High thermal conductivity diamond coated graphite by microwave plasma chemical vapour deposition

Diamond film was grown on high thermal conductivity graphite substrate using microwave plasma chemical vapour deposition method. Nanodiamond particles were uniformly seeded on the substrate to generate high nucleation density by a spray gun. The continuous and high purity diamond film was obtained, and growth rate was up to 2.7 μm h^??1. The thickness, surface morphology, quality and composite phase of the film were analysed by SEM, Raman and X-ray diffraction. It was shown that graphite coated with diamond presented a higher thermal conductivity (520?W?m^??1 k^??1) than copper. Furthermore, this coated material with high thermal conductivity, good strength and non-conductive surface will make it possible to be widely used in thermal management field.     

Effect of temperature and substrate surface texture on wettability and morphology of IMCs between Sn–0.7Cu solder alloy and copper substrate

In the present work, the effect of soldering temperature (270 and 298?°C) and substrate surface texture (0.02 and 1.12?μm) on wetting characteristics and morphology of intermetallic compounds (IMCs) between Sn–0.7Cu lead-free solder on copper substrates was investigated. It was found that increase in temperature and substrate surface roughness improved the wettability of solder alloy. However, the effect of surface roughness on wettability was significant as compared to that of temperature. The spreading of solder alloy was uniform on smooth substrate, whereas spreading of the alloy on rough substrate resulted in an oval shape. The morphology of IMCs transformed from long needle shaped to short and thick protrusions of IMCs with increase in surface roughness of the substrate. Needle shaped and thick protruded intermetallics formed at the solder/Cu interface were identified as Cu₆Sn₅ compounds. The formation of Cu₃Sn IMC was observed only for the spreading of solder alloy at 298?°C which contributed to improvement in the wettability of solder alloy on both smooth and rough substrate surfaces.     

Electrodeposition and morphology analysis of Bi-Te thermoelectric alloy nanoparticles on copper substrate

Nanoscale Bi-Te particles with thermoelectric properties on copper substrate were investigated. The substrate was prepared by electroplating copper layer on a copper zinc alloy plate in a copper sulfate solution. Electrodeposition of the Bi-Te alloy particles was then performed in a nitrate bath. The electrolyte is composed of 0.05 M bismuth nitrate and 0.01 M tellurium dioxide dissolved in 2.0 M HNO₃. Cyclic voltammetry and quartz microbalance tests associated with the electrodeposition process were conducted to show the mechanism and kinetics of the deposition. The morphology and compositional analysis of Bi-Te was obtained using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) respectively. The morphology analysis suggested that nanoscale Bi-Te particles were obtained and the EDS results indicated that the surface of the copper substrate contained Cu₂O. The atomic ratio 1:1 for Bi:Te in the alloy, which is equivalent to the weight percentage of Bi:Te = 62%:38% was confirmed from the data obtained.     

Nucleation and growth of copper particles on Pt and Pt/poly-3-methylthiophene modified electrode in presence of Cl complexing agent

The kinetics of electrochemical deposition of copper particles from Cu²⁺ solution on platinum and poly-3-methylthiophene modified platinum electrode was studied in potentiostatic conditions in presence of Cl⁻ anions. The complex behavior of current transients suggests that the deposition process involves several stages with different kinetics. Results obtained on platinum show that after an initial adsorption process, the copper deposition is accomplished through two different models: a three-dimensional nucleation and growth under diffusive control (3DPD model) and a progressive nucleation and two-dimensional growth (2DP model). The analysis of current transients recorded on platinum poly-3-methylthiophene modified electrode (Pt/PMT) shows a very different behavior. On Pt modified electrode a process of growth related to a semi-infinite diffusion to a planar surface was accompanied by two different mechanisms of nucleation and growth: a three-dimensional nucleation and growth with no diffusive control (3DP model) and an instantaneous nucleation with two-dimensional growth (2DP model).     

