Oxidation Of Boron Carbide Research Articles

Evaluation of boron evaporation kinetics from stainless-steel–B4C alloy during steam oxidation at high temperatures

To understand the core degradation process at the Fukushima Daiichi Nuclear Power Station, the oxidation of boron carbide–stainless steel alloy under steam starvation condition was studied at temperatures in the range of 1,288–1,573 K. Low steam supply led to swift Fe–O layer formation, embedding Fe–B–O and Fe–Cr–O, and boron evaporation mainly as oxides was observed through the Fe–B–O phase precipitated in the Fe–O layer. The rate constant of boron evaporation kB was derived from the measured data as kB = 0.0157 exp (–79.8 × 103/RT) for T ≥ 1,423 K and kB = 8.69 × 10−5exp (–44.4 × 103/RT) for T < 1,423 K where R and T are the gas constant and temperature, respectively. The obtained constant was comparable to the reaction rate of B4C oxidation. In addition, a test with an even more decreased steam supply was conducted to examine the impact of steam quantity on the boron evaporation kinetics. Consequently, it was confirmed that decreasing the oxygen supply resulted in a slowdown of outer Fe–O layer formation, which enhances the outwards diffusion of B and allows greater evaporation of B oxides.

Mechanism of Boron Carbide Oxidation in a Dry Environment

ABSTRACT We analyze in detail the published experimental data on the high-temperature oxidation of compact boron carbide in dry oxygen in the range of 600–1200°C. The results of the analysis show that the oxidation of boron carbide occurs with the accumulation of excess (compared to stoichiometry) boron. Using experimental data, the time dependences of the thickness of the boron oxide melt layer formed on the sample surface during its oxidation and the mass of boron oxide evaporated from the sample surface are determined. For different temperatures, the evaporation rates of boron oxide are determined. Based on the analysis, a model of the high-temperature oxidation of compact boron carbide in dry environment was developed. Using the developed model, calculations of the process of boron carbide oxidation at different temperatures were carried out. Based on the results of calculations, the time dependences of the mass of the sample, the mass of the resulting carbon dioxide, and the thickness of the boron oxide melt layer were determined. The calculation results are compared with the experimental data and with the theoretical temperature dependence of the boron oxide melt evaporation rate, calculated using the Hertz – Knudsen equation. Based on the simulation results, it is concluded that the rates of chemical reactions are finite, and the process of B4C oxidation cannot be reduced only to oxygen diffusion through the boron oxide layer.

Comparison of residual carbon content and morphology of B4C powders synthesized under different conditions

In this article, the impact of different B4C synthesis methods on the amount of residual carbon and the final morphology of the prepared ceramic particles was investigated. The main materials for the synthesis of B4C were glucose and boric acid, and the effects of adding tartaric acid and performing mechanical activation were studied. For this purpose, two methods of carbon dissolution and boron carbide oxidation were used to determine the amount of residual carbon in the ceramic products. The results of the investigations on the sample synthesized in optimal conditions showed that if additives and mechanical activation are not used, about 7 wt% of carbon will remain in the synthesized powder. The amount of carbon decreased to 5.7 wt% with mechanical activation, but the best result was obtained with the addition of tartaric acid, in which the amount of impurity dropped to 3.3 wt%. Finally, the size and morphology of B4C particles and carbon impurities were observed and compared using a scanning electron microscope.

The preparation and property analysis of B4C modified inorganic amorphous aluminum phosphates-based intumescent flame retardant coating

In response to high environmental requirements, an incombustible inorganic intumescent flame retardant coating was prepared from amorphous aluminum phosphate emulsion as the matrix, with boron carbide as the modifier. The influence of boron carbide (B4C)on the fire resistance performance and physic-chemical properties of coatings were studied by means of large panel combustion method, smoke density test, Fourier transformed infra-red analysis (FTIR), thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), specific surface area (BET method) and pore size analysis. With the addition of 0.75 wt% B4C, the backside temperature of APB3 coating (aluminum phosphate-based coating modified with 0.75 wt% B4C) after an hour was only 140 ℃, 33.4 % lower than that of blank APB0 coating. And the smoke density (Ds) of APB3 was only 64.73 %, 36.3 % lower than that of blank APB0. The foaming expansion caused by decomposition reactions, the structural improvement caused by boron carbide oxidation, and the formation of ablative resistant ceramics and glass were the main reasons for excellent flame retardant and thermal insulation performance. Remarkably, the melting B2O3 generated from the oxidation of B4C could repair cracks, modify pore structure, improve the integrity of closed pores, increase the specific surface area of porous structure, and promote the formation of borosilicate glass. Due to the structural integrity of the outermost coating surface caused by the oxidation of B4C and the construction of borosilicate glass, much gas was shielded inside the coating, making the coating more eco-friendly. Besides, APB3 coating has good water resistance. After immersion in water for 360 h, the reduction rate of fire resistance performance of the coating was only 17.1 %. This work provides theoretical guidance for broadening the types of intumescent coatings and promotes the development of inorganic green coatings.

