presented as a new technique for testing the mechanical properties of thin films. Single-layer microbeams of Au and SiO2 have been fabricated using conventional silicon micromachining techniques. Typical thickness, width, and length dimensions of the beams are 1.0,20, and 30 μm, respectively. The beams are mechanically deflected by a Nanoindenter, a submicron indentation instrument that continuously monitors load and deflection. Using simple beam theory and the load-deflection data, the Young’s moduli and the yield strengths of thin-film materials that comprise the beams are determined. The measured mechanical properties are compared to those obtained by indenting similar thin films supported by their substrate.

Synthesis, characterization, and properties of nanophase TiO 2

Abstract: Ultrafine-grained, nanophase samples of TiO2 (rutile) were synthesized by the gas-condensation method and subsequent in situ compaction. The samples were studied by a number of techniques, including transmission electron microscopy, Vickers microharness measurements, and positron annihilation spectroscopy, as a function of sintering temperature. The nanophase compacts with average initial grain sizes of 12 nm were found to densify rapidly above 500 °C, with only a small increase in grain size. The hardness values obtained by this method are comparable to or greater than those for coarser-grained compacts, but are achieved at temperatures 400 to 600 °C lower than conventional sintering temperatures and without the need for sintering aids.

Nucleation of a new phase from the interaction of two adjacent phases: Some silicides

Abstract: The reactions of metal layers with their silicon substrates resulting in the formation of various silicides are considered generally not only as phenomena common to all diffusion couples where new phases are formed, but also as typical of all transitions from two to three phases. The conditions under which such transitions will display the same characteristics as encountered in the usual one-to-two phase transitions (condensation, crystallization, boiling) are analyzed by comparison to the classical theory of nucleation. Because of the lack of knowledge about the exact values of the relevant parameters, the discussion is carried out mostly in descriptive thermodynamic terms. Although nucleation effects are analyzed in general terms, the main focus of attention is a class of reactions where nucleation dominates the formation of a new phase; a salient feature of these reactions is the absence of any equilibrium temperature, although the nucleation temperatures are relatively well defined within narrow limits. Nucleation effects are correlated to such material characteristics as the stability of the nucleated phases, and to such kinetic characteristics as the sequence of phase formation. The modification of the energy levels of the different phases brought about by stress, ion bombardment, or the replacement of usual phases by metastable ones, are considered with respect to their effect on nucleation processes. The nearly total absence of literature references to nucleation in metal-metal diffusion couples is discussed with respect to some specific aspects of the metal-silicon reactions.

Simple embedded atom method model for fcc and hcp metals

Abstract: A procedure based on the embedded atom method (EAM) is presented for developing atomistic models for use in computer simulation calculations, with an emphasis on simple but general schemes for matching experimental data with fitting parameters. Both the electron density function and the two-body potential are taken as exponentially decreasing functions and the model is derived for any choice of cutoff distance. The model has been applied successfully to seven fcc and three hcp metals, but the extension to bcc metals was unsuccessful because of difficulty in matching the shear anisotropy ratio.

Growth mechanism of vapor-deposited diamond

Abstract: An elementary-reaction mechanism of diamond growth by a vapor deposition process is proposed. The central postulate is that the main monomer growth species is acetylene. The mechanism basically consists of two alternating steps: surface activation by H abstraction of a hydrogen atom from a surface carbon and the addition of one or two acetylene molecules. During the addition reaction cycle a number of solid C–C bonds is formed and hydrogen atoms migrate from a lower to an upper surface layer. The mechanism is in general agreement with the macroscopic views of the Russian researchers and is consistent with the numerous experimental observations reported in the literature.

