Universal character of relaxation of the intensity of light scattering by boron oxide melt in the temperature range 300–340°C

It has been established that the process of stabilization of the boron oxide melt in the temperature range 300–340°С is accompanied by universal changes in the intensity of the polarized V _v component of light scattering characterized with the formation of a minimum. It has been demonstrated that the above feature does not depend on the direction of the intensity approaching to the stationary state for the selected temperature; i.e., it is observed in the process of intensity relaxation from both high and low temperatures. Upon attainment of the minimal values, the increase of the V _v intensity with the stabilization time was found to be satisfactorily described by the empirical dependence of the exponential type. The characteristic times of intensity changes calculated in accordance with the respective equation significantly exceed the times of structural relaxation and increase along with the temperature decrease in accordance with the exponential dependence. It has been established that the characteristic times of intensity changes as its values approach the stationary one from a higher temperature are larger than when approaching from a lower temperature. It has been shown that under these conditions, changes in the intensity of the depolarized H _v component are characterized with the formation of a maximum registered for both modes (sample cooling and heating). It has been revealed that the increase of the H _v intensity in the maximum does not exceed 10% of its regular value, which allows relating the formation of a minimum of the V _v component to the decrease of the isotropic light scattering intensity.     

Light scattering by boron oxide in the temperature range of 225–330°C

It is found that the process of variation in the intensity of the polarized component of the light scattered by boron oxide in the temperature range of 250–290°C after cooling from temperatures of 330 and 450°C possesses the peculiarities similar to those found earlier for the step-by-step mode of cooling for temperatures higher than 295°C and is characterized by the formation of a minimum. It is also detected that at lower temperatures the minimum possesses an asymmetric shape. It is shown that the process of increasing the intensity in time is satisfactorily approximated by an empirical equation of an exponential form. It is found that the change in the relaxation times with the temperature corresponds to an exponential law for the entire set of the data that we have obtained. It is found that the change in the depolarized component intensity in the region of low temperatures is relatively small, which makes it possible to associate the behavior of the polarized component with the change in the value of isotropic scattering. It is shown that in agreement with the results obtained earlier the time dependence of the intensity of the polarized component at a higher temperature for a sample preliminarily annealed at a low temperature is characterized by the formation of a maximum. The experimental results that we have obtained indicate the universal character of the manifestation of a peculiarity in the behavior of the scattered light intensity in the process of cooling and heating of boron oxide in the glass transition range.     

Oxidation of the zirconium boride-silicon nitride composite in the temperature range 1100–1300°C in air

Composite materials and coatings based on zirconium boride and silicon nitride have been prepared by heat treatment of mixtures of the initial components in air. The chemical reactions result in the formation of the glass melt encapsulating initial particles, which provides an increased heat resistance of the material. The kinetics of high-temperature oxidation of the composite based on the ZrB₂-Si₃N₄ system has been studied. The interaction between solid components of the ZrB₂-Si₃N₄ system, atmospheric oxygen, and the glass melt has been described in terms of the formal kinetic equations for heterogeneous processes. Thermogravimetric, thermal, and X-ray powder diffraction analyses have been carried out, and the electrical resistivity of the compact samples has been determined.     

Oxidation processes in a silicon-boron-zirconium boride composite in the temperature range 1000–1300°C

Silicon, boron, and zirconium boride coatings on graphite have been obtained by thermal treatment of the initial components in air. During the course of the chemical reactions, a glass melt encapsulating (wrapping) the initial particles is formed, which makes it possible to create an air barrier preventing the diffusion of oxygen into the sample. The interactions between the components of the Si-B-ZrB₂ system with oxygen and the glass-forming melt have been investigated.     

Thermogravimetric investigation of the oxidation of the ZrB2-SiO2 composite in the temperature range 800–1300°C

The kinetics and mechanism of high-temperature oxidation of the components of the ZrB₂-SiO₂ system are investigated using thermogravimetric and X-ray powder diffraction analyses. It is shown that the interaction between solid components of the ZrB₂-SiO₂ system, atmospheric oxygen, and the glass-forming melt can be adequately described by the equations of the formal kinetics of heterogeneous processes. The previously revealed effect of encapsulation of a high-melting compound by the glass-forming melt is confirmed. This effect manifests itself in the fact that, upon introduction of a silicon-containing component, the increment of the weight of a sample decreases as a result of the oxidation and the weight losses in the form of gaseous products decrease to almost zero.     

