Henry Preiss

Carbothermal synthesis of vanadium and chromium carbides from solution-Derived precursors

Abstract Vanadium and chromium tartrate precursors prepared from aqueous solutions have been used as preceramic materials for carbothermal reactions with and without simultaneous nitridation. Their thermal behaviour has been investigated by TG/DTA, X-ray diffraction and measurement of surface areas. Under pyrolysis up to about 600°C, reactive composites consisting of intimately mixed carbon and amorphous M 2 O 3 oxides are formed by salt decomposition. The subsequent processes of crystallization of oxides and carbothermal reduction render both the composites and the reduction products porous. The carbothermal reactions leading either to V 8 C 7 , a V(C,N,O) solid solution, and Cr 3 C 2 , respectively, proceed at moderate temperatures between 800 and 1100°C. The final products result as assemblages of fine carbide or carbonitride particles, however, with extensive heat treatment the particles grow and the surface areas diminish. Air oxidation of the final products has been studied by simultaneous TG/DTA.

Formation of black glasses and silicon carbide from binary carbonaceous/silica hydrogels

Binary hybrid gels consisting of a carbonaceous hydrogel and a silica gel were prepared as preceramic materials. The pyrolytic conversion into carbon-containing glasses under inert conditions were followed by TG analysis, FTIR spectroscopy and nitrogen adsorption. On pyrolysis up to 700 °C, the functional groups of the carbonaceous gel constituent were destroyed and the porosity of the binary gels is nearly completely reduced. Black glasses consisting of amorphous silica and carbon are formed between 700 and 1400 °C. TG analysis showed that the dense glasses with a carbon content < 10 wt.% are stable up to 1000 °C in air. Heat treatment at 1300–1400 °C renders the glasses porous and instable against oxidation by air. On heating above 1400 °C, the SiO2 crystallizes and converts into SiC by a carbothermal reduction. At 1600 °C, the glass with a molar ratio CSi = 3 has transformed into SiC crystallites with grain sizes of 1 μm and less.

Thermal treatment of binary carbonaceous/zirconia gels and formation of Zr(C,O,N) solid solutions

Abstract Binary organic—inorganic gels, in which a carbonaceous gel obtained from a coal tar pitch and a zirconia gel are combined, were prepared as precursors for carbothermal reduction with and without simultaneous nitridation to form Zr(C,O,N) solid solutions. Zirconium(IV) propoxide as the starting material for zirconia was hydrolyzed in the presence of acetylacetone in order to avoid undesirable precipitation. After pyrolysis at 800 °C, a precursor consisting of reactive carbon and nanometersized ZrO2 was obtained. Its behavior on heat treatment was investigated by TG, X-ray analysis, and nitrogen adsorption. Microporosity was generated by crystallization of the zirconia constituent in the temperature range 800–1100 °C. The carbothermal conversion into Zr (C,O,N) begins under nitrogen at about 1100 °C and under argon at about 1200 °C; mesoporosity is additionally formed in both cases. The final products at 1500 or 1600 °C—solid solutions of cubic Zr(C,O) or Zr(C,O,N)—are assemblages of fine crystallites with grain sizes of l μm and less. The formation of these sub-micrometer crystallites may be explained by the high homogeneity and reactivity of the gel-derived carbon reactant. Nitrogen adsorption is shown to be a useful technique to follow the reduction reactions in gel precursors.

Formation of NbC and TaC from gel-derived precursors

Abstract Binary hydrogels, in which a niobium or tantalum oxide gel and a pyrolysable organic compound are combined, were prepared as preceramic materials. Carbonaceous gel or saccharose is used as the organic gel constituent, and alkoxides or peroxo acids are the starting materials for the transition metal component. Under pyrolysis at 600–700 °C, the gels are transformed into carbide precursors in which a reactive carbon and finely dispersed oxide particles are mixed intimately. The subsequent carbothermal reduction proceeds at lower temperatures compared to physical mixtures of oxides and carbon black. The high-temperature treatment was monitored by TG/DTA, X-ray diffraction and nitrogen adsorption. Microporosity is generated in the temperature range 700–800 °C, and mesoporosity is additionally formed at higher temperatures. The changes in porosity are controlled by the processes of crystallization, carbothermal reduction and sintering. The final products at 1400–1500 °C are assemblages of fine particles of the face-centred cubic carbides with grain sizes of 1 μm and less.

