Formation Of Volatile Species Research Articles

Work-Function-Dependent Reduction of Transition Metal Nitrides in Hydrogen Environments.

Amidst the growing importance of hydrogen in a sustainable future, it is crucial to develop coatings that can protect hydrogen-sensitive system components in reactive hydrogen environments. However, the prediction of the chemical stability of materials in hydrogen is not fully understood. In this study, we show that the work function is a key parameter determining the reducibility (i.e., denitridation) of transition metal nitrides (TMNs) in hydrogen radicals (H*) at elevated temperatures. We demonstrate that, when the work function of a TMN system drops below a threshold limit (ϕTH), its reduction effectively stops. We propose that this is due to the preferential binding of H* to transition metal (TM) atoms rather than N atoms, which makes the formation of volatile species (NHx) unfavorable. This finding provides a novel perspective for comprehending the interaction of hydrogen with TM compounds and allows prediction of the chemical stability of hydrogen-protective coatings.

Probing the Reactivity of ZnO with Perovskite Precursors.

To achieve more stable and efficient metal halide perovskite devices, optimization of charge transport materials and their interfaces with perovskites is crucial. ZnO on paper would make an ideal electron transport layer in perovskite devices. This metal oxide has a large bandgap, making it transparent to visible light; it can be easily n-type doped, has a decent electron mobility, and is thought to be chemically relatively inert. However, in combination with perovskites, ZnO has turned out to be a source of instability, rapidly degrading the performance of devices. In this work, we provide a comprehensive experimental and computational study of the interaction between the most common organic perovskite precursors and the surface of ZnO, with the aim of understanding the observed instability. Using X-ray photoelectron spectroscopy, we find a complete degradation of the precursors in contact with ZnO and the formation of volatile species as well as new surface bonds. Our computational work reveals that different pristine and defected surface terminations of ZnO facilitate the decomposition of the perovskite precursor molecules, mainly through deprotonation, making the deposition of the latter on those surfaces impossible without the use of passivation.

Thermal Decomposition and Kinetic Parameters of Three Biomass Feedstocks for the Performance of the Gasification Process Using a Thermogravimetric Analyzer

Thermogravimetric analysis (TGA) is a powerful technique and useful method for characterizing biomass as a non-conventional fuel. A TGA apparatus has been utilized to experimentally investigate the impact of biomass feedstock diversity on the performance of the gasification of hardwood (HW), softwood (SW) pellets, and refuse-derived fuel (RDF) materials. The solid conversion rate and the volatile species formation rate have been estimated to quantify the rates of devolatilization for each material. In addition, the combustion kinetic characteristics of the three biomass feedstocks were investigated using TGA at different heating rates, and a thermal kinetic analysis was conducted to describe the gasification process. Therefore, the kinetic parameters have been evaluated for different thermal reactions and non-isothermal kinetic models that depend on the relationships between heating rates and temperature profiles. The results show that the amount of tar content from the RDF was higher than that of pure hardwood and softwood feedstocks. Hence, tar removal must be part of any process using syngas produced from RDF feedstocks in a gas engine to produce electricity.

Chemical Interaction of Hydrogen Radicals (H*) with Transition Metal Nitrides.

Transition metal nitrides (TMNs) are reported as protective coatings in reactive hydrogen environments. Although the permeation of H2 through TMN coatings is well reported, their reducibility in H* environments is less investigated. In this work, we categorize the interaction of H* with ambient exposed TiN, ZrN, HfN, VN, NbN, and TaN thin films at 700 °C into three classes. We find that in TiN and VN samples, H*-induced reduction was limited to the surface (≈ top 2 nm). Significant denitridation was observed in ZrN and HfN samples beneath the surface, along with an increase in the transition metal oxide (TMOx) fraction. Denitridation was observed in NbN and TaN samples as well, but the increase in the TMOx content was less than for ZrN and HfN. We propose a model in three steps: hydrogenation, formation of volatile species, and diffusion of subsurface atoms to the surface. We show that the interaction of H* with TiN, ZrN, HfN, VN, NbN, and TaN with partially oxidized surfaces can be explained using the preferred hydrogenation pathway (based on the work functions) and the thermodynamic driver for forming volatile species (NH3 and H2O; based on the change in Gibbs free energy).

