Study of high-temperature reaction of Ti with B by the method of electrothermal explosion

Chernogolovka. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 1, pp. 82–87, January–February, 1993.     

Rapid electrothermal response of high-temperature carbon nanotube film heaters

Electrothermal materials, which convert electric energy into heat have been known for many years now. They are the heart of plethora of devices surrounding us, but their isotropic structure does not allow for tailoring of operating conditions. The carbon nanotube film heaters developed in house were built for the first time on a counter-intuitive concept to pass electric current against the alignment axis. They render superior and time-independent performance reaching terminal temperature without lag. Just 1 mg of these ultra-light free-standing films outperforms 120 g of nichrome, most commonly used resistive material. They are envisioned to become the next generation heating materials particularly in the applications, in which weight (aviation and aerospace industry), size (microreactor technology) or speed (kinetic systems) is of importance.     

Macrokinetics of toluene and benzene nitration under laminar conditions

Mononitration of toluene and benzene has been carried out in a laminar jet using aqueous mixtures of nitric and sulphuric acids. With acid mixtures containing about 30 mole % H₂SO₄, the rate data obtained are in good agreement with values predicted using a model, assuming fast reaction in a zone in the aqueous phase adjacent to the interface, in conjunction with diffusivities measured under laminar conditions. At low H₂SO₄, concentrations, the rate data fit a simple kinetic model. It is shown that the model based on mass transfer with simultaneous chemical reaction can explain previously reported results for the competitive nitration of toluene and benzene in the heterogeneous liquid phase regime.     

Dense high-temperature ceramics by thermal explosion under pressure

Near fully dense in-situ particulate reinforced ceramic matrix composites (CMCs) were fabricated from fine Ti–B₄C, Ti–BN, Ti–Al–BN, Ti–SiC, Ti–B₆Si and Al–TiO₂ powder blends with or without the addition of Ni. Two reactive synthesis techniques were employed: thermal explosion/TE (SHS) under pressure, where the compacted reagent blend was placed and rapidly heated in a pressure die preheated slightly above the ignition temperature, and reactive hot pressing/RHP. In both approaches, the processing or preheating temperature (≤1250°C) was considerably lower than those typical of the current methods used for the processing of ceramic matrix composites. Partial to full conversion of reagents into products was achieved during TE, and a moderate external pressure of ≤150 MPa was sufficient to ensure full density of the final products. Rapid cooling from the combustion temperature due to the ‘heat sink’ action of the pressure die resulted in the fine/micronsize microstructures of the in-situ composites synthesized. RHP processing yielded dense materials with even finer microstructures, however full conversion of reagents into products has not been achieved.     

Redistribution of adsorbed VOCs in activated carbon under electrothermal desorption

Electrothermal desolption is an electricity-promoted desolption technology developed only in the last decade. It is extremely eficient and straightforward when the adsorbent is electrically conductive, since heating can be achieved by the Joule effect. The volatile organic compound (VOC) vapors desorbing from micropores might redistribute and condense in mesopores with high concentration, which is possible since no dilution occurs. To study this problem, benzene and activated carbon were used as the working system, and a theoretical analysis was developed. In a wide temperature range up to 400°C, no VOC vapor could be condensed in mesopores with the strong micropore adsorption effect. With the weak micropore adsorption effect, however, mesopore condensation will occur, but it only takes place in mesopores smaller than 3 nm in diameter, and the amount is generally negligible. To prevent any possible condensation, the desorption temperature should at least equal the liquid boiling point calculated in a 2-nm capillary tube.     

Macrokinetics of obtaining functional products based on polystyrene and polymethyl methacrylate under nonisothermal conditions

Thermal conditions of the process of manufacturing large-block, optically pure products based on polystyrene and polymethyl methacrylate (scintillators, light concentrators, bulletproof glass, etc.) have been experimentally and theoretically studied. The flow sheet of the process includes two sequentially connected reaction apparatuses, i.e., a tubular reactor for frontal polymerization and a semi-continuous molding reactor. Prepolymer is formed in the tubular reactor and is fed to the molding reactor. As this reactor is being filled, polymerization continues under autothermal heating conditions. After filling, the molding reactor is replaced by a similar one, while the filled reactor is kept in a temperature-controlled oven for final polymerization followed by cooling and removal of the product.     

Macrokinetics of reaction and thermal explosion in Ni and Al powder mixtures

Chernogolovka. Translated from Fizika Goreniya i Vzryva, Vol. 24, No. 3, pp. 67–74. May–June, 1988.     