Liquidus relations in Y–Ba–Cu oxides

Abstract: The liquidus relations in the system YO1.5–BaO–CuOx in air in the compositional region near the superconducting oxide YBa2Cu3Ox were studied by differential thermal analysis, x-ray diffraction, electron microprobe analysis, and visual observation. The temperatures of 11 invariant points and the corresponding reactions were determined. YBa2Cu3Ox was found to melt incogruently at 1015 °C to Y2BaCuO5, which in turn melts incongruently to Y2O3 at 1270 °C. These reactions mean that preparing the superconducting phase by melting and rapid cooling will result in the presence of these two phases as well. The peritectic reaction YBa2Cu3Ox + CuO⇉Y2BaCuO5 + liquid at 940 °C accounts for the observation of partial melting, improved synthesis purity, and grain growth at temperatures of 950 °C. The determination of these invariant temperatures and reactions provide insight into optimal processing conditions.

The cracking and decohesion of thin films

Abstract: The cracking and decohesion of thin films can be characterized by critical values of a nondimensional parameter governed by the residual stress, the film thickness, and a fracture resistance. This article reviews the status of understanding concerning the magnitude of this number for various types of adherent film on either brittle or ductile substrates. Important effects of elastic properties, substrate thickness, and yield strength are described.

A defect model for ion-induced crystallization and amorphization

Abstract: Extensive experimental investigations have been reported on the ion-induced motion of the interface between the crystalline and amorphous phases of silicon. The crystal grows into the amorphous phase at low ion fluxes and high temperatures. The amorphous phase grows into the crystal at high ion fluxes and low temperatures. The experimental observations are shown to fit a model based on a single defect. The concentration of this defect decays by binary recombination, this is, two of the defects annihilate one another. The model accounts for the linear relationship between interface motion and reciprocal temperature, for the Arrhenius temperature dependence of the flux at which no interface motion occurs, and for the temperature independence of the crossover frequency observed in beam pulsing experiments. The defect on which this model is based has a motion energy of 1.2 eV. Assuming that the same defect is also responsible for thermal recrystallization of the amorphous phase gives a formation energy of 1.5 eV for the defect. The defect is believed to be a dangling bond in the amorphous phase.

An investigation of hardness and adhesion of sputter-deposited aluminum on silicon by utilizing a continuous indentation test

Abstract: The hardness of aluminum films on silicon are measured as functions of depth of the indenter. The films have thicknesses of 0.25,0.5, and 1.0μm. The adhesion between one film and the substrate has been reduced through the prior deposition of a 10 nm layer of carbon. In each case the hardness is found to increase as the indenter approaches the film-substrate interface, but the rate of increase is greater for a film with good adhesion than for one with poor adhesion. It is suggested that this increase results from the constraint on deformation of the film by the substrate. A physical model is proposed whereby the yield stress of the film, σo, and an average effective shear strength τ of the indenter-film and film-substrate interfaces, may be determined from the data.

Zirconium oxide crystal phase: The role of the pH and time to attain the final pH for precipitation of the hydrous oxide

Abstract: Precipitated hydrous zirconium oxide can be calcined to produce either a monoclinic or tetragonal product. It has been observed that the time taken to attain the final pH of the solution in contact with the precipitate plays a dominant role in determining the crystal structure of the zirconium oxide after calcination at 500 °C. The dependence of crystal structure on the rate of precipitation is observed only in the pH range 7–11. Rapid precipitation in this pH range yields predominately monoclinic zirconia, whereas slow (8 h) precipitation produces the tetragonal phase. At pH of approximately 13.0, only the tetragonal phase is formed from both slowly and rapidly precipitated hydrous oxide. The present results, together with earlier results, show that both the pH of the supernatant liquid and the time taken to attain this pH play dominant roles in determining the crystal structure of zirconia that is formed after calcination of the hydrous oxide. The factors that determine the crystal phase are therefore imparted in a mechanism of precipitation that depends upon the pH, and it is inferred that it is the hydroxyl concentration that is the dominant factor.