Temperature-Dependent Gassing Analysis By on-Line Electrochemical Mass Spectrometry of Lithium-Ion Battery Cells with Commercial Electrolytes

Li-ion batteries (LiBs) are known to evolve gases during their first few formation cycles due to the formation of a solid electrolyte interphase (SEI) at the anode active material particle surface. Simultaneously, the electrolyte can react by other mechanisms leading to the formation of volatile species. 1,2 At a high state-of-charge, the cathode can also release reactive oxygen which causes the formation of CO and CO2 from the oxidative decomposition of the electrolyte.3,4 Consequently, by the analysis of evolved gas species one can study the effect of electrolyte additives, reveal substantial degradation processes within LiBs, and rationalize optimized formation strategies.4–6 The evolution of gaseous species is typically quantified in-situ by monitoring the cell pressure (e.g., via the so-called Archimedes principle) or by online mass spectrometry.5,7 Concerning the latter, an on-line electrochemical mass spectrometer (OEMS) was developed for the application in LiB research and proved to be particularly powerful, since it allows a quantitative analysis and the identification of the evolved and consumed gas during the operation of a battery.4,7 The quantification of OEMS data, however, is challenging when the vapor pressure of the electrolyte is high, which, e.g., is the case for linear carbonates, particularly at elevated temperatures. Mixtures of linear and cyclic carbonates are widely used in LiB applications due to their low viscosity, high salt solubility, and high conductivity. In contrast to the cyclic ethylene carbonate (EC) with a vapor pressure of 0.25 mbar, the vapor pressure for the typically used linear carbonates dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) can vary from 23 to 80 mbar at room temperature.8 Their vapor pressure further increases by a factor of ~3 when increasing the temperature from room temperature to 45 °C.9 Thus, a significant share of the gas in the head-space of the OEMS cell consists of electrolyte vapor. As they undergo fragmentation in the mass spectrometer, a background signal is superimposed on the relevant mass traces related to hydrogen, ethylene, carbon monoxide, oxygen, and carbon dioxide.1 Furthermore, the cell pressure inherently decreases due to the continuous leak through the capillary (with a leak rate of ~1 µL/min) that connects the OEMS cell (~10 mL volume) with the mass spectrometer. Because of the liquid reservoir of electrolyte in the OEMS cell and its constant vapor pressure, this results in an enrichment of the gas phase with electrolyte vapor. Analyzing the pressure-dependent flow of gaseous species into the mass spectrometer allowed us to introduce corrections for signal normalization, electrolyte background change, and signal quantification.In the present study, we applied these corrections in the investigation of the influence of LP57 (1 M LiPF6 in EC:EMC 3:7wt/wt) and LP47 (1 M LiPF6 in EC:DEC 3:7wt/wt) electrolyte on the gassing characteristics during the formation of full-cells with a graphite anode and a Ni-rich cathode from low (10 °C) to high (45 °C) temperatures. From that, we were able to conclude how the formation temperature influences the gas evolution related to SEI formation as well as the trans-esterification of the electrolyte and the lattice oxygen reactivity towards different electrolytes. A comparison to a model electrolyte (1.5 M LiPF6 in EC) with a low vapor pressure (< 0.25 mbar) allowed us to assess the detection limits of all the signals depending on the electrolyte’s vapor pressure and to evaluate the validity of our proposed signal corrections. Acknowledgment: The authors thank BMW AG for their financial support.

Synthesis and Morphological Control of UO2F2 Particulates.

Uranium-based microspheres are of interest due to their potential applications as targets for medical isotopes production, as fuel for nuclear reactors, and as standardized materials for nuclear forensics. Here, for the first time, UO2F2 microspheres (1-2 μm) have been prepared from the reaction between UO3 microspheres and AgHF2 in an autoclave. In this preparation, a new fluorination method has been applied, and HF(g)-produced in situ from the thermal decomposition of AgHF2 and NH4HF2-was used as the fluorinating agent. The microspheres were characterized by powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM). Diffraction results indicated that the reaction performed with AgHF2 at 200 °C led to anhydrous UO2F2 microspheres, while at 150 °C, hydrated UO2F2 microspheres were obtained. Meanwhile, NH4HF2 led to the formation of contaminated products as driven by the formation of volatile species.