Substrate-diffusion-controlled film growth: Silver and copper on lead (111)

Silver and copper were vapour deposited onto atomically clean lead (111) surfaces under ultrahigh vacuum conditions. Low energy electron diffraction, reflection high energy electron diffraction and Auger electron spectroscopy measurements in the same chamber showed that, for a 0.1 nm min^-1 impingement of silver or copper, the deposit completely re-evaporated. At 10 nm min^-1 both silver and copper condensed onto a lead (111) surface at room temperature. The silver deposit was a continuous-layer single crystal which had an atomically flat upper surface covered by an equilibrium monolayer of substrate atoms epitaxed to it. The results for the copper experiments were similar to those for silver, except for the presence of small amounts of misoriented copper (111) and polycrystalline copper.It is proposed that substrate diffusion during the condensation of silver or copper onto lead (111) is crucial in determining the mode of film growth. It is thought that two-dimensional nucleation results from a mixed mobile layer of adsorbate and substrate atoms.     

Study of the crystallization kinetics in amorphous Fe73.5Cu1Nb3Si13.5B9 alloy

The kinetics of the crystallization of Fe_73.5Cu₁Nb₃Si_13.5B₉ amorphous alloy was studied using Mössbauer spectroscopy and X-ray diffractometry. During isothermal annealing of the samples, two phases were observed: a crystalline D0₃-FeSi alloy with fine grains and an amorphous phase enriched with niobium, boron and copper. It was found that the growth rate of the particles of the crystalline alloy was controlled by diffusion. For longer annealing time or at higher annealing temperature the growth process was found to be suppressed, probably by the niobium atoms. The activation energy obtained for the crystallization was about 143 kJ mol^–1.     

Structure evolution on annealing of copper-doped carbon film

Thin copper-doped (8 at.% Cu) carbon film was deposited by direct current magnetron sputtering of composite graphite/copper target in argon plasma. The evolution of film structure on annealing at 600 °C in a vacuum has been studied by transmission electron microscopy and electron diffraction. The as-deposited film was amorphous with copper atoms uniformly distributed over the film volume. Annealing resulted in precipitation of copper particles within carbon film followed by the decrease in the density of copper particles and increase in particle average size with annealing time due to diffusion coalescence within the ensemble of copper particles. The coalescence occurred by the mixed mechanism of bulk and surface diffusion of copper atoms within carbon film that contained a large number of structural defects. As a result, the mean radius of copper particles in ensemble changed as R¯⁵ ∼ t.     

Sintering of silver and copper nanoparticles on (001) copper observed by in-situ ultrahigh vacuum transmission electron microscopy

The sintering of copper and silver nanoparticles with single crystal copper substrates has been studied using a novel in-situ ultrahigh vacuum transmission electron microscope (UHV TEM). The system is equipped with a UHV DC sputtering attachment enabling metal nanoparticles to be generated in-situ and transferred directly into the microscope in the gas phase. In both cases, we find the particles to be of initially random orientation on the substrate. Upon annealing, however, the particles reorient and assume the same orientation as the substrate. The process apparently occurs by a mechanism involving sintering and grain growth. In the case of silver on copper, grain growth cannot occur since the metals are immiscible. Our observations show that, upon annealing, the particles wet the substrate surface and form epitaxially oriented islands by surface diffusion and grain boundary migration. The post-anneal islands exhibit the orientation relationship (111)_Ag∥001)_Cu, [110]_Ag∥[110]_Cu.     

Wettability and spreading kinetics of liquid aluminium on boron nitride

The wettability and spreading kinetics of liquid AI on CVD-BN were investigated by the sessile drop method in a vacuum of about 1.1×10⁻³ Pa at 1070 to 1430 K. The wettability on the Al-BN system was different from that of the Al-SiC system reported in the literature. The wetting angle of Al-BN linearly decreased with an increase of temperature in high temperature range, and tended towards 15° at 1430 K. Complete wetting could be obtained at about 1470 K. The interface energy between liquid Al and the reaction layer and the surface energy of CVD-BN were calculated by means of Warren thermodynamics analysis. The surface of CVD-BN was examined by XRD. The results show that the surface of CVD-BN was chiefly composed of those low energy planes parallel to (001). According to the relationship of the spreading kinetics from the experiments, the apparent activation energy of the liquid Al spreading on the surface of CVD-BN was calculated. The result shows that the activation energy of interfacial reaction accounts for about 54% of the spread activation energy. The spread of liquid Al on CVD-BN was affected by the interfacial reaction, diffusion and liquid movement on the surface.     