Divacancy control of the balance between ion-beam-induced epitaxial cyrstallization and amorphization in silicon

Abstract: The ion-bombardment-induced reversible movement of a planar amorphous/crystalline interface in silicon has been studied between 100 and 400 °C. The temperature dependence of the ion dose rate at which there is zero interface movement has an activation energy of 1.2 eV, the dissociation energy of divacancies. Scaling of this dose rate for different ion species exhibits a quadratic dependence on the density of displaced atoms in the collision cascade of individual ions, giving further evidence for divacancy control of the interface movement.

Direct measurement of surface forces between sapphire crystals in aqueous solutions

Abstract: Measurements are presented of the electrical double layer and van der Waals forces between the (0001) surfaces of two single-crystal sapphire platelets immersed in an aqueous solution of NaCl at pH values from 6.7 to 11. The results fit the standard Deryaguin-Landau-Verwey-Overbeek (DLVO) theory, with a Hamaker constant of 6.7 × 10−20 J. These are the first measurements made using the Israelachvili surface forces apparatus without mica as a substrate material, and they demonstrate the possibility of using this technique to explore the surface chemistry of a wider range of materials.

Mechanism of plastic deformation of Mn-added TiAl L 1 0 -type intermetallic compound

Abstract: Titanium aluminum intermetallic compound is a possible candidate for a high-temperature structural material, except for a problem of lack of room-temperature ductility. Recently, this problem was found to be overcome possibly by the addition of Mn, but this mechanism has not been fully understood yet. In order to understand the fundamental mechanism of the ductility improvement by Mn addition, microanalyses have been carried out. The results are as follows. Twin structures in a TiAl intermetallic compound in the as-cast state can be climinated by high-temperature annealing, while those in Mn-added TiAl are thermally more stable and exist even after annealing for 86.4 ks at 1273 K. The reason for this thermal stabilization of twin structures is considered to be due to the pinning effect of twin dislocations by Mn addition. The enhancement of twin deformation in TiAl by Mn addition is regarded to be caused by two factors. One is the stabilization of twin partial dislocations, becoming the nucleation sites for twin formation. The other is the decrease in stacking fault energy, which makes twin deformation energetically easier.

Decoupled bulk and surface crystallization in Pd 85 Si 15 glassy metallic alloys: Description of isothermal crystallization by a local value of the Avrami exponent

Abstract: Isothermal devitrification of Pd85Si15 amorphous alloys has been analyzed using differential scanning calorimetry (DSC) and x-ray diffractometry. Both as-quenched and aged amorphous ribbons were investigated. Crystallization of aged samples starts from the surface and proceeds several micrometers into the bulk. The product of this process is a layer of strongly textured palladium (111) followed by a mixture of Pd2Si, Masumoto MSI phase, and untextured palladium. Next, the crystallization occurs via a different (bulk) mechanism, resulting in a mixture of Masumoto MSII phase and untextured palladium. The bulk mechanism is the only one observed in as-quenched samples. The surface and bulk crystallization mechanisms are spatially decoupled and, therefore, the corresponding DSC data can be analyzed separately. This has been done according to the Kolmogorov–Johnson–Mehl–Avrami model and also using the recently developed concept of local value of Avrami exponent n. For both the surface and bulk crystallization the phase transition process cannot be characterized by a single value of n. Observed variation of n with the crystallized fraction x is explained by a considerable variation of the nucleation rate that takes place during devitrification.

Phase selection in electrohydrodynamic atomization of alumina

Abstract: Electrohydrodynamic atomization’has been adapted to produce Al2O3 powders ranging in size from 10 nm to 300 μm. Microstructural characterization using x-ray diffraction, scanning, and transmission electron microscopy reveals changes in phase selection as a function of particle size, hence supercooling. Amorphous powders are common below 100 nm in diameter. Cubic spinel γ is found in single phase form between 100 nm and 2μm, and partially transformed to δ between 2 and 20 μm. There is also evidence of σ and θ or a precursor of θ forming directly from the liquid above 5 μm. The stable corundum structure is consistently found above 20 μm but exhibits three different morphologies: faceted, dendritic, and cellular. Phase selection is examined on the basis of fundamental thermodynamic and kinetic considerations and results from computer models predicting the thermal history of the powders. It is concluded that metastable phases require the elimination of catalytic sites for the nucleation of a and are thus more likely to form in the smaller powders. Furthermore, submicron powders achieve sufficiently high cooling rates to preserve the metastable phases formed (γ), but those higher than ∼ 1 μm experience a thermal excursion long enough to transform γ to more stable forms of Al2O3.

