High-temperature Transformation Research Articles

High-temperature treatment of functional Heusler alloy Ni46Mn41In13 thin foils for microsystem devices and electronics

In this study, the high-temperature transformation in a thin foil of non-stoichiometric Heusler alloy based on Ni-Mn-In is investigated using in-situ TEM. The highly ordered cubic L21 phase undergoes decomposition upon heating, forming a phase with a composition close to Ni75Mn25, which is identified as disordered FCC-Ni3Mn and secondary phases, mainly manganese oxides and sulfides. All phases formed upon heating to a temperature of 1173 K are preserved during the in-situ cooling experiment to a temperature of 123 K.

Phase transitions, baro- and piezocaloric effects in single crystal and ceramics of ferroelectric NH4HSeO4

A study of heat capacity, thermal dilatation and sensitivity to hydrostatic and uniaxial pressure was carried out on single-crystal and ceramic samples of NH4HSeO4. The main parameters of low-temperature successive phase transitions B2 (T1) ↔ incommensurate IC (T2) ↔ ferroelectric P1 (T3) ↔ non-ferroelectric did not depend on the type of samples. The behavior of the volumetric strain and the results of direct measurements of T3(p) contributed to the resolution of the longstanding problem associated with the ambiguity of the sign of the corresponding volumetric baric coefficient. The role of thermal expansion anisotropy in the formation of the piezocaloric effect (PCE) near the ferroelectric phase transition at T3 has been studied. Due to the strong difference in the linear baric coefficients, the main contribution to the barocaloric effect (BCE) comes from the inverse intensive and extensive PCE associated with the a-axis. Compared to a single crystal, ceramics demonstrate lower BCE values, which, however, exist in a wider temperature range, which leads to close values of integral caloric parameters. The strong decrease in both BCE and PCE at low-temperature transformations in NH4HSeO4 compared to the ferroelectrics NH4HSO4 and NH4NH4SO4 is associated with a small change in entropy during three low-temperature phase transitions, ΣΔSi = 2.52 J/mol∙K, which is a consequence of a high degree of structural ordering in selenate as a result of a high-temperature transformation at T0 between the superionic and B2 phases, accompanied by a giant change in entropy, ΔS0≈Rln21.

Possible hazardous components in dental alginates: Physicochemical properties by a mineralogical and spectroscopic investigation

Alginates are products used as impression materials in dentistry and prosthetics. They consist of polymers, calcium alginates mixed with diatomite and additives. Recently, the occurrence of severe silicosis associated with exposure to respirable dust of such materials has increased the scientific interest in understanding how these materials may pose a toxicological problem to workers. The primary objective of this study is to improve the understanding of both the existence and the characteristics of the toxicant(s) contained in these materials, with the goal of better defining the risk assessment for this occupational setting.Two commercial dental alginates were subjected to a mineralogical, microchemical and spectroscopic investigation. The results indicate the presence of a significant amount of diatomite, clearly identified by micromorphology and formed mainly by cristobalite. The respirable fraction of the dust represents at least 30 % of the total number of particles, and this fraction contains a relevant amount of crystalline silica particles. Conversely, the investigated alginate materials do not exhibit the presence of radical species.The results obtained confirm that the cristobalite detected originates from the high-temperature transformation of amorphous silica during the calcination process of diatomite, prior to mixing with the other components. The same process also produces wollastonite (CaSiO3), which, like cristobalite, is a crystalline phase known for its toxicological effects. The present findings call for a rethinking of dental alginates with regard to the definition of their health risks for technical operators.

The effect of Fe addition on high temperature transformations of specific silica sources

The effect of Fe addition on high temperature transformations of specific silica sources

High temperature decomposition of polymeric carbon monoxide at pressures up to 120 GPa.

While polymeric carbon monoxide (pCO) has been experimentally found to remain amorphous and undecomposed at room temperature up to 50GPa, the question of whether crystalline counterparts of it can be obtained naturally raises. From different computational studies, it can be inferred that either the crystallization of amorphous pCO (a-pCO) or its decomposition into a mixture of CxOy suboxides (x > y) or carbon and CO2 may occur. In this study, we report experimental investigations of the high temperature (700-4000K) transformation of a-pCO in the 47-120GPa pressure range, conducted by x-ray diffraction in laser heated diamond anvil cells. Our results show the formation of no crystalline phases other than CO2 phase V, thus indicating the decomposition of the pristine a-pCO into CO2 and, likely, a mixture of amorphous CxOy suboxides and amorphous carbon hardly detectable at extreme conditions. These results support the theoretical picture of the pCO decomposition. We also show that the pressure-temperature kinetic border for this decomposition is very steep, thus indicating a strongly pressure-dependent kinetic barrier.