Swelling, intercalation and reactivity of sulfonated pitches and mesophase pitches

Abstract Sulfonation of pitches and mesophase pitches renders them hydrophilic and gives rise to swelling, intercalation and ion-exchange properties of the sulfonation products because of their partly lamellar structure. The swelling of a sulfonated coal tar pitch (SP) and a sulfonated mesophase pitch (SMP) by alcohols and water was studied by X-ray diffractometry, measurement of swelling ratio and differential calorimetric analysis (d.c.a.). The X-ray patterns of the alcohol-swollen products show that the alcohol molecules are extensively sorbed in laminar fashion in the gaps between the aromatic lamellae by a mechanism resembling intercalation. The interlaminar expansion, swelling ratio and heat of immersion increase with carbon number for aliphatic n -alcohols. The reactivity of the carbonaceous matrix is enhanced by swelling. Oxidation of SP and SMP by hydrogen peroxide converts both pitches into mainly water-soluble carbonaceous materials forming hydrosols and hydrogels. FT-i.r. spectroscopy and chemical analysis indicate that a high concentration of oxygen functional groups, especially carboxylic groups, is produced by oxidation.

Pore formation during the pyrolysis of C-Nb2O5 and C-Ta2O5 xerogels

Binary organic-inorganic gels have been prepared by mixing a carbonaceous hydrosol and a Nb2O5 or Ta2O5 sol derived by hydrolysis of the alkoxides. The gels are pyrolyzed under an inert atmosphere into precursors in which carbon and the metal oxides are mixed very intimately. High temperature treatment converts the precursors into the cubic face centered carbides. The precursors as well as the carbides have been shown to be micro- and mesoporous materials. Measurements of nitrogen adsorption reveal a characteristic change of the shapes of the isotherms (Type I → Type IV) and of the hysteresis loops (H4→H2→H1) during the thermal processes. Pore widening has been observed with rising temperature. The phenomena of crystallization, carbothermal reduction and sintering were found to control the pore shape and size. The results of the adsorption measurement correlate well with those of the thermoanalytical and X-ray diffraction studies.

Preparation of carbon catalysts by hydrothermal treatment of a carbonaceous hydrogel

Abstract The effects of hydrothermal treatment (liquid water) at 300–470°C on a carbonaceous hydrogel containing acidic functional groups were studied. Unlike the pyrolysis of the dried hydrogel in the absence of water, the hydrothermal treatment converts the gel into a solid carbonaceous product characterized by a substantial pore system and residual functional groups on the pore surface. Total pore volumes and specific surface areas, determined by Hg porosimetry and N 2 adsorption, increase with the increase of the hydrothermal temperature. Surface acidity, however, decreases with rising hydrothermal temperature. As was shown by the dehydration of n -butanol to form ethers and butenes, the hydrothermally prepared products are effective acidic catalysts. Their catalytic efficiency can be raised by an increase of the area and the acidity of the surface. Thus, the sample hydrothermally prepared at 350°C shows the highest degree of catalytic conversion.

Characterization of pitches by X-ray scattering and differential scanning calorimetry

Abstract In this study X-ray investigations of distilled and partly pyrolysed pitches were performed at moderately small scattering angles. The average molecular diameters, determined from the peak maxima at moderately small angles, increase with the temperature of heat treatment. In the case of pitch distillation, the increase of the molecular diameter correlates with the increase of the glass transition temperature measured by differential scanning calorimetry (d.s.c.). It was found that both X-ray and d.s.c. methods are well adapted to check pitch transformation during heat treatment.

Structure and swelling of halogenated mesophase pitches

Abstract An attempt has been made to characterize the structure of the reaction products resulting from the halogenation of mesophase pitch. FT-i.r. spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction show the presence of halogenated aromatics and hydroaromatics which are of a lower lamellar orientation than in non-halogenated mesophase pitches. The halogenated pitches swell in contact with solvents acting as electron donors or acceptors. Basic organic solvents are the most active swelling agents. A crosslinked structural model is proposed which comprises ordered domains of lamellar aligned aromatics and a disordered network. The crosslinks are bonding regions to join the ordered domains and the disordered material. It is assumed that halogenated hydroaromatic molecules assist in producing the crosslinks.

Structural investigations of graphite intercalation compounds of antimony chloride fluorides by Moessbauer spectroscopy

Abstract Natural graphite flakes were intercalated by SbCl 5 , SbCl 4 F and SbCl 2 F 3 at T ≤ 125°C for a reaction time of t ≤ 2 hours. The 121 Sb Moessbauer spectra of the stage 2 intercalation compounds show only one intense line in the Sb(+ 5) region. The spectra can be fitted both in terms of two inequivalent Sb(+ 5) sites and of a single Sb(+ 5) site. The isomer shifts 6 increase by replacing Cl by F in the intercalates as well as by intercalation of the halides in the graphite lattice. The isomer shifts of the intercalation compounds are attributed to polymeric species of the type (mSbX 5 · SbX 6 ) − . Because the spectra provide no evidence for Sb(+ 3), a significant oxidation of graphite is ruled out. Intercalation compounds heated for a long time at 150°C were found to exhibit a dramatically high concentration of Sb(+ 3) which is interpreted as the result of a thermal decomposition of the pentahalides.