Thermomechanical model of crystalline elastoplastic media

A model is proposed for a solid whose mathematical form is similar to the Ginzburg-Landau theory for superconductors. An effect involving loss of shear stability of a crystalline medium in a field of shear stresses is discussed and the crystalline media are classified into those of the first and second kind depending on the nature of the defect penetration in the bulk of the material. Pis’ma Zh. Tekh. Fiz. 25, 31–38 (October 26, 1999)     

Influence of nonmetallic inclusions on the fatigue strength of metals

A model of growth of short fatigue cracks in the vicinity of nonmetallic inclusions in crystalline metallic materials is proposed. We establish the influence of inclusions on the fatigue strength of materials of this type. A conclusion is made that the inclusions whose typical size is smaller than the grain size exert practically no influence on the fatigue strength of the material. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 2, pp. 5–9, March–April, 2007.     

Heat treatment of wollastonite-type Y–Si–Al–O–N glasses

Cumulative work over the last twenty years has defined the glass-forming regions in several M–Si–Al–O–N systems (M = Mg, Ca, Y, Ln) with the resulting crystalline products identified after heat treatment. Glass-forming regions in nitrogen-rich sialon glasses have been recently reported and heat treatment of some of these glasses in the Y–Si–Al–O–N system has been performed. The crystallization of yttrium-containing glasses is particularly sensitive to small variations in composition and heat treatment temperature and in the current work the results of three series are discussed: (1) a single composition, Y15.2Si14.6Al8.7O54.6N6.9 (16 e/oN), treated at 30 °C intervals between 875–1410 °C; (2) compositions of a constant Y: Si:Al ratio of 3:3:2 and up to 32 e/oN and (3) selected compositions lying on the 28 e/o N plane. Two different sets of crystalline products are found to form above and below 1200 °C. This revised version was published online in November 2006 with corrections to the Cover Date.     

Study of structural transformations and phases formation upon calcination of Zn–Ni–Al hydrotalcite nanosheets

In this paper, we describe a general process for the synthesis of highly crystalline Zn–Ni–Al hydrotalcite-like materials. The structure and thermal decomposition of the prepared samples are studied by XRD, FT–IR, TG–DSC, SEM, TEM and N₂ adsorption/desorption. The morphology of large-sized, porous and hexagonal platelike Zn–Ni–Al hydrotalcite is affected by calcination temperature. BET specific surface area and pore volume are observed to increase with increase of the calcination temperature up to 700°C followed by a further decrease with increasing temperature.     

Electron microscopy study of chemically deposited Ni-P films

The structure of electroless thin films of Ni-P has been studied. The microstructure and the selected area diffraction pattern of the samples reveal that certain samples transform to crystalline Ni with P in solid solution by nucleation and growth, whereas others transform to crystalline state by growth alone. The former set of thin films having a P-content of 19–21 at.% is characterized as amorphous. Films with a P-content of 13–15 at.% fall in the latter category and are characterized as microcrystalline. Those with a P-content of 16–18 at.% contain both amorphous and microcrystalline regions.     

Effect of process conditions on phase mixtures of sol–gel-synthesized nanoscale orthorhombic, tetragonal, and monoclinic zirconia

Zirconia particles synthesized under ambient conditions are frequently amorphous, requiring heat to produce a crystalline phase. Synthesized nanoparticles are generally found in the tetragonal phase. In this article, a room temperature sol–gel synthesis of crystalline sub-10-nm zirconia particles is described. By adjusting the acid concentration of the reaction, it is found that the particles’ crystalline phase can be modified. Under acidic conditions with moisture present, the tetragonal phase is produced, whereas under acidic conditions with low water content, 2–5-nm particles suggestive of the metastable orthorhombic phase are produced. Subsequent heat treatment of all powders produced with this technique resulted in their transformation first to the tetragonal phase, and ultimately to the monoclinic phase. The extent of the transformation to the monoclinic phase depends upon the atmosphere, however, suggesting that oxygen vacancies play a significant role in the stabilization and determination of the resulting phase.     

