O. A. Shlyakhtin

Recent progress in cryochemical synthesis of oxide materials

Recently conventional cryochemical processes have been complemented by cryoextraction, cryoprecipitation, cryoimpregnation and freeze casting techniques. Considerably more attention is being paid to colloidal solution processing and the application of sol-gel procedures like ion exchange treatment and hydrolysis of organometallic compounds. New preparation methods for nanopowders of various oxides and metals have been developed while further treatment of cryochemical powders using powder engineering techniques allows the powder grain size to vary from 0.2 to 5-6 µm. Cryochemical methods are also suitable for preparation of various porous media, like porous silica or cryogels with surface areas close to those of aerogels, and for the synthesis of HTSC powders for different applications. Fundamental and applied aspects of cryochemical processing are discussed.

Particle size control of LiCoO2 powders by powder engineering methods

The directed influence on the micromorphology of precursors during preparation of lithium cobaltate powders by chemical synthesis methods allows to control the size of LiCoO2 crystallites from 4–6 mm to 60–100 nm. Localization of melting during thermolysis by mixing acetate precursors with foaming agents promotes the reduction of grain size from 4-6 down to 1–1.5 mm. More efficient prevention of grain coalescence can be performed by introducing a thermally stable inert encapsulation agent (K2SO4) removed by dissolution after thermal processing. The combination of this method with intense deagglomeration of precursor mixtures by planetary milling results in keeping the grain size at the level of primary LiCoO2 particles (60–80 nm) even at T=800 C. Intense mechanical processing of as-formed LiCoO2 powders is undesirable due to negative influence on the crystallographic ordering processes. # 2003 Elsevier Science Ltd. All rights reserved.

Enhancement of low field magnetoresistance by chemical interaction in bulk composites La0.7Sr0.3MnO3/SrMeO3 (Me=Ti, Zr)

In order to enhance low field magnetoresistance (LFMR) of bulk manganite materials, two series of composites, La0.7Sr0.3MnO3/SrTiO3 and La0.7Sr0.3MnO3/SrZrO3, have been prepared by thermal processing of mixed powders. For both series annealing of pelletized mixtures over certain critical temperature leads to the substantial enhancement of LFMR for compositions close to theoretical percolation threshold. At higher processing temperatures enhanced MR values are observed for composites with higher La0.7Sr0.3MnO3 content, while compositions with a lower amount of manganite become insulating. The magnitude of enhanced MR remains almost the same within the same series, but the value of enhancement is substantially different for composites, containing SrTiO3 and SrZrO3 (1.5% and 2.5% at H=100 Oe, respectively). Similar dependencies are observed for coercive field values. Observed shift of the enhancement boundary during thermal processing and significant dependence of SrZrO3 lattice parameters versus components ...

New manganite-based mediators for self-controlled magnetic heating

La?Ag and La?Na perovskite manganites could be considered as smart mediators for various biomedical applications requiring self-controlled isothermal heating including magnetic hyperthermia. Dissipation of the alternating magnetic field energy causes heating of their aqueous suspensions to terminate at 42?48??C without external temperature control. Termination temperatures of manganite suspensions are close to Tc and demonstrate, unlike other proposed mediator materials, a negligible dependence on the concentration of mediator particles in suspension.

In situ study of the ZnO?NaCl system during the growth of ZnO nanorods

Synchrotron x-ray diffraction and high temperature SEM examinations were used to study the crystallochemical and morphological evolution of a ZnO–NaCl system during thermal processing. Decomposition of ZnO precursor (Zn2(OH)2CO3 ∗ xH2O) at T < 400 ◦ Ci saccompanied by shrinkage and destruction of its fibre-lik ep articles into nanosized isotropic ZnO crystallites. Intensive intergrowth of ZnO nanoparticles and its interaction with coarse NaCl crystallites leads at T = 650–700 ◦ Ct o the formation of a continuous sponge-like framework with an elementary unit size which is far below the initial size of NaCl crystallites. The higher temperature processing at T 700 ◦ Ci saccompanied by the appearance of nanorods. The role of the metastable liquid phase in the formation of ZnO nanorods at T < 800 ◦ Ci s discussed. Luminescence properties of ZnO nanorods synthesized at 500 and 700 ◦ Cw ere demonstrated.

