Measured Activation Energy Research Articles

Insights into the reaction pathway of n-butane conversion over HZSM-5 zeolite at low temperature

The kinetics of n-butane conversion over HZSM-5 zeolites at low temperature were studied. The results show that n-butane conversion mainly occurs via monomolecular pathway under low n-butane pressure (<0.1 bar), while quickly transforms to bimolecular pathway with the increase of n-butane pressure. This can be explained by the lower apparent activation energy of bimolecular reaction compared with the monomolecular one. With the increase of n-butane pressure, the reaction orders for the formation of bimolecular products with respect to the n-butane pressure decrease from 2.0 to 1.0, which is caused by the change in the kinetic regime in which the kinetic parameters are being measured. Upon an increase in pressure, the Brønsted acid sites go from low coverage to high coverage. At low coverage of n-butane, the initial point for the measurement of apparent activation energies is the gas phase of the hydrocarbon and the highest point in energy is butyl carbonium ions. While at full coverage, the initial point is the carbenium ions and the highest point is the hydride transfer reaction, which follows a first order kinetic rate law with respect to the n-butane pressure.

Identification of an Active NiCu Catalyst for Nitrile Synthesis from Alcohol

Development of heterogeneous catalysts for alcohol transformation into nitriles under oxidant-free conditions is a challenge. Considering the C–H activation on α-carbon of primary alcohols is the rate-determining step, decreasing the activation energy of C–H activation is critical in order to enhance the catalytic activity. Several NiM/Al2O3 bimetallic catalysts were synthesized and scrutinized in catalytic transformation of 1-butanol to butyronitrile. Ni–Cu was identified as a suitable combination with the optimized Ni0.5Cu0.5/Al2O3 catalyst exhibiting 10 times higher turnover frequency than Ni/Al2O3 catalyst. X-ray absorption spectroscopy (XAS) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) revealed that the NiCu particles in the catalyst exist in the form of homogeneous alloys with an average size of 8.3 nm, providing an experimental foundation to build up a catalyst model for further density functional theory (DFT) calculations. Calculations were done over a series of NiM catalysts, and the experimentally observed activity trend could be rationalized by the Brønsted–Evans–Polanyi (BEP) principle, i.e., catalysts that afford reduced reaction energy also feature lower activation barriers. The calculated activation energy (Ea) for C–H activation with coadsorbed NH3 dropped from 63.4 kJ/mol on pure Ni catalyst to 49.9 kJ/mol on the most active NiCu-2 site in NiCu bimetallic catalyst, in good agreement with the experimentally measured activation energy values. The Ni0.5Cu0.5/Al2O3 catalyst was further employed to convert 11 primary alcohols into nitriles with high to near-quantitative yields, at a Ni loading 10 times less than that of the conventional Ni/Al2O3 catalyst.

Electron density measurements in shock tube using microwave interferometry

Microwave interferometry (MWI) is a nonintrusive diagnostic technique, capable of measuring small quantities of electrons present in a flame plasma. In this paper, a 94 GHz microwave interferometer is characterized and validated to perform robust and reliable measurements of electron concentrations in thermal and nonthermal plasmas in a shock tube. The MWI system is validated first by measuring the refractive index of a dielectric material. Subsequently, the system is used for measuring electron densities during the thermal ionization of argon and krypton in shock tube experiments. The measured activation energies are in good agreement with both the measured values from previous studies and theoretical values. The MWI system is finally used for measuring electron density time-histories in fuel oxidation experiments in the shock tube. The electron density profile of methane combustion shows a peak at the ignition time which agrees with pressure measurements. Experimental electron histories are also in overall agreement with predictions of the methane ion chemistry model.

