Mass spectrometric study of the vaporization of lithium metasilicate

The vaporization of Li₂SiO₃(c/1) has been studied by the mass spectrometric Knudsen effusion method. The vaporization process has been found to be incongruent. Partial pressures of Li(g), LiO(g), Li₂O(g), SiO(g) and Li₂SiO₃(g) over Li₂SiO₃(c/1) have been determined in the temperature range 1166–1762 K. Partial pressures of O₂(g) have also been calculated from the reaction Li₂SiO₃(1) = 2 Li(g) + SiO(g) + O₂(g). The enthalpies of formation and the atomization energies for LiO(g) and Li₂O(g) have been evaluated from the partial pressures to be ΔH^o_f0(LiO,g) = (84.5 ± 12.8)kJ/mol, ΔH^o_f0(Li₂O, g) = (?148.1 ± 15.8)kJ/mol, D₀^o(LiO) = (321.4 ± 12.8)kJ/mol and D₀^o(Li₂O) = (713.2 ± 15.8)kJ/mol, respectively. The value of D₀^o(Li₂O) is somewhat greater than twice that of D₀^o(LiO).     

Mass Spectrometric Study of Ions Formed from Cesium Metaborate Vapor under Electron Impact

An ionization behavior of cesium metaborate vapor under electron impact has been studied by a mass spectrometric method. Formations of Cs²⁺, CsB⁺, CsO⁺, Cs⁺ ₂, Cs₂O⁺, B⁺ and BO^? ₂ ions have been identified in addition to the well known ions of Cs⁺, CsBO⁺, CsBO⁺ ₂ and Cs₂BO⁺ ₂. Ionization processes and vapor precursors for these ions have been given from ionization efficiency curves, appearance energies, temperature dependence of ion intensities and energetics of the ionization processes as follows: the process for the formations of Cs⁺ with AE(Cs⁺)=3.9± 1.0 eV and BO^? ₂ ions is the ion pair formation from CsBO₂(g), that for CsBO⁺ ₂ ion is the simple ionization of CsBO₂(g), that for Cs⁺ with AE (Cs⁺) =9.1±0.5 eV, Cs²⁺, CsBO⁺, CsB⁺, CsO⁺ and B⁺ ions is the dissociative ionization from CsBO₂(g) and that for Cs⁺ ₂, Cs₂O⁺ and Cs₂BO⁺ ₂ ions is the dissociative ionization from Cs₂(BO₂)₂(g). The knowledge of the ionization behavior of cesium metaborate vapor under electron impact is very useful in the mass spectrometric study of vaporization behaviors of CsBO₂(s) and simulated radioactive waste borosilicate glasses.     

Thermal conductivity of Na2O–B2O3–SiO2 melts

Borosilicate glasses are candidate materials for the immobilization of high-level radioactive waste. The values of thermal conductivity of different borosilicate melts are thus indispensable information when optimizing the temperature distribution in a glass melting furnace. In this study, the thermal effusivity of Na₂O–B₂O₃–SiO₂ melts was measured using a front heating–front detection laser-flash method. The thermal conductivity, which can be obtained by combining the measured thermal effusivity with the specific heat capacity and density, was calculated using the least-squares method; the values for the Na₂O–B₂O₃–SiO₂ melts either slightly decreased linearly with increasing temperature or remained almost constant over the investigated temperature range. The values of thermal conductivity of the Na₂O–B₂O₃–SiO₂ melts were higher than those of B₂O₃–SiO₂ melts and lower than those of CaO–B₂O₃–SiO₂ melts. Furthermore, the thermal conductivity of the Na₂O–B₂O₃–SiO₂ melts was compared with those of the B₂O₃–SiO₂ and CaO–B₂O₃–SiO₂ samples.     

Determination of molecular, atomic, electronic cross-sections and effective atomic number of some boron compounds and TSW

The transmission of gamma-rays of some boron compounds (H₃BO₃, Na₂B₄O₇) and the trommel sieve waste (TSW) have been measured by using an extremely narrow-collimated-beam transmission method in the energy range 15.74-40.93 keV. Molecular, atomic and electronic cross-sections and effective atomic numbers have been determinated on the basis of mixture rule and compared with the results obtained from theory.     

