Knudsen Cell Research Articles

Transmission Electron Microscopy-Based Analysis of Electrically Conductive Surface Defects in Large Area GaSb Homoepitaxial Diodes Grown Using Molecular Beam Epitaxy

We investigate a mechanism causing shorting of large area GaSb diodes grown on GaSb substrates using molecular beam epitaxy (MBE). The source of these shorts is determined to be large crystallographic defects on the surface of the diodes that are formed around droplets of gallium ejected from the gallium Knudsen cells during MBE. The gallium droplets cause defects in the crystal structure, and, as the epitaxy continues, the gallium is incorporated into the surrounding material. The shape of the defects is pyramidal with a central void extending from the epi-surface to the gallium core. Processing a GaSb diode with these surface defects results in the top-side contact metal migrating into the defect and shorting the diode. This prevents realization of large area diodes that are critical to applications such as photovoltaics and detectors. The diodes in this study are electrically characterized and the defect formation mechanism is investigated using cross-section transmission electron microscopy and electron dispersive spectroscopy.

Interpretation of Knudsen Cell Experiments to determine the Instant Release Fraction in Spent Fuel Corrosion Scenarios by using a Mechanistic Approach: the Caesium Case

ABSTRACTThe Knudsen Effusion Mass Spectrometer (KEMS) and the mechanistic MFPR (Module for Fission Product Release) code are tools which seem particularly interesting to support studies of the Instant Release Fraction (IRF) of Cs from spent nuclear fuel in a final repository. With KEMS, the thermal release of 137Cs and 136Xe were analysed by annealing up to total vaporization (2500K) of high burn-up (60 GWd/tU) Spent Nuclear Fuel (SNF) samples. Powder samples from the centre of the fuel, without high burn-up structure, were used. To determine the IRF, samples were analysed before and after being submitted to corrosion experiments in bicarbonated aqueous media.MFPR was applied to determine the localization of Cs and fission gases in the SNF at the end of irradiation; the results are compared and supported by dedicated thermodynamics calculations performed for equilibrium conditions at various temperatures and fuel oxygen potentials by the non-ideal thermodynamic MEPHISTA (Multiphase Equilibria in Fuels via Standard Thermodynamic Analysis) database. A possible mechanism for Cs release during thermal annealing is proposed, taking into account inter-granular release and Cs oxide vaporization, atomic diffusion, ternary oxide phase formation and bubble release.Differences in KEMS release profiles before and after submitting the samples to aqueous corrosion are attributed to the IRF and to changes in the vaporisation mechanism because of differences in the oxygen potential (pO2). The IRF of Cs estimated from the KEMS spectra, consisting on the part located at the grain boundaries and in inter-granular bubbles, is not significantly different from that corresponding to the experimental results found using classical static leaching experiments.New experimental campaigns are being designed to confirm our interpretation proposed after this first run.

Thermodynamic Measurements of Alloys and Compounds by Double Knudsen Cell Mass Spectrometry and Their Application to Materials Processing.

For the development and optimization of materials processing a collection of thermodynamic information concerning substances that participate in the reactions is important. One fundamental way to obtain such information is to measure the vapor pressure of gas species under conditions where they are in equilibrium with the condensed phases. Over the past 60 years Knudsen cell mass spectrometry has been used to identify and quantitatively determine gas species at high temperatures. This article describes thermodynamic foundation and examples of measurements in order to demonstrate the use of mass spectrometry focusing on the field of process metallurgy and recycling processes.

Study of vaporization of LiI, LiI/C70, LiI/LiF/C70 from the Knudsen cell located into ionization chamber of the mass spectrometer

The vaporization of LiI, LiI/C70 and LiI/LIF/C70 were studied using a Knudsen cell located into ionization chamber of a magnetic sector mass spectrometer in a temperature range from 350 ?C to 850 ?C. ?s the ion species, LinI+ (n = 2, 3, 4, and 6) were identified from a mixture LiI/C70. While the clusters LinI+ and LinF+ (n = 2 - 6) were detected from a mixture LiI/LiF/C70. The intensities of LinI+ were higher than the emission of LinF+ cluster when the ratio of LiI to LiF was 2:1. By contrast, the emission of the LinF+ is favored when the ratio of LiI to LiF was 1:2. These results show that the vaporization of a mixture LiI/LIF/C70 from the Knudsen cell located into ionization chamber of the mass spectrometer represents an efficient and simple way to obtain and investigate clusters of the type LinX, X-F, I. In this work it has also been shown that the trend of the ln (Intensity, arbit. units) versus temperature for all LinI+ clusters before and after the melting point of LiI was not same. It suggested that the way of the formation of these clusters can be different due to changes in temperature.

