High-melting Phase Research Articles

Metastability of dimiristoylphosphatidylethanolamine as studied by FT-IR and the effect of α-tocopherol

The metastability of dimiristoylphosphatidylethanolamine (DMPE) has been studied by means of Fourier transform infrared spectroscopy (FT-IR), both in the absence and in the presence of α-tocopherol. Two different methods of hydration were used to prepare the samples, poorly hydrated and well hydrated, and the results have been compared with anhydrous DMPE. Poorly hydrated DMPE gave place to a high-melting phase formed upon melting from gel to L α at approx. 49°C, with a new transition to L α at approx. 55°C. However, well hydrated DMPE incubated at 4°C for 49 days gave place to a subgel phase which was transformed by heating into a L β phase at about 40°C and this into a L α phase after further heating at 52°C. The subgel phase was more hydrated and less rigid than the high-melting phase. On the other hand, α-tocopherol, when included in poorly hydrated DMPE, stabilized a high-melting phase, which was transformed by heating, directly into a L α phase. However, when a sample containing α-tocopherol was incubated for 49 days at 4°C a dehydrated solid phase different from the subgel and the high-melting phases was formed.

Hot-Pressed ceramics of ultradisperse composite powders

The regularities of variation of the microstructure, phase composition, degree of crystallinity, and properties of hot-pressed ceramics prepared from ultradisperse composite powders of the Si3N4 - Y2O3 system depending on the β/α phase ratio in Si3N4 and the concentration of Y2O3 are discussed. An increase in the concentration of Y2O3 in composite powders up to 15 – 18% and a decrease in the β/α ratio of Si3N4 lead to a change of the composition of the secondary crystalline phases with an increase in their refractoriness (from silicon oxynitride and yttrium silicates to complex yttrium — silicon oxynitrides), a lower concentration of the vitreous phase, a higher degree of crystallinity, and a higher content of stretched grains in the structure of the ceramics. The presence of the high-melting phase of yttrium — silicon oxynitrides improves the phase and granular homogeneity of the structure that forms by the type of solid solutions, with an essentially similar composition of the grains and the intergranular phase. The formation of a high-melting fine-grained self-toughening structure with a coefficient of grain elongation up to 8 – 9 ensures a high and practically stable level of the mechanical properties of the synthesized materials at temperatures up to 1400 – 1500°C.

Effect of molybdenum impurities on the electronic properties of high-melting phases

High-melting carbides and nitrides of transition metals are used intensively for the creation of composites with enhanced thermomechanical characteristics. X-ray microanalysis data show [i] that such melts contain a solid kernel (which may consist of carbides, nitrides, and their mutual solid solutions), a plasticizing bundle (intermetalide), and a socalled transition zone of the circular structure (K-phase), surrounding particles of the solid base and consisting of a solid solution of the elements of the bundle in an outer layer of the high-melting base. It is known that one of the most effective plasticizers of non-tungsten composites is the nickel-molybdenum alloy, andthe K-phase is formed mainly owing to the dissolution of one of the bundle components, molybdenum, in the solid base of the material.

Certain regularities of phase formation and structure evolution during hot-pressing of ultrafine powders

We studied the processes of structure evolution and phase formation in the material during hot-pressing of ultrafine (highly dispersed) composite powders of the [Si3N4-10% Y2O3] system. Densification of the powder occurs under the conditions facilitating chemical interaction in the complex heterogeneous system. The sintering mechanism of UFCP consists of a stage of self-activated rapid grain growth in the solid phase mainly in the 1120–1150°C range and a stage of liquid phase sintering in the 1150–1650°C range under the conditions of varying composition, viscosity, and content of the liquid phase. It was established that densification is initiated by a low-melting yttrium silicate type liquid phase that disappears after participating in the process leading to the formation of a high-melting liquid phase having a composition close to that of yttrium-silicon oxynitrides. The revealed regularities in the behavior of UFCP during the process of hot-pressing offer the possibility of controlling the structure evolution in them purposefully in order to obtain the required set of properties. The results of our studies are being used for obtaining engineering components.