Effective atomic numbers of some composite mixtures including borax

Effective atomic numbers for (PbO and Na₂B₄O₇10H₂O) and (UO₂(NO₃)₂, and Na₂B₄O₇10H₂O) mixtures against changing contents of PbO, Na₂B₄O₇10H₂O, and UO₂(NO₃)₂ were measured in the X-ray energy range from 25.0 to 58.0 keV. The gamma rays emitted by a ²⁴¹Am annular source have been sent on the absorbers which emits their characteristic X-rays to be used in transmission arrangement. The X-rays were counted by a Si(Li) detector with a resolution of 146 eV at 5.90 keV. The changing compositions of the compounds were assigned to be 0, 0.167, 0.333, 0.500, 0.666, 0.833 and total masses of the mixtures were adjusted to be identical. Also, the total effective atomic numbers of each mixture were estimated by using the mixture rule. The measured values were compared with estimated values for the mixtures.     

Review on the addition of boron compounds to radiation shielding concrete

The effects of the addition of three commercially available boric compounds (boric acid, boric frit, and borax) on the shielding properties of two radiation shielding concrete, made of carbonate and hematite aggregates, have been investigated. The results show that boric acid (H₃BO₃) and its frit have deleterious effect on the setting of ordinary cement in ratios 0.5-1% of the total weight of the concrete.Adding Borax (Na₂B₄O₇) has no significant effect on strength of concrete in the range up to 1% by wt, but it has significant effects on shielding efficiency in thick concrete shields (100 cm) as it reduces the capture gamma rays up to 80% better than unborated concretes.     

Thermal vaporization and deposition of gallium oxide in hydrogen

The thermodynamics of gallium oxide vaporization and deposition in Ar–6% H₂ at elevated temperatures are described. It is shown that Ga₂O₃ vaporizes in H₂ as Ga₂O(g) at elevated temperatures. During thermal processing the Ga₂O(g) moves to cooler zones of the furnace, back reacts with H₂(g) and H₂O(g) and condenses out as Ga(l) and Ga₂O₃(s). Upon removal from the furnace, the exposed Ga forms a ubiquitous surface oxide of Ga₂O₃. X-ray photoelectron spectroscopy (XPS) was used to examine heat treated Ga₂O₃ powders and vaporization products deposited onto SiO₂ and Cu substrates. In agreement with the thermodynamic predictions, these data demonstrate that the deposition product contained Ga₂O₃ and metallic Ga. Analysis of the XPS spectra also revealed an intermediate oxidation state for Ga. The precise bonding of this state could not be demonstrated conclusively, but it is suggested that it may be solid Ga₂O. For coherent product deposition on Cu the metallic Ga concentration increases and the Ga₂O₃ concentration decreases with sputtering depth, suggesting the metallic Ga in the outermost layers of the deposit is readily oxidized during air exposure.     

Vaporization study on the niobium monoxide and niobium metal systems by mass-spectrometric methods

The vapor pressures over single-phase NbO(s) and two-phase NbO(s) — Nb(s) were measured by the mass-spectrometric method in the temperature range 1948–2301 K. The main vapor species over both systems were observed to be NbO(g) and NbO₂(g). By applying the second and the third law treatments of thermodynamics to the partial vapor pressures of NbO(g) and NbO₂(g), the enthalpies of vaporization were calculated for the following reactions: NbO(inNbO(s)) = NbO(g), NbO₂(inNbO(s)) = NbO₂(g), 2NbO(s) = Nb(s) + NbO₂(g), 2NbO(s) = Nb(s)+NbO(g) + 0(g). The enthalpies of formation and the dissociation energies of NbO(g) and NbO₂(g) obtained from these reactions were in good agreement with the previous results obtained from the vapor pressure measurements on NbO₂(s) by the present authors. The partial pressures of oxygen were calculated as a function of temperature from the partial pressures of NbO₂(g) and NbO(g), from which the partial molar enthalpies and entropies of oxygen in the system of NbO(s) and Nb(s) were determined.     

Mass spectrometric study of the evaporation of LiCrO2 as a corrosion product in the compatibility experiment of Li2O pellets with Fe-Ni-Cr alloys

A mass spectrometric Knudsen effusion study of the vaporization of LiCrO₂ in the temperature range of 1673–1873 K has shown the following: (1) The major vapor species over solid LiCrO₂ are Li(g), Cr(g), CrO(g), CrO₂(g) and LiCrO₂(g). (2) The vaporization process involves a sublimation reaction, LiCrO₂(s) = LiCrO₂(g), and a dissociation reaction, $\text{LiCrO}{}_2\text{(s) =}\text{1}\text{2}\text{Cr}{}_2\text{O}{}_3\text{(s) + Li(g) +}\text{1}\text{4}\text{O}{}_2\text{(g)}$. (3) The standard enthalpy of solid LiCrO₂ at 298 K is derived to be (?935 ± 21) and (?966 ± 18) kJ/mol from the 2nd and 3rd law treatments, respectively.     

