Hf-C system

With few exceptions there have been no studies of the effect of oxygen and nitrogen impurities on the Hf-C system. It is not unreasonable, however, to expect this system to behave like the other members of Group 4 in this regard. An additional difficulty is created by the frequent presence of zirconium, which generally depresses the various properties. When the zirconium content is given, the reported measurements have been corrected assuming the mixed system to have a linear variation of the property between end members of equal carbon content. 

The unique advantage of the plasma chemical method is the ability to collect the condensate, which can be used for raw material decomposition or even liquid-liquid extraction processes. The condensate consists of a hydrofluoric acid solution, the concentration of which can be adjusted by controlling the heat exchanger temperature according to a binary diagram of the HF - H20 system [534]. For instance, at a temperature of 80-100°C, the condensate composition corresponds to a 30-33% wt. HF solution. 

The HF-SbFs system is known to be a superacid H34). The possible relevance of this to the intercalation process was pointed out by Vogel V12), who first reported on the extremely high electrical conductivity of graphite-SbFj measured normal to the crystallographic c-axis. The measured conductivity was approximately 40 times that of pristine graphite, and 50% greater than that of pure copper. Other workers... 

Protonated cyclopropane has been considered to be a real intermediate since the work of Aboderin and Baird (1964), and protonated alkylcyclopropanes have also recently come to be considered intermediates as opposed to transition states in many rearrangements occurring in strong acid media where SbFs is the Lewis acid. Thus Brouwer and Oelderik (1968a) suggested that protonated methyl-cyclopropane is an intermediate in the isomerization of sec-butyl-1- C to sec-butyl-2- cations in the HF-SbFj system, and Saunders et al. (1968) implicated this species in the rearrangement of... 

Halogen-donor Ligands. Xep2 reacts with excess VF5 at 90 °C to give XeF2,VF5 as colourless transparent crystals, m.p. 22—28°C. The i.r. spectrum of the vapour indicates that the compound is completely dissociated in this phase. The phase diagram for the HF—VF system has been presented. 

The 5 (or 6)HF Et3N system reacts with a variety of branched alkenes to give tertiary alkyl fluorides. Hydrofluorination of dienes proceeds at the more reactive C = C bond selectively. Thus, ethyl geranate (1) and ethyl nerolate (2) react with 5HF EtaN at the noncon-... 

Stream Number Stream Source Liquid Flowrate (GPD) Arseni c Rate (Ib/MO) Percent Arsenic to HF Treatment System and Recycled Solids Total Arsenic Concentration (mg/L) Percent Solids Soluble Arsenic Concentration (mg/L)... 

The isomerization of ra-pentane in superacids of the type RFS03H-SbF5 (RF = C F2 +i) has been investigated by Commeyras and co-workers.93 The influence of parameters such as acidity (A), hydrocarbon concentration (B), nature of the perfluoroalkyl group (C), total pressure (D), hydrogen pressure (E), temperature (F), and agitation has been studied. Only A, C, E, and F have been found to have an influence on the isomerization reaction in accordance with such reactions in the HF-SbFs system. 

Whereas the C2—C4 alcohols are not carboxylated under the usual Koch-Haaf conditions, carboxylation can be achieved in the HF-SbF5 superacid system under extremely mild conditions.400 Moreover, Olah and co-workers401 have shown that even methyl alcohol and dimethyl ether can be carboxylated with the superacidic HF-BF3 system to form methyl acetate and acetic acid. In the carboxylation of methyl alcohol the quantity of acetic acid increased at the expense of methyl acetate with increase in reaction time and temperature. The quantity of the byproduct dimethyl ether, in turn, decreased. Dimethyl ether gave the desired products in about 90% yield at 250°C (90% conversion, catalyst/substrate ratio =1 1, 6h). On the basis of experimental observations, first methyl alcohol is dehydrated to dimethyl ether. Protonated dimethyl ether then reacts with CO to yield methyl acetate [Eq. (5.154)]. The most probable pathway suggested to explain the formation of acetic acid involves the intermediate formation of acetic anhydride through acid-catalyzed ester cleavage without the intervention of CO followed by cleavage with HF [Eq. (5.155)]. 

Enthalpies of activation, transition-state geometries, and primary semi-classical (without tunneling) kinetic isotope effects (KIEs) have been calculated for 11 bimolecu-lar identity proton-transfer reactions, four intramolecular proton transfers, four nonidentity proton-transfer reactions, 11 identity hydride transfers, and two 1,2-intramole-cular hydride shifts at the HF/6-311+G, MP2/6-311+G, and B3LYP/6-311+-1-G levels.134 It has been found that the KIEs are systematically smaller for hydride transfers than for proton transfers. The differences between proton and hydride transfers have been rationalized by modeling the central C H- C- unit of a proton-transfer transition state as a four-electron, three-centre (4-e 3-c) system and the same unit of a hydride-transfer transition state as a 2-e 3-c system. 