Structure of the high-melting phase of petroselinic acid

L'acide cis-octadec-6-enoique, C 18 H 34 O 2 , cristallise dans Pbca avec a = 7,311, b = 5,565 et c = 88,01 A, Z = 8; affinement jusqu'a R = 0,054. Deux couches bimoleculaires formant une structure polytype a double couche existe dans une unite se repetant le long de l'axe c. Conformation moleculaire.

Microstructure and phase composition in high-strength aluminium alloys produced by a splat-cooling technique

Rapid solidification (RS) experiments have been carried out on high-strength AIZnMgCuX alloys (X = Zr, Ti, Cr or Mn) using the rotating-wing splat-cooling technique. The resulting particles (splats) were examined by light-microscopy, X-ray diffraction (Guinier-technique) and TEM. In comparison to industrially produced powders the microstructure was refined by about a factor of two. The alloying elements could be kept partially in solution and/or led to metastable phases which precipitated during solidification or handling before subsequent examination. Heat treatment of the rapidly solidified splats generated a relatively homogeneous and dense precipitation of several metastable and/or stable phases. In some cases the RS technique used could not prevent special high-melting phases from precipitating already in the melt. These phases acted as quite effective grain size stabilizers during subsequent heat treatment. The higher solidification rates are concluded to show potential for improving the mechanical properties of these alloys.

Chemical Vapor Deposition Techniques

Chemical vapor deposition (CVD) is one of the few deposition processes in which the deposited phase is produced in situ via chemical reaction(s). Thus the vapor source for CVD can consist of high vapor pressure species at moderate temperatures and yet deposit very high-melting phases. For example, pure TiB2, which melts at 3225°C, can be produced at 900°C from TiCl4, BC13, and H2.Chemical vapor deposition and its variants such as low pressure CVD (LPCVD), plasma-assisted CVD (PACVD), and laser CVD (LCVD) have been active areas of research for many years. Recent review articles have contained extensive lists of the phases deposited by CVD, which include most of the metals and many carbides, nitrides, borides, silicides, and sulfides. The techniques have found increased acceptance as commercial methods for the fabrication of films and coatings which are fundamental to the semiconductor device and the high-performance tool bit industries. They have been used to prepare multiphase-multilayer coatings, stand-alone bodies, and fiber-reinforced composites. As the demand increases for more complex and sophisticated materials, it is expected that CVD will play a still larger role.In CVD a solid material is deposited from gaseous precursors onto a substrate. The substrate is typically heated to promote the deposition reaction and/or provide sufficient mobility of the adatoms to form the desired structure. Chemical vapor deposition was performed for the first time when early humans inadvertently coated cooking utensils with soot from the campfire. In this CVD process, hydrocarbons generated by the heated wood pyrolyzed on the utensil surface, depositing carbon.

Electric erosion of the composite W(Mo)-Cu in electroerosion treatment of hard alloy in a carbon-containing liquid. I. Mechanism of wear of pseudoalloys W-Cu

The discovered mechanism of erosion of tungsten-copper pseudoalloys in electrospark treatment of the hard alloy VK20 in a carbon containing liquid Is characterized by the formation of sections of high melting phase on the working surface, the phase consisting of a mixture of tungsten and its carbides, and by the predominant action of the electric discharges on these sections. As a rule, copper is removed along the perimeter of the sections of high melting phase. The amount of removed phase is determined (with constant pulse parameters) by the ratio between the components of the pseudoalloy. The concept was advanced of a dynamic surface layer of pseudoalloys during mass effect on them of electrlc discharges in a carbon containing liquid.

