Potassium Atoms Research Articles

Band Structure of Bilayer Graphene Intercalated by Potassium Atoms. Ab Initio Calculations

Using the electron density functional theory, the electronic band structure of pure and potassium-intercalated bilayer graphene has been studied. It is shown that after the intercalation process, a band gap appears in the band structure of bilayer graphene. In addition, the energy gap changes nonlinearly depending on the intercalate concentration in the interlayer space of bilayer graphene. We also calculated the energy spectra of bilayer graphene containing vacancy defects, the presence of which leads to the appearance of mid-gap states.

Current transmission pathways in potassium intercalated graphene

Carbon-based conductors are the focus of considerable research investigating the material roles and transport mechanisms which determine current transmission performance. Ab initio analysis of the potassium–graphene system suggests that neither material constituent should be cast in a subordinate current transmission role. When the dopant density is low, the potassium atoms perform a conventional doping function, improving the conductivity of the current carrying carbon substrate via charge transfer. As the doping density increases, the potassium atoms begin to form a second transmission pathway, complimenting the carbon substrate pathway and contributing to the net current transport. At the highest modeled dopant density, corresponding to the well known KC8 structure studied in various experiments, the potassium atoms form a parallel and nominally independent current transmission network, whose contribution to electron transport approximately matches that of the underlying graphene. Consistent with published experimental data on doped nanowires, the modeling results show that conductivity is a sensitive function of potassium concentration. The sensitivity is due to an internal state transition, not subject to direct experimental measurement, which strongly influences nanocomposite conductor performance.

Density functional theory study on the catalytic reduction of nitric oxide over the bio-char surface during biomass reburning: The influence of inherent potassium

Active sites on char surface have been proven to be effective for nitric oxide (NO) reduction. In this work, density functional theory was used to study the catalytic reduction of NO over the zigzag char surface. Two bio-char models with or without potassium were established and the reaction paths during char-NO interaction were built. The effect of inherent potassium and active sites on NO reduction was taken into consideration. Mulliken charge results showed that the migration of electron from potassium(K) atom to carbon atom would increase the electron density on char surface, which further improved the electron-donating capacity. Orbital electron distribution and ESP results proved that char(K) was more likely to donate electron to NO than char-1 during char-NO interaction. Reaction potential energy results indicated that the bonded K atom on char surface would enhance the chemisorption of NO via electrical charge transfer and reduced energy barriers from 173.7 kJ/mole to 31.6 kJ/mole, which was consistent with the kinetic analysis results. The bonding of –O-K to carbon atoms on char surface greatly activated the NO reduction during char-NO interaction, which enhanced the adsorption and reduction of NO.

Detection of alkali path in a pilot-scale combustor using laser spectroscopy and surface ionization — From vapor to particles

Alkali species have been under intensive research in thermal conversion applications due to their abundance especially in biomass fuels. Alkali metals, sodium (Na) and potassium (K), are known to cause severe operational problems in combustion units, such as slagging, fouling, and corrosion. In this work, we present a monitoring method to follow alkali behavior from vapor to particles in a pilot-scale reactor. In our approach we combine Tunable Diode Laser Atomic Spectroscopy (TDLAS) for atomic potassium monitoring, Collinear Photofragmentation and Atomic Absorption Spectroscopy (CPFAAS) for KCl and KOH detection, and Surface Ionization Detection (SID) for monitoring of total flue gas and aerosol alkali content. Experiments were carried out in the Chalmers 100 kW oxy-fuel combustion unit that, during these experiments, used propane as fuel. Alkali species were injected as a water solution directly to the flame. In addition, SO2 was used to alter the conditions for alkali species formation injecting it directly to the combustion feed gas. Due to the alkali monitoring system described, we were able to monitor the alkali behavior during nucleation and sulfation processes. The conditions for dimer formation and heterogeneous nucleation were observed when the temperature conditions were changed by lowering the thermal input to the unit.

