Evaporation Of B2O3 Research Articles

Design and preparation of resin matrix composite coating with good ablation resistance performance under high-energy laser irradiation

High-energy lasers can cause severe damage to conventional materials in an extremely short time. Highly reflective coatings are effective against damage, but they cannot withstand high laser power densities. In this study, a composite coating with two characteristics, namely, reflection and energy consumption, was designed and prepared. TiO2 and BN powders, as light reflective media, were introduced into a boron-modified phenolic resin (BPF) to prepare an anti-laser composite coating. The results show that the initial reflectivity of the composite coating can reach 78.8% at a wavelength of 1064 nm. After damage-free protection for 0.4 s, ablation occurred in the irradiation region. Owing to the formation of residual char, which is the pyrolysis product of BPF, the coating maintained its protective performance. When the composite coating was irradiated at 1000 W/cm2 for 10 s, the back-surface temperature was 186 °C, which indicates decent anti-laser performance. We also studied the role of BN by setting a control experiment. The high conductivity and energy consumption caused by the evaporation of B2O3, which is the oxidation product of BN, helped in improving the anti-laser performance. The residual char with high porosity and degree of graphitization proved to be effective to isolate laser energy. This study provides a new design for protection of conventional materials against laser damage.

Crystallinity dependence of high-temperature oxidation of silicoboron carbonitride monoliths

Comparative investigation of performances of metastable materials in amorphous and crystalline states is pivotal to exploring their applications in engineering. The oxidation behavior and kinetics of the partially crystalline Si2BC3N (PCS) monoliths, i.e. amorphous Si2BC3N with precipitation of nanocrystallites (10−30 nm) of SiC and BN(C), were investigated at 1500−1700 °C with dwell time ranging from 0.5 h to 16 h. Oxidation leads to the formation of amorphous SiO2 and cristobalite, and more sufficient crystallization of PCS. Due to the release of gases including CO, CO2, N2 and evaporation of B2O3, PCS undergoes weight loss at 1500−1700 °C. Loose oxide scales with low adhesion strength are formed at 1500−1600 °C, while the scales are highly dense, adherent at 1700 °C indicating good oxidation resistance of PCS. Compared to completely amorphous Si2BC3N (CAS), PCS exhibits lower activation energy of ∼50.0 kJ‧mol−1 for 1500−1600 °C/≤16 h, and higher parabolic rate constants (42.5 μm2‧h−1 for 1500 °C/≤16 h, 172.2 μm2‧h−1 for 1600 °C/≤16 h, and 120.4 μm2‧h−1 for 1700 °C/≤4 h), which suggests the inferior oxidation resistance of PCS. One most possible reason is that the changes in superficial chemical compositions and reactivity during crystallization cause larger oxygen uptake on PCS surfaces.

Ignition delay kinetic model of boron particle based on bidirectional diffusion mechanism

Ignition delay time of B particles is one of the key factors that influence their burnout ratio in the afterburner. In this study, the micro/nanofabricated slice measurement of a combustion residue particle of B was carried out. By combining the experimental results obtained with previous experimental results, the surface diffusion mechanism of a single B particle was completely verified. Then, an ignition delay kinetic model of B particle was developed using the principles of semiempirical models. By ensuring the initial ignition temperature, the ignition delay of a single B particle can be divided into two stages: (i) heat transfer stage and (ii) low-temperature oxidation stage. The existence of both O2 diffusion and (BO)n diffusion (bidirectional diffusion) was confirmed during the low-temperature oxidation stage. Only heat transfer between the B particle and surroundings occurred during the heat transfer stage, whereas both heat transfer and oxidation occurred during the low-temperature oxidation stage. The oxidation involves four global reactions: (i) evaporation of B2O3, (ii) diffusion of O2, (iii) diffusion of (BO)n, and (iv) reaction of H2O. The final ignition delay time of a B particle is equal to the sum of the lasting times of heat transfer stage and low-temperature oxidation stage. The results of computed ignition delay time obtained by the model are consistent with the previous experimental data under O2/H2O atmosphere. According to the prediction of the model, the increase in the initial particle size will prolong the ignition delay time of a B particle. The ignition delay of a small B particle is dominated by the low-temperature oxidation stage, whereas the ignition delay of a large B particle is dominated by the heat transfer stage.