Anisotropy in thermal properties of boron carbide–graphene platelet composites

Dense boron carbide–graphene composites were prepared by hot-pressing method. Phase analysis revealed presence of small amount of tungsten borides contamination in all sinters. Microstructure of composites showed orientation of graphene platelets. The anisotropy of wave velocity in ultrasonic measurements reached 25%. The values of thermal diffusivity and thermal conductivity were determined in different directions using laser flash analysis methods (LFA 427). For the composite containing 4wt% of graphene platelets the anisotropy of thermal properties exceed 70%. Boron carbide–graphene composites were also examined in the field of thermal stability in air using DTA-TG apparatus. Oxidation of boron carbide started above 720°C. Boron oxide layer formed on the surface was observed under scanning electron microscope.

Oxidation of boron carbide powder

The oxidation of boron carbide powder was studied by using thermogravimetric technique. The oxidation was carried out by heating boron carbide powder in a stream of oxygen. Both isothermal and non-isothermal methods were used to study the kinetics of oxidation. Model-free isoconversional method was used to derive the kinetic parameters. A multistep (three steps) oxidation reaction was observed. The oxidation reaction did not reach to completion due to the formation of glassy layer of boric oxide on the surface of boron carbide powder. This acts as a barrier for further diffusion of oxygen into boron carbide particles and for the release of CO2 from boron carbide. The effective activation energy obtained using isoconversional method for boron carbide was 211 ± 8 kJ mol−1 for ‘α’ = 0.1–0.3. It was inferred that room temperature oxidation of boron carbide is a kinetically hindered process.

Steam oxidation of boron carbide–stainless steel liquid mixtures

In the framework of nuclear reactor core meltdown accidents studies, the oxidation kinetics of boron carbide–stainless steel liquid mixtures exposed to argon/steam atmospheres was investigated at temperatures up to 1527°C. A B–Cr–Si–O liquid protective layer forms on the surface of the mixtures in contact with steam. This protective layer gradually transforms into a Cr2O3-rich slag. Important quantities of liquid can be projected from the melt during oxidation. These projections are favoured by high B4C contents in the melt, high steam partial pressures and low temperatures.In addition to stainless steel–boron carbide melts, simpler compositions (pure 304L stainless steel, iron–boron, iron–boron carbide and stainless steel–boron) were studied, in order to identify the basic oxidation mechanisms.

Investigation on boron carbide oxidation for nuclear reactor safety: Experiments in highly oxidising conditions

The oxidation kinetics of boron carbide pellets were investigated in steam/argon mixtures in the temperature range 1200–1800°C for steam partial pressures between 0.2 and 0.8bar and total flows (steam+argon) between 2.5 and 10g/min resulting in gas velocities from 1.01 to 5.34m/s. A kinetic model for boron carbide pellet oxidation depending on temperature, steam partial pressure and flow velocity is obtained. The activation energy of the oxidation process was determined to be 163±8kJ/mol. The strong influence of temperature and steam partial pressure on the boron carbide oxidation kinetics is confirmed. The obtained data suggest the coexistence of two kinetic regimes, one at 1200°C and the other at 1400–1800°C, with different dependence on steam partial pressure.

