N. T. Kuznetsov

Derivatives of the closo-dodecaborate anion and their application in medicine

The paper presents a comparative analysis of the possibilities and characteristic features of the application of various polyhedral boron compounds, viz., the closo-decaborate anion [B10H10]2–, the closo-dodecaborate anion [B12H12]2–, the carba-closo-dodecaborate anion [CB11H12]–, carboranes C2B10H12, and the bis(dicarbollide) complexes [M(C2B9H11)2]– (M = Fe, Co, or Ni), in boron neutron capture therapy (BNCT) for cancer. The requirements on compounds used in BNCT are formulated and the advantages of the application of the closo-dodecaborate anion are considered. The data on the synthesis of various derivatives of the closo-dodecaborate anion, which either already found use in BNCT or are most promising in this field, are summarized. The possibilities of the application of agents derived from the closo-dodecaborate anion in medical diagnostics are discussed.

Promising ultra-high-temperature ceramic materials for aerospace applications

Some aspects of heat transfer upon the interaction between components with a sharp leading edge and high-enthalpy high-speed flow of dissociated air have been considered; some material characteristics, which should be primarily taken into account when prognosticating the behavior of materials that are promising for using as components of hypersonic flight vehicles, have been substantiated; specific features of the oxidation of materials based on zirconium and hafnium diborides have been touched briefly; the methods of increasing oxidation resistance of these materials that have been developed by various groups of researchers have been demonstrate; some works concerning the behavior of samples under the effect of high-enthalpy flows of dissociated air have been described, including those that simulate sharp leading domes and edges of wings of hypersonic flight vehicles.

Derivatives of closo-decaborate anion [B10H10]2− with exo-polyhedral substituents

Methods of introducing functional groups into the [B10H10]2− anion based on electrophilic, radical, or nucleophilic substitution for exo-polyhedral hydrogen atoms have been surveyed. Special attention has been focused on nucleophilic substitution reactions promoted by acids, including protonic acids, anhydrous hydrogen halides, metal halides, and carbocations. In addition, methods of tailored functionalization of the substituents in the cluster have been described.

Synthesis of highly dispersed super-refractory tantalum-zirconium carbide Ta4ZrC5 and tantalum-hafnium carbide Ta4HfC5 via sol-gel technology

Nanocrystalline powders of super-refractory complex carbides Ta4HfC5 and Ta4ZrC5 were synthesized using a hybrid method comprising sol-gel technology for preparing highly dispersed metal oxidescarbon starting mixtures and a relatively low-temperature (1300–1500°C) carbothermal synthesis under a dynamic vacuum (P = 1 × 10−3 to 1 × 10−5 MPa). The elemental and phase compositions of the products and average crystallite sizes were determined. TEM was used to study particle morphology and dispersion. Microstructures were observed by SEM. BET specific surface areas were determined for powders prepared at 1400°C.

Specifics of pyrohydrolytic and solid-phase syntheses of solid solutions in the (MgGa2O4)x(MgFe2O4)1 − x system

This work demonstrates the possibility of preparing solid solutions in the (MgGa2O4)x(MgFe2O4)1 − x system by pyrohydrolytic and solid-phase methods. It is shown that the products obtained have different specific surface areas depending on the ratio between metal nitrates and citric acid. The composition dependence of the unit cell parameter deviates considerably from the Vegard’s rule. The compounds obtained are found to be stable up to 300°C, which makes them candidate materials for electronics.

Low-temperature synthesis of nanodispersed titanium, zirconium, and hafnium carbides

Nanosized refractory titanium, zirconium, and hafnium carbides were manufactured from highly dispersed metal dioxide-carbon starting mixtures at moderate temperatures of 1200°C or lower. The products were characterized by powder X-ray diffraction, elemental analysis, and transmission electron microscopy. The average size of particles (in nanometers) manufactured at 1200°C was as follows: for TiC, 13 ± 4; for ZrC, 17 ± 3; and for HfC, 16 ± 3; the average crystallite size (in nanometers) was as follows: for TiC, 8 ± 2; for ZrC, 5 ± 2; and for HfC, 8 ± 3. Thermodynamic modeling was performed for the synthesis of Group IVB carbides via carbothermal reduction of the corresponding oxides. The results show that in the titanium dioxide-carbon system, for example, titanium monocarbide formation is possible at a temperature as low as 850°C (p = 10−4 MPa). Highly dispersed metal dioxide-carbon starting mixtures were prepared using solgel technology from metal alkoxyacetylacetonates in the presence of a polymeric carbon source.

Specific interactions in metal salts and complexes with cluster boron anions BnHn2− (n = 6, 10, 12)

Specific interactions that appear in metal salts and complexes with cluster boron anions BnHn2− (n = 6, 10, 12) have been discussed. These interactions, as well as chemical bonds, involve vertices, edges, or faces of boron polyhedra. Specific interactions have a considerable effect on the structure of compounds, making a significant contribution to the formation of the unit cell and forming supramolecular assemblies. Compounds containing BnHn2− cluster anions shed new light onto the nature of specific interactions owing to their many-center character and great variety.

Low-temperature synthesis of TaC through transparent tantalum-carbon containing gel

Thermodynamic modeling of the synthesis of tantalum monocarbide via the carbothermic reduction of tantalum(V) oxide indicates that TaC synthesis is thermodynamically plausible below 1000°C at pressures from 1 × 10−5 to 1 × 10−4 MPa. Using a Ta(OC5H11)5 solution in n-pentanol, we prepared a transparent tantalum-carbon containing gel and then a fine-particle Ta2O5 + C mixture, which was used to synthesize tantalum monocarbide at temperatures from 850 to 1200°C and pressures from 10−5 to 10−4 MPa. The elemental and phase compositions of the samples were determined, and the morphology of the TaC particles was examined by electron microscopy.

Coordination compounds of electron-deficient boron cluster anions BnHn2− (n = 6, 10, 12)

This survey concerns the coordination ability of BnHn2− (n = 6, 10, 12) boron cluster anions and their derivatives in complex formation. Boron cluster anions form four types of compounds: salts of organic cations and alkali-metal cations, including Cat2BnHn, where specific interactions can be observed between a cation Cat and a boron cluster anion; salts of protonated anions CatB6H7 and CatB10H11, analogues of Cat[MBnHn] complexes, where an extra hydrogen atom appears bound with the BBB face of a boron polyhedron and performs as a hard acceptor; metal complexes with outer-sphere boron cluster anions where specific ligand-ligand interactions may be observed between a boron cluster anion and an inner-sphere ligand; and true metal complexes with boron cluster anions that enter the inner coordination sphere. The last case characterizes closo-hydroborate anions as polydentate ligands whose denticity can vary widely under the effect of substituents or other ligands in the complex.

Synthesis of ultrafine yttrium aluminum garnet using sol-gel technology

Mesoporous yttrium aluminum garnet Y3Al5O12 powders were prepared using sol-gel technology proceeding from solutions of metal alkoxoacetylacetonates. Xerogel microstructure was studied by SEM, and the fact of mesopores being formed was established. The temperature range within which Y3Al5O12 crystallizes in a dynamic mode from the xerogel was determined to be 850–950°C using an SDT Q600 TGA/DTA/DSC analyzer. A 1-h isothermal treatment of the xerogel was shown to reduce the garnet phase formation temperature to 800°C. At lower temperatures (400, 450 or 500°C), even long-term (6-h) calcination yielded X-ray amorphous powders with developed surfaces (specific surface areas were 230–350 m2/g). Powder particle coarsening was studied upon sintering for 2 and 4 h at 1000, 1200, and 1400°C.