Features of polytherms of the viscosity of Fe–B melts

It is established that the formation of a viscous film on the surface of a melt is possible during the viscosimetry of Fe–B melts under normal conditions. It is noted that the film has a considerable effect on the results from measurements and can lead to the appearance of additional features on polyterms. Thermodynamic analysis of the composition of surface layers shows that a viscous film contains mainly boron oxide B2O3. It is concluded that the evaporation of B2O3 from the surface of the alloy produces additional anomalies on the polytherm of viscosity.

Ablation behavior and mechanism of SiCf/Cf/SiBCN ceramic composites with improved thermal shock resistance under oxyacetylene combustion flow

The ablation properties and mechanisms (under oxyacetylene combustion) together with thermal shock behavior of SiCf/Cf/SiBCN ceramic composites were investigated. The solid ablation products are primarily amorphous SiO2 and cristobalite. The primary ablation mechanisms include fiber and ceramic matrix oxidation, evaporation of B2O3 (l) and SiO2 (l), and mechanical exfoliation. SiCf/Cf/SiBCN has a significantly low mass ablation rate and a desirable linear ablation rate. The combination of crack deflection caused by SiC and carbon fibers, fiber pull-out and debonding improves thermal shock resistance and thus leads to the absence of surface macrocracks.

Ablation mechanism and properties of in-situ SiAlON reinforced BN–SiO2 ceramic composite under an oxyacetylene torch environment

Ablation mechanism and properties of in-situ SiAlON reinforced BN–SiO2 ceramic composites have been carefully studied under different ablation time. Results showed that the ablation properties depend on the ablation time. When the ablation time increases from 10s to 60s, the mass ablation rate increases from 0.20mg/s to 6.24mg/s and the linear ablation rate increases from 9.0μm/s to 28.1μm/s. The phases of ablation products mainly consist of amorphous SiO2, Mullite, Corundum and B2O. Four time-dependent ablation behaviors contributed to the various ablation regions: oxidation of matrix which includes BN and Si2Al3O7N; evaporation of B2O3(l) and softening of SiO2(s); scouring away of SiO2(l) and Al2O3(l) by high-speed gas stream; evaporation of SiO2(l) and Al2O3(l).

Oxidation of zirconium diboride with niobium additions

Oxidation of ZrB2 ceramics containing Nb additions at 1500 ◦ C resulted in the formation of a two-layer oxide scale. The outer surface was partially covered by a glassy layer containing B2O3 with smaller amounts of Nb and Zr oxides dissolved into it. With increasing exposure time, evaporation of B2O3 from the outer layer resulted in precipitation of oxide particles in the receding glassy phase. Between the outer layer and the unoxidized (Zr,Nb)B2 was a porous layer that consisted of particles containing Zr, Nb, and O. The formation of Nb2Zr6O17 was observed in the porous oxide layer. Since this compound is solid at the oxidation temperature, liquid phase sintering of the ZrO2 scale was not possible. However, dissolution of Nb into B2O3 increased the stability of the liquid/glassy layer, which acted as a barrier to the transport of oxygen at higher temperatures compared to the scale formed on nominally pure ZrB2. © 2013 Elsevier Ltd. All rights reserved.

High Temperature Oxidation Behavior of the B<sub>4</sub>C/BN Composites

The B4C/BN composites were fabricated by hot-pressing process. The microstructure, mechanical properties and oxidation resistances of the B4C/BN composites were investigated. It was shown that the h-BN particles were distributed in the B4C ceramics matrix. The mechanical properties of the B4C/BN microcomposites and the B4C/BN nanocomposites decreased gradually with the increasing content of h-BN. The mechanical properties of the B4C/BN nanocomposites were significantly improved in comparison with the B4C/BN microcomposites. The oxidation processes were performed at 1000oC, 1100oC, 1200oC, 1300oC for 20h. The oxidation curves of the B4C monolith, the B4C/BN microcomposites and the B4C/BN nanocomposites decreased gradually with the increase of oxidation temperature and oxidation time. The specimen’s weight and the oxidation resistance decreased gradually with the increase of oxidation temperature and oxidation time. The specimens remained good oxidation resistance at 1000oC; the oxidation resistance decreased remarkably at 1300oC. The decreasing specimen’s weight was attributed to the evaporation of B2O3 which produced by oxidation process of B4C and h-BN. The phase composition and microstructure of specimen’s surface after oxidation process were investigated by XRD and SEM.

