Amorphization Reaction Research Articles

Formation of Interface Layers between Corundum‐Based Refractory Ceramics with Carbon Additions and Molten 42CrMo4 Steel

The formation of reaction layers between corundum‐based (Al2O3 + C) filter materials covered with functional Al2O3C coatings having different carbon contents and molten steel 42CrMo4 is investigated at the reaction temperature of 1600 °C. Within the first 60 s, an amorphous reaction layer is formed at the interface between the original filter surface and the molten steel. This layer consists mainly of Fe and O with small, varying amounts of Al and Si. After a longer contact time (120 s), nanocrystalline phases, which are identified as iron‐based oxides, formed within this amorphous interlayer. After 5 min, the amorphous interface layer is replaced by a dense corundum layer, on which thin corundum platelets are attached. The formation of the amorphous interface layer, its transformation to dense corundum, and the effect of the carbon content in the functional Al2O3C coating on the nucleation and growth of the corundum platelets are discussed. The reaction experiments are carried out in a spark plasma sintering device to avoid liquid metal convection and as immersion tests in a steel casting simulator to obtain relevant information about the filtering process.

The role of nanoscale aggregation of ferrihydrite and amorphous silica in the natural attenuation of contaminant metals at mill tailings sites

Ferrihydrite, a hydrous ferric oxide, is ubiquitous in silica-rich mine drainage sites, although some uncertaity remains concerning its nanoscale texture and colloidal properties such as its association with amorphous silica and reactions with contaminant metals. To understand the mechanisms of natural attenuation of contaminants governed by ferrihydrite and amorphous silica aggregations, mine tailings material from the Ningyo-toge uranium deposit, Japan, has been investigated in detail utilizing many traditional solution and mass spectroscopy methods, as well as synchrotron-based methods the latest in FIB- and analytic TEM-based technologies. At this site, the past mining activity results in slightly acidic mine drainage and the contamination of a mill tailings pond with relatively high concentrations of toxic elements, including U, As, and Ra. We have systematically investigated the geochemistry of water, colloids, and sediments from the groundwater draining the former mine site (now remediated), down gradient to the mill tailings pond where the water pH remains ∼6.2. Groundwater with 0.18 mg/L dissolved oxygen (DO) is oxygenated rapidly to levels of 1.65 mg/L DO when exposed to the atmosphere along this flow path. Dissolved Fe2+ is oxidized instantly to form ferrihydrite nanoparticle aggregates > 200 nm in diameter, adsorbing ∼67% of the total As (3.36 µmol/L) in solution. For solutions and particles/aggregates passing through 200-nm filters, ∼17% of the 1.17 µmol/L As is sorbed on these smaller ferrihydrite aggregates. Simultaneously, varying amounts of amorphous silica with particle sizes as small as several nanometers were observed to adsorb onto ferrihydrite aggregates. Arsenic is present predominantly as arsenate, As(V), and preferentially adsorbs to ferrihydrite within these ferrihydrite–amorphous silica aggregates. Arsenate adsorption is not inhibited by silica adsoption on ferrihydrite when the Si/Fe composition is ∼0.25. Uranium is also adsorbed onto aggregates of colloids. After 24 h of static exposure to the ambient atmosphere, the zeta potential of the aggregate that formed in oxygenated water decreases from −16 to −24 mV, near the zeta potential of pure silica, potentially leading to increased colloidal stability. Some large-sized aggregates are deposited at the bottom of the collecting well and slowly adsorb minor amounts of Mn (1.45 wt.%); other aggregates are transported to the mill tailings pond downsteam, where the water is further oxidized (DO = 3.09 mg/L), and deposited onto the sediment together with birnessite, a hydrated Mn-oxide that appears to be biogenic in this case. During sedimentation, associations of amorphous silica within the aggregate play a key role in retaining the ferrihydrite structure that adsorbs toxic elements such as As. Consequently, the mechanism of natural attenuation at the nanoscale constrained by the chemical and physical properties of ferrihydrite–amorphous silica aggregates is of critical importance in the Ningyo-toge mill tailing site. It is shown that it is also important in other mine drainage and mill tailings sites due to the ubiquitous and dominant occurrence of both Si and Fe in these types of settings.