Bohr-Weisskopf effect in the potassium isotopes

The magnetic hyperfine structure constants have been calculated for low-lying levels in neutral potassium atom taking into account the Bohr--Weisskopf (BW) and Breit--Rosenthal (BR) effects. According to our results the $4p_{1/2}$ state of K~I is free from both BR and BW corrections on the level of the current theoretical uncertainties. Using this finding and the measured values of the $A(4p_{1/2})$ constants, we corrected the nuclear magnetic moments for several short-lived potassium isotopes. The BW correction is represented as a product of atomic and nuclear factors. We calculated the atomic factor for the ground state of K I, which allowed us to extract nuclear factors for potassium $I^\pi = 3/2^+$ isotopes from the experimental data. In this way the application range of the single-particle nuclear model for nuclear-factor calculation in these isotopes has been clarified.

High-Voltage Layered Manganese-Based Oxide Cathode with Excellent Rate Capability Enabled by K/F Co-doping

P-type layered manganese-based materials are prone to undergo lattice oxygen oxidation accompanied by oxygen layer slipping and even unfavorable phase transitions at around 4.2 V, giving rise to rapid discharge capacity decline and inferior structural stability, which restricts their operating voltage and energy density. Here we propose a modification strategy for layered Na0.67MnO2 material with K/F co-doping so as to reach the optimization of lattice oxygen oxidation behavior and structural stability at a high cut-off voltage of 4.4 V. Combining X-ray powder diffraction refinement results, ex situ X-ray photoelectron spectrometry analysis, high-resolution transmission electron microscopy, and electrochemical characterization, our work demonstrates that an appropriate amount of K/F co-doping has a favorable effect on the formation of well-reversible lattice oxygen oxidation behavior and the improvement of Na layer spacing. Owing to the synergetic effect of potassium and fluorine atoms, the initial discharge capacity is up to 210.2 mA h g–1 at 0.1 C and 140.2 mA h g–1 at 1 C with 73% capacity retention after 100 cycles and high discharge capacity of 115.0 mA h g–1 at 5 C, with 68.1 mA h g–1 retained after 200 cycles. The kinetic analysis shows that the optimal K0.05Na0.62MnO1.95F0.05 sample exhibits the largest sodium ion diffusion coefficient and the smallest electrochemical polarization, which paves a novel path for the application of layered manganese-based oxides in high-voltage cathode materials for sodium-ion batteries.

Reduced graphene oxide coated modified SnO2 forms excellent potassium storage properties

In this work, SnO2 and Sn nanoparticles adhered to the surface of rGO (SnO2/Sn/rGO) applied as potassium ion batteries (KIBs) anode materials were synthesized via thermal reduction. Preparing SnO2 material into a nanostructure for modification can reduce ion diffusion distance to improve the number of active sites appropriate for K+ adsorption, and efficiently reduce the volume change which is conducive to enhancing the potassium storage capacity. Besides, layered rGO inhibits SnO2/Sn aggregation, while increased surface area also reduces diffusion channel and electrolyte contact. However, larger specific surface area result in a lower initial Coulomb efficiency (ICE). The approach adopted here is to force the solid electrolyte interface (SEI) to fully emerge during the first charge-discharge cycle by using electrolytes containing 1 M KFSI in EC and DEC (1:1, v/v). According to density functional theory (DFT) analysis, the doping of rGO and SnO2 effectively enhanced the adsorption of potassium atoms and reduced the diffusion barrier of K+. Therefore, SnO2/Sn/rGO nanocomposites have a high specific capacity (325.8 mAhg−1 after 350 cycles at 0.1 Ag-1), an excellent ICE (66.57%), and a long cycle life (203.6 mAhg−1 after 1000 cycles at 0.5 Ag-1).