Vaporization study on UO2 and (U1−yNby)O2+x by mass-spectrometric method

The vapor pressures over UO_2.000 and (U_1?yNb_y)O_2+x (y = 0.01, 0.05, x = 0.000–0.022) were measured by the mass-spectrometric method in the temperature range 2025–2343 K. The main gas species over UO_2.000 were observed to be UO₃(g) and UO₂(g) and those over (U_1?yNb_y)O_2+x were NbO₂(g), NbO(g), UO₃(g) and UO₂(g). The partial vapor pressures of almost all gas species over (U_1?yNb_y)O_2+x increased with increasing O/M (M = U + Nb) ratio. With increasing Nb content in (U_1?yNb_y)O_2.000, the partial vapor pressures of UO₂(g) and UO₃(g) decreased and those of NbO(g) and NbO₂(g) increased. The congruently vaporizing composition in the (U_1?yNb_y)O_2+x phase was estimated to be ($\text{U}{}_{\mathrm{0.985\pm 0.005}}\text{Nb}{}_{\mathrm{0.015\pm 0.005}}\text{)O}{}_{2.000}$ from the compositional dependence of the total vapor pressures. The partial molar enthalpy and entropy of oxygen of (U_1?yNb_y)O_2+x calculated from the partial pressures of gaseous species NbO₂(g) and NbO(g) were in fairly good agreement with those previously obtained by the present authors with a thermobalance.     

Structural investigations of borosilicate glasses containing MoO3 by MAS NMR and Raman spectroscopies

High molybdenum concentration in glass compositions may lead to alkali and alkaline-earth molybdates crystallization during melt cooling that must be controlled particularly during the preparation of highly radioactive nuclear glassy waste forms. To understand the effect of molybdenum addition on the structure of a simplified nuclear glass and to know how composition changes can affect molybdates crystallization tendency, the structure of two glass series belonging to the SiO₂-B₂O₃-Na₂O-CaO-MoO₃ system was studied by ²⁹Si, ¹¹B, ²³Na MAS NMR and Raman spectroscopies by increasing MoO₃ or B₂O₃ concentrations. Increasing MoO₃ amount induced an increase of the silicate network reticulation but no significant effect was observed on the proportion of units and on the distribution of Na⁺ cations in glass structure. By increasing B₂O₃ concentration, a strong evolution of the distribution of Na⁺ cations was observed that could explain the evolution of the nature of molybdate crystals (CaMoO₄ or Na₂MoO₄) formed during melt cooling.     

Vaporization of uranium-carbon-nitrogen system

The vaporization of species from the uranium-carbon-nitrogen system has been investigated by mass spectrometry with the Knudsen cell technique. After calibration with gold, the vapor pressures of U and N₂ have been obtained over UC_1?xN_x in the temperature range of 1900–2300K. For uranium mono-nitride, values of enthalpies Δ$\text{H}$°₂₉₈ for overall reaction (1)$\text{UN(s) = U(g) + 0.5N}{}_2\text{(g)}$ and the partial reactions (2)$\text{UN(s) = U(L) + 0.5N}{}_2\text{(g)}$ and $\text{U(L) = U(g)}$ have been obtained with both second- and third-law treatments. For UC_1?xN_x the experimental vapor pressures were used to calculate activity of UN in the solutions. For three compositions ($\text{x = 0.69}$, 0.48, 0.30) measured, activity coefficients were found to be slightly smaller than unity.     

Defect volumes of BO2 doped Y2O3 (B = Ti, Zr, Hf and Ce)

Atomistic simulations have been employed to study the effect of BO₂ (fluorite) incorporation into the bixbyite oxide Y₂O₃. The energetically preferred defect mechanism and the associated lattice parameter changes that occur from BO₂ doping have been predicted. The addition of Group IV elements into Y₂O₃ can follow three different mechanisms. The energetically favourable method is through a mediated reaction for ZrO₂ and HfO₂ while for TiO₂ and CeO₂, reducing B⁴⁺ to B³⁺ provides the lowest energy reaction. ZrO₂ and HfO₂ doping results in the lowest volume changes.     