Understanding the Stability and Recrystallization Behavior of Amorphous Zinc Phosphate

Zinc phosphate, an important pigment in phosphate conversion coatings, forms protective films on rubbing surfaces. We have simulated the underlying reactions under shear by ball-milling zinc phosphate and monitored the reaction of hopeite (Zn3(PO4)2·4H2O) and the retarded recrystallization of the amorphous reaction product by powder X-ray diffraction (PXRD) and quantitative infrared (IR) spectroscopy. Abrasion of stainless steel was simulated by addition of pure 57Fe. The results provide insight into the chemistry of phosphate conversion coatings or during battery cycling of metal phosphates and give theoretical guidance for the preparation of amorphous phosphates. Thermal analysis revealed that the release of structural water is a key step during the reaction of hopeite under shear to ball-milled amorphous zinc phosphate. The back-reaction and associated recrystallization kinetics of amorphous zinc phosphate show a classical Langmuir behavior. Fe impurities inhibit the recrystallization of ball-milled amorphous zinc phosphate strongly. 57Fe Mössbauer spectroscopy and PXRD revealed that Fe is oxidized to Fe2+ and Fe3+ during ball-milling and incorporated locally at the tetrahedral and octahedral sites of the structure. Ball-milled amorphous zinc phosphate is metastable as γ-Zn3–xFex(PO4)2. EPR studies showed the incorporation of Fe3+ to be coupled with the formation of Zn2+ vacancies. The Fe3+ defect sites bind water because of their higher Pearson hardness (compared to Fe2+ and Zn2+), thereby reducing water mobility and inhibiting further reactions like the recrystallization to hopeite. Our findings reveal the amorphization mechanism of Zn3(PO4)2·4H2O in stainless steel ball mills at the atomic scale and highlight how the reactivity of amorphous products is affected by impurities associated with the processing method.

Effect and Progress of the Amorphization Process for Microscale Silicon Particles Under Partial Lithiation As Active Material in Lithium-Ion Batteries

Silicon electrodes in lithium-ion batteries undergo large volume changes during (de-)lithiation.[1] This leads to severe morphological changes over extended charge/discharge cycles, resulting in particle pulverization and surface area increase concomitant with a continuous growth of the solid-electrolyte-interphase (SEI).[2] To address this issue and to improve cycle-life, various mitigation strategies have been used in the literature, such as the use of nanostructured particles of silicon.[3] While this reduces particle pulverization, the high surface area of nanostructured silicon leads to high initial losses due to SEI formation. In this work, a concept presented by Jantke et al. is investigated using micro-scale silicon particles operated under capacity-limited conditions (i.e., partial utilization of its capacity) in order to diminish the above described phenomena.[4] Since crystalline silicon is irreversibly transformed into an amorphous phase upon lithiation, the purpose of the partial capacity utilization is to maintain a crystalline core and thus to prevent a full particle pulverization.In the present study, we investigate the amorphization process of micro-scale silicon particles in a silicon dominant anode containing 70 %wt silicon over extended charge/discharge cycling in half-cells with a lithium reference electrode, varying the lower cutoff potential of the Si electrode. While the capacity of the Si electrode after formation remains constant for cutoff potentials of ≥170 mV vs. Li+/Li (see green line in Fig. 1) of the lithiation step, the capacity continuously increases over cycling for cutoff potentials of <170 mV vs. Li+/Li (see purple line in Fig. 1), implying an ongoing amorphization of the crystalline core (cycling was conducted at C/5 with a CV step at the end of lithiation until a current of C/20, C-rates a referenced to the areal capacity of 1.0 mAh/cm2).[5] Interestingly, relaxation experiments show that the open circuit voltage (OCV) of the lithiation- and therefore amorphization reaction for crystalline silicon during the formation cycle is also observed at 170 mV vs. Li+/Li. We further validate the change of the amorphous/crystalline phase fraction over cycling by in-situ XRD measurements of Si/Li pouch-cells with a lithium reference electrode. Generating a specific amount of amorphous silicon during formation is achieved by using a defined capacity for subsequent cycling, which prevents the potential of the Si anode from dropping below the critical potential, and thus maintains the crystalline core. Finally, the cycling stability in full-cells with Si anodes and NMC622 cathodes will be presented. Our results offer insights into the effect and the progress of the silicon amorphization process when Si anodes are operated under capacity-limited conditions in order to avoid silicon particle pulverization and to enhance the performance and lifetime of silicon-based anodes. N. Obrovac, V. L. Chevrier, Chemical Reviews, 114, 11444 (2014).Wetjen, S. Solchenbach, D. Pritzl, J. Hou, V. Tileli, H. A. Gasteiger, J. Electrochem Soc., 165, A1503 (2018).Goriparti, E. Miele, F. De Angelis, E. Di Fabrizio, R. Proietti Zaccaria, C. Capiglia, Journal of Power Sources, 257, 421 (2014).Jantke, R. Bernhard, E. Hanelt, T. Buhrmester, J. Pfeiffer, S. Haufe, J. Electrochem Soc., 166, A3881 (2019).N. Obrovac, L. J. Krause, J. Electrochem Soc., 154, A103 (2007). Figure 1 Delithiation capacity of Si//Li pouch-cells (loading: 1.7 mgSi/cm2; utilized areal capacity: 1.0 mAh/cm2) with a lower cutoff limit for lithiation of either 140 mV vs. Li+/Li (purple) or 170 mV vs. Li+/Li (green). Formation cycles were conducted by cycling at C/10 (with constant-current (CC) lithiation and delithiation), while subsequent cycling was conducted at C/5 with a CV step at the end of lithiation until a current of C/20 (C-rates are referenced to the utilized areal capacity). Experiments were conducted at 25°C with 1M LiPF6 in FEC/DEC (2:8 v:v) and with two glassfiber separators (VWR). Figure 1

