Microstructure and thermal expansion behavior of diamond/SiC/(Si) composites fabricated by reactive vapor infiltration

Diamond/SiC/(Si) composites were fabricated by Si vapor vacuum reactive infiltration. The coefficient of thermal expansion (CTE) of composites have been measured from 50 to 400 °C. With the diamond content increasing, CTE of composite decreased, simultaneously, the microstructure of the composites changed from core–shell particles embedded in the Si matrix to an interpenetrating network with the matrix. The CTEs of composites versus temperature matched well with those of Si. The Kerner model was modified according to the structural features of the composites, which exhibited more accurate predictions due to considering the core–shell structure of the composites. The thermal expansion behavior of the matrix was constrained by diamond/SiC network during heating.     

喷墨打印头陶瓷喷孔的皮秒激光加工研究

喷墨打印头的喷孔直径一般为几十微米,材料一般是有机薄膜、不锈钢薄片等。针对有机膜等作为喷嘴板材料的缺陷,采用氧化锆陶瓷代替,以提高耐磨能力和耐腐蚀性。采用皮秒激光打孔方法,可在多种材料上加工出直径几十微米的小孔,包括聚酰亚胺（厚度50μm）、氧化锆陶瓷（厚度120μm）和铜片（厚度70μm）。通过控制不同的参数变化,可制备出孔径约20μm、孔间距为140μm的喷孔阵列。对比不同参数下喷孔尺寸和微观形貌发现,较高功率密度、较低单点停留时间有助于得到圆孔,减少重铸层。     

Hydrogen trapping in precipitation-hardened alloys

The ingress of hydrogen in three precipitation-hardened alloys (Inconel 718, Incoloy 925, and 18 Ni maraging steel) exposed to an acetate electrolyte (1 mol L⁻¹ HAc/1 mol L⁻¹ NaAc where Ac = acetate) was studied using a potentiostatic pulse technique. The data were shown to fit a diffusion/trapping model under interface control, and values were determined for the irreversible trapping constants (k) and the flux of hydrogen into the alloys. The density of irreversible trap defects in Inconel 718 and Incoloy 925 was calculated from k and found to be in excellent agreement with the concentration of NbTi(CN) and TiC particles, respectively. The maraging steel was characterized by two trapping constants; one is associated with quasi-irreversible traps that saturate, leaving only irreversible traps thought to be TiC/Ti(CN) particles. The irreversible trapping constants for these alloys are consistent with their relative susceptibilities to hydrogen embrittlement. Moreover, a comparison of the trapping constants with those for AISI 4340 steel and two other nickel-base alloys (Monel K-500 and MP35N) indicates that a strong correlation exists between hydrogen embrittlement susceptibility and trapping capability over all the alloys.     

Conduction mechanism of polyaniline: Effect of moisture

It has recently been shown that the emeraldine base polymer form of polyaniline undergoes a transition from an insulating to a metallic state upon protonation with HCl. The conductivity of the protonated polymer is sensitive to the moisture content of the polymer, varying by as much as a factor of five upon pumping the samples. We report here the temperature-dependent microwave-frequency (10¹⁰ Hz) conductivity and dielectric constant as a function of environmental history. The results are consistent with the effect of moisture on the barriers between small metallic polymer grains. Texture of granular metal particles and localization within metallic islands are decisive in the frequency- and temperature-dependence of the conductivity and dielectric constant.     

Influence of some anions on the stress corrosion of low alloy steels in aqueous solutions

Polarization and stress corrosion behaviour of two types of low alloy steels, AISI 4130X and AISI 4145, were studied in agitated 0.05 M sodium acetate and 0.05 M trisodium phosphate solutions saturated with natural gas at room temperature. The magnitude of currents, shown in potentiokinetic polarization curves in the range of —0.8 V(SCE) to +0.8 V(SCE) in these media, is very small compared with earlier results obtained in sulphate and chloride solutions.Stress corrosion experiments conducted using a slow strain rate technique under cathodic conditions indicate the onset of hydrogen embrittlement on both types of steel in sulphate, chloride, acetate and phosphate solutions. Under anodic conditions, the results in 0.05 M sodium acetate solutions indicate strong susceptibility to stress corrosion compared with no susceptibility in 0.05 M trisodium phosphate solutions if there is no onset of pitting. These results are in contrast with results showing strong anodic dissolution and consequent absence of stress corrosion under anodic conditions in sulphate and chloride media.Results of Auger and IR analyses of anodic surface films formed in acetate and phosphate solutions are presented and discussed in relation to their observed mechanical behaviour.     

The effect of temperature on hydrogen induced fracture in UNI 40NiCrM07 steel

The influence of the temperature on hydrogen embrittlement in UNI 40NiCrMo7 steel was evaluated by means of delayed failure tests. The tests were carried out in the temperature interval 2°–60°C using samples which had undergone two different heat treatments. Hydrogen was introduced into the samples in intensiostatic conditions (d.d.c. 10mA/cm²) by means of cathodic charge in a 0.1N sulphuric acid solution thermostatically maintained at the test temperatures. The results obtained showed a noticeable influence of the temperature on threshold load values and crack incubation times in the tests carried out on quenched material, whereas this influence was found to be negligible in the tests performed on tempered material. This different behaviour was attributed to the effect of the temperature on movement kinetics in the dislocations, responsible for the accumulation and transport of hydrogen into the zones of maximum stress, and on hydrogen diffusion in the metallic lattice.     

Metallographic cross sections and scanning electron microscopy as analytical tools in the failure analysis of rectifiers

The use of metallography for the assessment of the integrity of the ohmic bond of rectifiers has been investigated. With care, good cross sections can be prepared. However, the integrity of the silicon to metal contact bond cannot be reliably estimated from a two-dimensional section. Scanning electron microscopy was found to be less susceptible to ambiguities as a result of specimen preparation. The depth of field of SEM micrographs permits a clear identification of the ohmic contact.