Stability of Cr oxide formed on Fe-20Cr-2Si alloy in 1200 °C steam or oxygen

Fe-20Cr-2Si alloy, proposed as a high-temperature oxidation barrier for accident-tolerant fuel cladding in light water reactors, maintained oxidation resistance at 1200 °C under Ar-50%H2O and Ar-20%O2 environments for up to 24 h. However, Cr2O3 on the Fe-20Cr-2Si alloy volatilized above 1000 °C, degradation started under Ar-20%O2, and voids were observed at the metal/oxide interface. These were attributed to the copious formation of volatile species owing to the high oxygen partial pressure in Ar-20%O2. Thus, more vacancies were generated and moved to the metal/oxide interface. Contrastingly, degradation was not initiated under Ar-50%H2O, indicating sufficient resistance of Fe-20Cr-2Si to highly oxidizing environments.

Thermal Atomic Layer Etching of CoO, ZnO, Fe2O3, and NiO by Chlorination and Ligand Addition Using SO2Cl2 and Tetramethylethylenediamine

Thermal atomic layer etching (ALE) of CoO, ZnO, Fe2O3, and NiO was achieved using chlorination and ligand-addition reactions at 250 °C. This two-step process was accomplished by first chlorinating the metal oxide with SO2Cl2. Subsequently, ligand addition to the metal chloride was performed using tetramethylethylenediamine (TMEDA). In situ quadrupole mass spectrometry (QMS) studies on metal oxide powders revealed that CoO, ZnO, Fe2O3, and NiO all formed stable and volatile MCl2(TMEDA) compounds (M = Co, Zn, Fe, Ni) as etch products at 250 °C. These QMS studies of the sequential SO2Cl2 and TMEDA exposures were facilitated by a new reactor design with two nested inlet lines that transport the reactants separately to the powder substrate. The time-dependence of the reactants and products could also be monitored by the QMS investigations. The large SO2+ ion intensity observed at the beginning of the SO2Cl2 exposure was consistent with the chlorination reaction MO + SO2Cl2 → MCl2 + SO2 + (1/2)O2. The time-dependent QMS studies also observed the MClx(TMEDA)+ ion intensity peaking at the beginning of the TMEDA exposures. The subsequent decay of the MClx(TMEDA)+ ion intensity, while the (TMEDA)+ ion intensity remained constant, was evidence for a self-limiting ligand-addition reaction. The mass loss of the metal oxide powders was confirmed after sequential SO2Cl2 and TMEDA exposures. The etching of two of these metal oxides was also verified using separate experiments on flat substrates using SO2Cl2 and TMEDA exposures at 250 °C. For CoO thermal ALE, an etch rate of 4.1 Å/cycle at 250 °C was measured using X-ray reflectivity (XRR) studies. For ZnO thermal ALE, an etch rate of 0.12 Å/cycle at 250 °C was measured using quartz crystal microbalance (QCM) investigations. Other first row transition metal oxides were surveyed in addition to CoO, ZnO, Fe2O3, and NiO. QMS studies of TiO2, Cr2O3, and MnO2 showed no volatile species formation during sequential SO2Cl2 and TMEDA exposures at 250 °C. In contrast, V2O5 and CuO were spontaneously etched using SO2Cl2 at 250 °C, as determined by the observation of volatile VOCl3 and CuCl3 etch products, respectively. Calculated Gibbs free energy changes for the various etching reactions also supported the experimental observations for the first row transition metal oxides. These studies illustrate that the chlorination and ligand-addition reaction mechanism can provide a new avenue for the thermal ALE of a variety of transition metal oxides that have nonvolatile metal chlorides.

