Alkali Metal Impurities Research Articles

Size-Dependent Isovalent Impurity Doping for Ambipolar Control in Cu3N.

Substitutional doping, involving the replacement of a host with an aliovalent impurity ion, is widely used to attain ambipolar controllability in semiconductors, which is crucial for device application. However, its effectiveness for p-type doping is limited in monovalent cation compounds due to the lack of suitable aliovalent (i.e., zerovalent) impurities. We propose an alternative approach for p- and n-type doping, mediated by the sizes of isovalent alkali metal impurities in Cu(I)-based semiconductors, such as copper nitride with an electron concentration of ∼1015 cm-3. Doping of isovalent Li with a smaller size to interstitial positions improves n-type conductivity, and electron concentration is controllable in the range of 1015 to 1018 cm-3. In contrast, larger isovalent Cs and Rb impurities facilitate p-type conversion, resulting in a hole concentration controllability of 1014 to 1017 cm-3. First-principles calculations indicate that Li is placed as an interstitial impurity acting as a shallow donor in conjunction with the formation of a neutral impurity on Cu defects. As the impurity size increases beyond the capacity of the vacant space, the formation of multiple acceptor-type Cu vacancies is enhanced owing to the repulsion between host Cu+ and Cs+/Rb+ impurities. Consequently, the Cs or Rb impurity is located at the sites of the N accompanied by six neighboring Cu vacancies, forming acceptor defect complexes. This size-dependent isovalent impurity doping scheme opens up an alternative avenue for advancement in optoelectronic devices using monovalent cation-based semiconductors.

The regeneration process of FePO4 in electrochemical lithium extraction: The role of alkali ions

The rapid expansion of lithium battery applications has resulted in a shortage of lithium resources, prompting researchers to focus on the electrochemical extraction of lithium from water resources using FePO4 as the host material. However, a large amount of alkali metal impurity ions in brine leads to irreversible capacity loss, limiting the industrial application of FePO4 materials in lithium extraction. The mechanism of alkali metal ions’ influence on FePO4 materials remains unclear. To address this issue, the quaternary phase diagram of FePO4-LiFePO4-NaFePO4-KFePO4 and the diffusion barriers of lithium/sodium ions in FePO4 were obtained for the first time based on the theoretical calculation of density functional theory (DFT). DFT and X-ray diffraction (XRD) refinement revealed that the inability to remove Na2/3FePO4 from the FePO4 is a critical issue affecting electrode recyclability. An innovative electrode regeneration process was proposed to enhance the lifespan of FePO4 material. The Na2/3FePO4 impurity was converted to LiFePO4 using K2S2O8 abstersion and 0.1 mol/L LiCl lithiation regeneration process. After five rounds of regenerations, the total lithium extraction of the material could still reach 80.18 % of the extraction of the brand-new electrode, demonstrating the multiple reuses of the host material and cost savings. These innovative discoveries can advance the industrial application of electrochemical lithium extraction from FePO4 electrode materials.

High Performance Concretes with Highly Reactive Rice Husk Ash and Silica Fume.

The search for new sources of high-quality non-crystalline silica as a construction material for high-performance concrete has attracted the interest of researchers for several decades. Numerous investigations have shown that highly reactive silica can be produced from rice husk, an agricultural waste that is abundantly available in the world. Among others, the production of rice husk ash (RHA) by chemical washing with hydrochloric acid prior to the controlled combustion process has been reported to provide higher reactivity because such a process removes alkali metal impurities from RHA and provides an amorphous structure with higher surface area. This paper presents an experimental work in which a highly reactive rice husk ash (TRHA) is prepared and evaluated as a replacement for Portland cement in high-performance concretes. The performance of RHA and TRHA was compared with that of conventional silica fume (SF). Experimental results showed that the increase in compressive strength of concrete with TRHA was clearly observed at all ages, generally higher than 20% of the strength obtained with the control concrete. The increase in flexural strength was even more significant, showing that concrete with RHA, TRHA and SF increased by 20%, 46%, and 36%, respectively. Some synergistic effect was observed when polyethylene-polypropylene fiber was used for concrete with TRHA and SF. The chloride ion penetration results also indicated that the use of TRHA had similar performance compared to that of SF. Based on the results of statistical analysis, the performance of TRHA is found to be identical to that of SF. The use of TRHA should be further promoted considering the economic and environmental impact that will be achieved by utilizing agricultural waste.