Solid-phase regrowth of compound semiconductors by reaction-driven decomposition of intermediate phases

Abstract: The solid-phase epitaxial regrowth of a III–V compound semiconductor by a two-stage reaction between a two-layer metallization and a compound semiconductor substrate is described The regrowth process begins with a low-temperature reaction between a metal M (eg Ni, Pd, or Pt) and a compound semiconductor substrate, AB, to produce an intermediate M, AB or MB, phase A subsequent reaction at a higher temperature between an overlayer of Si, Ge, Al, or In and the intermediate phase results in the decomposition of the intermediate phase and the epitaxial regrowth of a layer of the compound semiconductor This regrowth mechanism is verified experimentally for the specific case of the Si/Ni/GaAs system Rutherford backscattering spectrometry and transmission electron microscopy data show that the ternary phase Nix GaAs, formed during the initial stage of the reaction, decomposes toNiSi and GaAs by reaction with the Si overlayer The incorporation of the overlayer element into the regrown semiconductor layer is proposed as a mechanism to explain the formation of Ohmic contacts in Si/Pd/n-GaAs, In/Pd/n-GaAs, In/Pt/n-GaAs, and similar two-layer metallization systems on n-GaAs

X-ray depth profiling of iron oxide thin films

Abstract: A nominally γ-Fe2O3 thin film (oxidized from an Fe3O4 film) has been structurally depth profiled using x-ray scattering in a grazing incidence, asymmetric Bragg geometry. By varying the grazing incidence angle, the x-ray penetration depth is varied from tens of Angstroms to several thousand Angstroms, slightly larger than the film thickness. At small incidence angles a diffraction pattern characteristic of α-Fe2O3 is observed, while at larger angles the pattern is predominantly from γ-Fe2O3, showing that there is a surface layer of α-Fe2O3. These results are quantified and the thickness of the α phase found to be 90 A. The presence of the α phase explains a nonferromagnetic layer observed previously. These data together with magnetic and chemical data suggest that the nonferromagnetic layer forms during oxidation of the Fe3O4 film due to outward diffusion of Fe ions and their subsequent oxidation to form α-Fe2O3

Reactions between palladium and gallium arsenide: Bulk versus thin-film studies

Abstract: Reactions between Pd and GaAs have been studied using bulk-diffusion couples of Pd (approx.0.6 mm thick)/GaAs and thin-film Pd (50 and 160 nm)/GaAs samples. The sequence of phase formation at 600 /sup 0/C between bulk Pd and GaAs was established. Initial formation of the solution phase ..mu.. and the ternary phase T does not represent the stable configuration. The stable configuration is GaAs chemically bondepsilonchemically bondlambdachemically bond..gamma..chemically bond..nu..chemically bondPd and is termed the diffusion path between GaAs and Pd. The sequence of phase formation for the bulk-diffusion couples is similar at 500 /sup 0/C. Phase formation for the thin-film Pd/GaAs specimens was studied at 180, 220, 250, 300, 350, 400, 450, 600, and 1000 /sup 0/C for various annealing times. The sequence of phase formation obtained from the thin-film experiments is rationalized readily from the known ternary phase equilibria of Ga--Pd--As and the results from the bulk-diffusion couples of Pd/GaAs. The thin-film results reported in the literature are likewise rationalized. The diffusion path concept provides a useful guide in understanding the phase formation in Pd--GaAs interface or any other M--GaAs interface. This information is important in designing a uniform, stable contact for the metallization of GaAs.