Effect of spinel content on hot strength of alumina-spinel castables in the temperature range 1000–1500°C

Hot modulus of rupture of Al₂O₃-spinel castables containing 5–15 wt% alumina-rich magnesia alumina spinel and 1·7 wt% CaO generally increases with increase in spinel content and temperature from 1000 to 1500°C. The magnitudes of hot modulus of rupture of castables containing 15 wt% spinel and 1·7 wt% CaO are 14·3 MPa at 1400°C and 15·6 MPa at 1500°C, while those of castables containing 20 wt% spinel and 1·7 wt% CaO are 12·5 MPa at 1400°C and 14·7 MPa at 1500°C. The former castables contained 15 wt% spinel of −75 μm size, while the latter contained 10 wt% spinel of +75 μm size and another 10 wt% spinel of −75 μm size. The bond linkage between the CA₆ and spinel grains in the matrix is believed to cause both the spinel content and temperature dependence of hot strength of Al₂O₃-spinel castables, as well as fine grain spinel even in amount less than coarser grain spinel to be more effective for enhancing hot strength. The trend of the magnitude of thermal expansion under load (0·2 MPa) above 1500°C of the castables is not necessarily indicative of the magnitude of hot modulus of rupture at 1400 or 1500°C. ©     

Interaction of ceria and erbia in air within temperature range 1500–600 °C

Phase equilibriums and structure transformations in the CeO₂–Er₂O₃ system have been studied within 1500–600 °C in the full concentration range using XRD and petrography methods. It was established that the system is characterized by the formation of solid solutions on the basis of cubic modification of Er₂O₃ (C-type) and fluorite CeO₂ (F-type). Solubility limits and concentration dependences of lattice parameters were determined for the phases forming in the system.     

Critical oxidation behavior of Ta-containing ZrB2 composites in the 1500–1650 °C temperature range

The microstructure evolution of ZrB₂ hot pressed with 15 vol% TaSi₂ was studied in the as-sintered state and after oxidation for 15 min at 1500 and 1650 °C in stagnant air. In the pristine material, the original ZrB₂ nuclei are surrounded by a mixed (Zr,Ta)B₂ solid solution. Refractory Ta-compounds are located at triple junctions and wetted grain boundaries are distinctive of this ceramic. After oxidation, the solid solution evolves into ZrO₂ grains encasing intragranular nano-structured TaB₂ particles. Here we show that the operating limit temperature of this composite is related to the critical oxidation of TaB₂ to Ta₂O₅ above 1600 °C, accompanied by large volume expansion and local liquid formation, which ruptures the ZrO₂ grains and structure.     

In-situ characterization of e-cigarette aerosols by 90°-light scattering of polarized light

The polarization ratio method is used for fast in-situ characterization of unimodal condensed aerosols of e-cigarettes. The method is based on 90°-light scattering of polarized 680 nm laser light by the droplet ensemble inside an optically defined measuring volume. Mass median droplet diameter (MMD) is derived from the ratio of scattered light from horizontally and vertically polarized incident light beams assuming a fixed value of the geometric standard deviation of the aerosol mass distribution. MMD is used to correct for the size dependence of the mass-based scattering signal of vertically polarized light to obtain the mass concentration if the sensor is calibrated once with an aerosol with a fixed MMD. The sensor uses commercially available aerosol photometers, and its application to e-cigarette aerosols was validated with an impactor for MMD and with a filter measurement for mass concentration. Good correlation (r² > 0.97) for both parameters was observed. Application ranges are mass concentration range 0.5–50 mg/L, MMD 0.2–1.2 µm, 100 ms time resolution, and 0.2–3 L/min flow rate. The usefulness of this simple sensor for e-cigarette aerosol characterization is demonstrated by developing a scaling law between MMD and operating parameters of an e-cigarette, i.e., puff flow rate and mass concentration.

Copyright © 2018 American Association for Aerosol Research     

Longitudinal tensile properties of H4LM graphite in the temperature range—253 to 500°C

Tensile strength data are presented for a molded H4LM graphite tested in the longitudinal orientation (“across the grain”) at a nominal engineering strain rate of 0·0005/min in liquid hydrogen at −253°C, in liquid nitrogen at −196°C, and in air at room temperature, 250 and 500°C. All ultimate strength values at all of these temperatures fall within the scattering of room-temperature strength measurements.     

Chemical and electrochemical behaviour of nickel ions in the ZnCl2-2NaCl melt at 450°C

The stability of nickel chloride and oxide as well as the electrochemical behaviour of the Ni²⁺ ion have been studied in ZnCl₂-2NaCl melts at 450°C by means of X-ray diffractions (XRD), potentiometry, cyclic voltammetry, chronoamperometry and chronopotentiometry. The standard potential of the redox couple Ni(II)/Ni(O) and solubility product of nickel oxide have been determined (E°Ni(s)/Ni(II) = ?1.006 ± 0.001 V (against Cl₂(1 atm)/Cl^?), pKs = 4.8 ± 0.1 in molality scale). These results have allowed the construction of E-pO^2? equilibrium diagrams. Nickel (II) reduction is close to the reversibility according to the scheme: $${\text{NiCl}}_{\text{2}} ({\text{solvated}}){\text{ + 2e}}^ - \Rightarrow {\text{Ni(s) + 2Cl}}^ - $$ with a diffusion coefficient, D _Ni, close to 3 × 10 ^?6cm²s^?1.     