Influence of chemical prehistory on the phase formation and electrochemical performance of LiCoO2 materials

Abstract Thermal decomposition of freeze-dried salt precursors leads to the formation of low-temperature (LT) modification of LiCoO 2 at 350–450 °C. The conversion rate of LT into high-temperature (HT) modification at 850 °C depends greatly on the anion composition of salt precursors and correlates quite well with the appearance of second step at thermogravimetric curves of their thermal decomposition related to the solid-state reaction between Li 2 CO 3 and Co 3 O 4 . Relationship between the appearance of Co 3 O 4 and preferential formation of LT/HT polymorphs at reduced temperatures is discussed. The consecutive formation of LT and HT modifications during solid-state reaction between Li 2 CO 3 and Co 3 O 4 at T >800 °C was observed. LiCoO 2 cathode materials with the domination of LT polymorph demonstrated a better initial discharge capacity while a greater amount of HT modification is accompanied by better reversibility of charge–discharge processes.

Preparation of dense La0.7Ca0.3MnO3 ceramics from freeze-dried precursors

Abstract Sinterability of La 0.7 Ca 0.3 MnO 3 precursors, obtained by the freeze-drying method, is studied in order to develop a technique for preparation of dense (>95%) ceramics for CMR measurements and sputtering applications. Single phase powders, obtained by thermal decomposition at 650°C, were subjected to deagglomeration by ultrasonic or mechanical treatment. Sintering of deagglomerated powders for several hours at T =1200–1300°C allowed to achieve densities up to 97–98%. The best sinterability is demonstrated by mechanically processed powder, but further sintering of ceramics, obtained from this precursor, results in significant dedensification (up to 85% at 1300°C). Analysis of precursors and dedensified samples shows at high temperature decomposition of carbonates in closed pores to be the most probable reason for the observed process.

On the synthesis and thermodynamic properties of SrLaGaO4-SrLaAlO4 solid solutions

Abstract Continuous solid solutions with a tetragonal structure of K 2 NiF 4 -type are formed in the SrLaGaO 4 -SrLaAlO 4 system. Samples of SrLaAl x Ga 1−x O 4 (0.0 ≤ x ≤ 1.0) were obtained by the freeze-drying method and their structural parameters and thermodynamic properties were studied by X-ray diffraction and heat flux Calvet calorimetry. Composition dependence of lattice constants was observed to follow Vegard’s low. Heats of solution of the SrLaAl x Ga 1−x O 4 samples in molten 2PbO·B 2 O 3 were measured, and enthalpies of formation and mixing were calculated. The possibility of eliminating the lattice mismatch of these phases and YBa 2 Cu 3 O x (x = 6.90–7.00) or Bi 2 Sr 2 CaCu 2 O 8 by proper selection of the Al/Ga ratio makes them perspective substrate materials for high-temperature superconducting thin films.

Enhanced low field magnetoresistance at room temperature in La0.7Sr0.3MnO3/SrTiO3(SrZrO3) composites

Abstract Enhanced room temperature magnetoresistance (1.5–2.5% at H=100 Oe ) is observed for composites La0.7Sr0.3MnO3/SrMeO3 (Me=Ti, Zr) with compositions close to the percolation boundary. Firing of the mechanical mixture of La0.7Sr0.3MnO3 (LSM) and SrMeO3 at T=900–1200°C results in significant interaction between constituents and in the shift of the percolation threshold to higher LSM content. The values of enhanced magnetoresistance, observed near percolation boundaries, are poorly dependent on firing temperatures but differ significantly for different insulating components.

A freeze-drying approach to the preparation of HgBa2Ca2Cu3O8+x superconductor

A freeze-dried mixture of metal nitrates was used to prepare the BaCaCuO precursor for the HgBa2Ca2Cu3O8+x (Hg-1223) superconducting phase. From this study we found that a highly homogeneous precursor is essential to increase the fraction of the Hg-1223 phase and to reduce the impurity phases. The quality of the samples prepared by using this method is highly improved in comparison with samples prepared by the conventional method of thermal decomposition of the nitrates.