Kinetic Monte Carlo simulation of Cu thin film growth

A three-dimensional kinetic Monte Carlo technique has been developed for simulating the growth of thin Cu films. The model involves incident atom attachment, surface diffusion of the atoms on the growing surface and atom detachment from the growing surface. A significant improvement in calculation of activation barriers for the surface atom diffusion on the growing film was made. The related effects caused by surface atom diffusion were taken into account. The results showed that there exist a transition temperature T_t at a certain deposition rate. When the substrate temperature approaches T_t, the growing surface becomes smoother and the relative density of the films increases. The surface roughness minimizes and the relative density saturates at T_t. The surface roughness increases with increased substrate temperature when the temperature is higher than T_t. T_t is a function of the deposition rate. The influence of the deposition rate on the surface roughness is dependent on the substrate temperature. The simulation results also showed that the relative density decreases with increasing deposition rate and average thickness of the film.     

Oxygen permeation through perovskite membranes and the improvement of oxygen flux by surface modification

Abstract

BSCF hollow fiber membranes possessing an asymmetric layered structure were prepared using a modified phase inversion process followed by subsequent sintering at temperatures from 1100 to 1175 ?C. The fibers were characterized by SEM, and tested for air separation at ambient pressure and temperatures between 650 and 950 ?C. Although the prepared hollow fibers resulted in self-supported asymmetric substrate with a very thin densified perovskite layer for mixed conduction, O₂ permeation was controlled by surface O₂ exchange kinetics rather than bulk diffusion. In order to improve O₂ flux, surface modification was carried out by the attachment of Pt particles on the surface of the hollow fiber. The maximum O₂ flux measured for pure perovskite hollow fiber was 0.0268 mol m^?2 s^-1 at 950 ?C whilst O₂ fluxes increased up to 25% after the surface modification using Pt micro-particles.     

Transient liquid phase diffusion bonding of 6061-13 vol.% SiCp composite using Cu powder interlayer: mechanism and interface characterization

Transient liquid phase diffusion bonding of an extruded 6061-13 vol.% SiCp composite at 560 °C, 0.2 MPa, using a 50-μm thick copper powder interlayer with 20 min, 1 h, 2 h, 3 h, and 6 h hold times has been investigated. The isothermal solidification and homogenization of the bond region occurred with 3 and 6 h holding, respectively. During isothermal solidification, smaller SiC particles (size range: 11–13 μm) were pushed by the moving solid/liquid interface and segregated around the bond centerline, whereas bigger SiC particles (size range: 24–33 μm) were engulfed. During isothermal solidification, the bigger SiC particles locally hindered the solidification front movement causing grain refinement. The kinetics of isothermal solidification, representing the displacement of the solid/liquid interface (y, in μm) as a function of time (t, in s), followed a power-law relationship: y = 35 t ^0.22. According to this kinetic equation, the effective diffusivity of copper in composite system was found to be about 10⁵ times higher than the lattice diffusivity indicating the dominance of short circuit diffusion through the defect-rich particle/matrix interface. Ultrasonic investigation of the bond interface indicated that the signal attenuation was strongly correlated with the width of the segregated layer-a feature that decreased with the increasing bonding time. The completion of isothermal solidification was indicated by a sharp rise in the received signal amplitude with a negligible attenuation.     

Equilibrium and kinetic studies of copper adsorption by activated carbon

Copper adsorption by granular activated carbon is reported in this paper. The experimental section includes titrations of activated carbon, as well as equilibrium and kinetic studies of copper adsorption. The potentiometric titration results show that the point of zero charge is 9.5, and that the surface charge increases with decreasing pH. The adsorption of copper strongly depends on solution pH and increases from 10 to 95% at pH ranging from 2.3 to 8. A dramatic increase in pH and emission of small gas bubbles are observed during the experiments, which may result from adsorption of hydrogen ion and/or reduction-oxidation reactions. The two-pK triple-layer model is employed to describe copper adsorption. KINEQL, an adsorption kinetics algorithm, is used to represent the experimental data, and it is found that the model can describe reasonably well the experimental measurements of surface charge, adsorption equilibrium, and adsorption kinetics. Calculations show that formation of the surface-metal complexes SO⁻Cu²⁺ and SO⁻CuOH⁺ (a hydrolysis product of SO⁻Cu ²⁺) in the outer layer around the surface of carbon results in removal of copper ion. It is also found that mass transfer controls the adsorption rate, and that adsorption occurs in the micropore region where both external mass transfer and diffusion are important.     