Nonintrusive Energy Meter for Nontechnical Losses Identification

We present in this paper a method and apparatus for nonintrusive measurement of active energy in low-voltage ac installations. In the proposed method, the active power is calculated from the voltage and current waveforms, and the phase shift between them. No voltage amplitude measurement is required since the nominal voltage of the ac installation is considered as the actual voltage. This approach bypasses the main disadvantage of capacitive ac voltage probing: low accuracy in amplitude measurements due to variations on the sensor coupling capacitance. The energy meter that implements the technique is composed of a commercial nonintrusive current sensor (clamp-on current transformer) and a contactless capacitive voltage sensor—designed to measure the voltage phase and waveform. The voltage sensor includes a shield to isolate it from external electric fields, making the energy meter suitable for applications in polyphase systems. The developed energy meter does not require on-site calibration, galvanic contact to the phase conductors, neither electrical circuit opening, allowing for quick, safe, and easy installation on overhead service drop line. Due to these characteristics, it can be used by electricity distribution companies in the preinspection of consumer units suspected of fraud. Experimental results proved that the developed energy meter is insensitive to the characteristics of cables (width and insulation), external electrical fields, as well as to the voltage sensor capacitance variations. The error of the active energy measurement under real condition in a two-phase installation was 1%.

Non-Isothermal Sublimation Kinetics of 2,4,6-Trinitrotoluene (TNT) Nanofilms.

Non-isothermal sublimation kinetics of low-volatile materials is more favorable over isothermal data when time is a crucial factor to be considered, especially in the subject of detecting explosives. In this article, we report on the in-situ measurements of the sublimation activation energy for 2,4,6-trinitrotoluene (TNT) continuous nanofilms in air using rising-temperature UV-Vis absorbance spectroscopy at different heating rates. The TNT films were prepared by the spin coating deposition technique. For the first time, the most widely used procedure to determine sublimation rates using thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC) was followed in this work using UV-Vis absorbance spectroscopy. The sublimation kinetics were analyzed using three well-established calculating techniques. The non-isothermal based activation energy values using the Ozawa, Flynn–Wall, and Kissinger models were 105.9 ± 1.4 kJ mol−1, 102.1 ± 2.7 kJ mol−1, and 105.8 ± 1.6 kJ mol−1, respectively. The calculated activation energy agreed well with our previously reported isothermally-measured value for TNT nanofilms using UV-Vis absorbance spectroscopy. The results show that the well-established non-isothermal analytical techniques can be successfully applied at a nanoscale to determine sublimation kinetics using absorbance spectroscopy.

Investigation of optical and electrical properties of Cobalt-doped Ge-Sb-S thin film

Abstract Amorphous Germanium Antimony Sulphide (Ge-Sb-S) doped with Cobalt (Co) have been deposited on glass substrates by thermal evaporation technique on a glass substrate. The films deposited onto glass substrates are characterized by Energy Dispersive X-ray Fluorescence Spectrometer, UV–VIS spectrophotometer, Raman spectroscopy, and Capacitance-Voltage Keithley meter. The optical band gap was calculated from the UV–Visible spectrum and found to be 2.05 eV. Raman spectroscopy measurements reveal that a wide band spectrum from 300 to 410 cm−1 centered at 355 cm−1. The Raman shift peaks at 325 cm−1 and 350 cm−1 are as-signed to the bond stretching mode Sb-S and Ge-S, respectively. In addition, from the obtained Raman spectra it is concluded that the presence of Co doped with Ge-Sb-S. The capacitance and conductance versus voltage measurements were performed at different temperatures. The results show a slight increase in the capacitance with temperature and it reaches a maximum value around 150 °C, and eventually it becomes negative. This behavior is interpreted in terms of the nucleation growth process and the thermally activated conduction process with measured activation energy of 0.79 eV.

CO2 Capture via Crystalline Hydrogen-Bonded Bicarbonate Dimers

Summary Limiting global temperature rises is increasingly dependent on the development of energy-efficient carbon-capture methods. Here, we report a simple CO2-separation cycle using an aqueous bis(iminoguanidine) (BIG) sorbent that reacts with CO2 and crystallizes into an insoluble bicarbonate salt. X-ray diffraction analysis of the bicarbonate crystals revealed “anti-electrostatic” hydrogen-bonded (HCO3−)2 dimers, stabilized by guanidinium cations and water. Mild heating of the crystals releases the CO2 and regenerates the BIG sorbent quantitatively, thereby closing the CO2-separation cycle. Experimental and computational investigations support a CO2-release mechanism consisting of surface-initiated low-barrier proton transfer from guanidinium groups to bicarbonate anions with the formation of carbonic acid dimers, followed by CO2 and H2O release in the rate-limiting step, with a measured activation energy of 102 ± 12 kJ/mol. The minimum energy required for sorbent regeneration is 151.5 kJ/mol CO2, which is 24% lower than the regeneration energy of monoethanolamine, a benchmark industrial sorbent.