Removal of lead from aqueous solution by synergistic interaction between LiCoO2 and tourmaline: Synthesis, characterization and mechanism investigation

Tourmaline-LiCoO2 composites were synthesized for Pb2+ elimination. The adsorption capacity of Pb2+ was firstly evaluated with varied tourmaline additions and sintering temperatures, and the appropriate values of these two parameters were 0.375 g and 800 °C (TM0.375LCO800), in which the mass ratio of tourmaline/LiCoO2 was about 1:10. The effects of solution pH and foreign cations on the adsorption behavior of TM0.375LCO800 were further investigated, which demonstrate that solution pH and Mn2+ and Ni2+ had little effect, while Cu2+ and Zn2+ had larger effect. Pseudo-first-order kinetic model and Langmuir adsorption isotherm fitted well with the Pb2+ adsorption characteristics on TM0.375LCO800, indicating the chemical adsorption process with an adsorption capacity of 64.01 mg/g. Physicochemical properties of tourmaline, LiCoO2 and tourmaline-LiCoO2 composites were comparatively analyzed by XRD, ATR, SEM, TEM, UV–vis DRS and XPS. It is found that it is the high-temperature transformation of the separate tourmaline and LiCoO2 into interacting states via tourmaline embedment into the LiCoO2 framework that promoted Pb2+ removal. Finally, the Pb2+ adsorption routes are elaborated. This study suggests the potential and eco-friendliness of tourmaline-LiCoO2 composite for removing heavy metal ions from water phase, and also provides scientific implications for the development of tourmaline-LiCoO2 composite for other environmental applications.

High temperature transformation, O2 etching, and passivation of single SiOx nanoparticles: kinetics and optical properties as structure probes

The surface chemistry and optical properties of silica nanoparticles (NPs) were studied using a single nanoparticle mass spectrometer. Initially, silica NPs were transparent to 532 nm excitation, but after brief exposure to a CO2 laser (10.6 μm) their 532 nm absorptivity increased, and when heated they emitted blackbody-like radiation, signalling that their compositions had become more Si-rich. The transformation was also probed by sublimation rate measurements from 1600 K to 2350 K, and the chemistry of the transformed surface was probed by measuring the O2 etching kinetics of pre-heated silica NPs at 1200 and 1300 K. Sublimation was significant at TNP ≥ 1900 K, and the NP optical properties showed further changes as the NPs sublimed. The O2 etching kinetics for pre-heated silica NPs were similar to those previously reported for single Si NPs under identical conditions, with etching efficiency (EEO2) initially increasing with etching time, then dropping by three orders of magnitude as the NP surface passivated by growth of a thermally stable SiO2 layer. The etching and passivation kinetics at different temperatures and O2 pressures are compared, and comparisons are also made to the temperature and time dependence of EEO2 for pre-heated silica, Si, SiC, and carbon NPs.

Role of oxygen vacancies in the structural phase transformations of granular TiO2 thin films

Tailoring the structural phases of titanium dioxide (TiO2) is nowadays highly attracting for diverse applications, ranging from optical coatings, where rutile outperforms anatase, to photocatalysis, where a mix of the two is preferred. In this framework, the very high temperature transformation of anatase to rutile constitutes a big drawback. Here, we investigate the structural transformations of granular TiO2 thin films subjected to thermal treatments in different gaseous environments, such as air, oxygen, and vacuum, with the latter expected to enhance the formation of oxygen vacancies. To this aim, we used a combined approach of X-ray diffraction (XRD), Raman Spectroscopy (RS), Atomic Force Microscopy (AFM) and Field Emission Scanning Electron Microscopy (FESEM), and we demonstrate that the achievement of a crystallization temperature as low as 150 °C for the anatase and 250 °C for the rutile is possible. By using a Gibbs free energy minimization approach, the combination of granular morphology and oxygen vacancies is proposed to tune both the amorphous to anatase transition and the anatase to rutile transformation. Finally, we explore the possibility of a low-dimensional growth of anatase-TiO2 by studying phonon confinement-like effect and crystallization kinetics thanks to time-dependent RS experiments.