Correlation of boron content and high temperature stability in Si–B–C–N ceramics

The preparation and characterization of precursor derived Si–B–C–N ceramics with similar Si/C/N ratios but variable boron content are reported. The polymeric precursors were prepared via hydroboration of poly(methylvinylsilazane) using different BH₃·SMe₂/polymer stoichiometries. High temperature thermogravimetric analysis of as-pyrolysed ceramics as well as XRD studies of post-annealed samples display a retarding effect of boron on both crystallization of SiC and Si₃N₄ and stabilization of crystalline β-Si₃N₄.     

Dielectric properties of silver-doped nanoporous silicate glasses in the temperature range between –50 and +250°C

The results of measuring dielectric parameters of nanoporous silicate glasses with a nanopore size of 3.5 and 25.7 nm, either doped with silver or not, in the temperature range between–50 and +250°C and the frequency range of 0.1–10⁶ Hz are reported. It is demonstrated that when silver nanoparticles are formed in glass pores, some silver remains in the form of subnanosized molecular clusters Ag _n and molecular complexes Ag _n –(OH) _m . The key dielectric properties of the relaxation centers are determined for different temperature ranges.     

Semiconductor-conductor transition of pristine polymer-derived ceramics SiC pyrolyzed at temperature range from 1200°C to 1800°C

This paper studies the effect of pyrolysis temperature on the semiconductor-conductor transition of pristine polymer-derived ceramic silicon carbide (PDC SiC). A comprehensive study of microstructural evolution and conduction mechanism of PDC SiC pyrolyzed at the temperature range of 1200°C-1800°C is presented. At relatively lower pyrolysis temperatures (1200°C-1600°C), the carbon phase goes through a microstructural evolution from amorphous carbon to nanocrystalline carbon. The PDC SiC samples behave as a semiconductor and the electron transport is governed by the band tail hopping (BTH) mechanism in low pyrolysis temperature (1300°C); by a mixed mechanism driven by band tail hopping and tunneling at intermediate temperature (1500°C). At higher pyrolysis temperatures (1700°C-1800°C), a percolative network of continuous turbostratic carbon is formed up along the grain boundary of the crystallized SiC. The samples demonstrate metal-like conductive response and their resistivity increases monotonically with the increasing measuring temperature.     

Impact of preheating on the mechanical performance of different MgO–C bricks - Low temperature range

This work complements an initial study regarding the mechanical behavior of MgO–C bricks at 1000 °C. In this case, two bricks bonded with phenolic resin, one of them containing aluminum, were treated at 600 °C and mechanically tested at RT and 600 °C. The thermal treatments attempt to simulate the in-service steelmaking ladle preheating process. At low temperatures, the binder pyrolysis is one of the main transformations and the Al melting neither its chemical reactions occur on a large scale yet. To evaluate the effects as the pyrolysis progresses, the soaking time at 600 °C was varied from 1 to 3 h. Although without significant chemical activity, the presence of Al affected the mechanical behavior of the tested bricks. The consolidation of the C–C network coming from the binder pyrolysis was identified as the main factor responsible for counterbalancing the material's degradation by microcracking. The heating combined with the low compressive pre-load applied on the tested specimens appears to close the microcracks and pores.     

The effect of boron containing frits on the anorthite formation temperature in kaolin–wollastonite mixtures

In order to investigate the effect of boron containing frits on anorthite formation temperature in kaolin–wollastonite mixture, four different frit compositions containing boron were prepared according to Seger formulas. One of these compositions also contained lead. Four different batches composed of 40 mass% kaolin, 40 mass% wollastonite and 20 mass% frit were prepared. The linear dilatometric (LD) curves of the batches were determined and subsequently the firing schedule (FS) curves were obtained from the LD curves. Cylindrical pellets prepared from each of the batches were fired in an especially designed furnace up to respectively 950, 975, 1000, 1025, 1050 °C. The firing period including the cooling process was adjusted to 210 min. The variation of the bulk densities of the products as a function of temperature were examined. X-ray diffraction (XRD) patterns of the products were also determined and it was observed that the minimum anorthite formation temperature was 1000 °C. Since it was known that with batches not containing any frit, the minimum anorthite formation temperature was 1100 °C, it was understood that leaded or unleaded boron containing frits decreased the anorthite formation temperatures around 100 °C.     

Synthesis and characterization of aluminum oxide–boron carbide coatings by air plasma spraying

Aluminum oxide (Al₂O₃)–boron carbide (B₄C) composites have been proposed for use as cutting tools as well as in high temperature applications due to their high hardness and fracture toughness. The air plasma spraying method was used to fabricate the composite coatings of Al₂O₃ and B₄C. Three different Al₂O₃:B₄C composition ratios of 90:10, 80:20, and 70:30 by weight were plasma sprayed on plain carbon steel substrates. The effect of B₄C content on microstructure, hardness, porosity and thermal diffusivity of the coatings were studied using scanning electron microscopy (SEM), microhardness testing, X-ray diffraction (XRD), and the flash diffusivity method. The plasma spray parameters were optimized in order to achieve a theoretical density of approximately 90%.