Kinetic studies of oxidation of MgAlON and a comparison of the oxidation behaviour of AlON, MgAlON, O'SiAlON-ZrO2, and BN–ZCM ceramics

Abstract The kinetics and morphology of the oxidation process of magnesium-aluminium oxynitride (MgAlON), aluminium oxynitride (AlON), O'SiAlON–ZrO2, and BN-ZCM have been studied in the temperature range 1373–1773 K. Oxidation experiments with powder and plate samples of the above materials have been carried out in air. MgAlON shows the best resistance to oxidation at lower temperatures (<1473 K), whereas at higher temperatures (∼773 K), AlON shows the best resistance. O'SiAlON–ZrO2 shows very good oxidation resistance up to 1673 K. But its oxidation rate increases strongly above 1673 K, presumably due to the formation of liquid phase. BN–ZCM has the poorest oxidation resistance due to the evaporation of B2O3. The activation energies for the chemical oxidation reaction of AlON, MgAlON, and O'SiAlON–ZrO2 are 214, 330 and 260 kJ/mol, respectively. The overall diffusion activation energies for AlON, MgAlON, O'SiAlON–ZrO2 and BN–ZCM are 227, 573, 367 and 289 kJ/mol, respectively.

Boron doping in Si molecular beam epitaxy by co-evaporation of B2O3 or doped silicon

We report on controlled boron doping of silicon molecular beam epitaxy (MBE) films by two techniques: sublimation of saturated boron-doped silicon from a boron nitride crucible and evaporation of B2O3 from a tungsten crucible. Both methods were performed with the simultaneous evaporation of undoped silicon from an electron beam source. Sharp doping profiles and constant evaporation rates were obtained for both methods. Doping levels between 2×1017 and 1.5×1020 cm−3 were realized. The evaporations require only relatively low temperatures: 700–1150 °C for B2O3 and 1000–1400 °C for subliming silicon. These results make it possible to replace gallium as the p-type evaporative dopant in silicon MBE with the more desirable boron without using ion imbedding or a very high crucible temperature.

Phase Relation in the System MgO-B&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; and Effects of Boric Oxide on Grain Growth of Magnesia

An extremely pure MgO as one of the starting materials was prepared by decomposing magnesium nitrate solution of 99.9% magnesium. No boric compound in magnesia obtained was recognized by the emission spectral analysis. Many mixtures of magnesium nitrate solution and boric acid corresponding to the various mixing ratio of MgO and B2O3 were preheated at around 600°C for 10hrs and then heat-treated at 910°, 1050°, 1300° 1500°, 1600°, 1700° and 1750°±5°C for 24 to 72hrs.The lattice constant of MgO and the crystallite size of MgO were measured with the internal standard of 99.999% Si and phases presented in the heat-treated specimens were identified with a X-ray diffractometer using the radiation of Cu Kα.Particle size of periclase was observed with a scanning electron microscope. The shrinkage, apparent porosity of magnesia body after heat-treatment and evaporation of B2O3 were also measured for evaluating the effects of the addition of B2O3 on the physical properties of sintered magnesia.The compound MgO⋅B2O3 was not detected, but the compounds 3MgO⋅B2O3, 2MgO⋅B2O3 and MgO⋅2B2O3 were recognized. It was confirmed that there was no solid solubility of B2O3 in MgO up to the temperature of 1750°C, and the grain growth of periclase was extremely accelerated by the addition of B2O3 particularly at the heat treatments of 1500°C. The shrinkage of magnesia body was maximum with the addition of B2O3, 2 to 3%, and decreased rapidly with the associate of bubbling of B2O3 when the temperature was higher than 1500°C in specimens containing more than 5wt% B2O3. Apparent porosity of magnesia body after heat-treatment at 1750°C for 24hrs was minimum with the addition of 0.1wt% B2O3 to be about 8%. The porosity increased with the increase in the addition of B2O3 for the sintered bodies containing higher than 0.1wt% B2O3.