In-Situ Al-Mg Alloy Base Composite Reinforced by Oxides and Intermetallic Compounds Resulted from Decomposition of ZrW2O8 during Multipass Friction Stir Processing.

In the presented work, the effect of friction stir processing admixing the zirconium tungstate ZrW2O8 powder on the microstructure, mechanical and tribological properties of the AA5056 Al-Mg alloy stir zone has been studied. The FSP resulted in obtaining dense composite stir zones where α-ZrW2O8 underwent the following changes: (i) high-temperature transformation into metastable β'-ZrW2O8 and (ii) decomposition into WO3 and ZrO2 oxides followed by the formation of intermetallic compounds WAl12 and ZrAl3. These precipitates served as reinforcing phases to improve mechanical and tribological characteristics of the obtained fine-grained composites. The reduced values of wear rate and friction coefficient are due to the combined action the Hall-Petch mechanism and reinforcement by the decomposition products, including Al2O3, ZrO2, β'-ZrW2O8 and intermetallic compounds such as WAl12 and ZrAl3. Potential applications of the above-discussed composites maybe related to their improved tribological characteristics, for example in aerospace and vehicle-building industries.

Experimental determination of the solid phase density of thermal power plant ash

Recycling of ash and slag waste from thermal power plants is an important environmental and economic task. The widespread use of ash in construction is mainly hindered by the high content of unburned carbon. The fine granulometric composition of the ash makes most mechanical methods of recycling and flotation ineffective, and is the most effective, environmentally friendly, and cheapest method is hydraulic separation. It allows for the recovery of about 70% of construction raw materials, in which the carbon content is reduced to regulatory commercial values, from 20% in raw ash to 5–10%. A central element of hydraulic separation research is establishing the hydraulic characteristics of the fly ash, with a special focus on determining its actual density, considering that the ash includes more than 10 different mineral components that have also undergone high-temperature transformation. Theoretical determination of the solid phase density of a multi-component mixture does not yield a reliable result. This article examines methods for determining bulk, true, and actual density, experimental approaches to their measurement, and the impact of various factors on these parameters. Correct density determination underlies the calculation of hydraulic characteristics of ash suspensions, the speed of hydraulic flows, and the efficiency of extracting individual components by way of hydroclassification and hydroseparation. The purpose of the research is to refine the methodology and experimentally determine the density of the solid phase of fly ash in its natural state and ash classified by size -240+40 µm, -40+0 µm, using the sample of ash from Pridniprovska TPP. The results of this work are significant for engineering when creating new technologies, for hydraulic devices recycling ash aimed at minimizing environmental impact and optimizing the use of technogenic resources. It was found that the density of the solid phase of ash from the ash dump of Pridniprovska TPP varies from 1.783 g/cm³ to 2.3, averaging 2.0 g/cm³. Variations in density are related to the inhomogeneity of the chemical composition, the presence of closed cavities in the particles. Keywords: fly ash, ash and slag waste.

Thermal evolution of stillwellite, CeBSiO5, a natural prototype for a family of NLO-active materials

Stillwellite, ideally CeBSiO5, is a rare mineral, however, its discovery stimulated the intense studies of a new class of stillwellite-like materials with remarkable electrical and optical properties. With the use of the modern non–ambient single crystal X-ray diffraction, this work sheds some light on the evolution of stillwellite crystal structure during the process of high-temperature transformations noted for the most of these practically important compounds. Here a sample of natural stillwellite [stillwellite-(Ce)] from the Darai-Pioz alkaline massif (Tadjikistan) has been studied by means of electron microscopy, IR spectroscopy, single crystal and powder X-ray diffraction (XRD) in a wide temperature range from −180 up to 1200 ​°C. The dielectric properties are also measured in the range from −180 up to 500 ​°C. The low (LT) and the high temperature (HT) structure deformation and transformation are described from single XRD data including the detail analysis of deformation of cationic polyhedra on heating. The disorder-to-order phase transition occurs in the stillwellite structure at about 450–500 ​°C. Both LT (disordered) and HT (ordered) modifications are characterized as relatively low expansion materials: αvolume ​= ​15 and 19 ​× ​10-6 °C-1 for LT and HT phases, respectively. The high-temperature transformation of stillwellite involves some negative thermal expansion (αvolume ​= ​−11 ​× ​10-6 ​°C-1) caused by order-disorder process and presumably by the partial oxidation of Ce3+ to Ce4+. The dielectric properties show a maximum in the region of transformation as well, but the study of dielectric hysteresis loops did not confirm the ferroelectric order of stillwellite structure at room temperature. The high-temperature behavior and the decomposition process of stillwellite are dictated by the partial oxidation of cerium.