Upcycling of natural volcanic resources for geopolymer: Comparative study on synthesis, reaction mechanism and rheological behavior

The present study prepared geopolymers based on two kinds of volcanic ash, which were mined from Shanxi Province (central China) and Jinlin Province (northeast-China) and denoted as S1 and S2, respectively. The two different volcanic ash were characterized by quantitative X-Ray Diffraction (XRD), X-ray fluorescence (XRF), and particle size analysis. The geopolymer pastes were obtained with the two volcanic ash and 6, 8, and 10 mol/L of sodium hydroxide solution. The mechanical and rheological properties were tested, and the results were correlated with microstructural analysis performed by scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS), Fourier Transform Infrared Spectrometer (FTIR Spectrometer), and 27Al magic angle spinning-nuclear magnetic resonance (27Al MAS-NMR). The results showed that the two kinds of volcanic ash have similar chemical composition while S2 has a higher content of the amorphous phase and smaller median particle size compared with that of S1. S2 has higher geopolymerization activity than S1. The maximum 72-hour compressive strength of the S1 and S2 geopolymer 7.8 MPa and 44.8 MPa, respectively, was acquired when 10 mol/L and 8 mol/L NaOH solution were used. It can be concluded from the results of microscopic experiments that the less content of the amorphous phase, coarser particles, and side reaction between unreacted NaOH and CO2 in the air are the main causes of the lower mechanical strength of S1-based geopolymer. The rheological experiment showed the flow curves of both types of volcanic ash conform to the Bingham model. The yield stress and plastic viscosity coefficient of S2 are higher due to the finer particle size.

Monitoring a Mechanochemical Reaction Reveals the Formation of a New ACC Defect Variant Containing the HCO3– Anion Encapsulated by an Amorphous Matrix