Hole-Doping to a Cu(I)-Based Semiconductor with an Isovalent Cation: Utilizing a Complex Defect as a Shallow Acceptor.

p-Type doping in Cu(I)-based semiconductors is pivotal for solar cell photoabsorbers and hole transport materials to improve the device performance. Impurity doping is a fundamental technology to overcome the intrinsic limits of hole concentration controlled by native defects. Here, we report that alkali metal impurities are prominent p-type dopants for the Cu(I)-based cation-deficient hole conductors. When the size mismatch with Cu+ in the host lattice is increased, these isovalent impurities are preferentially located at interstitial positions to interact with the constituent Cu cations, forming stable impurity-defect complexes. We demonstrate that the Cs impurity in γ-CuI semiconductors enhances hole concentration controllability for single crystals and thin films in the range of 1013-1019 cm-3. First-principles calculations indicate that the Cs impurity forms impurity-defect complexes that act as shallow acceptors leading to the increased p-type conductivity. This isovalent doping provides an approach for controlled doping into cation-deficient semiconductors through an interaction of impurities with native defects.

Family of biomass-derived Ni and Ni–Mn catalysts of CO2 methanation

CO2 utilization nowadays is one of the most important challenges posed by global warming. CO2 conversion to methane is considered as a stage of the Power-to-X technology that uses green electricity for hydrogen production. Sustainable utilization of agricultural wastes is also a hot topic of the responsible consumption and production. In this work rice husk (RH) was used to produce Ni-based catalysts of CO2 methanation. We suggested different treatments of RH for the synthesis of various catalyst supports. The purity of the support was found to be a crucial parameter determining the catalyst structure, activity, and selectivity. Sulfur, phosphorous, alkali and alkaline-earth metal impurities decreased the CO2 conversion and methane selectivity, while promotion with manganese strongly enhanced the catalyst efficiency. Up to 86 % CO2 conversion and 100 % methane selectivity were observed even at 250 °C over the Mn-promoted Ni-based catalysts supported on amorphous silica and purified RH ash. The stability of the catalyst depended on its specific surface area and chemical composition. The oxide-based supports with the highest specific surface areas were the most stable and active in CO2 conversion. The RH-derived samples showed a promising activity at rather low operation temperatures and can be considered as green, cheap, and stable catalysts for the Power-to-X technology.

Comparative Study of Water-Leaching and Acid-Leaching Pretreatment on the Thermal Stability and Reactivity of Biomass Silica for Viability as a Pozzolanic Additive in Cement.

The present work aims to introduce a novel and eco-friendly method, i.e., a water-leaching pretreatment for extracting highly reactive biomass silica from rice husk (RH), for viability as a pozzolanic additive in cement. For comparison, the traditional acid pretreatment method was also employed throughout the experimental study. The silica from RH was extracted using boiled deionized water and acid solution as leaching agents to remove the alkali metal impurities, and then dried and submitted to pyrolysis treatment. The results indicated that potassium was found to be the major contaminant metal inducing the formation of undesirable black carbon particles and the decrease in crystallization temperature of amorphous RHA silica. The boiling-water-leaching pretreatment and acid-leaching pretreatment on RHs significantly removed the metallic impurities and reduced the crystallization sensitivity of RHA silica to calcination temperature. A highly reactive amorphous silica with purity of 96% was obtained from RH via 1 N hydrochloric acid leaching followed by controlled calcination at 600 °C for 2 h. The acid treatments increased the crystallization temperature of silica to 1200 °C and retained the amorphous state of silica for 2.5 h. In the case of water-leaching pretreatment, leaching duration for 2.5 h could yield an amorphous silica with purity of 94% and render the silica amorphous at 900 °C for 7 h. The RHA silica yielded by water-leaching pretreatment presented a comparable enhancing effect to that of acid leaching on hydration and improved the strength of cement. Furthermore, compared with the acid-leaching method, the water-leaching pretreatment method is more environmentally friendly and easier to operate, and hence more widely available.