Organic Telluride Formation from Paint Solvents Under Gamma Irradiation

The interactions between tellurium and organic material during a nuclear reactor accident are critical to source term estimations because of the possible formation of volatile species. Reactions taking place in the containment sump are of interest since these can lead to re-volatilization and increase the fission product source term. This paper presents results from experiments investigating the interaction of tellurium dioxide with three paint solvents—texanol ester, methyl isobutyl ketone, and toluene—under containment sump conditions. The experiments were performed by irradiating a mixed solution of tellurium dioxide and paint solvents at a dose rate of 4 kGy/h up to 300 kGy. The resulting samples were analyzed for tellurium concentration and speciation. Tellurium(IV) was found to reduce to metallic tellurium under irradiation when paint solvents were present. More importantly, several volatile organic tellurides were identified in the irradiated samples, which suggests that tellurium can form volatile species in sump conditions when in contact with dissolved paint solvents. This paper provides novel evidence of organic telluride formation in the sump and raises further interest in tellurium chemistry during a severe nuclear reactor accident.

Oxidative stabilities of grass carp oil: possible mechanisms of volatile species formation in hydroperoxylated metabolites at high temperature

Oxidative stabilities of grass carp oil: possible mechanisms of volatile species formation in hydroperoxylated metabolites at high temperature

Application of TPO/TPR methods in oxidation investigations of CoSb3 and Mg2Si thermoelectrics with and without a protective coating of “black glass”

One of the main problems in thermoelectric material research is degradation of thermoelectric materials caused by oxidation at elevated temperatures in air. In order to prevent these materials from oxidation, they can be covered with protective coating. A good candidate for such coating is layers made of so-called black glass, which can be successfully applied as a protective coating against oxidation. Thermal oxidation study can be demanding for measuring equipment used for in situ examination due to formation of volatile species which are corrosive for both metal and ceramic elements. This focused our attention on temperature-programmed reduction—temperature-programmed oxidation TPR/TPO methods in which the risk of damage of expensive parts of the equipment can be minimized. These methods have an important place in characterization of solid materials and can be used as the thermal analysis methods providing complementary results to TG and DTA. The aim of the study was to confirm the usefulness of TPR/TPO methods for the study of oxidation of modern thermoelectric materials and validation of “black glass” coatings as surface protection. Pure and doped (In; Ce) CoSb3 as well as pure Mg2Si were chosen for this study. XRD and SEM/EDX methods were used for material characterization, and TG/DTA measurements were carried out as reference for TPR/TPO. We succeeded to deposit “black glass” coating on Mg2Si and to show suppression of oxidation up to 495 °C. Experimental results show high usability of the TPR/TPO method in oxidation studies of thermoelectrics and protective coatings of thermoelectric materials.

Impact of biomass diversity on torrefaction: Study of solid conversion and volatile species formation through an innovative TGA-GC/MS apparatus

The objective of this work is to compare the kinetic behavior of a large set of European biomasses during torrefaction, both in terms of solid transformed and volatile species released, and to determine whether biomass behaviors can be classified according to main biomass families, namely deciduous wood, coniferous wood, agricultural coproducts and herbaceous crops. 14 biomasses representative of European diversity were torrefied in chemical regime following a non-isothermal procedure (200 to 300 °C, 3 °C min-1) in a thermogravimetric analyzer coupled with a gas-chromatograph mass spectrometer through a system of heated storage loops (TGA-GC/MS). Coniferous and deciduous wood were found to have similar behaviors in terms of solid evolution profile and species produced, while being different in terms of kinetics. On the contrary, agricultural biomass appeared to be a highly heterogeneous group where different biomass subtypes should be selected in order to represent the diversity of behaviors during torrefaction. Biomass macromolecular composition, together with the biological origin and the structural matrix of biomass, were shown to be determining factors of biomass behavior in torrefaction.

Preparation and flame retardancy of reactive flame retardant for cotton fabric

Monochlorotriazine aminopropyl silanol phosphate (MCASP) was synthesized as a novel flame retardant agent for cotton fabric. It was characterized by FTIR, 1H, 13C and 31P NMR. The flame retardancy of the cotton fabric treated with MCASP was characterized by limiting oxygen index (LOI), and the vertical burning test. The cotton fabric treated with 300 g L−1 MCASP obtained the best flame retardancy with a LOI value of 31.0%, and the char length decreased to 4.5 cm. Combustion behaviors of treated cotton fabric were tested by cone calorimeter. The ignition time increased, and the values of total heat release, heat release rate, mass loss decreased. The surface morphology of the residues and the thermal stability of cotton fabric were investigated by scanning electron microscopy and thermogravimetric analysis, respectively. The chemical structure of residue of treated cotton fabric was studied by FTIR. The results showed that MCASP played a protective role on the degradation of cotton fabric, hindered the formation of volatile species and favored the formation of char. What is more, the strength and durability of treated fabric were studied by tear force test and washing durability test, respectively.