Amorphous calcium carbonate (ACC) is an important precursor in the biomineralization of crystalline CaCO3. In nature, it serves as a storage material or as a permanent structural element, whose lifetime is regulated by an organic matrix. The relevance of ACC in materials science is primarily related to our understanding of CaCO3 crystallization pathways and CaCO3/(bio)polymer nanocomposites. ACC can be synthesized by liquid–liquid phase separation, and it is typically stabilized with macromolecules. We have prepared ACC by milling calcite in a planetary ball mill. Phosphate “impurities” were added in the form of monetite (CaHPO4) to substitute the carbonate anions, thereby stabilizing ACC by substitutional disorder. The phosphate anions do not simply replace the carbonate anions. They undergo shear-driven acid/base and condensation reactions, where stoichiometric (10%) phosphate contents are required for the amorphization to be complete. The phosphate anions generate a strained network that hinders ACC recrystallization kinetically. The amorphization reaction and the structure of BM-ACC were studied by quantitative Fourier transform infrared spectroscopy and solid state 31P, 13C, and 1H magic angle spinning nuclear magnetic resonance spectroscopy, which are highly sensitive to symmetry changes of the local environment. In the first—and fast—reaction step, the CO32– anions are protonated by the HPO42– groups. The formation of unprecedented hydrogen carbonate (HCO3–) and orthophosphate anions appears to be the driving force of the reaction, because the phosphate group has a higher Coulomb energy and the tetrahedral PO43– unit can fill space more efficiently. In a competing second—and slow—reaction step, pyrophosphate anions are formed in a condensation reaction. No pyrophosphates are formed at higher carbonate contents. High strain leads to such a large energy barrier that any reaction is suppressed. Our findings aid in the understanding of the mechanochemical amorphization of calcium carbonate and emphasize the effect of impurities for the stabilization of the amorphous phases in general. Our approach allowed the synthesis of new amorphous alkaline earth defect variants containing the unique HCO3– anion. Our approach outlines a general strategy to obtain new amorphous solids for a variety of carbonate/phosphate systems that offer promise as biomaterials for bone regeneration.

Characterization of amorphous and crystalline ASR products formed in concrete aggregates

Amorphous and crystalline alkali silica reaction (ASR) products formed in aggregates of two different concrete mixtures exposed to the concrete prim test both at 38 °C and 60 °C have been analysed by scanning electron microscope with energy dispersive X-ray spectroscopy and by Raman microscopy. Additionally, amorphous ASR products were synthesized and analysed with Raman microscopy and 29Si nuclear magnetic resonance. Amorphous ASR products display a higher Na/K-ratio than crystalline ones. Both types of products display a structure dominated by Q3-sites (Si-tetrahedra with three bridging oxygen atoms typical for a layer structure) with a secondary amount of Q2-sites (Si-tetrahedra with two bridging oxygen atoms typical for a chain structure). Temperature in the CPT alters the structure of the crystalline ASR. While the Raman spectra of the product formed at 38 °C is identical to the one formed in concrete structures, the one of the 60 °C product corresponds to K-shlykovite.

Spinel-structured high entropy oxide (FeCoNiCrMn)3O4 as anode towards superior lithium storage performance

High entropy oxide (HEO) is a new-type inorganic material composed of multiple principle metal elements with a single-phase structure and is proved to display many potential unexpected properties such as high structural stability and superionic conductivity. Herein, a novel spinel-structured high entropy oxide (FeCoNiCrMn)3O4 is prepared by high-temperature solid state reaction and evaluated as anode for lithium-ion batteries (LIBs). In-situ high-temperature X-ray diffraction (HT-XRD) is used to reveal structure evolution of mixed oxides with the calcination temperature increase and a single-phase spinel-structured (FeCoNiCrMn)3O4 is obtained at 900 °C. The effect of temperature on structure and electrochemical performance of HEO were investigated, and the HEO-900 anode with commercial mass loading exerts higher capacity (discharge/charge, 1034/680 mAh g−1) and better rate capability (182 mAh g−1 at 2 Ag-1) than HEO-950 and HEO-1000 for its moderate particle size, and all the three samples show excellent cycling stability. Ex-situ XRD and transmission electron microscope are applied to unravel the lithium-storage mechanism of (FeCoNiCrMn)3O4, an amorphization reaction process occurs during the initial discharging and the amorphous structure is maintained in subsequent cycles. The synergetic effect of multiple metal cations with different radius, valence states and reaction potentials and entropy stabilization effect make the HEO display a superior electrochemical performance in LIBs. This work provides a new concept to design multi-element transition metal oxide anode materials by high entropy strategy.

Amorphous versus Crystalline in Water Oxidation Catalysis: A Case Study of NiFe Alloy.