Structure changes of MPECVD-grown carbon nanosheets under high-temperature treatment

Vertically-aligned carbon nanosheets (CNSs), which were fabricated by microwave plasma-enhanced chemical vapor deposition in Ar and CH₄ system, have been annealed at high temperatures in the range of 1200–3000 °C. The morphologies and microstructures of the treated CNSs were analyzed by scanning and transmission electron microscopes, and Raman spectroscopy. High temperature treatment process efficiently removed the amorphous carbon and some defects and improved the graphitization of the CNSs. The graphitized grains increase and the interlayer spacing decreases with increasing heat temperatures. Heat treatment of the CNSs at temperatures from 1500 to 2000 °C was found to achieve the edges consisting of many single-layer graphene sheets. Annealing at temperatures above 2100 °C, the edges of nanosheets consist of 2–5 layer graphene with many zigzag junctions. The mechanism of reconstruction for the edges in the CNSs ascribes possibly to the carbon atom vaporization at high temperatures.     

Macrokinetics of thermal explosion in a niobium-aluminum system. II. Phase-formation dynamics

The thermal explosion in a gas-free mixture of metal powders has been studied for the first time using the method of dynamic x-ray structural analysis. The dynamics of formation of new crystalline phases at all stages of the process is determined. It is shown that melting of aluminum is followed by dissolution of niobium in the melt and crystallization of the intermediate product NbAl₃, and the melt does not wet niobium particles. A dramatic improvement in wetting is observed at a temperature of ≈ 1040 K (apparently, due to the destruction of oxide films). This results in a thermal explosion, and the Nb₂Al phase is formed. The reaction during the thermal explosion also follows the dissolution-crystallization mechanism. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 2, pp. 45–50, March–April, 2000. This work was supported by the Russian Foundation for Fundamental Research (Grant No. 98-03-32137).     

Methanolysis of amides under high-temperature and high-pressure conditions with a continuous tubular reactor

The methanolysis of amides, which is widely employed in the synthetic organic chemistry, hardly occurs under mild conditions. To safely and controllably intensify the methanolysis reaction with hightemperature and high-pressure environment, a continuous tubular reaction technology is developed,whose space–time yield is over twice of that of the conventional batch reaction. The methanolysis of acetanilide is selected as the representative reaction, and the influences of temperature, pressure, rea...     

Direct interaction of high-temperature melt with water

The interaction of high-temperature melts formed upon combustion of F₂O₃-Al thermit in a closed volume with water was explored at initial pressures of 101 kPa and 7.5 MPa using a specially designed high-pressure SHS reactor. A maximum force recorded by a force gauge installed on the outer reactor wall was around 750 and 150 N, respectively. Upon direct contact with water, the combustion product was found to undergo dispersion into submicron particles. After completion of combustion, the gas cushion of the reactor was found to contain 70–90% hydrogen gas. The observed system behavior was associated with steam explosions arising in the reactor.     

CO高温变换催化剂浸渍条件研究

采用γ-Al₂O₃作载体,Ni^2＋、Cu^2＋和Mn^2＋水溶液为浸渍液,进行制备无铬CO高温变换催化剂浸渍条件的研究。由实验数据回归出Ni^2＋、Cu^2＋和Mn^2＋活性组分在载体上的浸渍动力学方程,得到Ni^2＋、Cu^2＋和Mn^2＋组分在载体上浸渍的速率常数为：k_Ni2＋= 0.000 2,k_Cu2＋=0.000 2,k_Mn2＋=0.001 6。Freundlich等温吸附表达式为：lnC_Ni2＋=－0.000 2 t＋4.447 4,lnC_Cu2＋=－0.000 2 t＋4.590 9和lnC_Mn2＋=－0.001 6 t＋3.589 5。考察了催化剂中活性组分含量与共浸渍组分浓度的关系,并得出从浸前液浓度来预测和控制催化剂成品中Ni^2＋、Cu^2＋和Mn^2＋活性组分含量的经验式。     

Coalescence of single-walled carbon nanotubes and formation of multi-walled carbon nanotubes under high-temperature treatments

Electric arc-discharge single-wall carbon nanotubes are annealed between 1600 and 2800 °C under argon flow. Their stability and evolution are studied by coupling TEM, X-ray diffraction and Raman spectroscopy. The first modifications appear at 1800 °C with a significant decrease of the crystalline order. It is due to SWNTs coalescence leading to smaller bundles but with an increase of the tube diameters from 2 to 4 nm. From 2200 °C, SWNTs progressively disappear to the benefit of MWNTs having at first two to three carbon layers then reaching 7 nm external diameter. The possible mechanisms responsible for the SWNTs coalescence and instability and their transformation in MWNTs are discussed.     

Thermal explosion of an unbounded plate under third-kind asymmetric boundary conditions

An analytical solution is given for the problem of a thermal explosion of a plane reagent layer under third-kind asymmetric boundary conditions. The critical Frank-Kamenetskii parameters are determined for the general and particular cases. Calculation results are compared with the literature data. It is shown that the critical parameters, determined by direct and inverse methods, do not coincide for small values of the Biot criterium and depend on the scale temperature. The particular feature of the approach is the application of the inverse method to the solution of the thermal-explosion problem. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 3, pp. 88–91, May–June 1998.