Microwave annealing for preparation of crystalline hydroxyapatite thin films

A sol was spun on single crystal silicon substrates at a spin-rate of 3000–5000 rpm followed by a low temperature cure to form a stable sol–gel/silicon structure. Good quality crystalline HA films of thickness ∼300–400 nm were obtained by annealing the sol–gel/Si structure in a conventional cavity applicator microwave system with a magnetron power of 1300 W, frequency of 2.45 GHz, and at a low processing temperature of 425 °C for annealing times ranging from 2–60 min. X-ray Diffraction and FTIR analysis confirmed that the crystalline quality of the thin films were comparable or better than those heat-treated under the same processing conditions (temperature and time) in a Rapid Thermal Annealing (RTA) system. The RBS data suggests a composition corresponding to stoichiometric hydroxyapatite Ca₁₀(PO₄)₆(OH)₂, the major inorganic component of bone. The results showed that the HA film thickness decreases with increasing sol spin-rate. The HA films showed good biocompatibility because little monocyte adhesion occurred and hence no inflammatory response was activated in vitro. The potential of microwave annealing for rapid and low temperature processing of good crystalline quality HA thin films derived from sol–gel is demonstrated.     

Comparative study of magnesium ferrite nanocrystallites prepared by sol–gel and coprecipitation methods

Magnesium ferrite particles consisting of nanocrystallites were synthesized by sol–gel and coprecipitation methods. Their mean crystalline size increased with increasing calcination temperature. At the same calcination temperature, the sol–gel-derived sample always had bigger mean crystalline size than the coprecipitation-derived sample, implying that the sol–gel method facilitated the formation of magnesium ferrite crystallites. Most of the sol–gel-derived magnesium ferrite particles had a lamellar structure consisting of nanocrystallites, which were probably derived from the porous dried gel precursor. The magnesium ferrite particles had superparamagnetic properties at 27 °C, and their saturation magnetization increased with increasing size.     

Effect of ZnO addition on properties of cordierite-based glass-ceramics

The influences of addition of ZnO on sintering, crystallization behavior and properties of cordierite-based glass-ceramics were investigated. Results show that with increasing ZnO content, the batch melting temperature, glass transition temperature and crystallization temperature all decrease. Addition of ZnO can greatly improve the sinterability of the glass powders and alter the type of the crystalline phases. Addition of 1.5 wt% ZnO seems to be reasonable. Thermal expansion coefficients (TCEs) of samples increase with increasing ZnO contents. The density was found to be an important factor affecting the dielectric loss of the samples. Dielectric constant and TCEs of the sintered bulk samples were found to depend on their relative densities and crystalline phases. The samples doped with 1.5–3.0 wt% ZnO sintered at 950 °C has a low dielectric constant (5.0–5.2), a low dielectric loss (≤0.0018) and a low thermal expansion coefficient (3.6–4.8 × 10⁻⁶ K⁻¹), which are promising electronic packaging materials.     

Glass–ceramic glazes for ceramic tiles: a review

Glass–ceramics are ceramic materials produced through controlled crystallisation (nucleation and crystal growth) of a parent glass. The great variety of compositions and the possibility of developing special microstructures with specific technological properties have allowed glass–ceramic materials to be used in a wide range of applications. One field for which glass–ceramics have been developed over the past two decades is that of glazes for ceramic tiles. Ceramic tiles are the most common building material for floor and wall coverings in Mediterranean countries. Glazed tiles are produced from frits (glasses quenched in water) applied on the surface of green tiles and subjected to a firing process. In the 1990s, there was growing interest in the development of frits that are able to crystallise on firing because of the need for improvement in the mechanical and chemical properties of glazed tiles. This review offers an extensive evaluation of the research carried out on glass–ceramic glazes used for covering and pavement ceramic tile is accomplished. The main crystalline phases (silicates and oxides) developed in glass–ceramic glazes have been considered. In addition, a section focused on glazes with specific functionality (photocatalytic, antibacterial and antifungal activity, or aesthetic superficial effects) is also included.     