Insights into direct plasmon-activated eletrocatalysis on gold nanostar via efficient photothermal effect and reduced activation energy

The field of plasmonics brings new opportunities in various fields such as photocatalysis, nanophotonics, photovoltaics and photothermal therapeutics. In particular, the generation of hot carriers (hot holes and electrons) as well as the phtothermal effect via excitation of localized surface plasmon resonance (LSPR) can be used to activate chemical reactions. Understanding the underlying mechanism requires quantitative measurements of catalytic enhancement on the plasmonic nanostructures. In this work, the plasmonic gold nanostar (AuNSs) activated electrocatalysis was investigated using ascorbic acid (AA) as a reaction model. We found that AuNSs shows significantly enhanced electrochemical performance upon LSPR excitation. By measurement of the reaction activation energy, open circuit potential (OCP), and the wavelength and light intensity-dependent current responses, it is proposed that the efficient hot carriers separation, reduced activation energy and considerate photothermal effect contribute to the promoted electrocatalysis. Based on the study, a direct plasmon-improved electrochemical sensor towards sensitive detection of AA is constructed, and AA concentration as low as 10.0 μM can be successfully detected. The present work would shed new mechanistic insights into the plasmon-mediated electrochemical processes.

In Situ Attachment of Acrylamido Sulfonic Acid-Based Monomer in Terpolymer Hydrogel Optimized by Response Surface Methodology for Individual and/or Simultaneous Removal(s) of M(III) and Cationic Dyes.

Herein, grafting of starch (STR) and in situ strategic inclusion of 2-(3-(acrylamido)propylamido)-2-methylpropane sulfonic acid (APMPS) via solution polymerization of 2-(acrylamido)-2-methylpropanesulfonic acid (AMPS) and acrylamide (AM) have resulted in the synthesis of smart STR-grafted-AMPS-co-APMPS-co-AM (i.e., STR-g-TerPol) interpenetrating terpolymer (TerPol) network hydrogels. For fabricating the optimum hydrogel showing excellent physicochemical properties and recyclability, amounts of ingredients and temperature of synthesis have been optimized using multistage response surface methodology. STR-g-TerPol bearing the maximum swelling ability, along with the retention of network integrity, has been employed for individual and/or simultaneous removal(s) of metal ions (i.e., M(III)), such as Bi(III) and Sb(III), and dyes, such as tris(4-(dimethylamino)phenyl)methylium chloride (i.e., crystal violet) and (7-amino-8-phenoxazin-3-ylidene)-diethylazanium dichlorozinc dichloride (i.e., brilliant cresyl blue). The in situ strategic protrusion of APMPS, grafting of STR into the TerPol matrix, variation of crystallinity, thermal stabilities, surface properties, mechanical properties, swellability, adsorption capacities (ACs), and ligand-selective superadsorption have been inferred via analyses of unadsorbed and/or adsorbed STR-g-TerPol using Fourier transform infrared (FTIR), 1H/13C NMR, UV–vis, thermogravimetric analysis, differential scanning calorimetry, X-ray diffraction, field emission scanning electron microscopy, energy-dispersive X-ray, dynamic light scattering, and rheological analyses and measuring the lower critical solution temperature, % gel content, pH at point of zero charge (pHPZC), and network parameters, such as ρc and Mc. The prevalence of covalent, ionic (I), and variegated interactions between STR-g-TerPol and M(III) has been understood through FTIR analyses, fitting of kinetics data to the pseudosecond-order model, and by the measurement of activation energies of adsorption. The formation of H-aggregate type dimers and hypochromic and hypsochromic shifts has been explained via UV–vis analyses during individual and/or simultaneous removal(s) of cationic dyes. Several isotherm models were fitted to the equilibrium experimental data, of which Langmuir and combined Langmuir–Freundlich models have been best fitted for individual Bi(III)/Sb(III) and simultaneous Sb(III) + Bi(III) removals, respectively. Thermodynamically spontaneous chemisorption processes have shown the maximum ACs of 1047.39/282.39 and 932.08/137.85 mg g–1 for Bi(III) and Sb(III), respectively, at 303 K, adsorbent dose = 0.01 g, and initial concentration of M(III) = 1000/30 ppm. The maximum ACs have been changed to 173.09 and 136.02 mg g–1 for Bi(III) and Sb(III), respectively, for binary Sb(III) + Bi(III) removals at 303 K, adsorbent dose = 0.01 g, and initial concentration of Bi(III)/Sb(III) at 30/5 and 5/30 ppm.