Catalytic water splitting driven by renewable electricity offers a promising strategy to produce molecular hydrogen, but its efficiency is severely restricted by the sluggish kinetics of the anodic water oxidation reaction. Amorphous catalysts are reported to show better activities of water oxidation than their crystalline counterparts, but little is known about the underlying origin, which retards the development of high-performance amorphous oxygen evolution reaction catalysts. Herein, on the basis of cyclic voltammetry, electrochemical impedance spectroscopy, isotope labeling, and in situ X-ray absorption spectroscopy studies, we demonstrate that an amorphous catalyst can be electrochemically activated to expose active sites in the bulk thanks to the short-range order of the amorphous structure, which greatly increases the number of active sites and thus improves the electrocatalytic activity of the amorphous catalyst in water oxidation.

Quantitative phase analysis of slag hydrating in an alkaline environment

An X-ray diffraction (XRD)-based evaluation of the crystalline and amorphous phases in slag hydrating in an alkaline environment is presented. A method is developed for the quantification of the amorphous phases present in hydrating slag in a sodium hydroxide solution. In hydrating slag, the amorphous reaction product is identified as calcium aluminosilicate hydrate. A water-soluble sodium-based amorphous reaction product is also produced. The XRD-based quantification method relies on the direct decomposition of the XRD intensity pattern of the total amorphous phase present in partially hydrated slag into the intensity patterns of the amorphous unreacted slag, the hydrate and the sodium-based product. The unreacted slag content in partially hydrated slag is also determined from the decomposition of the intensity signature of the total amorphous phase. An independent verification of the amorphous unreacted slag content in hydrating slag is obtained from measurements of blends of unhydrated and partially hydrated slag. The XRD-based phase-quantification procedure developed here provides a basis for evaluating the extent of reaction in hydrating slag.

Reversible Li Insertion in Guest Free Type II Si Clathrates for Li-Ion Batteries

Stable intercalation anodes for Li-ion batteries are comprised of two groups: graphite and Ti/Nb oxides. Graphite electrodes are reversible but can’t be operated at high current rates because of particle fracture and dendrite formation as a result of volume expansion and a low reaction potential (0.1 V). Li4Ti5O12 is a popular insertion anode that demonstrates good stability (zero strain) and fast charging but its high reaction voltage (1.55 V) lowers the energy density of the full cell. A crystal framework that allows low strain Li-insertion at a lower potential (i.e. 0.2 -1.0 V) could allow for fast charging with less propensity for dendrite formation while also increasing the energy density of the cell. Intermetallic compounds have reaction voltages with Li between 0.2 V – 1.0 V, which makes them interesting candidates for insertion anodes. However, Li alloying accompanied by a large volume expansion is common in these materials (e.g. Si, Ge, Sn) so host frameworks that allow bulk Li insertion into the structure before a Li alloying reaction are needed. Intermetallic clathrates are crystal structures that are comprised of a group IV framework of cages which host alkali guest atoms in the center of the cages (e.g. Ba8Si46, K8Ge44). This structure type has led to many interesting materials properties such as thermoelectricity, superconductivity, hydrogen storage, and tunable optical properties. Recently, our group has been studying the electrochemical reactions of clathrates with Li to understand how the defects and characteristics of clathrates affect the electrochemical properties and if reversible Li insertion into the crystal structure is possible. Our previous work on Type I clathrates with filled cages indicate that guest atoms are frustrating Li mobility and that bulk Li diffusion between cages requires guest atom vacancies1,2. Type II Na-Si clathrates (NaxSi136 (0< x < 24)), composed of Si20 and Si28 cages (Figure 1a), are well known for having tunable guest atom occupancy based on the thermal treatment. Previous work has proven with nuclear magnetic resonance spectroscopy that Li can be inserted into the empty clathrate cages of Si136 when the guest atom occupancy is low (x = 0-1)3. This insertion process manifests as a potential plateau at 300 mV prior to the amorphization of the Si lattice. Lithiation of the clathrate cages has been demonstrated but delithiation and cycling of Si136 has not been investigated. In addition, the Li pathways and positions in the lithiated Si136 structure have not been concluded. In this work, the synthesis and electrochemical and structural characterization of guest free type II Si clathrate is reported. The type II Si clathrates (Si136) were synthesized via thermal decomposition of Na4Si4 under vacuum and then characterized with synchrotron powder XRD and transmission electron microscopy to confirm the low Na content. By applying a voltage cutoff (~260 mV) prior to the onset of the amorphization reaction, reversible Li insertion is demonstrated for the first time (Figure 1b). The delithiation shows uniquely asymmetric behavior indicating a different delithiation mechanism than from the lithiation. The Si136 structure can insert ~32 Li before beginning the Li alloying reaction which corresponds to a capacity of around 220 mAh/g (470 Ah/L). When cycled with a voltage cutoff prior to the Li alloying reaction, the electrode demonstrates a repeatable electrochemical profile demonstrating the reversible Li chemistry. In addition, we find that the capacity depends sensitively on the voltage cutoff used (Figure 1c). After 50 cycles, the electrode maintains the clathrate crystal structure, evidence of a reversible insertion reaction. Ex-situ XRD of the fully lithiated clathrate (Figure 1d), shows that the clathrate structure has a very low volume expansion of around 0.1%, indicating that the lithiation process could be considered zero-strain. Synchrotron powder XRD and X-ray pair distribution function analysis is used to confirm the positions of Li during insertion and elucidate the lithiation mechanism. Since the Li insertion voltage is 300 mV for Si136, the risk of dendrite formation at higher current rates could be reduced while maintaining higher energy density cell due to a low reaction voltage. These results are interesting for the future design of insertion anodes that maintain a higher full cell energy density similar to that of graphite while still providing rapid and reversible Li insertion. Zhao, R. et al. ACS Appl. Mater. Interfaces 9, 41246–41257 (2017).Dopilka, A. et al. ACS Appl. Mater. Interfaces acsami.8b11509 (2018). doi:10.1021/acsami.8b11509Langer, T. et al. J. Electrochem. Soc. 159159, 1318–1322 (2012). Figure 1