Mechanisms of ductility improvement in L12 compounds

Abstract: The present article first describes some characteristics of structure, chemistry, and electronic (bond) nature for grain boundaries in the A3B Li2-type intermetallic compounds. Next, the phenomenological aspects for the grain boundary brittleness of the Li2-type intermetallic compounds are reviewed with respect to the combination of the constituent atoms, the alloying effect, the stoichiometry effect, and a role of impurity or gaseous atoms. It is emphasized that the brittleness of grain boundaries in the intermetallic compounds is directly controlled by the atomistic and electronic structures at grain boundary regions. Based on these systematic investigations, it is suggested that the brittleness of the Li2-type intermetallic compounds can be manipulated by appropriate control of composition and the corresponding electrochemical bond environment at grain boundary planes and by control of test environment. Furthermore, some examples of the materials development are described.

Variable strain energy in amorphous silicon

Abstract: Different Raman experiments on structural relaxation of a-Si and a-Ge are reviewed and discussed in relation to calorimetric measurements on a-Ge On the basis of the correlation found between results from Raman spectroscopy and results from calorimetry in the case of a-Ge and of the strong similarity between a-Si and a-Ge in terms of their Raman spectra, it is suggested that the strain energy in a-Si may vary considerably with preparation conditions and subsequent treatments Under this assumption the a-Si Gibbs free energy versus temperature has been constructed for material in different initial states of relaxation It is shown that the melting temperature of amorphous silicon should increase when relaxation occurs during the heating phase prior to melting Thus differences in apparent melting temperature, as observed under different laser heating conditions, may be explained

Interface modifications for improving the adhesion of a -C:H films to metals

Abstract: The adhesion of diamondlike hard carbon films to silicide forming metals was improved by using an interfacial silicon film several atomic layers thick. The use of thicker (> 10nm) silicon layers results in a decrease in the adhesion, probably due to a degradation of the structural integrity by excessive silicide formation.

An investigation of a nonspiking Ohmic contact to n-GaAs using the Si/Pd system

Abstract: A low-resistance nonspiking Ohmic contact to n-GaAs is formed via solid-state reactions utilizing the Si/Pd/GaAs system. Samples with Si to Pd atomic ratios greater than 0.65 result in specific contact resistivity of the order of 10 −6 Ω cm 2 , whereas samples with atomic ratios less than 0.65 yield higher specific contact resistivities or rectifying contacts. Rutherford backscattering spectrometry, cross-sectional transmission electron microscopy, and electron diffraction patterns show that a Pd, Si layer is in contact with GaAs with excess Si on the surface after the Ohmic formation annealing. This observation contrasts with that on a previously studied Ge/Pd/GaAs contact where Ohmic behavior is detected after transport of Ge through PdGe to the interface with GaAs. Comparing the Ge/Pd/GaAs system with the present Si/Pd/GaAs system suggests that a low barrier heterojunction between Ge and GaAs is not the primary reason for Ohmic contact behavior. Low-temperature measurements suggest that Ohmic behavior results from tunneling current transport mechanisms. A regrowth mechanism involving the formation of an n + GaAs surface layer is proposed to explain the Ohmic contact formation.

Improved stability of the ZnO varistor via donor and acceptor doping at the grain boundary

Abstract: The ZnO varistor degradation has been attributed to the field-assisted, temperature-activated diffusion of interstitial zinc in the depletion layer. To improve stability, one approach is to reduce the formation of interstitials, and then further, to prevent their migration through empty interstitial sites. Based on this concept, an amphoteric dopant, such as Na or K, has been incorporated in the ZnO varistor grain boundary wherein a dopant is substituted both in the lattice and in the interstitial sites. A grain boundary defect model has been developed for this dual mode of substitution, with the dopant acting as an acceptor at the lattice site and as a donor at the interstitial site. Under these conditions, and given a desired neutrality range, the concentration of zinc interstitial is indeed shown to be reduced and stability greatly improved. The experimental data presented here validate the grain boundary defect model presented in this and in an earlier paper [J. Mater. Sci. 20, 3487 (1985)].