Radiative lifetime of triplet excitation recombinations mediated by spin–photon interaction in semiconductors

The radiative recombination of triplet exciton, is studied here theoretically in inorganic amorphous and organic crystalline semiconductors. A new time-dependent exciton–spin–orbit–photon interaction operator derived recently is used to calculate rates of radiative recombination of triplet excitons and the corresponding radiative lifetimes. It is illustrated that the new operator gives rise to a first order non-zero term responsible for the triplet photoluminescence. Results agree quite well with the known experimental results.     

TEM characterization of metal and metal oxide particles supported by multi-wall carbon nanotubes

Composites of Sn–Pd particles and multi-walled carbon nanotubes (MWNTs), and of Ni–Co particles and MWNTs were investigated by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The Sn–Pd particle composites were prepared by impregnation deposition, and the Ni–Co particle composites were prepared by electroless deposition on MWNTs synthesized by a chemical vapor deposition method using thermal catalytic decomposition of hydrocarbon. The morphologies and the microstructure of the deposited particles were investigated upon annealing the composites with and without hydrogen. Both the crystalline particle structure and the particle morphology are observed to be different for the samples annealed with and without hydrogen.     

Structural and morphological properties of RE3+ doped sesquioxide Y2O3 spherical nanoparticles

Effects of rare earth (Er³⁺, Yb³⁺, Sm³⁺, Ce³⁺) doping of yttrium oxide nanoparticles on the crystal structure and morphology have been investigated. X-ray diffraction, thermal analysis and scanning electron microscopy show that chemical and structural transformations are taking place in as-prepared carbonate spherical particles in the temperature range of 400–1200 K. In the final form crystalline agglomerates are compositionally homogeneous with a diameter range of 165–185 nm and an average crystalline grain size of 45 nm. Rare earth (RE) doped yttria particles are found to be up to 40% smaller in crystal grain size. The development of surface texture is due to nanocrystalline substructure which is related to the applied thermal treatment: carbonate decomposition and yttria crystallization processes.     

Properties of amorphous and crystalline titanium dioxide from first principles

We used first-principles methods to generate amorphous TiO₂ (a-TiO₂) models and our simulations lead to chemically ordered amorphous networks. We analyzed the structural, electronic, and optical properties of the resulting structures and compared with crystalline phases. We propose that two peaks found in the Ti–Ti pair correlation correspond to edge-sharing and corner-sharing Ti–Ti pairs. Resulting coordination numbers for Ti (∼6) and O (∼3) and the corresponding angle distributions suggest that local structural features of bulk crystalline TiO₂ are retained in a-TiO₂. The electronic density of states and the inverse participation ratio reveal that highly localized tail states at the valence band edge are due to the displacement of O atoms from the plane containing three neighboring Ti atoms; whereas, the tail states at the conduction band edge are localized on over-coordinated Ti atoms. The -point electronic gap of ∼2.2 eV is comparable to calculated results for bulk crystalline TiO₂ despite the presence of topological disorder in the amorphous network. The calculated dielectric functions suggest that the amorphous phase of TiO₂ has isotropic optical properties in contrast to those of tetragonal rutile and anatase phases. The average static dielectric constant and the fundamental absorption edge for a-TiO₂ are comparable to those of the crystalline phases.     

Crystallization and void formation in ZnO–B2O3–SiO2–MgO sintered solder glasses

The evolution of crystallization and porosity changes with firing temperature were studied in ZnO–B2O3–SiO2–MgO glasses. Those glasses presintered at 610 °C to a low porosity were crystallized in the temperature range of 690–870 °C. The glasses were crystallized by a surface crystallization mechanism. The porosity increased with the crystallization temperature. In the temperature range of 710–790 °C, several crystalline phases, such as 3ZnO–B2O3, willemite (2ZnO–SiO2), 5ZnO–2B2O3, and another form of zinc silicate (2ZnO–SiO2), produced at relatively low temperatures, were produced, while above 800 °C only the 2ZnO–SiO2 phase co-existed with a glass phase. Only an observed density difference between the glass and the crystallized glass cannot be attributed to the void formation during the crystallization reaction. Due to the crystallization the composition of the remaining glass around the crystalline phases is expected to change. The depletion of a certain component in the remaining glass, probably the SiO2 due to the production of the 2ZnO–SiO2 phase, might result in the increase in the vapour pressure of the remaining glass and lead to the observed increase in porosity. Below 800 °C, at which temperature the crystallization rate is fast and only a small amount of the glass phase remained, the porosity remained constant after the completion of the crystallization. Contrarily at 860 °C the porosity continuously increased with firing time. This revised version was published online in November 2006 with corrections to the Cover Date.     