Relativistic optical potential approach for electrons collision by potassium atom

This work includes the determination of the differential cross sections (DCSs), total cross sections (TCSs), and momentum transfer cross sections (MTCSs) for electron scattering by potassium atoms, as well as the derivation of the spin polarisation parameter S (). These calculations are made to describe the interaction between an electron and a target atom in the energy range (5–500 eV). Theoretical work was done by combining the static potential with the polarisation potential over large distances and the Perdew -Zunger correlation potential over short ones. "The total results show close agreement when compared to the experimental and theoretical conclusions provided by the others".

Effect of hydrogen atoms on potassium-ion electrets used in vibration-powered generators

Potassium-ion electrets, which are mainly composed of amorphous silica and potassium atoms, can accumulate negative charges and are expected to be useful for future vibration powered generators. According to previous research, the origin of the accumulation of negative charges was revealed to be related to the presence of SiO5 structures in amorphous silica. In general, Si–O bonds are so stable that potassium-ion electrets are expected to retain a high electric potential longer than previously studied materials and are therefore being studied intensively for practical use. However, charge decay phenomena were observed in previous experiments, and this presents serious problems. In this study, we focus on the role of hydrogen which is considered to be one of the origins of charge decay and have investigated the behavior of hydrogen in potassium-ion electrets and the mechanism of the charge decay using first-principles calculations. As a result, we found that hydrogen atoms can create a positively charged characteristic local structure or break SiO5 structures and therefore cause degradation of the negative charge storing ability of potassium-ion electrets.

Numerical Study and Experimental Verification of Biomass Conversion and Potassium Release in a 140 kW Entrained Flow Gasifier.

In this study, a Eulerian-Lagrangian model is used to study biomass gasification and release of potassium species in a 140 kW atmospheric entrained flow gasifier (EFG). Experimental measurements of water concentration and temperature inside the reactor, together with the gas composition at the gasifier outlet, are used to validate the model. For the first time, a detailed K-release model is used to predict the concentrations of gas-phase K species inside the gasifier, and the results are compared with experimental measurements from an optical port in the EFG. The prediction errors for atomic potassium (K), potassium chloride (KCl), potassium hydroxide (KOH), and total potassium are 1.4%, 9.8%, 5.5%, and 5.7%, respectively, which are within the uncertainty limits of the measurements. The numerical model is used to identify and study the main phenomena that occur in different zones of the gasifier. Five zones are identified in which drying, pyrolysis, combustion, recirculation, and gasification are active. The model was then used to study the transformation and release of different K species from biomass particles. It was found that, for the forest residue fuel that was used in the present study, the organic part of K is released at the shortest residence time, followed by the release of inorganic K at higher residence times. The release of inorganic salts starts by evaporation of KCl and continues by dissociation of K2CO3 and K2SO4, which forms gas-phase KOH. The major fraction of K is released around the combustion zone (around 0.7-1.3 m downstream of the inlet) due to the high H2O concentration and temperature. These conditions lead to rapid dissociation of K2CO3 and K2SO4, which increases the total K concentration from 336 to 510 ppm in the combustion zone. The dissociation of the inorganic salts and KOH formation continues in the gasification zone at a lower rate; hence, the total K concentration slowly increases from 510 ppm at 1.3 m to 561 ppm at the outlet.

Quantitative Tomographic Laser Absorption Imaging of Atomic Potassium during Combustion of Potassium Chloride Salt and Biomass.

Gaseous potassium (K) species play an important role in biomass combustion processes, and imaging techniques are powerful tools to investigate the related gas-phase chemistry. Here, laser absorption imaging of gaseous atomic K in flames is implemented using tunable diode laser absorption spectroscopy at 769.9 nm and a high-speed complementary metal oxide semiconductor (CMOS) camera recording at 30 kfps. Atomic K absorption spectra are acquired for each camera pixel in a field of view of 28 × 28 mm at a rate of 100 Hz. The technique is used to determine the spatial distribution of atomic K concentration during the conversion of potassium chloride (KCl) salt and wheat straw particles in a laminar premixed CH4/air flame with an image pixel resolution of up to 120 μm. Due to axisymmetry in setup geometry and, consequently, atomic K distributions, the radial atomic K concentration fields could be reconstructed by one-dimensional tomography. For the KCl sample, the K concentration field was in excellent agreement with previous point measurements. In the case of wheat straw, atomic K concentrations of around 3 ppm were observed in a cylindrical flame during devolatilization. In the char conversion phase, a spherical layer of atomic K, with concentrations reaching 25 ppm, was found within 5 mm of the particle surface, while the concentration rapidly decreased to sub-ppm levels along the vertical axis. In both cases, a thin (∼1 mm) layer without any atomic K was observed in close vicinity to the particle, suggesting that the potassium was initially not released in its atomic form.

