Oxidation of Low-Oxygen Silicon Carbide Fibers (Hi-Nicalon) in Carbon Dioxide

Polycarbosilane-derived low-oxygen SiC fibers, Hi-Nicalon, were heat-treated for 36 ks at temperatures from 1273 to 1773 K in CO₂ gas. The oxidation of the fibers was investigated through the examination of mass change, crystal phase, resistivity, morphology, and tensile strength. The mass gain, growth of β-SiC crystallites, reduction of resistivity of the fiber core, and formation of protective SiO₂ film were observed for the fibers after heat treatment in CO₂ gas. SiO₂ film crystallized into cristobalite above 1573 K. Despite the low oxygen potential of CO₂ gas ( p _O2= 1.22 Pa at 1273 K − 1.78 × 10² Pa at 1773 K), Hi-Nicalon fibers were passively oxidized at a high rate. There was a large loss of tensile strength in the as-oxidized state at higher temperatures because of imperfections in the SiO₂ film. On the other hand, the fiber cores showed better strength retention even after oxidation at 1773 K.     

Fe-Ni-Si基奥氏体时效合金的马氏体相变和形状记忆效应

It is well known that the morphologies of the α martensite formed from the γ phase in ferrous alloys are classified into five types of lath, butterfly, (225)A type plate,lenticular and thin-plate. Among those α martensites, onlythe thinplate martensite, which is characterized by containing a high density of transformation twins, has a potential of exhibiting a perfect shape memory (SM) effect.Recently the present authors found in Fe-Ni-Si alloys that the thin-plate martensite is formed by the introduction of fine and coherent γ-(Ni,Fe)3Si particles with a L12 ordered structure in the austenite matrix due to ausaging. In the present study, the SM properties of the ausaged Fe-Ni-Si alloys with the thin-plate martensite are investigated by a conventional bending-test. The effects of the addition of Co to the Fe-Ni-Si alloys on the martensitic transformation and the SM properties are also investigated. It is shown that while the ausaged Fe-Ni-Si ternary alloys exhibit an imperfect SM effect due to reverse transformation from stress-induced thin-plate martensite to austenite, the SM properties are improved by the addition of Co. An almost perfect SM effect is confirmed in the Fe-Ni-Si-Co alloys by heating to 1 100 ℃ after deformation at -196 ℃.     

Co41Ni32Al27-xSix合金的马氏体相变和磁性转变

利用金相显微组织分析技术、示差扫描量热法(DSC)和振动磁力计(VSM),考察了Co41Ni32Al27-xSix合金中Si元素含量x对马氏体相变和铁磁性转变的影响,用X射线衍射方法分析马氏体相的结构类型.增加x能够显著提高合金的马氏体相变温度,并且同时提高铁磁性转变Curie点;在x≤5的范围内,x增加1可以造成马氏体相变温度提高50-60 K,同时Curie点提高大约10 K;马氏体相的晶体结构仍然是L10型有序结构,但是随着x的增加,单胞体积减小.讨论了马氏体相变温度和Curie点同时提高的原因.     

β基Ni52Ga28Fe20-xCox合金的马氏体相变和磁性转变

利用金相和SEM，XRD，DSC和VSM研究了Co含量x对Ni52Ga28Fe20-xCox合金马氏体相变和铁磁性的影响。结果表明：x≤6％时（原子分数，下同），Co代替Fe显著提高合金的马氏体相变温度，每1％Co可以使马氏体相变温度提高50K～60K，但对Curie点的影响不大。淬火温度从1423K降低到773K对马氏体相变温度的影响不大，但使Curie点提高20K-30K，其原因是母相的B2→L21转变导致的有序化程度提高。粉末试样X射线衍射发现1423K和773K2种温度处理后都只出现L10（2M）晶体结构类型的马氏体，并且合金粉末化后降低了马氏体相变温度和增加了γ相的析出，但块状Ni52Ga28Fe17Co3合金773K淬火后为14M型马氏体。     