Microstructure and crystallographic properties of Cu77Zn21 alloy under the effect of heat treatment

The influence of heat treatment process on microstructure and physiochemical properties of Cu₇₇Zn₂₁ alloy was studied. Samples were heated from 300 to 700°C for 15 minutes and gradually cooled to room temperature. Samples were characterized using mechanical testing and surface analysis techniques. It was observed that the grain size increases with the increasing temperature. The decrease in hardness is due to composition change of the Cu-Zn alloy attributed to zinc diffusion.XRD shows the presence of only α brass with a preferential orientation along the (111) plane. The analyses show that temperature affects the crystalline parameters. The rms microstrain was reduced mainly by thermal activation. The heat treatment promotes atomic diffusion and leads to a better crystallinity of the particles. The results of Raman investigations confirmed the changes in the Cu-Zn alloy surface composition with a clear enrichment in CuO. This behavior correlates with the results obtained by mechanical testing.     

Interfacial morphology and concentration profile in the unleaded solder/Cu joint assembly

A joint assembly of lead-free solder/intermetallic layers/copper was prepared by hot-dipped solder coated on a copper substrate and then by thermal ageing at 100, 125, 150, and 170°C for 50, 100, 200, and 600 h, respectively. Results of interfacial morphologies and concentration profiles on the solder/copper joint were presented. Optical and scanning electron microscope (OM and SEM) were used to measure the thickness of intermetallic layers and then to illucidate the development of microstructure at the joint assembly. The phases of intermetallic compound were identified to be Cu₃Sn and Cu₆Sn₅ by both X-ray mapping in electron probe micro-analysis (EPMA), and X-ray diffraction. The intermetallic layers, subtracted from the initial thickness formed by hot dipping, showed a linear dependence on the square root of ageing time at various ageing temperatures. The diffusion coefficients of intermetallic compounds are estimated by an Arrhenius equation, and the pre-exponential terms of Cu₃Sn layer and Cu₆Sn₅ layer are 7.10×10^-7cm²s^-1 and 6.1×10^-3cm²s^-1, respectively. The associated activation energies of Cu₃Sn layer and Cu₆Sn₅ layer are 57.03kJmol^-1 and 83.76kJmol^-1, respectively. A model of a diffusion-controlled mechanism is used to fit the concentration profiles of the joint assembly, and exhibits a fairly good quantitative agreement with the measured data. The initial thickness formed as soldering proceeds is also taken into account to evaluate the apparent thickness by introducing a term of corrected ageing time. ©1999 Kluwer Academic Publishers     

Low temperature interdiffusion in Cu/Ni thin films

Interdiffusion in Cu/Ni thin films was studied by means of Auger electron spectroscopy in conjunction with Ar⁺ ion sputter profiling. The experimental conditions used aimed at simulating those of typical chip-packaging fabrication processes. The Cu/Ni couple (from 10 μm to 60 nm thick) was produced by sequential vapor deposition on fused-silica substrates at 360, 280 and 25 °C in 10^-6 Torr vacuum. Diffusion anneals were performed between 280 and 405 °C for times up to 20 min. Such conditions define grain boundary diffusion in the regimes of B- and C-type kinetics. The data were analyzed according to the Whipple-Suzuoka model. Some deviations from the assumptions of this model, as occurred in the present study, are discussed but cannot fully account for the typical data scatter. The grain boundary diffusion coefficients were determined (for nickel through copper, Q_b = 33.7 kcal mol^-1 (1.46 eV), D_b⁰ = 4.2 × 10^-2 cm²s^-1; for copper through nickel, Q_b = 30.2 kcal mol^-1 (1.3 eV), D_b⁰ = 7.6 × 10^-5 cm²s^-1) allowing calculation of respective permeation distances.     