A mass spectrometric investigation of the vaporisation behaviour in the (U + Pu + O) system

The vaporisation behaviour of (U,Pu)O2 mixed oxides (Pu/M=0.25, 0.50 and 0.75, with M=U+Pu) was studied by means of mass spectrometry. Hyperstoichiometric samples were heated in a Knudsen cell up to T=2300K. The evolution of the uranium and plutonium bearing gaseous species was studied as a function of time in order to evaluate the congruent vapour composition. Ionisation efficiency measurements were performed and the partial pressures of the gaseous species involved in the vaporisation process were determined. The vapour pressure has also been calculated using a thermochemical model for the (U+Pu+O) system. A quasi congruent composition with respect to the O/M ratio has been assumed, in agreement with the experiments. Nevertheless, the evaluation of all the experimental and calculated results shows that a total congruent composition exists for a single composition of the mixed oxide (MOX) samples with a Pu/M content slightly lower than 0.50. A good agreement is obtained between the calculated and experimental vapour pressure data, as well as the quasi congruent vaporisation compositions.

Adsorption calorimetry during metal vapor deposition on single crystal surfaces: Increased flux, reduced optical radiation, and real-time flux and reflectivity measurements

Thin films of metals and other materials are often grown by physical vapor deposition. To understand such processes, it is desirable to measure the adsorption energy of the deposited species as the film grows, especially when grown on single crystal substrates where the structure of the adsorbed species, evolving interface, and thin film are more homogeneous and well-defined in structure. Our group previously described in this journal an adsorption calorimeter capable of such measurements on single-crystal surfaces under the clean conditions of ultrahigh vacuum [J. T. Stuckless, N. A. Frei, and C. T. Campbell, Rev. Sci. Instrum. 69, 2427 (1998)]. Here we describe several improvements to that original design that allow for heat measurements with ~18-fold smaller standard deviation, greater absolute accuracy in energy calibration, and, most importantly, measurements of the adsorption of lower vapor-pressure materials which would have previously been impossible. These improvements are accomplished by: (1) using an electron beam evaporator instead of a Knudsen cell to generate the metal vapor at the source of the pulsed atomic beam, (2) changing the atomic beam design to decrease the relative amount of optical radiation that accompanies evaporation, (3) adding an off-axis quartz crystal microbalance for real-time measurement of the flux of the atomic beam during calorimetry experiments, and (4) adding capabilities for in situ relative diffuse optical reflectivity determinations (necessary for heat signal calibration). These improvements are not limited to adsorption calorimetry during metal deposition, but also could be applied to better study film growth of other elements and even molecular adsorbates.

Thermodynamic measurement of Dy + Fe binary system by double Knudsen cell mass spectrometry

The activities of Dy in Dy–Fe alloys with Dy concentrations ranging from (7.0 to 89)at% were determined at (1273 to 1573)K by measuring the ion current of the vapor of Dy in equilibrium with Dy–Fe melts, Dy2Fe17+γFe, etc. The standard Gibbs energy of formation of Dy2Fe17, Dy6Fe23, DyFe3, and DyFe2 were calculated from the activities of Dy obtained from these measurements.

Knudsen cell mass spectrometric study of the Cs2IOH(g) molecule thermodynamics

The gas phase of the CsI+CsOH system is analyzed by high temperature Knudsen cell mass spectrometry in order to confirm the existence of the Cs2IOH(g) complex molecule. The mass spectrometric analysis is quite complex since such molecules undergo dissociative ionization into fragment ions that mix with the same ions from dimers of the pure compounds in the same vapor phase. Varying the chemical conditions for vaporization by using different CsI+CsOH mixture contents showed that the ionization of the Cs2IOH(g) molecule led to five different fragment ions, Cs2OH+, Cs2I+, Cs+, CsOH+ and CsI+. This complex ionization pattern was studied in relation with previous assessed values for the vaporization of CsOH and CsI pure compounds in which monomer and dimer molecules are predominant. The equilibrium constant for the reaction CsI(g)+CsOH(g)=Cs2IOH(g) was determined and, after modeling the structure of the Cs2IOH molecule, the enthalpy of formation was determined using the third law of thermodynamics, as follows: ΔfH°(Cs2IOH, g, 298.15K)=−578±14.7kJ·mole−1.