Preparation and properties of cotton fabrics treated with a novel antimicrobial and flame retardant containing triazine and phosphorus components

A novel antibacterial and flame-retardant agent, monochlorotriazine triethylphosphite guanidine (MCTPG) was synthesized successfully. The chemical structure of MCTPG was characterized by FTIR and 1H NMR. The flame retardancy of treated cotton fabric was evaluated by limiting oxygen index (LOI), the vertical burning test and cone calorimeter, respectively. The treated cotton fabric obtained good flame retardancy with a LOI value of 31.2%, and the char length decreased to 8.5 cm. The ignition time increased, and the values of total heat release, heat release rate, mass loss decreased. The thermal stability and surface morphology of treated cotton fabric were investigated by thermogravimetric analysis and scanning electron microscope (SEM), respectively. The results showed that MCTPG played a protective role in the degradation of cotton fabrics, hindered the formation of volatile species and favored the formation of char. Furthermore, the antimicrobial activity of treated cotton fabrics was tested. And it showed that the inhibition zone of the treated cotton fabrics base to Escherichia coli and Staphylococcus aureus reached to 2.9 mm and 2.8 mm, respectively.

Properties of flame-retardant cellulose fibers with ionic liquid

Flame-retardant cellulose fibers were prepared by dissolving cellulose in tetrabutylammonium acetate and dimethyl sulfoxide, and blending with amino silicone oil (ASO). The ASO was used as a novel fabric softener and flame retardant for cellulose fibers. Fourier-transform infrared spectroscopy showed that blending with ASO did not adversely affect the cellulose fibers. The flame retardancy of the cellulose fibers blended with ASO was determined based on the limiting oxygen index (LOI). Cellulose fibers blended with 8 wt% (add-on) ASO gave the best flame retardancy, with an LOI of 28, which was higher than that of the virgin fibers. The thermal properties of the flame-retardant cellulose fibers were investigated using differential scanning calorimetry and thermogravimetric analysis. The results showed that ASO prevented degradation of the cellulose fibers, hindered the formation of volatile species, and favored char formation. The mechanical properties of the flame-retardant cellulose fibers were better than those of virgin cellulose fibers.

Laser-Ablation Sampling for Accurate Analysis of Sulfur in Edible Salts.

We evaluated the performance of laser ablation analysis techniques such as laser-induced breakdown spectroscopy (LIBS), laser ablation inductively coupled optical emission spectrometry (LA-ICP-OES), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), in comparison with that of ICP-OES using aqueous solutions for the quantification of sulfur (S) in edible salts from different geographical origins. We found that the laser ablation based sampling techniques were not influenced by loss of S, which was observed in ICP-OES with aqueous solutions for a certain salt upon their dissolution in aqueous solutions, originating from the formation of volatile species and precipitates upon their dilution in water. Although detection of S using direct laser sampling with LA-ICP-MS has well-known isobaric and polyatomic interferences, LIBS and LA-ICP-OES showed good accuracy in the detection of S for all salts. LIBS also provided the ability to identify the dominant chemical form in which S is present in salts. Correlation between S and oxygen, observed in LIBS spectra, provided chemical information about the presence of S2- or [Formula: see text], which are associated with the origin and quality of edible salts.