A novel approach for mechanical alloying amorphisation in magnetic Fe–Co alloy system

The aim of this work is to investigate the possibility to extend the formation of amorphous phase in Fe–Co based system without compromising their magnetic properties. For this purpose, a novel approach was used in which different elements were added during mechanical alloying (MA) with a specific trend. Elements such as Ti, Al, Si, B, P and C enhance amorphisation of iron-based alloy by mechanical alloying (MA) based on thermodynamic and kinetic considerations. In contrast, Co obstructs the amorphisation reaction by MA. Therefore to obtain fully amorphous structure in Fe–Co magnetic alloy chronological addition of elements was applied. In first step, (Fe50Ti25Al25)70 was ball milled to obtain (Fe, Ti)3Al intermetallic compound, then, (B15P7C6Si2)30 was added to (Fe, Ti)3Al and an amorphous-nanocrystalline state was achieved. Finally 20 at% Co was added to incorporate Co in the amorphous alloy through continued ball milling for 60 h. In this way a full amorphous structure was achieved in (Fe50Ti25Al25)70B15P7C6Si2–Co20 composition with superior magnetic properties. Structural analysis verified experimentally with x-ray diffraction (XRD), energy-dispersive x-ray spectroscopy (EDS) mapping and transmission electron microscopy (TEM). Further, magnetic properties was investigated by vibrant sample magnetometer (VSM).

Impurity-enhanced solid-state amorphization: the Ni–Si thin film reaction altered by nitrogen

Solid-state amorphization, the growth of an amorphous phase during annealing, has been studied in a wide variety of thin film structures. Whereas research on the remarkable growth of such a metastable phase has mostly focused on strictly binary systems, far less is known about the influence of impurities on such reactions. In this paper, the influence of nitrogen, introduced via ion implantation, is studied on the solid-state amorphization reaction of thin (35 nm) Ni films with Si, using in situ x-ray diffraction (XRD), ex situ Rutherford backscattering spectrometry, XTEM, and synchrotron XRD. It is shown that due to small amounts of nitrogen (<2 at.%), an amorphous Ni–Si phase grows almost an order of magnitude thicker during annealing than for unimplanted samples. Nitrogen hinders the nucleation of the first crystalline phases, leading to a new reaction path: the formation of the metal-rich crystalline silicides is suppressed in favour of an amorphous Ni–Si alloy; during a brief temperature window between 330 and 350 °C, the entire film is converted to an amorphous phase. The first crystalline structure to grow is the orthorhombic NiSi phase. We demonstrate that this impurity-enchanced solid-state amorphization reaction occurs only under specific implantation conditions. In particular, the initial distribution of nitrogen upon implantation is crucial: sufficient nitrogen impurities must be present at the interface throughout the reaction. Introducing implantation damage without nitrogen impurities (e.g. by implanting a noble gas) does not cause the enhanced solid-state amorphization reaction. Moreover, we show that the stabilizing effect of nitrogen on amorphous Ni–Si films (with a composition ranging from 40% to 50% Si) is not restricted to thin film reactions, but is a general feature of the Ni–Si system.