Scalable Synthesis of Collagenic-Waste and Natural Rubber-Based Biocomposite for Removal of Hg(II) and Dyes: Approach for Cost-Friendly Waste Management.

For initiating a prosperous cost-friendly waste management of small-scale industries, cow buffing dust (CBD), one of the abundantly available semisynthetic collagenic solid wastes, has been used as a nonsulfur cross-linker of natural rubber (NR) for fabricating an NRCBD–biocomposite superadsorbent. The as-prepared reusable biocomposite bearing variegated collagenic and noncollagenic N-donors, along with the O-donors, has been reported for ligand-selective preferential superadsorption from waste water. Thus, a CBD and NR-based scalable biocomposite bearing optimum cross-linking, excellent physicochemical properties, and reusability has been developed via systematic optimization of the torque and reaction time for cost-friendly adsorptive exclusion of dyes, such as 2,8-dimethyl-3,7-diamino-phenazine (i.e., safranine, SF) and (7-amino-8-phenoxazin-3-ylidene)-diethylazanium dichlorozinc dichloride (i.e., brilliant cresyl blue), BCB, and Hg(II). The CBD-aided curing of NR has been achieved through the formation of a cross-linked chromane-ring originated via reaction between the methylol-phenol ring of phenol-formaldehyde resin and isoprene unit of NR. The partial disappearance of unsaturation in cured-NRCBD, relative variation of crystallinity, surface properties, elevated thermal stabilities, and ligand-selective superadsorption have been studied by advanced microstructural analyses of unadsorbed and/or adsorbed NRCBD using Fourier transform infrared (FTIR), 13C nuclear magnetic resonance, ultraviolet–visible, and O 1s-/N 1s-/C 1s-/Hg 4f7/2,5/2-X-ray photoelectron spectroscopies, thermogravimetric analysis, differential scanning calorimetry, X-ray diffraction, field emission scanning electron microscopy, energy-dispersive spectroscopy, and pHPZC. Response surface methodology-based optimization has been employed to attain the optimum potential of NRCBD, considering the interactive effects between pHi, temperature, and concentration of the dye. H-aggregate and time-dependent hypochromic effect has been observed during individual adsorption of dyes. Moreover, the prevalence of chemisorption via ionic interaction between NRCBD and SF, BCB, and Hg(II) has been realized by FTIR, fitting of kinetics data to the pseudosecond-order model, and measurement of activation energies. The Brunauer–Emmett–Teller and Langmuir isotherms fit the best to BCB and SF/Hg(II), respectively. Thermodynamically spontaneous chemisorption have shown the maximum adsorption capacities of 303.61, 46.14, and 166.46 mg g–1 for SF, BCB, and Hg(II), respectively, at low initial concentration of Hg(II)/dyes = 40 ppm, 303 K, and adsorbent dose = 0.01 g.

Crystallographically Determined Etching and Its Relevance to the Metal-Assisted Catalytic Etching (MACE) of Silicon Powders.

Metal-assisted catalytic etching (MACE) using Ag nanoparticles as catalysts and H2O2 as oxidant has been performed on single-crystal Si wafers, single-crystal electronics grade Si powders, and polycrystalline metallurgical grade Si powders. The temperature dependence of the etch kinetics has been measured over the range 5–37°C. Etching is found to proceed preferentially in a 〈001〉 direction with an activation energy of ~0.4 eV on substrates with (001), (110), and (111) orientations. A quantitative model to explain the preference for etching in the 〈001〉 direction is developed and found to be consistent with the measured activation energies. Etching of metallurgical grade powders produces particles, the surfaces of which are covered primarily with porous silicon (por-Si) in the form of interconnected ridges. Silicon nanowires (SiNW) and bundles of SiNW can be harvested from these porous particles by ultrasonic agitation. Analysis of the forces acting between the metal nanoparticle catalyst and the Si particle demonstrates that strongly attractive electrostatic and van der Waals interactions ensure that the metal nanoparticles remain in intimate contact with the Si particles throughout the etch process. These attractive forces draw the catalyst toward the interior of the particle and explain why the powder particles are etched equivalently on all the exposed faces.