Investigations on boron carbide oxidation for nuclear reactors safety—General modelling for ICARE/CATHARE code applications

The present paper deals with the problem of boron carbide pellet oxidation which might occur during a severe accident. A basic correlation, involving global variables, has been developed for the simulation of boron carbide oxidation with the ICARE/CATHARE code. This modelling has been based on available experimental data, including the VERDI separate effects experiments performed by IRSN at low pressures and high temperatures. According to the agreement between the measured and the calculated bundle temperatures as well as hydrogen release and oxidized B 4C, the ICARE/CATHARE code simulates rather well QUENCH experiments involving B 4C control rod degradation, Zircaloy oxidation under starvation and cooling with steam. Based on simulations results, it has been noticed that the B 4C degradation has a slight direct effect on global bundle degradation but a non-negligible influence on Zircaloy oxidation through power release, material melting and flowing down.

Oxidation of B 4C by steam at high temperatures: New experiments and modelling

The oxidation kinetics of various types of boron carbides (pellets, powder) were investigated in the temperature range between 1073 and 1873 K. Oxidation rates were measured in transient and isothermal tests by means of mass spectrometric gas analysis. It was found that oxidation is strongly influenced by the thermohydraulic boundary conditions and in particular by the steam partial pressure and flow rate. On the other hand, the microstructure of the B 4C samples has a limited influence on oxidation. Very low amounts of methane were produced in these tests. On the basis of the experimental data obtained in the isothermal tests, a new model on boron carbide oxidation was developed. The model self-consistently simulates surface reaction kinetics and mass transport of various species in the multi-component gas phase as rate determining steps of the oxidation process. The model was implemented in the SVECHA/QUENCH code and verified against the transient tests.

Modeling of boron carbide oxidation in steam

A simple parametric model has been developed for the simulation of boron carbide oxidation tests at high temperatures done at Forschungszentrum Karlsruhe. The model predicts that an equilibrium oxide film thickness is reached, whose value is only determined by the two rate parameters of the model. Applying the principles of convective mass transfer in coolant channels, one is able to establish the link from the parametric model to a more generalized consideration, which allows to identify the hydrogen surface concentration as a parameter, which is independent of the flow conditions.

Oxidation of boron carbide at high temperatures

The oxidation kinetics of various types of boron carbides (pellets, powder) were investigated in the temperature range between 1073 and 1873 K. Oxidation rates were measured in transient and isothermal tests by means of mass spectrometric gas analysis. Oxidation of boron carbide is controlled by the formation of superficial liquid boron oxide and its loss due to the reaction with surplus steam to volatile boric acids and/or direct evaporation at temperatures above 1770 K. The overall reaction kinetics is paralinear. Linear oxidation kinetics established soon after the initiation of oxidation under the test conditions described in this report. Oxidation is strongly influenced by the thermohydraulic boundary conditions and in particular by the steam partial pressure and flow rate. On the other hand, the microstructure of the B 4C samples has a limited influence on oxidation. Very low amounts of methane were produced in these tests.

Oxidation behaviour of a multi-layered ceramic-matrix composite (SiC) f/C/(SiBC) m

The oxidation behaviour of a 2.5D multi-layered ceramic-matrix composite (SiC) f/C/(SiBC) m was investigated in a dry atmosphere O 2 (20 vol.%) CO 2 (5 vol.%) He and in the presence of water vapour H 2O (2.3 vol.%). The matrix denoted (SiBC) m is constituted of three phases: silicon carbide, boron carbide and a SiBC phase. The aim of boron incorporation is to improve the oxidation resistance of the composite by boron oxide or borosilicate formation. The transformations were followed by thermogravimetry during isothermal experiments of 20 h exposure in the range 600–1200°C and the changes of the specific area of the samples were measured by krypton adsorption at 77 K. The results of this study show that effective protection occurs in the range 650–900°C and is mainly related to the oxidation of boron carbide. At higher temperatures, boron oxide is no longer protective because of its volatilisation and consumption by reaction with water vapour. However, the two other constituents of the matrix, SiC and SiBC, lead to self-healing behaviour by both borosilicate and silica formation.

Characterization by transmission electron microscopy of debris formed on plasma spray and hard PVD ceramic coatings

In this paper the structural investigation by TEM of debris generated in ball-on-disc tests is discussed. A 200 μm thick B 4C-Cr 2O 3-TiO 2 coating deposited by air plasma spraying on a steel disc was worn in a sliding wear (ball-on-disc) test, and a steel disc coated with a 2 μm thick (Ti, Nb)N layer deposited by PVD was tested under similar conditions. After a discussion of the preparation of TEM samples, the TEM analysis of the debris is discussed. Evidence is given for the fracture and pull-out of carbide particles, and for the oxidation of boron carbide and (Ti, Nb)N during the wear test.