Effect of active components on strength development in alkali-activated low calcium fly ash cements

The compressive strength achieved in alkali-activated low-calcium fly ash depends on the total reactive oxide ratios in the activated system, the reactive alumina content in fly ash and the initial molarity of NaOH. The total reactive silica in the system is the reactive silica contributed by the fly ash and soluble silica from the alkaline solution. A minimum molarity of NaOH is required to ensure complete dissolution of the glassy phase present in fly ash. The maximum compressive strength depends on the amorphous reaction product content formed, which is determined by the reactive alumina content in the fly ash. The highest compressive strength is achieved for the ratio of the total reactive silica to sodium ratio in the activated system equal to 4.72 and lower. More Na is incorporated in the reaction product on increasing the Na2O content relative to the reactive oxides.

Mechanochemistry allows carrying out sensitive organometallic reactions in air: glove-box-and-Schlenk-line-free synthesis of oxidative addition complexes from aryl halides and palladium(0).

Organic reactions that employ moisture- and/or oxygen-sensitive reagents or intermediates usually involve the use of glove-box or Schlenk-line techniques as well as dry and degassed solvents. Unfortunately, these requirements may greatly reduce the utility of the targeted organic molecules. Herein, we demonstrate that solvent-free mechanochemical synthetic techniques allow using highly oxygen-sensitive palladium(0) species in air for the stoichiometric oxidative addition of aryl halides. The low diffusion efficiency of gaseous oxygen in crystalline or amorphous solid-state reaction mixtures should be the main reason for the low impact of the presence of atmospheric oxygen on the sensitive oxidative addition reactions under the applied conditions. This study thus illustrates the outstanding potential of mechanochemistry to serve as an operationally simple, glove-box-and-Schlenk-line-free synthetic route to organometallic compounds and other valuable synthetic targets, even when sensitive reagents or intermediates are involved.

Interdiffusion in as-deposited Ni/Ti multilayer thin films analyzed by atom probe tomography

The nanoscale design of metallic multilayer thin films is one crucial factor that greatly influences the kinetics, often inducing unusual phase transformations. Metastable or amorphous phases may directly form in as-deposited films of certain thicknesses, which is common for Ni/Ti multilayers. Atom probe tomography and X-ray diffraction analyses are performed here to study the interdiffusion and structural changes as a function of the bilayer thickness in Ni/Ti multilayers. The films are deposited by DC magnetron sputtering with near 50:50 compositions. Multilayers with 5 nm bilayer thickness are found to be highly intermixed, with compositions up to ∼25 at.% for both diffuser metals, inducing amorphization reactions during deposition.

Influence of processing temperature on the reaction product and strength gain in alkali-activated fly ash

The roles of temperature and curing periods on product formation and strength gain in alkali activation of low-calcium siliceous fly ash are evaluated. Higher temperature favors the kinetics of amorphous reaction product formation and inclusion of Si in the reaction product, which accompanies the densification of the microstructure. Following initial curing at 60 °C, the reaction product continues to form in the activated system on decreasing the temperature to 25 °C. Initial curing at 25 °C does not lead to formation of reaction product. Reaction product forms in the system for initial curing temperature of 40 °C and higher. The rate of the solid-based reaction, after hardening, is increased by temperature. The total amorphous reaction product content formed in the system depends on the water to solids ratio of the activated mix. The amorphous product composition, however, varies with temperature. With time there is an increase in the Si/Al ratio in the reaction product formed 40 °C and higher. For initial curing at 60 °C, there is no increase in the Si/Al ratio in the reaction product when the temperature is decreased to 25 °C. Reaction product with a higher Si/Al atomic ratio achieves a higher ultimate compressive strength. On increasing the Si/Al ratio in the reaction product from 2.5 to 3.3, the ultimate compressive strength achieved increased from 60 MPa to 74 MPa. Early curing, immediately after mixing, at a higher temperature is critical in achieving a higher Si/Al atomic ratio in the reaction product. Optimum initial curing time at high temperature is based on achieving a high Si/Al atomic ratio in the reaction product.