A versatile chemical vapor synthesis reactor for in situ x-ray scattering and spectroscopy.

We describe a versatile reactor system for chemical vapor synthesis of nanoparticles, which enables in situ investigations of high temperature gas phase particle formation and transformation processes by x-ray scattering and x-ray absorption spectroscopy. The system employs an inductively heated hot wall reactor as the energy source to start nanoparticle formation from a mixture of precursor vapor and oxygen. By use of a modular set of susceptor segments, it is especially possible to change solely the residence time of the gas mixture while keeping all other process parameters (temperature, gas flow, pressure) constant. Corresponding time-temperature profiles are supported by computational fluid dynamics simulations. The operation of the system is demonstrated for two example studies: tin oxide nanoparticle formation studied by small angle x-ray scattering and iron oxide nanoparticle formation by x-ray absorption spectroscopy.

Structure and properties of aluminized alloys of the Fe–Cr–Al system

The formation features of the aluminide coating structure on Cr15Al5 and Cr23Al5 alloys after aluminizing by immersing in aluminum and silumin melt are studied. The influence of Si on the transformation of the structure and phase composition of the aluminide coating under high-temperature heating conditions are studied. It is shown that aluminizing of the Fe–Cr–Al system alloys by immersion in an aluminum melt ensures the formation of a defect-free continuous coating on their surface, which is a crystallized eutectic mixture of Al + FeAl3 and a layered diffusion zone at the boundary with the substrate. Replacing the aluminum melt with silumin with 12 % Si leads to the appearance of the Al7Fe2Si ternary intermetallic compound in the coating composition, which almost completely replaces FeAl3. In the process of hightemperature transformation of the coating structure, the presence of Cr and Si at the initial stage leads to the appearance in the coating composition intermetallic compounds (Cr,Fe)5Al8, Cr3Si, Al7Fe2Si, Fe3(Si,Al)5 and Cr5Si3 along with Fe–Al binary, and at the final stage – does not affect the phase composition, forming solid solutions based on iron aluminides. It is shown that the oxidation of aluminide coating on the Fe–Cr–Al system alloys at its early stages is accompanied by the formation of a stable modification of α-Al2O3 aluminum oxide and (Fe,Cr)2O3 ternary oxide, and with long-term oxidation leads to the appearance of δ-Al2O3 metastable modification and FeAl2O4. It has been established that the Si-alloyed aluminide coating on the Cr15Al5 alloy has a lower corrosion penetration depth at 1100 °C. At the same time, the retention time of heatresistant properties for the aluminide coating on the Cr23Al5 alloy is ~20000 h, and on the Cr15Al5 alloy it is ~5000 h for the aluminide and 6000 h for aluminosilicide coatings.The study was supported by the Russian Science Foundation grant No. 21-79-10246, https://rscf.ru/project/21-79-10246/.

Transformation of ZnS microspheres to ZnO, their computational (DFT) validation and dye-sensitized solar cells application

ZnO spheres were synthesized by solid state oxidation of ZnS microspheres at different oxidation temperatures of 600 and 700 °C in an ambient atmosphere. The high temperature transformation of ZnS microspheres into hexagonal structured and highly crystalline ZnO microspheres comprising interconnected nanoparticles facilitating efficiently electron transport as well as charge collection through an intended path in the solar cell. The comparative physico-chemical and photovoltaic studies were done for synthesized ZnO microspheres. The structural study by x-ray diffraction of ZnO confirmed the hexagonal ZnO; the UV–Visible spectroscopy study showed λmax varying from 410 to 413 nm and 397 to 407 nm for ZnO microspheres synthesized by heating at 600 and at 700 °C, respectively. Additionally, crystalline and electronic structures were validated by density functional theory studies. The computational studies also revealed growth of hexagonal ZnO, where the bandgap varied with the oxidation temperature. The photovoltaic properties of ZnO microspheres synthesized at 600 °C exhibited better performance than the ones synthesized at 700 °C due to high surface roughness leading to enhanced dye loading and favorable charge collection. Dye-sensitized solar cells fabricated from ZnO microspheres synthesized at 600 and 700 °C exhibited the maximum power conversion efficiency of 3.38% and 3.06% correspondingly.