Ion-Transport Engineering of Alkaline-Earth Hydrides for Hydride Electrolyte Applications

The heavier alkaline-earth hydrides (AeH2; Ae = Ca, Sr, Ba) are promising materials for hydrogen energy applications, due to their excellent ionic conductivity and thermal stability. We use first-principles calculations to elucidate their defect chemistry and ion-transport mechanisms. On the basis of calculated formation energies, we find that hydrogen vacancies (VH) are present in large concentrations, and hence, transport is vacancy-mediated. The vacancy migration barrier depends strongly on the charge state of the vacancy, with VH+ yielding the lowest barriers. The activation energy for ionic transport, which is the sum of the migration barrier and formation energy, can be further reduced by lowering the formation energy of the vacancies. This goal can be achieved by doping with acceptors, such as alkali-metal impurities, which induces larger concentrations of VH+. We show that, with optimized conditions for doping, ionic conductivities can be improved by several orders of magnitude; for BaH2, the cond...

The Effect of Alkali Metal and Alkaline Earth Metal Impurities on the Iodine-Induced Corrosion of CANDU Fuel Sheathing

During normal operation in Canada deuterium uranium (CANDU®) reactors, the stress corrosion cracking (SCC) of fuel sheathing is mitigated effectively, in part, using a thin graphite-based coating known as CANDU lubricant (CANLUB). Mechanisms typically proposed for the demonstrated SCC mitigation offered by CANLUB include lubrication and/or chemical interactions. An additional possibility, that was recently suggested, involves the sequestering of iodine through its interaction with alkali metal and/or alkaline earth metal impurities in the CANLUB coating. This possibility is supported by the systematic analysis and testing in this paper, wherein three prevalent impurities (Na, Ca, and Mg) found in CANLUB were incorporated into SCC slotted ring experiments as metal oxides. When the amount of metal oxide (Na2O, CaO, or MgO) matched or exceeded the amount of iodine (6 mmol = 16 mg/cm3), Na2O and CaO protected the rings from corrosion whereas MgO enhanced their corrosion. When Zircaloy-4 sheathing is subjected to mechanical stress, high temperature, and high concentrations of iodine vapor, it is better protected by siloxane coatings than by graphite-CANLUB coatings. Consequently, since metal impurities (Na, Ca, and Mg) are found more abundantly in siloxane coatings than in graphite-CANLUB coatings, Zircaloy-4 slotted rings were coated with graphite-CANLUB containing Na, Ca, and/or Mg at those more abundant concentrations. Since these concentrations remain below 6 mmol, SCC test results suggest that the siloxane's superior adhesion is an essential first step in preventing corrosion induced by 6 mmol of iodine.

Effect of Alkali Metal Atom Doping on the CuInSe2-Based Solar Cell Absorber

The efficiency of Cu(In,Ga)Se2 (CIGS)-based solar cells can be markedly improved by controlled introduction of alkali metal (AM) atoms using post-deposition treatment (PDT) after CIGS growth. Previous studies have indicated that AM atoms may act as impurities or agglomerate into secondary phases. To enable further progress, understanding of atomic level processes responsible for these improvements is required. To this end, we have investigated theoretically the effects of the AM elements Li, Na, K, Rb, and Cs on the properties of the parent material CuInSe2. First, the effects of the AM impurities in CuInSe2 have been investigated in terms of formation energies, charge transition levels, and migration energy barriers. We found that AM atoms preferentially substitute for Cu atoms at the neutral charge state. Under In-poor conditions, AM atoms at the In site also show low formation energies and are acceptors. The migration energy barriers show that the interstitial diffusion mechanism may be relevant only f...