Preparation and characterization of geopolymers based on a phosphoric-acid-activated electrolytic manganese dioxide residue

Electrolytic manganese dioxide residue (EMDR) is a type of industrial solid waste that contains many soluble heavy metal ions. To reduce the hazard to environment, EMDR was proposed as raw material to fabricate phosphoric-acid-activated geopolymers in the present work. Relevant characteristics of these materials such as the compressive strength, bulk density, crystalline components, microstructure, leaching characteristic, high temperature resistance and corrosion resistance were investigated in this work. The results showed that EMDR was highly reactive with phosphoric acid and could be cured to form geopolymer gel materials with compressive strength up to 96.3 MPa after curing for 2 days at 80 °C with river sand added as an aggregate. Based on the analysis of X-ray diffraction and microstructure, the high compressive strength was due to the amorphous reaction products of phosphoric acid and magnetite in EMDR. The results of the leaching test of the geopolymer samples from EMDR showed that the manganese stabilization efficiency was 95.4%, and other measured elemental concentrations were all within the limits of Integrated Wastewater Discharge Standard in China when the CaO to wet EMDR ratio was 0.10. In addition, the geopolymers from EMDR showed good resistance to high temperature and aggressive environment except strong acid. Therefore, the results obtained in this study reveal that this method for the fabrication of geopolymers is a promising way to recycle EMDR and other iron-rich industrial solid wastes.

Separation of arsenic and antimony from dust with high content of arsenic by a selective sulfidation roasting process using sulfur

The separation of arsenic and antimony from dust with high content of arsenic was conducted via a selective sulfidation roasting process. The factors such as roasting temperature, roasting time, sulfur content and nitrogen flow rate were investigated using XRD, EPMA and SEM–EDS. In a certain range, the sulfur addition has an active effect on the arsenic volatilization because the solid solution phase ((Sb,As)2O3) in the dust can be destroyed after the Sb component in it being vulcanized to Sb2S3 and this generated As2O3 continues to volatile. In addition, an amorphization reaction between As2O3 and Sb2O3 is hindered through the sulfidation of Sb2O3, which is also beneficial to increasing arsenic volatilization rate. The results show that volatilization rates of arsenic and antimony reach 95.36% and only 9.07%, respectively, under the optimum condition of roasting temperature of 350 °C, roasting time of 90 min, sulfur content of 22% and N2 flow rate of 70 mL/min. In addition, the antimony in the residues can be reclaimed through a reverberatory process.

Kinetics of Silver Photodiffusion Into Amorphous Ge20S80 Films: Case of Pre‐Reaction

Silver photodiffusion into amorphous chalcogenide has attracted much attention because of its potential applications for example in memory devices. For its development, it is important to know how Ag ions diffuse in chalcogenide layers upon light exposure. In this paper, the photo‐induced effect on “pre‐reacted” films before light exposure, originating from Ag/Ge20S80/Si substrate and Ge20S80/Ag/Si substrate is investigated, using neutron reflectivity, X‐ray reflectivity, and X‐ray diffraction. Two types of “pre‐reacted” films according to the original stacking order are obtained. In both cases, a pure Ag layer almost disappeared, and there is a small amount of monoclinic Ag2S. The reaction time for the photodiffusion is shorter, in both cases, than that in Ag/Ge20S80 with a pure Ag layer, indicating a different reaction process. After prolonged light exposure, a uniform amorphous reaction layer is produced in the films originating from the Ag/Ge20S80/Si substrate, while both an amorphous reaction product and the Ag2S fragments exist in the films originating from Ge20S80/Ag/Si substrate. The mechanism of the specific photo‐reaction is discussed in terms of the role of the Ag2S fragments.