Mass Spectrometric Studies of Oxides

Current studies at NASA Glenn on oxide thermodynamics are discussed. Previous studies on the vaporization of B2O3 in reducing atmospheres led to inconsistent studies when B was used as a reductant. It is shown that liquid B2O3 does not wet B and a clear phase separation was noted in the Knudsen cell. This problem was solved by using FeB and Fe2B to supply a different and constant activity of B. The thermodynamic data thus derived are compared to quantum chemical composite calculations. A major problem in high temperature mass spectrometry is the determination of accurate ionization cross sections, particularly for molecules. The method of Deutsch and Mark shows promise and some sample calculations are discussed. Finally current studies on the thermodynamics of rare earth silicates are discussed. Here the problems are obtaining a measurable signal from SiO2 vaporization and non-equilibrium vaporization. The use of a Ta reducing agent provides a stronger signal, which is related to silica activity. The Whitman-Motzfeld relation adapted to KEMS measurements is applied to obtain equilibrium pressures.

The Design, Construction, and Testing of a Knudsen Effusion Mass Spectrometric System Suitable for Studies of Nuclear Fuel Materials

Benefiting hugely from many years of experience of conducting vaporization studies with a commercially bought magnetic sector instrument (VG Micromass 30 BK), we designed and constructed a KEMS system having a quadrupole mass spectrometer. It essentially consisted of two chambers, one housing the Knudsen cell furnace assembly and the other housing the QMS with an indigenously designed and fabricated vacuum-isolation valve between these two chambers. The QMS initially used was from Extrel, but later that from Hiden was installed. Systematic vaporization experiments were undertaken on iron, nickel, chromium, zirconium, a Fe-Al alloy, MnO, uranium, and a U-Zr alloy. Electron bombardment heating was employed and the temperatures were measured by sighting the optical pyrometer onto a blackbody hole drilled at the bottom of the Knudsen cell. The results on the elements and the Fe-Al alloy were satisfactory while those on MnO, U, and U-Zr alloy were disconcerting but interesting and educative.

The Study of Cerium Tetrafluoride Vaporization by Knudsen Effusion Mass Spectrometry

The evaporation and decomposition of cerium tetrafluoride CeF4(s) at high temperatures were studied by means of Knudsen Effusion Mass Spectrometry (KEMS). The saturated vapor pressure over CeF4(s) was determined by the isothermal evaporation of the sample from Pt Knudsen cell. The enthalpy of CeF4(s) sublimation was calculated using the second and the third laws of thermodynamics.

Transformation of a Double Focusing B-E Mass Spectrometer for Multiple-Cell Knudsen Effusion – Part I: Scientific Concepts

Knudsen Effusion Mass Spectrometry (KEMS) is a powerful method in the field of experimental thermodynamics at high temperatures. Since 2005, the long term project of transforming and upgrading a double-focusing VG Micromass 54-38K mass spectrometer for multiple-cell effusion has started at the IM2NP in Marseille/France. The scientific concepts underlying the transformation are presented in this first paper. The molecular beam effusing from the Knudsen cells must be sampled under restricted collimation conditions. The resulting geometric constraints are taken into account in the design of the large main vacuum chamber necessary to house a multiple-cell effusion furnace. Decrease of the molecular transmission between the cell and the ionization source needs to be counterbalanced by an increase in the sensitivity of the ionization source. Other modifications of the instrument are reviewed. The first tests and results will be the subject of a second paper to be published.

Mass Spectrometric Studies of Non-Equilibrium Vaporizations in Knudsen Cells: A Way to the Determination of the Nature of Kinetic Processes

Non-equilibrium vaporizations in Knudsen cells occur when the net rate of vaporization at the surface of the sample is slower than the rate of effusion. In such a situation, the description of the molecular flow exchanges in an effusion cell according to the Motzfeldt equation allows the determination of the evaporation coefficients from different pressure determinations of the same sample performed with different cell geometries. Further, the chemical signification of the value of the evaporation coefficients was not very often explained. Different thermodynamic analyses are presented on experimental results in order to explain the origin of retarded vaporizations of Al2O3 , SiO2 , AlN , BN and mixtures Si-SiO2 as well as amorphous SiO.

Composition and Properties of Saturated Vapor over NaBr-LnBr3 (Ln = La, Lu) Binary Systems

The NaBr−LnBr3 (Ln = La, Lu) binary systems and their individual components NaBr, LaBr3 and LuBr3 were studied by the Knudsen effusion cell mass spectrometry (KEMS) and the density functional theory (DFT). The complex molecules NaLnBr4, Na2LnBr5 and both positive Na2LnBr4 +, Na3LnBr5 + and negative NaLnBr5 − complex ions have been observed in saturated vapor along with the molecules and ions inherent to the pure components. Molecular parameters and energy characteristics of species were obtained by quantum-chemical calculations. The partial pressures of molecules have been obtained. The equilibrium constants of dissociation reactions of complexes were determined and the enthalpies of reactions have been calculated using the second and third laws of thermodynamics. The enthalpies of formation of complex molecules and ions have been obtained.