Intense pulsed light annealing of copper zinc tin sulfide nanocrystal coatings

A promising method for forming the absorber layer in copper zinc tin sulfide [Cu2ZnSnS4 (CZTS)] thin film solar cells is thermal annealing of coatings cast from dispersions of CZTS nanocrystals. Intense pulsed light (IPL) annealing utilizing xenon flash lamps is a potential high-throughput, low-cost, roll-to-roll manufacturing compatible alternative to thermal annealing in conventional furnaces. The authors studied the effects of flash energy density (3.9–11.6 J/cm2) and number of flashes (1–400) during IPL annealing on the microstructure of CZTS nanocrystal coatings cast on molybdenum-coated soda lime glass substrates (Mo-coated SLG). The annealed coatings exhibited cracks with two distinct linear crack densities, 0.01 and 0.2 μm−1, depending on the flash intensity and total number of flashes. Low density cracking (0.01 μm−1, ∼1 crack per 100 μm) is caused by decomposition of CZTS at the Mo-coating interface. Vapor decomposition products at the interface cause blisters as they escape the coating. Residual decomposition products within the blisters were imaged using confocal Raman spectroscopy. In support of this hypothesis, replacing the Mo-coated SLG substrate with quartz eliminated blistering and low-density cracking. High density cracking is caused by rapid thermal expansion and contraction of the coating constricted on the substrate as it is heated and cooled during IPL annealing. Finite element modeling showed that CZTS coatings on low thermal diffusivity materials (i.e., SLG) underwent significant differential heating with respect to the substrate with rapid rises and falls of the coating temperature as the flash is turned on and off, possibly causing a build-up of tensile stress within the coating prompting cracking. Use of a high thermal diffusivity substrate, such as a molybdenum foil (Mo foil), reduces this differential heating and eliminates the high-density cracking. IPL annealing in presence of sulfur vapor prevented both low- and high-density cracking as well as blistering. However, grain growth was limited even after annealing with 400 flashes. This lack of grain growth is attributed to a difficulty of maintaining high sulfur vapor pressure and absence of alkali metal impurities when Mo foil substrates are used.

Influence of alkali metal impurities on properties of yttrium aluminum garnet doped with cerium

We consider the influence of Li, Na, and K metal dopants on the luminescent characteristics of Y2.97Ce0.03Al5O12 powders. It is shown that the photoluminescence intensity of yttrium aluminum garnet under excitation with blue light with a wavelength of 460 nm decreases with an increase of the content of these metals. The largest effect is observed for Li, the addition of which in amount of 0.05 wt.% leads to a decrease of intensity by 45%. The addition of the indicated metals does not influence the microstructure and the phase composition of the powders.

Photoconductivity of CdS films, undoped and doped with alkali-metal impurity ions

The nonequilibrium electron processes that occur during photoconductivity increase and relaxation in pyrolytic CdS films, both nominally undoped and doped with alkali metals, are studied. The photocurrent increase and relaxation are slow. In some cases, the experimental curve describing the photoconductivity increase is S-shaped or exhibits several thresholds. The dependence of the steady-state photocurrent on the excitation intensity is superlinear. The introduction of alkali-metal impurities increases the photocurrent in the CdS films by up to an order of magnitude compared to the photocurrent in the undoped films. The experimentally observed features of the photoconductivity kinetics of the structures under study are attributed to the profound effect of trapping centers of several types on the behavior of nonequilibrium charge carriers.