Effect of gas flow rate on oxidation behaviour of alloy 625 in wet air in the temperature range 900–1000 °C

In many industrial processes, the flow rate of hot gaseous service environments may be in the range of several m/s. In the present paper, the effect of gas flow rate on the high temperature oxidation behaviour of alloy 625 in wet air is presented. The gas velocity was varied from near static conditions to linear gas flow rates up to 6 m/s. The oxidation kinetics were studied by gravimetry during cyclic/discontinuous testing at 900 and 1000 °C. The oxide scales and the subsurface depletion zones formed during exposure were studied by light optical microscopy, scanning electron microscopy with energy dispersive X‐ray analysis and, for selected specimens, by electron backscatter diffraction. It was found that Cr loss due to formation of volatile species is substantially enhanced by high gas flow rates thus significantly influencing the oxidation limited life time of the oxidising component. Within the studied range of flow rates no plateau value was reached, the Cr loss being substantially larger at a flow of 6 m/s than at 0.7 m/s. Additionally, it was found that geometrical factors of the test specimen substantially affected the extent of volatile species formation. Especially the leading edge of the specimen exhibited more extensive Cr loss than the other specimen areas.

Preparation and properties of cotton fabrics treated with a novel guanidyl- and phosphorus-containing polysiloxane antimicrobial and flame retardant

Guanidyl- and phosphorus-containing polysiloxane (GPPDMS) was synthesized as a novel antimicrobial and flame retardant agent for cotton fabrics and it was characterized by FT-IR and 1H NMR. The flame retardancy of the cotton fabrics treated with GPPDMS was characterized by limiting oxygen index (LOI) and the vertical burning test. The cotton fabric treated with 18.6% (add-on) of GPPDMS obtained the best flame retardancy with a LOI value of 31.9, which was 13.9 units higher than that of the untreated one. The thermal decomposition behavior of cotton fabrics was investigated by thermogravimetric (TG) analysis and the results showed that GPPDMS played a protective role on the degradation of cotton fabrics, hindered the formation of volatile species and favored the formation of char. Furthermore, the antimicrobial activity of the treated cotton fabrics was tested and it showed that the antibacterial efficiency of Escherichia coli and Staphylococcus aureus was 97% and 96% respectively.

In Situ Gas Analysis - Method Development and Application to Study Cathode Performance in Li-O2 Cells

New insights into the electrochemical formation of volatile species in Li-O2 cells are provided by combining differential electrochemical mass spectrometry with online pressure analysis. In situ gas analysis during Li-O2 battery cycling is crucial to identify parasitic side-reactions and determine reversibility of the cell. The figure shows the potential and gas evolution profiles of O2, CO2, H2, H2O, CO, DME during the 1st and 3rd galvanostatic charge (0.1 mA/cm2) of a Li-O2 cell (20:80 PTFE:BP2000 carbon, 0.2 M LiTFSI in Diglyme) after a shallow discharge (~5% of total capacity). On the 1st and 3rd discharge, oxygen is consumed at a lower rate (~2.12 and ~2.28e-/O2, respectively, as probed by ΔP(O2)) than expected for Li2O2 formation (2e-/O2), clearly indicating influence of parasitic side-reactions during cycling. On charge, O2 evolves slightly above 3 V initially with a rate expected for Li2O2 oxidation. At 3.2 V, O2 evolution rate gradually drops and CO, H2, H2O appear. At 3.8 V, CO2 evolves possibly as results of oxidation of carbonates, which formed during discharge. Above 4.5 V, DME is observed, which is related to direct electrolyte oxidation. In subsequent cycles, the cell over-potentials increase, the maxima of O2 evolution rate gradually shift to higher potentials, while the gas evolution associated with parasitic side-reactions becomes more pronounced. In our contribution we demonstrate how development and application of in situ gas analysis with high sensitivity provide detailed insights into Li-O2 cell chemistry. BASF SE is greatfully acknowledged for financial support.

Gas-Phase Interactions as Sources of Contamination in Solar Silicon

Avoiding contamination of silicon for solar cells during high-temperature processing steps is a key issue. Contamination of silicon via direct contact due to diffusion processes has been recognized. Here, it is shown that interactions with the gas phase also are a potential source of contamination. Thermodynamic calculations performed for a temperature range of 373 K to 1873 K (100 °C to 1600 °C) and total pressure of 10 kPa predict the formation of volatile species that are harmful for photovoltaic properties. Volatile species form due to graphite–moisture interaction and the interaction of this gas mixture with compounds commonly present in silicon production units: graphite, quartz, alumina, boron nitride, and iron. The results of the calculations are supported by the data of the surface contamination of exposed graphite furnace parts.