Crystalline transformations in spherulites of poly(vinylidene fluoride)

Parts of poly(vinylidene fluoride) spherulites of the α-phase undergo a transformation to the higher-melting γ-form when crystallized at high temperatures. As a rule, this transformation originates at the periphery of α-spherulites where their lamellae are in contact with, and oppositely directed to, lamellae of γ-spherulites; the latter are formed only at high temperatures. The transformation on then retreats towards the nuclei of α-spherulites at a slow (∼10 −4 μm s −1), linear rate, which increases with temperature. At very high temperatures, this transformation is also initiated at some α-nuclei; a few of the α-spherulites also exhibit areas of the high-melting phase irregularly dispersed within their interiors. In samples crystallized below ∼154°C, the transformation is of very limited extent, even after prolonged annealing at higher temperatures.

The polymorphism of poly(vinylidene fluoride) IV. The structure of high-pressure-crystallized poly(vinylidene fluoride)

Abstract A detailed study of high-pressure-crystallized poly(vinylidene fluoride) has indicated that a mixture of low-melting phase II and high-melting phase I is present, rather than a new crystalline phase (phase III) as originally suggested. The relative amounts of phase I and phase II resulting from crystallization under pressure are a function of pressure and the degree of supercooling. Pressure crystallization at 285°C and 5500 atm results in samples which were pure phase I with an increased melting point of 187°C.

Inverse Segregation of MgO and MgAI2O4 in Fusion‐Cast Bodies

Rapidly cooled castings formed from certain liquids in the system Mg0‐MgA1204 are observed to have an enrichment of the low‐melting spinel in the early solidified skin of the casting. Periclase, the high‐melting phase, is concentrated in the core of the casting. Following the metallurgical literature, an explanation for this nonequilibrium phenomenon, known as inverse segregation, is advanced which involves volume contraction upon solidification and flow‐back of residual, spinel‐rich liquids along interdendritic channels to the contraction volumes. The dependence of phase segregation on cooling history is discussed. Inverse segregation is negligible near the eutectic and extreme near the 70 wt% MgO concentration.

Effect of basic open-hearth slag on magnesium-silicate ladle brick

Magnesium-silicate refractories — dunite, talcomagnesite and olivinite — react with basic open-hearth slag, forming both low-melting phases (pyroxeme-type mineral and monticellite) and high-melting phases (periclase and composite spinel). The preliminary test on magnesite-silicate refractories as lining for steel-casting ladles have shown that the slag adheres and becomes welded to the refractory to an appreciable extent. The hypothesis has been expressed that this adhesion is due to the appearance of periclase and composite spinel in the contact zone — slag from high-melting interacting products — and that these two minerals apparently increase the viscosity of the melted slag.

Ternary alloys formed by aluminium, transitional metals and divalent metals

A general survey has been made, by metallographic and X-ray methods, and by the examination of extracted crystals, of the constitutions of the alloys of aluminium and zinc with iron, cobalt and nickel, of aluminium and magnesium with cobalt, and of aluminium and beryllium with manganese and iron. In the latter two systems, new ternary compounds Mn 3Be2Al 15 and Fe 3Be7Al 7 have been discovered in equilibrium with the aluminium-rich solid solution. No ternary compounds in equilibrium with the aluminium-rich alloys occur in the other systems studied; an unidentified ternary phase separates from zinc-rich alloys in the system Al-Ni-Zn, but does not enter into equilibrium with the zinc-rich solid solution. The outstanding feature of the system Al-Co-Zn is the deposition from zinc-rich melts, at low temperatures, of the high-melting phases CoAl and Co 4Al 13 in addition to Co 2Al 9; these phases persist in equilibrium with the zinc-rich solid solution. Similarly in the system Al-Ni-Zn, Ni 2Al 3 as well as NiAl 3 is found in zinc-rich alloys, while in the system Al-Co-Mg the high-melting phase CoAl enters into equilibrium with the magnesium-rich solid solution. By contrast, only a phase derived from FeAl 3, and none of the more iron-rich aluminium-iron compounds, is found in equilibrium with zinc in the system Al-Fe-Zn. The main features of the equilibria are discussed in terms of the free energies of phases present, and the data, taken together with the results of previous work, lead to a better understanding of the factors affecting the formation of aluminium-rich ternary compounds.