Formation of ceramics from K-activated FeOx-(Al2O3)-SiO2 inorganic polymers: Effect of Al/K and Si/K molar ratio

The high-temperature transformations and crystallization of K-based inorganic polymers made from FeO-SiO2 and FeO-SiO2-Al2O3 iron-rich model slags were investigated, with particular focus on the effect of the Al/K and Si/K molar ratios. Upon firing to 1100 °C, the main crystalline phases were hematite and (iron) leucite. Addition of corundum decreases the crystallization rate of leucite. Increasing the Al/K molar ratio increased the thermal stability due to the substitution of Al3+ for Fe3+ in leucite. The Si/K molar ratio impacted the crystallization of leucite. Upon firing to 1100 °C and 1350 °C there was a linear correlation between the amount of leucite and the Si/K molar ratio. Higher Si/K molar ratio resulted in ceramics with higher glass content and lower thermal stability. Thus, the phase assemblage can be adjusted by changing the amount of activator.

Particle Safety Assessment in Additive Manufacturing: From Exposure Risks to Advanced Toxicology Testing.

Additive manufacturing (AM) or industrial three-dimensional (3D) printing drives a new spectrum of design and production possibilities; pushing the boundaries both in the application by production of sophisticated products as well as the development of next-generation materials. AM technologies apply a diversity of feedstocks, including plastic, metallic, and ceramic particle powders with distinct size, shape, and surface chemistry. In addition, powders are often reused, which may change the particles’ physicochemical properties and by that alter their toxic potential. The AM production technology commonly relies on a laser or electron beam to selectively melt or sinter particle powders. Large energy input on feedstock powders generates several byproducts, including varying amounts of virgin microparticles, nanoparticles, spatter, and volatile chemicals that are emitted in the working environment; throughout the production and processing phases. The micro and nanoscale size may enable particles to interact with and to cross biological barriers, which could, in turn, give rise to unexpected adverse outcomes, including inflammation, oxidative stress, activation of signaling pathways, genotoxicity, and carcinogenicity. Another important aspect of AM-associated risks is emission/leakage of mono- and oligomers due to polymer breakdown and high temperature transformation of chemicals from polymeric particles, both during production, use, and in vivo, including in target cells. These chemicals are potential inducers of direct toxicity, genotoxicity, and endocrine disruption. Nevertheless, understanding whether AM particle powders and their byproducts may exert adverse effects in humans is largely lacking and urges comprehensive safety assessment across the entire AM lifecycle—spanning from virgin and reused to airborne particles. Therefore, this review will detail: 1) brief overview of the AM feedstock powders, impact of reuse on particle physicochemical properties, main exposure pathways and protective measures in AM industry, 2) role of particle biological identity and key toxicological endpoints in the particle safety assessment, and 3) next-generation toxicology approaches in nanosafety for safety assessment in AM. Altogether, the proposed testing approach will enable a deeper understanding of existing and emerging particle and chemical safety challenges and provide a strategy for the development of cutting-edge methodologies for hazard identification and risk assessment in the AM industry.

Heating-Induced Transformation of Anatase TiO2 Nanorods into Rock-Salt TiO Nanoparticles: Implications for Photocatalytic and Gas-Sensing Applications.

