Interaction of atomic hydrogen with the graphite single-crystal surface

The reaction of atomic hydrogen (or atomic deuterium) with highly orientated pyrolytic graphite surfaces has been studied by means of thermal desorption spectroscopy. In some cases atomic deuterium instead of atomic hydrogen, was used solely to assign the desorbed masses unambiguously to the different hydrocarbons. The desorption of D₂ and fourteen hydrocarbons was observed. D₂ desorbed at higher temperatures than the CH-(CD) compounds, the desorption spectra of the hydrocarbons contained two peaks. The dependence of the desorption spectra of several hydrocarbons on the heating rate, the atomic hydrogen exposure and the composition of the desorption products was investigated in detail. The kinetic parameters of the desorption process were determined for CH, C₂H₂, and CD₄. The spectra showed that there must be a first order desorption process for all the hydrocarbons, the values for the activation energy and the frequency factor were the same within experimental errors. The results were discussed by means of a simple model.     

Massenspektrometrische untersuchungen der kondensation von dünnen schichten auf fremden unterlagen: I. Kinetik der adsorption und der desorption von Ag auf W

The adsorption and desorption kinetics of silver on clean polycrystalline tungsten were investigated with a mass-spectrometric technique. The deposition up to about 2 monolayers occurred without two-dimensional phase transformation. The thermal accommodation coefficient was found to be unity. The desorption energy and frequency factor for different coverages were determined. The bonding of silver atoms in the first monolayer was found to be localized. Additionally, thermal desorption experiments with linear heating rate were carried out.     

Hydrogen TDS spectra and their relation to the conditions of implantation and retention of hydrogen in graphite materials

Based on the literature thermal desorption spectroscopy (TDS) data, TDS spectra of hydrogen were analyzed and classified by graphite materials and implantation conditions. Using our experimental results, all the spectra were recalculated to the equal heating rate of 5K/s. The peak positions in TDS spectra have been found to be related to irradiation conditions and to the type of hydrogen traps. An example of using the established regularities of the TDS spectra is given to obtain the data on hydrogen trapping and retention in graphite materials.     

AES and LEED studies of xenon adsorption on (100)NaCl

The thermal and electro impact behaviour of NO adsorbed on Pt(111) and Pt(110) have been studied by LEED, Auger spectroscopy, and thermal desorption. NO was found to adsorb non-dissociatively and with very similar low coverage adsorption enthalpies on the two surfaces at 300 K. In both cases, heating the adlayer resulted in partial dissociation and led to the appearance of N₂ and O₂ in the desorption spectra. The (111) surface was found to be significantly more active in inducing the thermal dissociation of NO, and on this surface the molecule was also rapidly desorbed and dissociated under electron impact. Cross sections for these processes were obtained, together with the desorption cross section for atomically bound N formed by dissociation of adsorbed NO. Electron impact effects were found to be much less important on the (110) surface. The results are considered in relation to those already obtained by Ertl et al. for NO adsorption on Ni(111) and Pd(111), and in particular, the unusual desorption kinetics of N₂ production are considered explicitly. Where appropriate, comparisons are made with the behaviour of CO on Pt(111) and Pt(110), and the adsorption kinetics of NO on the (110) surface have been examined.     

Intercalation of potassium in graphite studied by thermal desorption spectroscopy

Thermal desorption spectra obtained after multilayer potassium depositions on graphite at T_s = 160 K reveal information about the formation and decomposition of potassium/graphite intercalation structures. Up to six desorption peaks were observed, of which three could be attributed to decomposition of three different intercalation-like surface structures. These structures are formed as the deposited potassium diffuses into the graphite substrate during the recording of the thermal desorption spectra. At submonolayer coverages, potassium desorbs in a sharp peak around 500 K, attributed to decomposition of a single intercalated potassium layer positioned between the two uppermost graphite layers.     