Effect of carbide on Al2O3/B4C burnable poison sintered in Ar

Al₂O₃–14 wt%B₄C composites with 0–1 wt% C addition were sintered in Ar at 1550–1650 °C. The influence of the C additive on the B loss, densification behavior, and microstructure of the Al₂O₃–B₄C composites were investigated. The results show that there are B₂O₃, H₃BO₃ and Al₁₈B₄O₃₃ exist between Al₂O₃ and B₄C interface, which result in B loss because of B₂O₃'s high vapor tension at above 1500 °C. The presence of Al₁₈B₄O₃₃ grains formed by chemical reaction of Al₂O₃ with surface oxides on B₄C inhibit the densification of pellets by reducing the specific free surface energy of the Al₂O₃. However, the added C eliminates those oxides to reduce B loss because of its higher activity than B₄C, and it also coarsens Al₂O₃ grain although the density of pellets is decreased by gas products.     

Solidification of “Cement-Glass”

Cement-glass, which is a mixture of sodium silicate (kNa₂O·mSiO₂nH₂O), silicon phosphate (P₂O₅·2SiO₂) and cement, was developed to solidify radioactive waste pellets in containers. The optimum molar ratio of Si(OH)₄, NaOH and P₂O₅2SiO₂ was found to be 2:2:1, based on compressive strength measurement of solidified samples. The compressive strength of solidified sodium silicate with silicon phosphate was increased when the amount of solidified SiO₂ per unit volume was increased by reducing the water content. Cement-glass had a higher Cs distribution factor than ordinary Portland cement.     

Vaporization study on nonstoichiometric NbO2 ± x by mass-spectrometric method

The vapor pressures over nonstoichiometric NbO_{2 ± x}(s) (1.972?2.037) were measured by mass-spectrometric method in the temperature range 1958–2326 K. The congruently vaporizing composition in the NbO_{2 ± x} phase was determined to be stoichiometric NbO_2.000 from the composition dependence of the total vapor pressures. The partial pressures of oxygen were calculated as a function of temperature and O/Nb composition from the partial pressures of the gaseous species NbO₂(g) and NbO(g) over nonstoichiometric NbO_{2 ± x}, from which the partial molar enthalpies and entropies of oxygen were calculated as a function of O/Nb composition. The composition dependence of the partial molar enthalpy and entropy obtained suggested the existence of some kind of short-range ordering in the nonstoichiometric NbO_{2 ± x}(s) phase. The enthalpies of formation of nonstoichiometric NbO_{2 ± x}(s) were also determined as a function of composition by combining the partial molar enthalpies of oxygen with the enthalpy of formation of stoichiometric NbO_2.00(s). The phase diagram around NbO_{2 ± x} at high temperatures was determined from the vaporization study.     

The standard molar enthalpy of formation of CdMoO4

The molar enthalpies of solution of CdMoO₄(s), CdO(s), Na₂ MoO₄(s) and NaF(s) in (10 mol HF(aq) + 4.41 mol H₂O₂(aq)) dm⁻³ have been measured using an isoperibol type calorimeter. From these results and other auxiliary data, the standard molar enthalpy of formation of CdMoO₄(s) has been calculated to be Δ_fH°(298.15 K) = −(1034.3 ± 5.7) kJ mol⁻¹. This value of enthalpy of formation of CdMoO₄(s) agrees well with the estimated enthalpy of formation of this compound. There is no other report on the thermodynamic property measurements on this compound.     

Vaporization behavior and Gibbs energy of formation of Rb2ThO3

The thermodynamic stability of rubidium thorate, Rb₂ThO₃(s), was determined from vaporization studies using the Knudsen effusion forward collection technique. Rb₂ThO₃(s) vaporized incongruently and predominantly as Rb₂ThO₃(s)=ThO₂(s) + 2Rb(g) + 1/2 O₂(g). The equilibrium constant K=p_Rb²·p_O2^1/2 was evaluated from the measurement of the effusive flux due to Rb vapor species under the oxygen potential governed by the stoichiometric loss of the chemical component Rb₂O from the thorate phase. The Gibbs energy of formation of Rb₂ThO₃ derived from the measurement and other auxiliary data could be given by the equation, Δ_fG°(Rb₂ThO₃,s)=−1794.7+0.42T ± .