Deposition of Corrosive Alkali Salt Vapors on the Blades of Gas Turbines Fueled by Coal-Derived Syngases

Many proposed clean coal technologies for power generation couple a gasification process with a gas turbine combined cycle unit. In the gasifier, the coal is converted into a syngas which is then cleaned and fired before entering the turbine. A problem is that coal-derived syngases may contain alkali metal impurities that combine with the sulfur and chlorine from the coal to form salts that deposit on the turbine blades, causing corrosion. This paper describes a new model, applicable to most types of coal, for predicting the dewpoint temperatures and deposition rates of these sodium and potassium salts. When chlorine is present the main alkali species in the mainstream gas flow are the chlorides; but when chlorine is absent, the superoxides dominate. However, because the high-pressure turbine blades are film-cooled, they are at much lower temperatures than the mainstream gas flow and analysis then shows that the deposit is composed almost entirely of the sulfates in either liquid or solid form. This is true whether or not chlorine is present. Detailed calculations using the new model to predict the alkali salt deposition rates on three stages of an example utility turbine are presented. The calculations show how the dewpoint temperatures and deposition rates vary with the gas-phase chlorine and sulfur levels as well as with the concentrations of sodium and potassium. It is shown that the locations where corrosion is to be expected vary considerably with the type of coal and the levels of impurities present.

Efficiency of generation of optical centers in KS-4V and KU-1 quartz glasses at neutron and gamma irradiation

Pure quartz glasses of KS-4V and KU-1 types are candidates for optical plasma diagnostic system in ITER. The purpose of experiment was to study the efficiency of defect production in these glasses under irradiation with 60Со γ-quanta (5.7Gy/s) dose range of 102–107Gy and the fission reactor neutrons in the fluency range of 1020–1023n/m2 and gammas simulating the plasma influence. In KU-1 (1000ppm OH) the accumulation kinetics of E′-(5.75eV) and NBO-(1.9eV) centers at γ-doses⩾5×105Gy and neutron fluencies <1021n/m2 is faster, than that in KS-4V glasses (<0.1ppm OH) that is caused by rupture of hydrogen bonds. At fluencies >1021n/m2 the NBO accumulation kinetics is slower in KU-1 than in KS-4B, because highly mobile hydrogen atoms access to the generated NBO centers. In KS-4V irradiated to γ-doses102–5×103Gy a new unstable absorption band at 1.8eV was found, which is caused by the glass synthesis conditions and alkali metal impurities. The transparency at 3.5–6.2eV at fluencies 1020–5×1021n/m2 is higher in KS-4V than KU-1. However at fluencies >1021n/m2 in KS-4V the photoluminescence band at 2.7eV is more intensive and distorts a diagnosed signal. The transparency in 3.5–1.2eV at fluencies >1021n/m2 is higher in KU-1 than KS-4V.

Ionization Thresholds of Alkali Metal Atoms on Helium Droplets

Superfluid helium droplets (HeN) have attracted strong interest as cold hosts for the investigation of weakly bound molecules. Whereas the host–dopant interaction is weak for neutral molecules, ion impurities may be surrounded by frozen shells of polarized helium atoms. An extreme example of the different behavior is given by alkali metal impurities that stay at the surface of the droplet but immerse into the droplet as cations releasing a considerable amount of binding energy. We report measurements of the photoionization efficiency for the rubidium-HeN and cesium-HeN systems and find that the ionization threshold is lowered compared with the free atoms. The corresponding energy shift increases when going from heavy to light alkali metals and from small to large helium droplets. Both effects can be explained by the difference in polarization energies associated with submerged alkali metal cations. The findings agree qualitatively well with recent calculations of helium snowball formation around alkali metal cations.

Effect of Alkali Metal Impurities on Co–Re Catalysts for Fischer–Tropsch Synthesis from Biomass-Derived Syngas

The effect of alkali impurities on Co-based Fischer–Tropsch (FT) catalysts is important for processing biomass-derived synthesis gas containing inorganic ash impurities. The effects of Na, K, Li, and Ca impurities on γ-Al2O3-supported Co–Re powder catalysts were studied at impurity loadings between 25 and 1,000 ppm. Impurity addition did not have any effect on H2 chemisorption, but the catalyst activity decreased during FT synthesis experiments. The impurities were also found to slightly increase the reduction temperatures of Co. Carbon selectivities to CH4 decreased with increasing impurity loading, while CO2 and C5+ hydrocarbon selectivities increased. Catalyst behavior was attributed mostly to electronic effects from the alkali addition, leading to decreased surface H concentrations and increased CO adsorption and dissociation. Site-time yield (STY) as a function of metal impurity loading at constant conversion of CO.

