Subsequent High-temperature Treatment Research Articles

Effects of Interactions Between Low and High Temperature Treatments on Flowering of Spring Rape (Brassica napusvar.annua)

Exposure to high temperature (30 °C) before or after exposure to low temperature (0, 4 or 8 weeks at 4 °C) consistently increased the number of leaf nodes at flowering and delayed flowering in a range of genotypes of spring rape(Brassica napusvar.annuaL.). Four days of prior exposure to high temperature had more effect than 2 d, and the effect of subsequent exposure to high temperature was maximized when exposure commenced 1 week after the end of the low-temperature treatment. In genotypes that showed a vernalization response (i.e. in which the number of leaf nodes at flowering was reduced or flowering was advanced by low temperature), this response was reduced or eliminated by either prior high-temperature treatment (antivernalization) or subsequent high-temperature treatment (devernalization). A biochemical model to account for these effects is proposed.

Metallographic investigation of DIGM in FeNi system

Diffusion-induced grain boundary migration (DIGM) and diffusion-induced recrystallization (DIR) in FeNi system have been studied by an application of an electroless nickel coating on a pure iron substrate and by subsequent high temperature treatment. Using optical and scanning electron microscopy, DIGM and DIR have been observed in the FeNi system for the first time. Mechanisms responsible for these phenomena have been discussed in terms of diffusion processes occurring at high temperatures.

Sulphur release from pyrites in relation to coal pyrolysis

In a study of the fixation of coal sulphur in the coke-char formed during heat treatment, the role of the mineral/inorganic sulphur and the influence of subsequent high temperature treatment on the release of the sulphur were examined with carbon/ferrous sulphide systems prepared by mixing pyrites with organic materials prior to carbonization and by heating mixtures of pyrites and graphitic and non-graphitic carbons. These carbon/ferrous sulphide systems and a series of commercial cokes were heated up to 2000 °C and the residues were analysed for total sulphur and sulphide sulphur. The carbonization of organics mixed with pyrites gave carbon residues with 3–5 wt% sulphur and in the non-graphitic poly(vinylidene chloride) carbon some 40% of this was present in organic form. The sulphur was progressively released by heat treatment above 1000 °C until at 1800 °C there was less than 0.1 wt% in the residual carbon. Heating non-graphic carbons with pyrites again led to the fixation of some organic sulphur in the carbon and such materials tended to retain their sulphur to higher temperatures than the carbon/ferrous sulphide systems based on graphitic carbons. It is concluded that the presence of carbon facilitates the thermal decomposition of ferrous sulphide. The fixation of sulphur by the carbon is, however, to some extent dependent on the nature of the carbon. Such ‘organic’ sulphur is more resistant to high temperature decomposition than the inorganic/mineral sulphur.

Mechanisms of orientation and graphitization of hard-carbon matrices in carbon/carbon composites

It now appears generally accepted that the process of stress-induced orientation and graphitization of a thermoset-resin-derived matrix in a carbon-fiber/carbon-matrix (C/C) composite is principally a result of molecular orientation induced during the pyrolysis process as a consequence of the restraint of pyrolysis shrinkage at the fiber/matrix interface by attractive or frictional forces between fiber and matrix. We hypothesize that the critical factor for the formation of lamellar graphite (by subsequent high-temperature heat treatment), instead of fibrillar or isotropic glassy carbon, is a state of multiaxial deformation in the pyrolysis step. Finite-element stress analyses of the relative stresses in the regions of interfilament matrix, as the matrix pyrolyzes from polymer to carbon, reveal patterns of biaxial and triaxial stress consistent with experimental observations of lamellar graphite formation in the matrix by the techniques of optical microscopy, scanning electron microscopy, and transmission electron microscopy. The implications of localized matrix orientation and graphitization for C/C composite properties are discussed in terms of a “duplex” system of composite reinforcement. An example is presented showing crack deflection and blunting at the matrix/matrix-sheath interface produced as a result of such orientation and graphitization.

MAS n.m.r. and i.r. studies of pillared clays

The silica- or alumina-pillared clays with a definite surface area and a larger basal spacing may be prepared by the gel dispersion method. The changes of i.r. and MAS n.m.r. spectra of these pillared clays before and after calcination at a higher temperature showed that the clays were exchanged with metal-hydroxide sols, that no characteristics changes were observed in the clay structure, and that it is only a cation-exchange process, but on subsequent high-temperature treatment at 450°C, obvious changes in MAS n.m.r. 29Si or 27Al spectra and i.r. framework vibration spectra appeared, so some changes in the clay structure did take place. It is suggested that during the calcination, a chemical reaction between tetrahedral sheets of the clays (with or without AI IV substitution) and the pillaring agents (pillars) takes place, forming the three-dimensional network structure.