Other actinide(VI) oxide fluorides exist. The thermal decomposition of UOF4 at 290°C (226) and the reaction of uranyl fluoride with SeF4 (275) yield a yellow solid, which is U203F6. The compound U205F2 is formed as the dihydrate in the U03-HF-H20 system (276) and when UF6 reacts with a small quantity of water, U305F8 is the product (275). 

The Hf-B-C system presents a situation that falls somewhat between the Ti and Zr systems [60]. Although the HfB phase is stable in the Hf-B binary system, it melts at 2100°C, below the melting point of the Hf parent metal (2227°C). During a directed metal oxidation reaction of molten Hf with B4C at just above the melting point of Hf, e.g., 2400°C, the Hf-B-C isothermal ternary cross section (Fig. 18) indicates that the molten metal is... 

Measurements of this type were carried out for the methylbenzene-(HF + BFg) system at 0°C by MacCaulay and Lien (1951). However, the authors used the method only in order to demonstrate the formation of the proton addition complex. They used partition measurements to determine the basicity. 

High molecular weight PAGs, 12-30K amu, are found to be particularly suitable for the formulation of the HF-C type of fluid. PAG, water (35% minimum) and glycol, and together with anticorrosion, anti-wear and antifoam additives, are formulated to conform to manufacturers specifications for particular systems. Stringent flammability requirements, corrosion protection, low levels of wear, foaming and other demands such as viscosity and air release must all be achieved. 

Nikolaev et a/. have reviewed the thermodynamic properties of the alkali-metal and alkaline-earth-metal hydrofluorides. Boinon et aC have reinvestigated the NaF-HF system for 30—45 wt. % NaF and discussed their results in the light of earlier work. Examination of the 10 C isotherm of the RbF-HF-H2O system confirms the existence of a number of hydrates and hydrofluoride phases, and provides evidence for the formation of a new solvate RbF,4HF. ... 

The author has used liquid-liquid extraction with thenoyltrifluoroacetone (TTA) and potentiometry using a fluoride selective electrode to determine the stoichiometiy and equihbrium constants of the fluoride complexes of Th(lV) and U(1V) only the Th(IV) data will be discussed here as the U(1V) data have been reviewed previously in [1992GRE/FUG]. The experiments have been made in a 4 M HCIO4 ionic medium at 20°C. The experimental data are described in detail and this study provides very useful information on the quality of the two experimental approaches used. The same author has previously studied the Zr(lV) and Hf(lV) systems using the same experimental approach. The slope of the ion selective electrode was determined experimentally and equilibrium was assumed to be attained when the emf changed by less than 0.1 mV/h. The experiments were made at three different total concentrations of Th(lV), 0.90, 1.79 and 4.48 mM and with the total concentration of HF varying from 0.406 to 225 mM. This corresponds to a concentration of free fluoride, 8 x 10 < [F ] < 5 x 10 M. The analysis of the experimental emf data is straightforward and the conditional equilibrium constants for the two reactions ... 

The reaction has been carried out by om group on aU small alkanes from Cl to Cs. l A comparison between exchange and ionisation rates shows that reversible protonation is much faster than ionisation and thus is directly related to the relative basicity of the various sigma-bonds, as expected from inductive effects (Table 3). Extensive reversible-protonation is not accompanied by intramolecular atom scrambling in protonated alkanes, as shown in a study involving C labelled propane and propane 1,1,1,3,3,3 de in the HF-SbFs system. This demonstrates that skeletal rearrangement does not occur via carbonium ions and necessitates carbenium intermediates. 

Benzene in the HF—TaFj system is hydrogenated by molecular hydrogen in the presence of isoalkanes at 50 °C to yield cyclohexane which is further partly converted into methylcyclopentane It is assumed that the first formed benzenium ion takes a hydride-ion from the Hj molecule or isoalkane. For example, mesitylene has... 

Another possible explanation is the hydrogenase system. In T. saccharolyticum, adaptation for improved growth after elimination of acetate production resulted in mutations in both adhE and the hfs hydrogenases. In C. thermocellum, the same treatment resulted in adhE mutations, but no mutations in the hydrogenases (Holwerda et al., unpubUshed). It is possible that there are properties of the hfs hydrogenase system in T. saccharolyticum that make it uniquely suited for high-yield ethanol production. If this is true, part or all of this system may need to be transferred to C. thermocellum. 

Besides the f.c.c. 8HfNj the Hf-N system contains two subnitride phases [41], riHf3N2 ( and Hf4N3 j. Only a few phase equilibria studies have been performed for this system and they are cited in the ASM Handbook [10]. The decomposition temperatures of the subnitride phases need to be re-investigated. 

G. D. Brundiers, Herstellung, Aufbau und Eigenschaften von Hafniumverbindungen im System Hf-C-N-O. Kernforschungszentrum Karlsruhe GmhH, KfK 2161, 1975, p. 7. 

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