Anatase TiO2 nanocrystals (NCs) play a vital role in photocatalytic applications due to their high catalytic activity and in gas-sensing applications due to their high chemical sensitivity. Here, we report the transformation at elevated temperature of anatase nanorods (NRs) with a length of 25 nm into rock-salt TiO nanoparticles with an average size of 9.2 ± 2.1 nm investigated by means of in situ heating in the transmission electron microscope. The NRs were completely transformed to titanium monoxide NCs after heating to a temperature of 1200 °C. We also identified an intermediate stage in the temperature range of 950–1200 °C, during which not only the anatase and rock-salt phases were found but also the brookite phase. Understanding of the phase and morphology evolution at high temperatures is of essence to the functionality of the NRs in various applications, as discussed in this work. Moreover, the high-temperature transformation to titanium monoxide is of interest as rock-salt TiO (γ-TiO) is known to exhibit superconducting properties. We propose the heating-induced transformation as a physical route to synthesize TiO NCs of very small size.

Superlattice ordering transitions driven by short-range structure in barium calcium carbonates.

Balcite (BaxCa1-xCO3) is a synthetic analog of rhombohedral carbonate minerals like calcite and dolomite that is disordered on both the cation and anion sublattices. Here, we show that multiple exotic superlattice structures, including a dolomite analog that we call balcomite, can form from balcite at elevated temperatures. The second-order balcite-to-balcomite conversion at temperatures between 150-600 °C is driven by the preference of barium and calcium for different oxygen coordination numbers and facilitated by local carbonate reorientation. At elevated pressure, further superlattice order arises from cation segregation in all three dimensions, producing a supercell with the same R3̄m symmetry as balcite but 6× larger. This highly ordered structure relaxes back to the balcomite structure upon returning to ambient conditions. None of the three naturally occurring polymorphs of Ba0.5Ca0.5CO3 (alstonite, paralstonite, barytocalcite) formed from balcite despite being putatively energetically favored. Instead, alstonite transformed to a balcomite-like structure via a first-order process after transiently converting to a paralstonite-like structure via a second-order process. Together, these results show that high temperature transformation pathways between structures in the barium calcium carbonate system can be driven by coarsening and are facilitated by similarity in short-range order, conceptually analogous to previously described low-temperature transformations. Many of the exotic high temperature carbonate structures are unstable, but may participate in transformation pathways between naturally observed metastable mineral phases, suggesting important roles for ephemeral phases in shaping past and current mineral distributions.

High temperature transformation of iron-bearing minerals in basalt: Mössbauer spectroscopy studies

Abstract The high temperature decomposition of basalt from Lower Silesia (Poland) was followed by Mössbauer spectroscopy investigation. The Fe content of the sample was ~9.0 at.%. The X-ray diffraction analysis shows that augite (37%) and olivine (12%) are major Fe-bearing mineral components. The sample also contains significant amount of anorthite (22%) and nepheline (17%). The sample was heated at various temperatures between 200°C and 1100°C for three hours. Up to a temperature of 500°C changes in contribution of Fe-bearing minerals are insignificant. Heating in the temperature range from 500°C to 1100°C leads to a systematic increase in contribution of iron oxides at the cost of contribution of silicate minerals, like augite and olivine. Mössbauer spectrum obtained after heating at 1100°C showed hematite as the main iron oxide phase. The ratio of Fe3+/Fetot in the non-heated sample was equal to 0.51 and after heating at 1100°C this ratio amounted to 0.89.

Novel extraction route of lithium from α-spodumene by dry chlorination.

Processing spodumene for lithium is challenging as it requires a high temperature transformation of the natural α-monoclinic form to β-tetragonal form, usually followed by acid baking and digestion. This three-step extraction process requires significant heat energy, acid, process complexity and residence time, leading to both operating and capital costs. An approach which helps to eliminate this challenge will therefore be a milestone in processing spodumene. This study, thus, investigates a direct chlorination of α-spodumene using calcium chloride followed by water leaching of the residue to recover lithium, which reduces the energy requirement and number of unit operations. HSC Chemistry software was used to simulate the process using both phases (α and β) of the mineral up to 1100 °C prior to experimental investigation. The α-form was the only polymorph identified in residues after leaching, suggesting that the extraction is directly from the α-phase. However, an initial formation of a metastable β-form followed by a fast synthesis of lithium chloride from it is also suspected. Under optimal conditions of calcium chloride/spodumene molar ratio of 2.0, and 1000 °C treatment for 60 minutes, almost 90 percent lithium chloride was extracted and 85 percent was recovered to the leach solution with the remainder exiting with the off-gas. An apparent activation energy of about 122 ± 6 kJ mol−1 was obtained at temperatures ranging from 800 to 950 °C during the process.