Magnetic hardening mechanism in sintered R–Fe–B permanent magnets

Abstract: After a short review on temperature dependence of the intrinsic coercivity in sintered Nd–Fe–B-type magnets is given, recent experimental results concerning the coercivity-anisotropy (H C–HA) correlation in B-rich Pr–Fe–B and Nd–Fe–B sintered magnets and the influence of the surface conditions of the sintered Nd–Fe–B magnets on the coercivity are reported. The results are interpreted in terms of the μo H c vs cμoHA – NIs plot, where I s is the spontaneous magnetization of R 2Fe14B (R = Pr or Nd) and N is the effective demagnetization field coefficient.

Rapidly solidified Al3Ti-base alloys containing Ni

Abstract: Two Ni-modified Al3Ti alloys (Al65Ni10Ti25 and Al62Ni8Ti30) were rapidly solidified by melt spinning. The resulting microstructure was studied using light microscopy and analytical electron microscopy. Significant variations in the microstructure and phases were observed between the two ribbons and through the thickness of each ribbon.A single-phase γ-TiAl structure was seen near the wheel side of the Al62Ni8Ti30 ribbon, having microcrystalline grains ∼ 100 nm in diameter. Second-phase particles of Λ-AlNiTi were found in the remaining regions of that ribbon as the structure became columnar due to reduced rates of cooling. The Al65Ni10Ti25 alloy exhibited a primary phase of π-Al6.5 NiTi2.5. A second phase of μ-Al2NiTi formed with morphology and distribution varying through thickness. Microchemistry measurements on the phases indicated substantial deviations (up to 14 at. %) from the stoichiometric compositions. Further, the π, γ, and μ are low-temperature phases that do not form by solidification under equilibrium conditions. The observation of these phases thus suggests significant undercoolings achieved during the melt-spinning processing of the present alloys. Both ribbons are brittle as spun.

Interfacial phenomena involving liquid metals and solid oxides in the Mg–Al–O system

Abstract: The wetting of ceramic surfaces by aluminum alloys has been reexamined using a chemical system where interfacial reactions and oxide film effects could be isolated. The system Al–Mg–O was chosen since it is technologically important and high-purity, well-characterized materials are readily available. Magnesium alloyed with the aluminum sessile drop and silicon picked up from the experimental apparatus cause an initial reduction in contact angle by altering the protective nature of the oxide film formed on the sessile drop. Evidence of spreading is observed as an intermediate process in the reactive sessile drop pairs. Reaction products formed between the Al–Mg alloys and sapphire (Al2O3), spinel (MgAl2O4), or periclase (MgO) can be interpreted with predicted phase equilibria and the measured loss of magnesium from the sessile drop. Only the rate of the periclase alloy interaction was rapid enough to result in a continuous product layer after 24 h at 800 °C. The volatilization of all of the magnesium from the sessile drop resulted in the formation of a true Al–Al2O3 interface. The contact angle for a true Al–Al2O3 interface is 88 ± 5 deg at 800 °C. The liquid-solid interfacial energy is 1688 ergs/cm2.