Concerning the incorporation of potassium in the crystal structure of combeite (Na2Ca2Si3O9)

Potassium incorporation in the structure of combeite has been studied in detail. Since natural combeites are known to contain only small amounts of potassium focus was laid on the Na-rich part of a hypothetical solid-solution series with composition Na2-xKxCa2Si3O9. Samples were prepared from mixtures of silica and the corresponding carbonates for nominal compositions with x = 0.2, 0.3 and 0.5, heated from ambient temperature to 1350 °C and slowly cooled to 1000 °C. After disintegration of the carbonates, the platinum capsules used as sample containers were welded shut in order to avoid losses of the volatile K2O and Na2O components. From all three batches potassium containing combeite crystals could be retrieved. Single-crystal diffraction experiments revealed the following compositions: Na2.10(1)K0.11(1)Ca1.90(1)Si3O9, Na2.09(1)K0.18(1)Ca1.91(1)Si3O9 and Na2.13(1)K0.18(1)Ca1.87(1)Si3O9. Consistently, the trigonal crystals (space group P 31 2 1) contained (i) (K + Na):Ca ratios larger than 1:1 and (ii) potassium concentrations lower than those in the starting mixtures. Since the K-contents of the samples obtained from the runs with x = 0.3 and 0.5 were almost identical, the solid-solution seems to be rather limited with an upper boundary of about one potassium atom per unit cell. The structure of the K-containing combeites is very close to the K-free structures reported in the literature. It can be described as a mixed tetrahedral-octahedral network in which additional K, Na and Ca cations are incorporated for charge compensation. A detailed analysis of the topological features of the net is presented. From the six observed extra-framework sites only the M22 position showing a coordination environment with ten next oxygen neighbours is involved in the K-substitution. Potassium uptake is also reflected in increasing values for the lattice parameters a and c as well as the unit-cell volumes. Actually, the c-axis is more affected from the incorporation of the comparatively large K+-cations.

Systematic Theoretical Study of CO Activation over Clean and Potassium-Modified Transition Metals

By using the periodic density functional theory (DFT) method, the adsorption and activation of CO on clean and potassium-modified transition metals, which includes Fe(100), Co(0001), Ni(111)/(100), Cu(111), Ru(0001), Rh(111)/(100), Pd(111), and Pt(111) surfaces, have been investigated systematically. The calculation results show that the additive of potassium atoms greatly enhances the binding strength of CO and O while only slightly improving the adsorption of C atoms. Moreover, the activation energy of CO dissociation is decreased in the presence of potassium additive atoms. Our results indicate that the CO activity enhancement by the series of alkali metals (Li, Na, K, Rb, and Cs) on Ni(100) increases in the order of Li < Na < Cs < Rb = K. On the basis of the electronic structure and geometric analysis, the physical origin of the promotion effect has been clarified, and it can be attributed to the direct electron transfer between potassium and O atoms involved in CO. The effect of an external electric field on CO activation was also investigated, in which it was shown that a positive electric field accelerated CO activation by elongating the C–O bond and transferring electrons. It is obvious that CO adsorption is enhanced when the orientation of the external electric field is the same as the direction of charge transfer. At last, the promoting effect of potassium on CO activation drops as the oxide state of potassium increases due to the steric effect and direct chemical bonding.