Study on the transdermal penetration mechanism of ibuprofen nanoemulsions

Objective: The purpose of this study was to research the mechanism of percutaneous penetration of Ibuprofen (IBU) nanoemulsion.Method: Transdermal penetration mechanism of IBU nanoemulsion was investigated by using Fourier transform infra-red spectral analysis (FTIR), differential scanning calorimeter thermogram (DSC), and activation energy (Ea) measurement. The in vivo skin penetration test of rats was carried out using Rhodamine B nanoemulsion to simulate the process of drug penetration into the skin, and the frozen section of the skin was observed by confocal laser scanning microscopy (CLSM).Result: FTIR spectra and DSC thermogram of rat skin treated with IBU nanoemulsion showed that infiltration occurred due to disruption of the stratum corneum (SC) protein–lipid structure and increasing of fluidity, hydration, and disruption of the lipid bilayer structure of the SC. The significant reduce in Ea (1.255 kcal/mol) for IBU permeating rat skin suggested crucial disruption of the SC lipid bilayers (P < 0.05), which is speculated that nanoemulsion may create new pathways to promote drug penetration. CLSM revealed that Rhodamine B penetrated into the SC in a shorter period of time and it accumulated around the sebaceous glands.Conclusion: The study of skin penetration mechanism indicated that nanoemulsion can be perfectly well used as the transdermal penetration of poorly soluble drugs.

On-line measurement of activation energy of ground bamboo using near infrared spectroscopy

Abstract On-line measurement of activation energy (Ea) is very important in supporting the thermal conversion process. The main objective of this study was to evaluate the Ea of ground bamboo using near infrared spectroscopy in real time. 80 bamboo samples with different diameters were selected using random sampling. Ea was determined using the Coats-Redfern method, and Ea of reaction order (n) at n = 1 and n≠1 was investigated. The performance of on-line measurement predicted by PLS modelling for Ea at n = 1 and Ea at n≠1 showed coefficients of determination of 0.781 and 0.714, respectively; standard error of prediction of 5.249 and 6.858 kJ/mol, respectively; and bias values of −1.0628 and −1.871 kJ/mol, respectively. Both PLS models were found to be fair and could be applied toward screening. The results showed that the vibration bands of lignocellulosic components (CH2, hemicellulose, cellulose, and lignin) highly influenced model development. Moreover, internal relationships were identified among Ea, the pre-exponential factor (A), and n, such as A (1/min) = 63251 × e0.2200×Ea (at n = 1), A (1/min) = 33719 × e0.2267×Ea (at n≠1), and n = 0.008 × Ea+0.254. These relationships can be used to evaluate A and n if Ea is known. In the case of this study, Ea was forecasted using an NIR model.

Which Activation Energy Do We Measure? Analysis of the Kinetics of Propene-3-13C Double-Bond-Shift Reaction on Silicalite-1 by 1H MAS NMR In Situ

Correct assignment of the experimental activation energy of chemical reactions on solid catalysts to either apparent (Eapp) or intrinsic (Eint) activation barriers represents a long-standing problem in the kinetic measurements performed with the use of magic-angle spinning (MAS) NMR spectroscopy in a closed microreactor. Here, the transformation of propene on silicalite-1 has been investigated. It is established that a double-bond-shift reaction represents the main route for propene transformation at 296–333 K with the involvement of hydrogen-bonded silanol groups as active sites. The kinetics of a double-bond-shift reaction was followed with 1H MAS NMR spectroscopy in situ by monitoring the 13C-label transfer in adsorbed propene-3-13C from the CH3 group to the ═CH2 group. The experimentally measured activation energy Eexp for this reaction is discussed in terms of assigning Eexp to either apparent (Eapp) or intrinsic (Eint) activation barrier. For the correct assignment of Eexp, an approach that is based...