Influence of crystallization time on microstructures and dielectric properties of tungsten–bronze glass–ceramics

A glass with a composition of 22.5SrO–22.5BaO–15Nb₂O₅–40SiO₂ (mol %) was prepared by a melt-quenching method and then heat-treated at 950 °C for different crystallization time. Microstructure observations were carried out using scanning electron microscope and dielectric properties were measured by a LCR meter. The experimental results show that volume fraction of the crystalline phase increased, dielectric constant maximum enhanced, and Curie temperature shifted as the crystallization time is prolonged. The decrease in the Curie temperature for the sample crystallized at 950 °C for 1 h is considered to be caused by the clamping effect from the glass matrix or small compositional fluctuation. Impedance spectroscopy has been employed to study the polarization contributions arising from the glass and crystalline phases in the glass–ceramics for different crystallization time. With the increase in crystallization time, the magnitudes of impedance and modulus as well as the relaxation frequency changed significantly. The activation energy calculated from the relaxation frequency increased for the glass phase due to a denser network structure, while the crystalline phase showed a slight decrease implying there is no change in its polarization mechanism.     

Some regularities of heterogeneous reactions in laminar flows in channels

A gas-mixture flow in a channel on whose walls heterogeneous reactions take place is considered. Numerical simulation is performed on the basis of equations for a narrow channel. Results of a numerical solution of the equations are interpolated by a relation involving analytically obtained asymptotic formulas. Some regularities are obtained for the problem considered. Russian Scientific Center “Applied Chemistry,” St. Petersburg. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 69, No. 2, pp. 206–214, March–April, 1996.     

Characterisation of hollow Russian doll microspheres

Poly(1-methylpyrrol-2-yl)squaraine (PMPS) particles have been characterised using SEM. The PMPS particles were used as templates to prepare bare silica and iron–silica hollow spheres, which were characterised using TEM and SEM. The PMPS particles and the hollow spheres are not uniformly sized and are agglomerated. The hollow spheres with larger diameters (>900 nm) contain an internal ‘Russian doll’ structure. The iron–silica hollow spheres are fused to one another, and the hollow spheres have a heterogeneous wall thickness. The silica and iron–silica hollow spheres both aggregate by size. There are two different size populations (for the diameter) of the bare silica and iron–silica hollow spheres. The smaller silica spheres have thinner walls compared to the larger silica hollow spheres. The larger silica hollow spheres and the iron–silica hollow spheres have similar wall thicknesses. The iron compound in the iron–silica hollow spheres has an oxidation state of 3+ and is crystalline.     

Instruments for contactless monitoring of internal surface roughness

Instruments Dozor-V and Dozor-VK are described whose operation is based on a reflexometric method. Test results are presented for cylindrical surfaces with a diameter of 30–100 mm with various forms of machining and materials. The instruments replace visual monitoring for roughness specimens and they are intended for application directly under industrial conditions. __________ Translated from Izmeritel’naya Tekhnika, No. 3, pp. 29–32, March 2006.     

Ultra-sharp pointed tip Si nanowires produced by very high frequency plasma enhanced chemical vapor deposition via VLS mechanism

Needle-like silicon nanowires have been grown using gold colloid as the catalyst and silane (SiH₄) as the precursor by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD). Si nanowires produced by this method were unique with sharpness below 3 nm. High resolution transmission electron microscopy (HRTEM) and X-ray diffraction technique (XRD) confirmed the single crystalline growth of the Si nanowires with (111) crystalline structure. Raman spectroscopy also has revealed the presence of crystalline Si in the grown Si nanowire body. In this research, presence of a gold nanoparticle on tip of the nanowires proved vapor–liquid–solid growth mechanism.