Phase equilibria and phase transformation of Co−Ni−Ga ferromagnetic shape memory alloy system

Phase equilibria and martensitic and magnetic phase transformations of the β phase in the Co−Ni−Ga system have been investigated. It has been shown that the β phase is in equilibrium with the α-phase over a wide range compositions at 600–1100°C. The β phase exhibits both a paramagnetic-ferromagnetic transition and a thermoelastic martensitic transition from B2 to L1₀ structure. The Curie temperature T _C increases with decreases with decreasing Ni content and with increasing Ga content. The composition region of the β phase exhibiting the thermoelatstic martensitic transformation from ferromagnetic austenite is located near the α+β two-phase region. T _C and M _s of α+β two-phase alloys increase with increasing annealing temperature. This paper was presented at the International Symposium on User Aspects of Phase Diagrams, Materials Solutions Conference and Exposition. Columbus, Ohio, 18–20 October, 2004.     

Effect of atmosphere on pyrolysis of Nicalon

The pyrolytic behaviour of Nicalon under a N₂ atmosphere was investigated at temperatures from 1673 to 1973 K, and was compared with that under an Ar atmosphere. The pyrolytic rate was measured by thermogravimetry, and heat-treated Nicalon was examined by X-ray diffraction, scanning electron microscopy, Auger electron spectroscopy and tensile testing. The pyrolytic rate was smaller in N₂ than in Ar. The nitrided case retarded the crystallization into -SiC and retained its high strength. The effectiveness of the nitrided case disappeared on heating in Ar. The strength was related to the size of the -SiC crystal in Nicalon.     

Simulation study on molecular relaxation in ionomer melts

Relaxation behavior of ionomer molecule in melt state was investigated by Brownian simulation where motion of a bead-rod chain representing a linear ionomer molecule was calculated in two-dimension. Entanglement between the other molecules was assumed by a repulsive interaction between the chain and randomly located entanglement points. Effect of ion aggregates was also introduced by an attractive interaction between the chain and attractive sites randomly dispersed in the simulation box. Relaxation spectra of end-to-end motion and local bond orientation were obtained in equilibrium calculation for various chain lengths. The effect of the attractive site was discussed on the spectrum. It was found that on the end-to-end motion, the longer the chain was, the more discrepant the spectrum was, while on the bond orientation, the shorter it was, the more discrepant the spectrum was. On the basis of these results, a picture to describe the experimental phenomena was proposed.     

Electrorheological response of swollen silicone gels containing barium titanate

We have investigated storage moduli of silicone gels containing barium titanate in the presence of dc electric fields. The gels containing barium titanate swollen by silicone oil showed a storage modulus reduction, i.e. negative electrorheological effect. Contrary, no negative electrorheological effect was observed in the unswollen gels and silicon gels without barium titanate. Swollen silicone gels and most of silicone/BaTiO₃ gels obeyed empirical quadratic dependence in electric field strength. Although an apparent phase separation was not observed in the swollen gel, microscopic phase separation may occur due to the difference in electric conductivity between particles (∼10⁻¹⁰ S/cm) and silicone oil (10⁻⁹ S/cm), as a result, the negative electrorheological effect appears. The effects of frequency, degree of swelling, and the field strength have been discussed.     

Discharge behaviors and jet profiles during electrospinning of poly(vinyl alcohol)

Relations between beads formation and jet profiles, and beads formation and electric discharge during electrospinning were investigated by changing sample parameters and spinning conditions. The jet profiles were obtained as a function of the position along the spinning line by using photographs. As the molecular weight or concentration increased, the elongational viscosity of jet at the onset of bending instability was increased, and beads formation was suppressed. In addition, beads formation was enhanced with increasing voltage. The electric discharge spark from the apex of cone was observed by using image intensified high‐speed camera. It was found that the beads were also formed easily when the electric discharge generated. POLYM. ENG. SCI., 50:1788–1796, 2010. © 2010 Society of Plastics Engineers