Model-free kinetic analysis of thermal behavior of urea-formaldehyde microcapsules

In this study, experimental and analytical investigations were conducted concerning kinetics of thermal reactions of urea-formaldehyde microcapsules (UFM). Microencapsulated phase change materials were synthesized via in-situ polymerization using UF resin as shell and drying oil as core, and were characterized by scanning electron microscope (SEM), particle size analysis (PSA) and Fourier transform infrared analysis (FTIR). Accordingly, model-free kinetics (MFK) was used to determine the kinetics of the thermal behavior of UFM. Therefore, Thermogravimetry analysis was conducted at different heating rates of 3, 5, 7, and 10 °C/min. The apparent activation energy was obtained by MFK, which led to identify the reaction mechanisms. The results revealed that there are two stages for degradation of the synthesized microcapsules. Addition of the oil core in the UF microcapsules has hindered one stage of the suggested three stages of the pyrolysis process of UF based resins and the two stages were unified as a single extended stage. Additionally, the measured activation energy showed a low value of 20 kJ/mol due to the presence of oil in the UF microcapsules.

Effect of fluorine (F) on structural and electrical properties of sprayed ZnO thin films

Un-doped and F doped ZnO thin films have been deposited on glass substrate using spray pyrolysis technique at substrate temperature 350 °C. The structural and electrical properties of these films were investigated by X-ray diffraction (XRD) and Van der Pauw method. Electrical transport study confirms semiconducting nature of the films. Dopant F atom enhances electrical conductivity substantially with the increase of F content. Activation energy measurement confirms that electrical transport mainly occurs due to free carrier transition and free excitation of defect states to the conduction band. The carrier concentration of ZnO increases with F-doping as confirmed by Hall voltage measurement and the order of carrier concentration is 1019 cm−3.

Effect of alloy composition on catalytic performance and coke-resistance property of Ni-Cu/Mg(Al)O catalysts for dry reforming of methane

In CH4−CO2 reforming, the activity and coke resistance of hydrotalcite-derived Ni-Cu/Mg(Al)O alloy catalysts show strong dependence on Ni-Cu composition. At reaction conditions of T = 873 K, CH4/CO2/N2 = 1/1/2, SV = 60 000 mL h–1 g–1, and TOS = 25 h, the alloy catalysts with bulk Cu/Ni ratio of 0.25–0.5 exhibit good activity, stability, and coke resistance, while those with lower and higher Cu/Ni ratios deactivate due to severe coking. The optimized Ni-Cu/Mg(Al)O catalysts show graphitic carbon which is only 1/85–1/136 that of Ni/Mg(Al)O, highlighting the efficacy of Ni-Cu alloying for coke suppression. Moreover, the Ni-Cu/Mg(Al)O catalyst of highest activity performs well at temperature as low as 723 K, appearing as an effective non-precious catalysts for low-temperature CH4−CO2 reforming. It is disclosed that a Cu composition of 25–45% is appropriate for the alloy catalyst to perform well. The results of activation energy measurement, CH4-TPSR/O2-TPO, and CO2-TPSR/H2-TPR indicate that alloying Ni with Cu inhibits CH4 decomposition, and Cu provides sites for CO2 dissociation to yield more active oxygen species suitable for carbon gasification, consequently lowering coke deposition and enhancing catalytic stability.

Ab-Initio Investigation of Polarization and Ion Conduction Mechanisms in the Bone Mineral and Electret Material Hydroxyapatite