A numeric model for the kinetics of water vapor sorption on cellulosic reinforcement fibers

Some natural fibers like flax, hemp and others show excellent mechanical properties that make them a promising choice for the reinforcement of polymers. The increasing research on natural fiber reinforced composites has still left important questions open, mainly concerning the fiber–matrix interface. Compared to the well optimized glass fibers, cellulose fibers show very different interaction with matrix polymers and adhesion promoters. The hydrophilic cellulose structure allows for the penetration of a considerable amount of water into the amorphous regions of the fibers, eventually exceeding 20% by mass, depending on fiber type, preparation and environmental humidity. Even embedded in totally apolar polymers the cellulose partly retains its ability for water sorption, which results in unfavorable effects, such as dimensional changes, decrease in strength, roughening of the surface, etc. The interaction of differently prepared fibers with water vapor and the effect of surface treatment is investigated by measuring the dynamics of water vapor sorption. An exponential model is used for the numerical evaluation of the sorption and desorption kinetics. The model not only allows for an excellent fit of the experimental isotherms, but without any further assumptions it immediately gives evidence of the existence of two distinct mechanisms for the exchange of water vapor, related to different sorption sites. These specific mechanisms are represented by individual sorption–desorption isotherms as components of the total isotherms. The model provides a clearer differentiation of the effects of fiber preparation and modification with respect to interfacial interactions.     

Cell-level hazard evaluation of 18650 form-factor Lithium-ion battery with different cathode materials

The thermal runaway process was studied in a Fire Propagation Apparatus (FPA) for three types of Lithium-ion batteries (LIB) of 18650 form-factor. Cathode materials are lithium cobalt oxide (LiCoO₂, or LCO), lithium nickel manganese cobalt oxide (LiNi_1/3Mn_1/3Co_1/3O₂, or NMC), and lithium iron phosphate (LiFePO₄, or LFP). All batteries have a graphite anode and were at a 100% state-of-charge. Each LIB was externally heated to a thermal runaway event, with the heat input at constant values of 20.4 or 34.1 W, which yielded heating rates on the order of 1 K/s, representative of the thermal runaway propagation process. The mass loss fraction before the thermal runaway events and the maximum values are similar under different heat inputs for a given type of LIB. For different types of LIBs, the maximum mass loss fraction shows the trend of LCO>NMC>LFP. Under the same heating condition, NMC has the highest maximum surface temperature followed by LCO then LFP. A lumped heat transfer thermal runaway model is developed using two decomposition reactions and one internal short circuit reaction to model the internal heat generation. The effective model parameters are optimized using the measured surface temperature and mass loss fraction. The model is able to simulate the thermal runaway behavior of LIB under external heating conditions and reasonably matches the experimental data of LIBs with different cathodes. The model predicts that under the same heat input condition, the thermal runaway time of LCO is shorter than NMC and LFP; the effective average internal heat generations are 22.6, 20.2, and 11.5 kJ for LCO, NMC, and LFP, respectively. The thermal runaway model will be used to predict the thermal runaway propagation in a LIB module.     

Thermal desorption of deuterium from MPG-8 and NB31 carbon materials after plasma irradiation

Deuterium retention in fine-grained graphite MPG-8 and carbon fiber composite NB31 after irradiation in a magnetron discharge and plasma-beam discharge was investigated by thermal desorption spectroscopy. The thermal desorption spectra reveal deuterium yield in a wide temperature range: from 400 to 1400 K. The spectra of the materials studied are qualitatively similar; however, the relative amplitudes of the thermal desorption peaks are different.     

The adsorption of oxygen on a stepped platinum single crystal surface

The adsorption of oxygen on the Pt(S)-[12(111) × (111) surface has been studied by Auger electron spectroscopy, low energy electron diffraction and thermal desorption spectroscopy. Two types of adsorbed oxygen have been identified by thermal desorption spectroscopy and low energy electron diffraction: (a) atoms adsorbed on step sites; (b) atoms adsorbed on terrace sites. The kinetics of adsorption into these two states can be modeled by considering sequential filling of the two adsorbed atomic states from a mobile adsorbed molecular precursor state. Adsorption on the step sites occurs more rapidly than adsorption onto the terraces. The sticking coefficient for oxygen adsorption is initially 0.4 on the step sites and drops when the step sites are saturated. The heat of desorption from the step site (45 ± 4 kcal/mole) is about 15% larger than the heat of desorption from the terraces.     