Formation of ultra-thin buried CoSi 2 layers by ion implantation in (100) Si

The formation of ultra-thin buried CoSi 2 layers in (100) Si by ion beam synthesis is investigated. Layers as thin as 23 nm under 19 nm of Si are obtained by systematically lowering the dose and energy of the Co implant and continuously optimizing the implant conditions. The most critical parameter is found to be the temperature during implant. The disintegration of the CoSi 2 layers upon high temperature treatment is caused by the presence of small pin-holes. The pin-hole density seems to be directly dependent on the implant conditions whereas their size is mainly determined by the subsequent high temperature treatment.

Arsenic redistribution and out-diffusion in TiSi2-Si bilayered structures

The influence of both titanium silicide formation and a subsequent high-temperature treatment in the range 900-1100 degrees C on the arsenic profile in silicon has been investigated. As a result of the two-step process used for the silicide formation, the dopant contained in the reached silicon layer redistributes and segregates both at the sample free surface and at TiSi2-Si interface. The arsenic profile in the unreacted silicon is not modified. Subsequent rapid high-temperature annealing (900-1100 degrees C) induces a migration of arsenic atoms from the silicon into the silicide. The transport occurs for samples either capped with SiO2 or uncapped. In the latter, in addition, arsenic out-diffusion is also observed. The residual arsenic dose in silicon decreases linearly with the square root of time during the high-temperature anneal. The process is governed by a diffusion coefficient whose measured value, 6*10-14 cm2 s-1, is comparable to that of arsenic in silicon, thus indicating that diffusion is the limiting mechanism for the arsenic loss in uncapped samples.

Reacted amorphous layers: Tantalum and niobium oxides

Abstract The main rule governing the formation of amorphous reacted phases is that one of the components should have a mobility much lower than that of the others. This is based on the simple observation that the crystallization of a multicomponent system requires that all of the components should be mobile. The rule, which is general and independent of the initial state (solid or gas) of the reactants, is verified by comparing the mobilities of oxygen and of the metal atoms during the low-temperature oxidation and subsequent high-temperature heat treatment of bilayers initially composed of niobium and tantalum metal thin films. The high mobility of oxygen allows the oxides to be formed at low temperature in an amorphous condition, while the low mobility of the metal ions prevents the mixing of these ions except at high temperatures.

Zirconium carbide recrystallization

This paper studies the primary recrystallization process of the sintered polycrystalline zirconium carbide with a composition of ZrC /SUB 0.98/ . The properties of zirconium carbide samples deformed under compression are presented; the selected degree of deformation ensures a lower scatter of grain sizes at relative error of +/- 5% in the final deformation measurement. The established mechanisms of structural changes in zirconium carbide during plastic deformation and subsequent high temperature treatment indicate the possibility of using thermomechanical methods for the direct control of the structure of these mechanical methods for the direct control of the structure of these and obviously othe group IV and V carbides obtained by powder metallurgical methods.

Behaviour of chromium-coated steels in sodium polysulphide environments

Several chromium coatings on mild steels were characterized and placed in media containing sodium polysulphides for corrosion studies. Chemical corrosion was studied up to 440°C and electrochemical corrosion at 350°C. Chromium plating with subsequent high temperature treatment was found to give the best results but chromized steels were also found to be suitable for use in cells below 350°C.

Alloy formation and metal oxide segregation in PtRe/γ-AlO catalysts as investigated by temperature-programmed reduction

Temperature-programmed reduction has been used to characterize the finely dispersed metal compounds in a series of Pt Re/γ-Al 2 O 3 catalysts. Strong evidence has been obtained that zerovalent Pt and Re atoms are in intimate contact with each other after catalyst reduction. The formation of “bimetallic clusters” supports the “alloy” explanation for the improved performance of this type of bimetallic reforming catalyst. Treatment of the reduced catalysts with oxygen above about 200°C causes segregation of platinum and rhenium oxides. Adsorption of oxygen at temperatures up to 100°C leaves the bimetallic clusters largely intact, but subsequent high-temperature treatment in the absence of extra oxygen leads to segregation of Pt and Re species. This suggests that in the presence of adsorbed oxygen the Pt-Re clusters are thermodynamically unstable, but that under mild conditions the rate of segregation is slow.