Optimal Cold Atom Thermometry Using Adaptive Bayesian Strategies

Precise temperature measurements on systems of few ultracold atoms is of paramount importance in quantum technologies, but can be very resource-intensive. Here, we put forward an adaptive Bayesian framework that substantially boosts the performance of cold atom temperature estimation. Specifically, we process data from real and simulated release--recapture thermometry experiments on few potassium atoms cooled down to the microkelvin range in an optical tweezer. From simulations, we demonstrate that adaptively choosing the release--recapture times to maximise information gain does substantially reduce the number of measurements needed for the estimate to converge to a final reading. Unlike conventional methods, our proposal systematically avoids capturing and processing uninformative data. We also find that a simpler non-adaptive method exploiting all the a priori information can yield competitive results, and we put it to the test on real experimental data. Furthermore, we are able to produce much more reliable estimates, especially when the measured data are scarce and noisy, and they converge faster to the real temperature in the asymptotic limit. Importantly, the underlying Bayesian framework is not platform-specific and can be adapted to enhance precision in other setups, thus opening new avenues in quantum thermometry.

The radiative processes involving ion-atom alkali systems: The case of hydrogen and potassium

The importance of the symmetric as well as non-symmetric ion-atom absorption processes in broad spectral region within a model of solar photosphere and also within the corresponding sunspot models, has been previously emphasized. This motivated us to study the photo-association, charge-exchange and photo-dissociation absorption processes in the cases of non-symmetric and symmetric systems involving hydrogen and potassium atoms, ions, molecules and molecular-ions. We have calculated the data for molecules and molecular state characterizations needed for current computation. We determined the partial and average cross-sections and corresponding spectral absorption rate coefficients, as the functions of wavelengths, and temperatures (100 nm ⩽λ⩽1000 nm, 400 K ⩽T⩽10 000 K). Our calculations are performed for conditions that exist in different layers of weakly ionized stellar atmospheres. The obtained data are potentially of high impact and may have broad further applications, e.g., for modelling of laboratory plasmas and various astrophysical objects including geocosmical ones.

Comparative study of electronic, optoelectronic, optical, and thermodynamic properties of two ovalene molecules and their derivatives functionalized with potassium and chlorine atoms

Comparative study of electronic, optoelectronic, optical, and thermodynamic properties of two ovalene molecules and their derivatives functionalized with potassium and chlorine atoms

Interference patterns in ionization of Kramers–Henneberger atom

We combine IR pump and XUV probe laser pulses to visualize the Kramers–Henneberger (KH) state of the potassium atom. We demonstrate that ionization of such an atom exhibits some molecular-like features such as low order interference maxima in photoelectron momentum spectra. The locations of these maxima allow to estimate spatial dimensions of the KH atom and can be used for accurate calibration of high intensity laser fields. At the same time, we show that an analogy between the KH atom and a homo-nuclear diatomic molecule cannot be extended too far. In particular, higher order interference maxima are very difficult to observe in the case of the KH state. We attribute this to a particular structure of the KH potential which does not confine electron motion to a well-defined potential well unlike in real diatomic molecules.

Quantitative real-time in situ measurement of gaseous K, KOH and KCl in a 140 kW entrained-flow biomass gasifier