Adsorption and desorption kinetics of Cu and Au on (0001) graphite

The interaction of Au and Cu atoms with the (0001) plane of graphite was studied by mass spectrometric measurements of desorption flux both in the presence and absence of an incident atomic beam. For these systems the condensation coefficient increases from ～0.05 at θ = 0 to unity at θ = 1; furthermore the rate of thermal desorption has a kinetic order of one-half for both systems at low coverage. These observations are consistent with a kinetic model in which two-dimensional nucleation of mobile adsorbed atoms occurs upon adsorption, while the reverse process, loss of atoms from the edges of disc nuclei, is the rate controlling step for desorption. This model implies that bonding of metal atoms to the basal plane of graphite is weak and nonlocalized, with adsorption occurring only when two-dimensional nucleation permits metal-metal bonding.     

Zero-order desorption kinetics based on phase equilibrium

The zero-order desorption kinetics is described for adsorbate systems in which three phases are in equilibrium and first-order desorption kinetics is assumed for the desorption from the topmost phase. The calculated results represent typical features of the observed zero-order desorption spectra. The possibility of specifying the phase boundaries from the thermal or isothermal desorption spectra is proposed. The relationship between the thermal or isothermal desorption processes and trajectories in the phase diagram is also discussed.     

In situ X‐ray diffraction study on the growth kinetics of NiO nanoparticles

The growth kinetics of NiO nanoparticles have been studied by in situ X‐ray diffraction using two detection systems (conventional and imaging plate). NiO nanoparticles were formed by thermal decomposition after heating of an amorphous compound formed by the coprecipitation method. It was found that the detection method using an imaging plate is more efficient than the conventional detection mode for observing changes in the crystallite growth of nanocrystalline materials. Studies have been carried out to investigate the effects of the heating rates on the particles growth. The results suggest that the growth process of the particles is accelerated when the samples are treated at low heating rates. The evolution of particles size and the diffusion coefficient obtained from X‐ray powder diffraction patterns are discussed in terms of the thermal conditions for the two types of detection.     

Kinetics Study of the Thermal Decomposition of Post-consumer Poly(Ethylene Terephthalate) in an Argon Atmosphere

The thermal decomposition of post-consumer samples of a carbonated water bottle made of poly(ethylene terephthalate), PC-PET, was examined by linear temperature programing under an argon atmosphere to determine its mass loss kinetics. A simple kinetic model, called the first order pseudo single-component model, was used. The total weight-loss of each sample assumed to be in two periods, with each period corresponding to a one step decomposition of the PC-PET to volatiles. Three methods for determining the kinetic parameters by thermal gravimetric analysis were examined: differential analysis at a constant heating rate (differential), temperatures of a given conversion at a number of heating rates (isoconversional), and the maximum rate at multiple heating rates (peak temperature). The latter two multiple heating rates methods results were comparable to each other but they were not in agreement with the results from the differential method. The results of the differential method were insensitive to the heating rate and consistent with kinetics data reported in the literature for PET.     

The coadsorption of carbon monoxide and deuterium on potassium predosed Ni(100)

The effect of potassium on the coadsorption of carbon monoxide and deuterium on Ni(100), with particular attention given to the low temperature Σ-CO and Σ-D₂ desorption states, has been studied by thermal desorption spectroscopy. Potassium shifts the Σ state peak temperatures to higher values and attenuates their intensities. The effects of potassium on the higher temperature carbon monoxide and deuterium desorption states are similar to the effects observed when one or the other is absorbed alone. A model is presented for the conversion of carbide to graphite on transition metal surfaces and for a possible role which potassium may play.     