Combination of Thermal Analysis and Knudsen Effusion Mass Spectrometry for Study of Metal Materials on Macro- and Nano-Scale

The study of metal materials could not be possible without the use of sophisticated techniques often enabling us to do precise experiments of various types by means of multifunction equipments. One of these is an apparatus combining thermal analysis (TA) with Knudsen effusion mass spectrometry (KEMS). In the Netzsch STA 409 CD/3/403/5/G apparatus, a specially-adapted type of the commercial STA 409 CD – QMS 403/5 Skimmer Coupling Instrument, the holder for differential scanning calorimetry (DSC) or differential thermal analysis (DTA) can be replaced by another one with alumina Knudsen cell for vapour pressure measurement of activities of components. The instrument capabilities are demonstrated on several examples of TA and effusion measurements of bulk metal materials and nano-powders.

The Process of Developing a Multi-Cell KEMS Instrument

Multi-cell KEMS offers many advantages over single cell instruments in regard to in-situ temperature calibration and studies on high temperature alloys and oxides of interest to NASA. The instrument at NASA Glenn is a 90° magnetic sector instrument originally designed for single cell operation. The conversion of this instrument to a multi-cell instrument with restricted collimation is discussed. For restricted collimation, the ‘field aperture’ is in the copper plate separating the Knudsen Cell region and the ionizer and the ‘source aperture’ is adjacent to the ionizer box. A computer controlled x-y table allows positioning of one of the three cells into the sampling region. Heating is accomplished via a Ta sheet element and temperature is measured via an automatic pyrometer from the bottom of the cells. The computer control and data system have been custom developed for this instrument and are discussed. Future improvements are also discussed.

Knudsen Effusion Mass Spectrometry of Nuclear Materials: Applications and Developments

The nuclearised Knudsen Effusion Mass Spectrometry (KEMS) facility at ITU is a unique device to measure not only the partial vapor pressure and ionization efficiency curves of actinide compounds, but also the release of fission product from irradiated fuel. Those measurements provide fundamental data for the safety assessment of nuclear fuel in normal and accidental conditions as well as for long term storage. The system is installed in a glove box and operates at very high temperatures, and thus requires a specific setup. Currently a 20 years old KEMS system equipped with a quadrupole mass spectrometer and joule heating coil is used. For a future system, a prototype of a laser-heated furnace for the Knudsen cell has been developed. It presents many advantages in comparison to standard heating techniques and fulfills key requirements for use in a nuclear environment. All these subjects are discussed.

Thermodynamic Measurement of Phosphates by Double Knudsen Cell Mass Spectrometry

Double Knudsen cell mass spectrometry was adapted for thermodynamic measurement of phosphates to obtain the accurate thermodynamic information. The ion currents of gaseous phosphorus and phosphorus oxides in equilibrium with a mixture of AlPO4 and Al2O3 at 1473 – 1573 K were measured by double Knudsen cell mass spectrometry. The Gibbs energy change of formation of AlPO4 was estimated by using these ion currents.

Determination of Vanadium Valence State in CaO-MgO-Al2O3-SiO2 System By High-Temperature Mass Spectrometry

In the present study, the applicability of the high-temperature mass spectrometric method combined with Knudsen effusion cell for quantifying the valence states of V in the multicomponent system CaO-MgO-Al2O3-SiO2-VOx up to a maximum temperature of 2050 K (1777 °C) was examined. The valence ratio of V3+/V4+ in slag phase was derived from the partial pressures of VO and VO2 in the effused vapor phase. The results show good agreement with the literature values obtained by other techniques. A correlation between the valence ratio V3+/V4+ and the oxygen partial pressure as well as basicity was achieved based on the present results and accessed data in the literature. The results of the present study demonstrate that the Knudsen cell-mass spectrometric method can be a very effective tool in estimating the valence ratios for of transition metals in metallurgical slags.

In-situ heavily p-type doping of over 1020 cm−3 in semiconducting BaSi2 thin films for solar cells applications

B-doped p-BaSi2 layer growth by molecular beam epitaxy and the influence of rapid thermal annealing (RTA) on hole concentrations were presented. The hole concentration was controlled in the range between 1017 and 1020 cm−3 at room temperature by changing the temperature of the B Knudsen cell crucible. The acceptor level of the B atoms was estimated to be approximately 23 meV. High hole concentrations exceeding 1 × 1020 cm−3 were achieved via dopant activation using RTA at 800 °C in Ar. The activation efficiency was increased up to 10%.