Thermal conductivity of Cu/diamond composites prepared by a new pretreatment of diamond powder

Cu/diamond composites were fabricated by spark plasma sintering (SPS) after the surface pretreatment of the diamond powders, in which the diamond particles were mixed with copper powder and tungsten powder (carbide forming element W). The effects of the pretreatment temperature and the diamond particle size on the thermal conductivity of diamond/copper composites were investigated. It was found that when 300 μm diamond particles and Cu–5 wt.% W were mixed and preheated at 1313 K, the composites has a relatively higher density and its thermal conductivity approaches 672 W (m K)⁻¹.     

X-ray diffraction line profile analysis of diffusional homogenization in powder blends

A new nondestructive method — based on computer simulation of X-ray diffraction line profiles — is proposed to characterize homogenization in compacted binary powder blends. As a parameter to characterize the stage of homogenization the relative peak position is proposed. This parameter is easy to determine in practice: during homogenization the position of an X-ray diffraction maximum is traced. As compared to other methods the present one has the following advantages: (i) it is fast and simple and (ii) it allows a more severe test of the model of interdiffusion applied. Experiments were performed with compacted blends of copper and nickel powders at 800, 900 and 1000° C. At the start of homogenization diffusion was very fast. Experiments at lower temperatures revealed that this was due to surface diffusion at the contact places between the copper and nickel particles with an activation energy of about 12 kcal mol^–1. Because of the sensitivity of the relative peak position to the interdiffusion model adopted it was shown that the generally accepted concentric sphere model: nucleus of nickel and shell of copper, should be modified to include a pre-alloyed shell at the copper/nickel interface at t=0. Then good correspondence between theory and experiment is obtained. Finally it was found that in the temperature range applied one diffusion mechanism is dominant with an activation energy of 32 kcal mol^–1, indicating grain boundary diffusion.     

Spreading of gallium on tin surfaces

Summary A study of the spreading of gallium on the surface of tin at 60°–70° C showed that immediately after application of liquid gallium (within 3 min), side by side with a drop of liquid phase a solid diffusion zone (with a characteristic surface relief) is formed, which-according to X-ray diffraction data [5]-is a solid solution of gallium in tin. The rate of spreading of the liquid drop is very slow, seldom exceeding 1×10^–6 mm/min. The rate of spreading of the diffusion zone changes from 1.6×10^–2 mm/min (in the first few minutes after application of gallium) to 0.7×10^–6 mm/min (48 hr later), which evidently is caused by the gradual reduction of the quantity of gallium available. The spreading of the diffusion zone takes place in a step-like fashion, the number and length of arrests increasing as the quantity of liquid gallium diminishes.The extent of the diffusion zone is increased by the application of an additional quantity of gallium, but there is a marked time-lag (several hours) between the second gallium application and the movement of the diffusion zone boundary. The rate of spreading of the gallium-coated zone across a tin specimen surface may vary from grain to grain owing, most likely, to different crystal orientation of various grains.Thanks are due to T. M. Averyanova and L. L. Krapivin for their assistance in the experimental work, and to E. D. Shchukin, B. D. Summ, A. V. Pertsov, and N. V. Pertsov for their part in interpreting the results.     

Oxygen permeation through perovskite membranes and the improvement of oxygen flux by surface modification

BSCF hollow fiber membranes possessing an asymmetric layered structure were prepared using a modified phase inversion process followed by subsequent sintering at temperatures from 1100 to 1175 °C. The fibers were characterized by SEM, and tested for air separation at ambient pressure and temperatures between 650 and 950 °C. Although the prepared hollow fibers resulted in self-supported asymmetric substrate with a very thin densified perovskite layer for mixed conduction, O₂ permeation was controlled by surface O₂ exchange kinetics rather than bulk diffusion. In order to improve O₂ flux, surface modification was carried out by the attachment of Pt particles on the surface of the hollow fiber. The maximum O₂ flux measured for pure perovskite hollow fiber was 0.0268 mol m⁻² s⁻¹ at 950 °C whilst O₂ fluxes increased up to 25% after the surface modification using Pt micro-particles.