Hydroxyapatite [Ca10(PO4)6(OH)2] (HAp) comprises up to 50% of human bone and is a subject of intense study in the context of biocompatibility for, e.g., bone transplants. It is also known to exhibit ionic conductivity, as well as piezo-, pyro-, and ferroelectricity, and this fact has stimulated research towards its possible application in various in vivo and ex vivo energy harvesting and nanoelectronics devices [1]. On a related note, it has also been shown that HAp can be turned into remarkably good “electrets”, or materials with a semi-permanent dipole. Such electrets are now of interest due to its potential application in vibration energy harvesting for Internet of Things (IoT), in addition to biological tissue engineering such as in vivo tissue culture under an external electric field. HAp can be poled into an electret by applying a DC bias at elevated temperature and then quenching to room temperature. The electret formation, i.e., the freezing of the dipole, has been explained by the displacement of protons under bias at high temperature and subsequent quenching of its motion [2]. However, the microscopic polarization and ion conduction mechanisms remain unclear, hindering the optimization of this material for various applications mentioned above. In this work, we undertook a computational approach to this problem [3]. We first performed ab-initio molecular dynamics simulations in HAp with/without point defects to identify major polarization and ion conduction mechanisms. Then, the minimum energy paths and activation energies of those mechanisms were studied in detail using the climbing image nudged elastic band (CI-NEB) method [4]. VASP [5] code was used in combination with VTSTtools [6] for the calculations. The Figure shows the hexagonal structure of hydroxyapatite often observed at elevated temperature. In our simulations, we used a supercell where this unit cell was multiplied by three times in the c-axis direction. Based on previous works on defect formation energetics [7, 8], we considered three types of defects, a proton vacancy, OH- vacancy, and proton interstitial, which are all located within the column of OH- ions along the c-axis. We then performed molecular dynamics simulations at 1000-1500 K for > 20 ps, where we can expect elementary processes with activation energies less than 1 eV to occur several times during the simulation according to transition state theory. From these simulations, we found that the dominant polarization/diffusion mechanisms mainly occur in the column of OH- ions aligned along the c-axis, and that the main processes are the flipping of the direction of OH- ions, exchange of protons between oxide ions, and the hopping of OH- vacancies. When proton interstitials are present as in the case of Ca-deficient HAp, interstitial proton hopping occurs mainly on the OH- column, although diffusion in the ab plane is also viable under bias. The activation energies for these processes were calculated using the CI-NEB method and are tabulated in the Table. The variations in the values originate from different environments around the local hopping/flipping site, which we estimate to be less than few tenths of an eV from the calculations that we have performed. The calculated activation energies are consistent with processes that have been detected in various temperature stimulated depolarization current experiments and conductivity measurements in the literature. This allows us to make a correspondence between measured activation energies and the dominant polarization/diffusion mechanisms at various experimental conditions. With such insight, materials processing conditions can be tailored for utilizing specific microscopic mechanisms in various applications. [1] S. B. Lang, Phase Transitions 89, 678 (2016).[2] Y. Tanaka et al., J. Appl. Phys. 107, 014107 (2010).[3] S. Kasamatsu and O. Sugino, Phys. Chem. Chem. Phys. 20, 8744 (2018).[4] G. Henkelman et al., J. Chem. Phys. 113, 9901 (2000).[5] G. Kresse and J. Furthmueller, Phys. Rev. B 54, 11169 (1996).[6] http://theory.cm.utexas.edu/vtsttools/index.html Figure 1

The critical role of substrate disorder in valley splitting in Si quantum wells

Atomic-scale disorder at the top interface of a Si quantum well is known to suppress valley splitting. Such disorder may be inherited from the underlying substrate and relaxed buffer growth, but can also arise at the top quantum well interface due to the random SiGe alloy. Here, we perform activation energy (transport) measurements in the quantum Hall regime to determine the source of the disorder affecting the valley splitting. We consider three Si/SiGe heterostructures with nominally identical substrates but different barriers at the top of the quantum well, including two samples with pure-Ge interfaces. For all three samples, we observe a surprisingly strong and universal dependence of the valley splitting on the electron density (Ev ∼ n2.7) over the entire experimental range (Ev = 30–200 μeV). We interpret these results via tight binding theory, arguing that the underlying valley physics is determined mainly by disorder arising from the substrate and relaxed buffer growth.

Edge-Selective Gas Detection Using Langmuir Films of Graphene Platelets.

Recent advances in the large-scale production of graphene have led to the availability of solution-processable platelets on the commercial scale. Langmuir-Schaefer deposition is a scalable process for forming a percolating film of graphene platelets, which can be used for electronic gas sensing. Here, we demonstrate the use of this deposition method to produce functional gas sensors, using a chemiresistor structure from commercially available graphene dispersions. The sensitivity of the devices and the repeatability of the electrical response upon gas exposure have been characterized. Raman spectroscopy and Kelvin probe force microscopy show doping of the basal plane using ammonia (n-dopant) and acetone (p-dopant). The resistive signal is increased upon exposure to both gases, showing that sensing originates from the change in the contact resistance between nanosheets. We demonstrate that Arrhenius fitting of desorption response potentially allows measurements of desorption process activation energies for gas molecules adsorbed onto the graphene nanosheets.