Thermal conductivity of CVD diamond films

Diamond films 60 and 170 µm in thickness were grown by PACVD (plasma-assisted chemical vapor deposition) under similar conditions. The thermal diffusivity of these freestanding films was measured between 100 and 300 K using AC calorimetry. Radiation heat loss from the surface was estimated by analyzing both the amplitude and the phase shift of a lock-in amplifier signal. Thermal conductivity was calculated using the specific heat data of natural diamond. At room temperature, the thermal conductivity of the 60 and 170 m films is 9 and 16 W-cm^–1. K^–1 respectively, which is 40–70% that of natural diamond, The temperature dependence of thermal conductivity of the CVD diamond films is similar to that of natural diamond, Phonon scattering processes are considered using the Debye model, The microsize of the grain boundary has a significant effect on the mean free path of phonons at low temperatures. The grain in CVD diamond film is grown as a columnar structure, Thus, the thicker film has the larger mean grain size and the higher thermal conductivity. Scanning electron microscopy (SEM) and Raman spectroscopy were used to study the microstructure of the CVD diamond films. In this experiment, we evaluated the quality of CVD diamond film of the whole sample by measuring the thermal conductivity.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

An Atomistic Tomographic Study of Oxygen and Hydrogen Atoms and their Molecules in CVD Grown Graphene

The properties and growth processes of graphene are greatly influenced by the elemental distributions of impurity atoms and their functional groups within or on the hexagonal carbon lattice. Oxygen and hydrogen atoms and their functional molecules (OH, CO, and CO₂) positions' and chemical identities are tomographically mapped in three dimensions in a graphene monolayer film grown on a copper substrate, at the atomic part‐per‐million (atomic ppm) detection level, employing laser assisted atom‐probe tomography. The atomistic plan and cross‐sectional views of graphene indicate that oxygen, hydrogen, and their co‐functionalities, OH, CO, and CO₂, which are locally clustered under or within the graphene lattice. The experimental 3D atomistic portrait of the chemistry is combined with computational density‐functional theory (DFT) calculations to enhance the understanding of the surface state of graphene, the positions of the chemical functional groups, their interactions with the underlying Cu substrate, and their influences on the growth of graphene.     

Synthesis and electrochemical characterization of LixCoO2 for lithium-ion batteries

The cathode material Li_xCoO₂ was synthesized by preliminary mechanochemical activation of precursor oxide mixture powders, followed by thermal treatment at 800 °C for 5 h. The effects of the molar ratio of Li/Co on the electrochemical behavior were examined. The Li_xCoO₂ at Li/Co=1.07 showed superior cycling stability to the Li/Co=1.0 sample. This is attributed to the disappearance of a phase transition related to monoclinic distortion and the relatively lowered transport resistance in Li/Co=1.07 sample.     

Boundary element analysis of thermal stress intensity factors for interface griffith and cusp cracks

The thermal stress intensity factors for interface cracks of Griffith and symmetric lip cusp types under vertical uniform heat flow in a finite body are calculated by the boundary element method. The boundary conditions on the crack surfaces are insulated or fixed to constant temperature. The relationship between the stress intensity factors and the displacements on the nodal point of a crack-tip element is derived. The numerical values of the thermal stress intensity factors for an interface Griffith crack in an infinite body are compared with the previous solutions. The thermal stress intensity factors for a symmetric lip cusp interface crack in a finite body are calculated with respect to various effective crack lengths, configuration parameters, material property ratios and the thermal boundary conditions on the crack surfaces. Under the same outer boundary conditions, there are no appreciable differences in the distribution of thermal stress intensity factors with respect to each material property. However, the effect of crack surface thermal boundary conditions on the thermal stress intensity factors is considerable.     

Wedge type creep damage in low cycle fatigue

A model is proposed to quantify the accumulation of wedge type creep damage in low cycle fatigue. It is proposed that such damage is produced primarily during the ramp periods of the cycle. Equations are developed for estimating incremental accumulation of damage per cycle in fully reversed, multiaxial loading. The rate of accumulation of damage depends on the strain-rate, the temperature, and the microstructure. The analysis is kept simple by making physically reasonable assumptions. Cycles to failure are predicted by invoking a fracture criterion. The model is applied to two sets of data; one set is a well characterized life test data on an aluminum alloy, and the other is phenomenological data on austenitic stainless steels. In both cases the predictions are good enough to prompt further experimental evaluation of the model. This paper deals with only one mechanism of creep-fatigue interaction. Other mechanisms of failure,e.g., ‘r’ type cavitation, or fatigue crack initiation and propagation, are also viable. The model described here may be expected to apply only under those conditions when wedge damage is the dominant failure mechanism.     

A study on interfacial mechanisms and structure of poly(ethylene-co-methacrylic acid)/copper with reflection–absorption infrared spectroscopy

The interfacial mechanism and structure of poly(ethylene-co-methacrylic acid)/copper were investigated using reflection-absorption infrared spectroscopy (RAIR). Based on IR spectrum of EMAA/copper, a strong absorption peak appearing at approximate 1,600 cm⁻¹ is attributed to the asymmetric stretching vibration of COO^― (υCOO_as^―) and a relatively weak absorption band at 1,375 cm⁻¹ is assigned to the symmetric stretching vibration of COO^― (υCOO_s^―). Therefore, it can be determined that copper interacts with EMAA through a strong ionic interaction and carboxylate structure is formed in the interfacial regions. And, according to the band intensities of carboxylate stretching modes and different sensitivities of RAIR to perpendicular and parallel transition moments, it can be concluded that EMAA is absorbed onto a copper substrate with a configuration in which the twofold axis of the C_2υ point group for carboxylate group inclines certain degree from the normal to the surface. In addition, the interfacial carboxylate structure of EMAA/copper is confirmed to be a monodentate one through calculating the difference (Δυ)between the asymmetric carboxylate stretch (υCOO_as^―) and the symmetric stretch (υCOO_s^―).     

Multifunctional Ir–Ru alloy catalysts for reversal-tolerant anodes of polymer electrolyte membrane fuel cells

To address the problem of fuel starvation in fuel-cell electric vehicles,which causes cell voltage reversal and results in cell failure when repeated continuously,we developed a reversal-tolerant anode(RTA) to promote water oxidation in preference to carbon corrosion.Graphitized carbon-supported Ir-Ru alloys with different compositions are employed as RTA catalysts in an acidic polyol solution and are shown to exhibit composition-dependent average crystallite sizes of <5.33 nm.The adopted approach allows the generation of relatively well-dispersed Ir-Ru alloy nanoparticles on the carbon support without severe agglomeration.The activity of IrRu₂/C for the hydrogen oxidation reaction is 1.10 times that of the stateof-the-art Pt/C catalyst.Cell reversal testing by simulation of fuel starvation reveals that the durability of IrRu₂/C(~7 h) significantly exceeds that of the conventional Pt/C catalyst(~10 min) and is the highest value reported so far.Thus,the developed Ir-Ru alloy catalyst can be used to fabricate practical RTAs and replace Pt catalysts in the anodes of polymer electrolyte membrane fuel cells.