Challenges in Developing High Performance Al-Li Alloys

The goal of replacing ‘conventional’ Al alloys with Al-Li alloys to reduce weight of aerospace structures has had only limited success to date, despite one of the largest alloy-development programs ever undertaken. One of the reasons being, for some product forms and crack-plane orientations, fracture toughness is lower compared with conventional Al alloys due to a greater propensity for low-energy intergranular fracture. Proposed explanations for brittle intergranular fracture include: (i) planar slip resulting in high stresses where slip bands impinge on grain-boundaries, (ii) embrittlement due to alkali-metal-impurity phases, and (iii) grain-boundary structural changes associated with segregation of lithium. The present paper reviews evidence for and against these proposed mechanisms based on (i) studies of 8090 and 2090 alloys, and the more recently developed 2297 alloy, and (ii) comparisons of fracture of these alloys with other materials that exhibit grain-boundary weakness. It appears that lithium segregation to grain boundaries is the prime cause of low-energy intergranular fracture in Al-Li alloys. Embrittlement by alkali-metal impurities is not important providing that impurity levels are less than about 5 ppm (as is the case for most commercially produced alloys). Planar slip and other factors probably play only a minor role.

On the application of surface ionization detector for the study of alkali capture by kaolin in a fixed bed reactor

Alkali metals, mainly potassium, together with other ash forming inorganic components in biomass are believed to be responsible for bed agglomeration in fluidized bed boilers burning biomass. A solution to the problem is to capture alkali vapor released from the fuel during combustion by inorganic solid additives such as kaolin. In this study, the capture of vaporous potassium chloride was investigated in a fixed bed reactor equipped with an on-line alkali detector. The detector, which is based on surface ionization, is capable of operating at alkali metal concentrations as low as those encountered after reaction with kaolin during biomass combustion (of the order of 1 ppb). Various experiments of KCl capture by kaolin powder were made at a reactor temperature of 850 °C. Kaolin removed up to 99% of alkali species in gas phase. The efficiency slightly decreased when KCl concentration decreased. The effect of kaolin addition on the release of alkali metals during wood combustion was studied at 650 °C in air. During pyrolysis, the alkali metal release increased slightly when kaolin was mixed with the wood due to the release of alkali metal impurities in the kaolin additive. The alkali metal release during char combustion was reduced to approximately 50% of the original value. The reduction increased as the amount of kaolin addition increased. Overall, the addition of kaolin suppressed the alkali metal release from the wood by approximately 20%.

Formation of Radiation-Induced Defects in Silica Glasses at High Irradiation Temperatures

The mechanisms of formation of radiation-induced defects in silica glasses are investigated under both simultaneous and sequential exposure to γ-radiation and high temperatures. It is demonstrated that silica glasses contain E"(T) centers (T = Si and Al) in impurity-free regions andE"M (T) centers in the vicinity of alkali metal impurities. These centers can undergo transformations. The origin of a new center responsible for the absorption band at 360 nm is elucidated. This center is identified as a twofold-coordinated aluminum atom with a trapped electron.

Photoluminescence in beryllium ceramics

The photoluminescence properties of beryllium ceramic obtained by semidry molding with and without deliberately introduced alkali metal impurities are considered. It is assumed that heat treatment of samples in hydrogen and the introduction of lithium and sodium impurities in small quantities lead to the formation of additional luminescent centers increasing the intensity of photoluminescence. In contrast, large concentrations of the impurity phase suppress photoluminescence. In heterovalent replacement of Be2+ with Li+ or Na+ ions, additional oxygen vacancies are formed, in which electrons can be localized under radiation and F+ centers can be formed.