Influence of oxidation on the composition and structure of the surface layer of hot-pressed boron carbide

The oxidation of hot-pressed boron carbide under isothermal conditions and under conditions of programmed heating up to 1500°C was investigated. Oxidized samples were studied by secondary-ion mass spectrometry, X-ray photoelectron spectroscopy, scanning electron microscopy, energy-dispersive X-ray microanalysis, X-ray diffraction, and other methods. It has been demonstrated that oxidation starts above 600°C and results in the formation of a thin transparent B2O3 film that is cracked after cooling. Up to 1200°C, the oxidation process is limited by the diffusion of reagents through the oxide layer; at higher temperatures, it is determined by the rate of chemical reaction of carbide with oxygen in the air. During boron carbide oxidation the etching of grain boundaries occurs, it results in strength degradation at higher temperatures.

Oxidation of hot-pressed boron carbide

This article investigates the kinetics of boron carbide oxidation at high temperatures in air and in water vapor from a chemical and crystallographic viewpoint, identifies the oxidation products as various forms of boron oxides, carbon dioxide, hydrogen, and hydrogen borates, and explores the roles played by porosity, temperature, and boron content on the oxidation of the carbide.

Kinetics of high-temperature oxidation of boron carbide in oxygen

Thermogravimetry and gas-adsorption chromatography were used to study the kinetics of formation of solid and gaseous products during the hightemperature oxidation of compact boron carbide in oxygen at 740 Torr. Oxidation resistance was observed at temperatures up to 1200°C. The main oxidation products were B2O3 and CO2. Oxidation was paralinear; the carbon consumption exceeded the consumption of boron as compared to the ratio of these elements in the compound B4C. This difference resulted in carbon depletion of the carbide layer in the substrate near the scale>.

Reaction of boron carbide with oxygen and the effect of preirradiation in a nuclear reactor

The kinetics of the oxidation of boron carbide between 503 and 528° have been studied by continuous measurement of weight change. The rate of oxidation reached a maximum at about 12% weight increase and this was maintained up to about 35% weight increase; the rate then decreased, the weight increase becoming proportional to the square root of time. At this stage the rate was controlled by a diffusion process. The activation energies for the initial and maximum rates were 37·2 (±1·5) and 39·9 (±0·5) kcal./mole respectively. Preirradiation of the boron carbide in BEPO enhanced the subsequent rate of oxidation. The initial acceleration of the reaction rate is attributed to splitting of crystals by oxidation along grain boundaries. Preirradiated crystals are more easily split than unirradiated crystals.

The oxidation of boron carbide

1. A study was made of the oxidation of boron carbide and carbon black by different oxidizers: mixtures of sulfuric acid with potassium bichromate, sulfuric acid with potassium permanganate, sulfuric, nitric, and perchloric acids with potassium bichromate, etc. It is shown that the best oxidizing mixture for the removal of free carbon from boron carbide is one containing sulfuric, nitric, and perchloric acids with potassium bichromate. 2. A study was made of the oxidation of boron carbide powders with different amounts of total carbon and boron by oxygen at different temperatures. It is shown that the oxidation of boron carbide (and free carbon) begins at 700°C. The formation of a B2O3 oxide film prevents the full oxidation of boron carbide at 1100°C. The complete oxidation of boron carbide by oxygen is only observed at 1200–1300°C. In the case of two boron carbide samples subjected to oxidation by oxygen, the higher oxidation intensity will be shown by the sample with the smaller value of the coefficient k=Bcomb./Ctotal.

Oxidation of Boron Carbide by Air, Water, and Air‐Water Mixtures at Elevated Temperatures

A study of the oxidation of boron carbide powder has shown measurable reaction at temperatures as low as 250 °C with water vapor and 450 °C in dry air. Removal of the oxidation product by water vapor occurs at a rate in excess of the oxidation rate below 550°–600°C. The presence of on the surface was found to inhibit the oxidation by but not by air. Linear dependence of rate on the partial pressure of water was observed. The activation energy for the water‐ reaction was 11 kcal/mole‐°C, while that for the air‐ reaction was 45 kcal/mole. Oxidation by dry air occurs at a lower rate than with water vapor until approximately 700°C (for 235 mm water partial pressure).