Structural, reactive and kinetic properties of acetylene on clean Mo(100)

Acetylene adsorbs onto Mo(100) at 80 K via precursor state kinetics, saturating the chemisorbed overlayer at a coverage of 1.0. Further exposure to acetylene adsorbs multilayers, which are desorbed by heating to 100 K. Two surface species are identified using angle-resolved ultraviolet photoelectron spectroscopy after adsorbing acetylene at 80 K. One species is undistorted chemisorbed acetylene, the other is an acetylenic species which is rehybridized to approximately sp² and is adsorbed with its C-C axis oriented at 45° with respect to the surface normal. On heating this surface to above 110 K, all the chemisorbed acetylene converts into the rehybridized species. This is stable up to a surface temperature of 180 K, following which it starts to decompose. Thermal decomposition is complete by 300 K. Only hydrogen is detected in thermal desorption spectroscopy, and the spectra are characteristic of hydrogen desorption from a carbided surface.     

The kinetics of hydrogen (deuterium) sorption by thin palladium layers studied with a piezoelectric quartz crystal microbalance

The kinetics of the hydrogen (deuterium) Sorption processes in the α-phase region of a thin electrodeposited Pd layer (thickness 3 × 10^?5 cm) are reported. Measurements have been performed with a piezoelectric quartz crystal microbalance using an AT-cut crystal with a resonance frequency f⁰_q = 5.27 MHz and a sensitivity of ～10¹⁰ digits g^?1, at a constant temperature 80.3 ± 0.05 °C and different pressures. Both the absorption and the desorption of H₂(D₂) on Pd layers are controlled successively by a chemisorption step (second order kinetics) in the early stage of the processes and by a surface migration step (first order kinetics), in the subsequent stage of the processes. Consistent with the reported piezoelectric quartz crystal microbalance measurements, thermal desorption and electrochemical desorption studies, a mechanism is suggested which takes into account the adsorption ofH(D) atoms on two different surface states: (a) a “pre-dissolved” state which does not depend on the surface nature; (b) and adsorption state, the density of which depends upon the structure and nature of the Pd sample. The kinetic control of the entire processes (sorption and desorption) depends on the ratio of the density of sites (a) to (b).     

Thermal desorption of interacting molecules from mixed adlayers

A Monte Carlo simulation method is used to study thermal desorption of gas molecules from mixed adlayers containing two species. The effects of lateral interactions among adatoms and initial surface coverage on the desorption spectra are examined. It is shown that attractive lateral interactions lead to sharper peaks and that desorption occurs at high temperatures, whereas repulsive interactions lead to multiple peak spectra. It is found that interactions between unlike molecules affect the spectra for species with lower desorption energy only, and that the two species desorb together only in certain cases. Lateral interactions also affect the desorption kinetics significantly and very different behavior for the two species may be obtained.     

Adsorption and diffusion of cyclopentane in silicalite-1: a thermodynamic and kinetic study

The adsorption isotherm, differential thermal gravimetry (DTG), thermal gravimetry (TG), diffusion and thermodynamic parameters of cyclopentane at various adsorption coverages have been investigated using microgravimetric technique over a range of temperature. The differences in equilibrium adsorption capacity and in diffusion coefficient at different temperature are discussed in terms of the characteristics of silicalite-1 and the features of cyclopentane molecule. The heat of sorption Q_st varies significantly with sorption coverage indicating that the adsorption and diffusion mechanism is complex. The dependences of thermodynamic properties like free energy change (ΔG) and entropy change (ΔS) on sorption coverage show a sharp decrease and increase suggesting that the adsorption sites of the silicalite-1 are not energetically uniform to cyclopentane. Two desorption peaks in the DTG and TG curves suggest that two heterogenous binding sites surely exist in silicalite-1 to cyclopentane.     

Quantitative analysis of thermally induced desorption during halogen-etching of a silicon (1 1 1) surface

Thermal desorption at a chlorine-adsorbed Si(1 1 1) surface was measured with high precision. High-sensitivity measurements of the temperature dependence of the isothermal process, and thermal desorption spectra (TDS) with various parameters, heating rates and levels of surface coverage, indicated that the desorption is a second-order reaction with an activation energy of 2.2 eV. The wide dynamic-range data throw light on the ability of various methods of thermal desorption measurement to describe quantitatively the surface reaction. It is important to obtain a precise energy value, which can be done by considering the whole TDS shape, as well as isothermal data, in order to distinguish various reaction processes. Our results are consistent with model calculations.