Response of a stringlike dislocation loop to an external stress

Abstract: The dynamics of a very thin dislocation loop under the influence of an externally applied, time dependent, stress field is studied in the context of continuum elasticity, where very thin means that the dislocation core is small compared to the loop’s typical radius of curvature as well as to any relevant acoustic wavelengths. This is done using energy and momentum conservation as derived from a variational principle for conservative motion of the loop. Energy conservation alone does not suffice, since it is insensitive to forces that do no work. The idea is to have a theory of sources (dislocation loops) interacting with a field (particle displacement) in the same sense that classical electrodynamics is a theory of point-charged particles interacting with the electromagnetic field. The sum of elastic strain and particle velocity generated by a dislocation loop and those generated by external agents are replaced in the action functional whose extrema give the equations of classical dynamic elasticity, thus obtaining a functional of the loop’s trajectory. Extrema of the action with respect to variations of the dislocation history select the trajectory that will be followed by the loop under prescribed external stresses. In general, the evolution of the loop will be governed by an integrodifferential equation. Differential equations are obtained when the work done by external forces is much greater than the elastic energy radiated, and the motion of any one point of the loop is affected only by those other loop points in its immediate neighborhood (local approximation). These equations are explicitly written down. They describe the dynamics of a string with mass and line tension of purely elastic origin. The cutoff procedure needed to give meaning to logarithmically divergent expressions is carefully described. The main ideas can be understood in the case of a screw dislocation, which is worked out in detail. The general case with two characteristic velocities, although algebraically more cumbersome, is not essentially different physically. Additional examples include the gliding edge, pinned dislocation segments, and kinks. Results presented are valid in a homogeneous, isotropic, infinite elastic solid, and ways in which these various restrictions might be lifted is discussed.

Oxygen transport through high-purity, large-grain Ag

Abstract: The permeation of oxygen through high-purity, large-grain Ag membranes has been studied over the temperature range of 400–800 °C. The permeability was found to be linear and repeatable, but the magnitude was 3.2 times smaller than that determined by past research. This factor may be due to negligible grain boundary diffusion that exists in this work. Auger electron spectroscopy (AES) does, however, suggest the importance of grain boundaries since intragranular oxygen was virtually undetectable and since AES line scans show substantial oxygen signals around the grain boundaries. The diffusivity measurements were found to exhibit two distinct linear regions, one above and one below a critical temperature of 630 °C. The high-temperature data have an activation energy (11.1 kcal mol−1) similar to that reported by others, but the low-temperature data have a comparatively larger activation energy (15.3 kcal mol−1). Vacuum desorption of the oxygen-saturated Ag was found to occur at the critical temperature of 630 °C, which is consistent with the increased mobility of oxygen atoms in the higher temperature regime. The higher activation energy observed in the lower temperature regime is probably due to the higher efficiency of traps.

Ion implantation in β-SiC: Effect of channeling direction and critical energy for amorphization

Abstract: Damage in single-crystal ..beta..-SiC(100) as a result of ion bombardment has been studied using Rutherford backscattering/channeling and cross-section transmission electron microscopy. Samples were implanted with Al (130 keV) and Si (87 keV) with doses between 4 and 20 x 10/sup 14/ cm/sup -2/ at liquid nitrogen and room temperatures. Backscattering spectra for He/sup +/ channeling as a function of implantation dose were initially obtained in the (110) direction to determine damage accumulation. However, the backscattered yield along this direction was shown to be enhanced as a result of uniaxial implantation-induced strain along (100). Spectra obtained by channeling along this latter direction were used along with the computer program t-smcapsr-smcapsIm-smcaps to calculate the critical energy for amorphization. The results for amorphization of ..beta..-SiC at liquid nitrogen and room temperature are approx.14.5 eV/atom and approx.22.5 eV/atom, respectively.

Anisotropic shrinkage of cordierite-type glass powder cylindrical compacts

Abstract: The pronounced anisotropy of shrinkage in the axial and the radial direction, observed in earlier studies on the sintering behavior of cordierite-type glass powder, is investigated in detail. It is shown that a planar orientation of the pore/solid interface exists that gives rise to an oriented shrinkage in the radial direction. The results show that the axial-to-diametral shrinkage ratio increases from a value of 0.3 to approximately 0.7 with increasing density, and that particle size and particle shape of the glass powders have little influence on shrinkage anisotropy.