Photofragmentation tunable diode laser absorption spectroscopy (PF-TDLAS) was used to simultaneously measure the concentrations of gas phase atomic potassium (K), potassium hydroxide (KOH) and potassium chloride (KCl) in the reactor core of a 140 kWth atmospheric entrained-flow gasifier (EFG). In two gasification experiments at air-to-fuel equivalence ratio of 0.5, the EFG was first run on forest residues (FR) and then on an 80/20 mixture of FR and wheat straw (FR/WS). Combustion at air-to-fuel equivalence ratio of 1.3 was investigated for comparison. A high K(g) absorbance was observed in gasification, requiring the photofragmentation signals from KOH(g) and KCl(g) to be recorded at a fixed detuning of 7.3 cm−1 from the center of the K(g) absorption profile. In combustion, the fragments recombined instantly after the UV pulse within around 10 µs, whereas in gasification, the K(g) fragment concentration first increased further for 30 µs after the UV pulse, before slowly decaying for up to hundreds of µs. According to 0D reaction kinetics simulations, this could be explained by a difference in recombination kinetics, which is dominated by oxygen reactions in combustion and by hydrogen reactions in gasification. The K species concentrations in the EFG were stable on average, but periodic short-term variations due to fuel feeding were observed, as well as a gradual increase in KOH(g) over the day as the reactor approached global equilibrium. A comparison of the average K species concentrations towards the end of each experiment showed a higher total K in the gas phase for FR/WS, with higher K(g) and KCl(g), but lower KOH(g), compared to the FR fuel. The measured values were in reasonable agreement with predictions by thermodynamic equilibrium calculations.

Phase transition of potassium atoms on silicene- (3×3)/Ag(111)

Potassium (K) atoms are predicted to form a stable and ordered structure on silicene with high coverage. However, not much is known about the atomic structures of K atoms on silicene/Ag(111). Here, the adsorption of K atoms on monolayer silicene-$(3\ifmmode\times\else\texttimes\fi{}3)/\mathrm{Ag}(111)$ has been studied by combined scanning tunneling microscopy and density functional theory (DFT) calculations. An intriguing structural phase transition from a dispersed phase to an ordered close-packed phase is found with the increasing of K density. DFT calculations reveal three metastable bridge adsorption sites for K on silicene-$(3\ifmmode\times\else\texttimes\fi{}3)$. K atoms show high mobilities in both the dispersed phase and around the vacancy defects or domain boundaries in the ordered phase, indicating a weak interaction between K and silicene-$(3\ifmmode\times\else\texttimes\fi{}3)$. A nearly $\mathsf{V}$-shaped density of states has been found in scanning tunneling spectroscopy obtained on the ordered K/silicene-$(3\ifmmode\times\else\texttimes\fi{}3)$, which is suggested to be the Dirac signature of silicene-$(3\ifmmode\times\else\texttimes\fi{}3)$ by electronic band structure calculations. This study not only reveals the adsorption mechanism of K atoms on silicene, but also provides an approach to study the adsorption of alkali-metal atoms on other graphenelike two-dimensional materials.

Insight into KOH and KCl release behavior of burning wood and straw pellets using quantitative in situ optical measurements

Two-dimensional laser-induced photofragmentation fluorescence (LIPF) was employed to quantitatively visualize the potassium hydroxide (KOH) and potassium chloride (KCl) vapor in the plume above burning wood and straw pellets. In the LIPF measurement, two excitation lasers at 266 and 193 nm were adopted to discriminate KOH and KCl. Meanwhile, tunable diode laser absorption spectroscopy (TDLAS), laser-induced breakdown spectroscopy (LIBS) and two-color pyrometry were used to measure the atomic potassium concentration, total elemental potassium concentration and surface temperature of the burning pellets, respectively. The combustion environment had a temperature of 1550 K and an oxygen concentration of 4.6 vol.%. Two peaks were observed from the temporal potassium release profile of the burning wood, corresponding to the devolatilization and char oxidation stage, while only a single release peak was observed from the burning straw attributed to its high ash content. During the char oxidation and ash cooking stages, KOH was observed to be the dominant potassium species released from the wood, while only KCl was observed for the straw which had a high content of chlorine. About 45% of the total potassium in the wood samples and about 10% in the straw samples were measured to be released during the combustion process. The high content of silicon in the straw retained a considerable amount of potassium in the ash. The wood had the potassium release mainly in the char oxidation stage (∼53% of the total release), while the straw had the main release during the ash cooking stage (∼49% of the total release). During the char oxidation and ash cooking stages, about 32% of Cl was released from the straw pellets in KCl, while the other part of Cl was considered to be released during the devolatilization stage in other Cl species form, such as HCl.