Polytype Transformation Research Articles

Co‐immobilization of a PuO2 surrogate and contaminated stainless steel within a zirconolite matrix

AbstractThe aqueous reprocessing of spent nuclear fuel generates a considerable amount of plutonium alongside contaminated stainless steel, necessitating meticulous handling for safe decommission and long‐term management. The present work investigated the co‐immobilization of CeO2 (as an inactive surrogate for PuO2) with metallic Fe and Cr (representing a simplified stainless steel) within a zirconolite ceramic wasteform, nominally targeting Ca1−xCexZrTi2−2xFexCrxO7. After sintering at 1400°C under an air atmosphere, the zirconolite phase constituted 90.8–95.1 wt.% of the product across the solid solution range of 0.05 ≤ x ≤ 0.25, alongside perovskite and baddeleyite secondary phases in varying proportion; no evidence of the unincorporated CeO2 or metallic and oxidized Fe or Cr were identified. Above x = 0.30 CeO2 was detected inferring, the solubility limit was reached. A polytype transformation from zirconolite 2M to 3T was confirmed by X‐ray diffraction and selected area electron diffraction results, with the relative fraction of the 3T phase gradually increasing to 49.5 wt.% at x = 0.30. Deconvolution of X‐ray photoelectron spectroscopy data revealed the partial reduction of Ce4+–Ce3+, whereas Fe and Cr species maintained trivalent, in agreement with the targeted substitution scheme. Benefitting from the excellent chemical flexibility of zirconolite structured compounds, the co‐immobilization approach may be an effective disposal pathway for Pu‐containing wastes and contaminated stainless steel residues.

Deciphering the Polymorphism of CaSi2: The Influence of Heat and Composition.

The Zintl phase CaSi2 is a layered compound with stacking variants known as 1P, 3R, and 6R. We extend the series by the 21R polytype formed by rapid cooling of the melt. The crystal structure of 21R-CaSi2 (space group R3̅m) was derived from HRTEM images, and the atomic positions were optimized by using the FPLO code (a = 3.868 Å, c = 107.276 Å). We explore polytype transformations by powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron backscattering diffraction (EBSD), and thermal analysis. While 6R-CaSi2 is thermodynamically stable at ambient conditions, nanosized impurities of silicon stabilize 3R-CaSi2 as a bulk phase.

Unravelling multiple thermotectonic events accommodated by the Highland Boundary Fault: Insights from K–Ar dating

Multiple reactivation episodes and long slip histories lead to complex fault structures, whose unravelling remains challenging in the absence of absolute time constraints. We apply K–Ar isotopic dating of illitic fault rocks, coupled with X-ray diffraction and microstructural analyses to constrain, for the first time, timing of illite-producing brittle fault movements accommodated by the Highland Boundary Fault (HBF), Scotland. Illite Age Analysis (IAA) plots indicate multiple fault reactivation events on the HBF. IAA plots for the red foliated chaotic fault-breccia conform to a ‘normal’ IAA pattern with younger ages recorded for ‘authigenic’ 1 M illite (306-276 and 300-272 Ma) and older dates for ‘detrital’ 2M1 illite (554-502 and 471-427 Ma). Conversely, IAA plots for the superimposed blue fault-gouge reverse the ‘normal’ trend, with older ‘authigenic’ 1 M illite (348-314 and 276-25 Ma) and younger ‘detrital’ 2M1 illite (258-234 and 250-226 Ma) ages. We propose this results from heterogeneous shearing and strain localisation. In localised pods of the blue fault-gouge, strain accelerated clay mineral growth, increased crystallite-size, and facilitated polytypic transformation from 1 M to 2M1 illite via defect migration. Elsewhere in the blue fault-gouge, and in the red foliated chaotic fault-gouge, low strain regimes allowed the 1 M polytype to remain unaltered.

Polytypic transformation behavior from 4H-SiC to 3C-SiC during high-speed scratching of single crystalline silicon carbide

The phase transformation behavior and associated mechanism of 4H-SiC induced by dynamic processing at room temperature conditions can be investigated with high-speed scratching tests. A dedicated high-speed scratching setup is developed to achieve the speed range over 10 m/s to 30 m/s while microscopic detection is performed to study the microstructural alteration of 4H–SiC. Raman spectra analyses indicate that the polytypic transformation from 4H to 3C occurs during high-speed scratching of single crystalline silicon carbide. Transmission electron microscopy demonstrates a shift in the stacking order of the atomic layers to face-centered cubic at the subsurface of the scratched groove, which reveals the formation mechanism of 3C-SiC. Furthermore, the 4H to 3C polytypic transformation exhibits a sensitive scratching speed correlation. The research can enhance the fundamental understanding of the mechanisms of damage formation during manufacturing of 4H-SiC components and provide g guidance for high-quality processing of 4H-SiC crystals.

4H to 3C Polytypic Transformation in Al+ Implanted SiC During High Temperature Annealing

4H to 3C Polytypic Transformation in Al+ Implanted SiC During High Temperature Annealing

High-pressure Raman scattering and X-ray diffraction study of kaolinite, Al2Si2O5(OH)4

Kaolinite is formed by weathering of continental crustal rocks and is also found in marine sediments in the tropical region. Kaolinite and other layered hydrous silicate minerals are likely to play a vital role in transporting water into the Earth's interior via subducting slabs. Recent studies have experimentally documented the expansion of the interlayer region by intercalation of water molecules at high pressures i.e., pressure-induced hydration. This is counter-intuitive since the interlayer region in the layered silicates is quite compressible, so it is important to understand the underlying mechanism that causes intercalation and expansion of the interlayer region.To address this, we explore the high-pressure behavior of natural kaolinite from Keokuk, Iowa. This sample is free of anatase impurities and thus helps to examine both low-energy (0–1200 cm−1) and high-energy hydroxyl (3000–4000 cm−1) regions using Raman spectroscopy and synchrotron-based powder X-ray diffraction.Our results show that the pressure dependence of the hydroxyl modes exhibits discontinuities at ∼3 GPa and ∼ 6.5 GPa. This is related to the polytypic transformation of Kaolinite from K-1 to K-II and K-II to K-III phase. Several low-energy Raman modes' pressure dependence also exhibits similar discontinuous behavior. The synchrotron-based powder X-ray diffraction results also indicate discontinuous behavior in the pressure dependence of the unit-cell volume and lattice parameters. The analysis of the bulk and the linear compressibility reveals that kaolinite is extremely anisotropic and is likely to aid its geophysical detectability in subduction zone settings. The K-I to K-II polytypic transition is marked by the snapping of hydrogen bonds, thus at conditions relevant to the Earth's interior, water molecules intercalate in the interlayer region and stabilize the crystal structure and help form the super-hydrated kaolinite which can transport significantly more water into the Earth's interior.

Off-stoichiometric softening and polytypic transformations in the plastic deformation of the C14 Fe[formula omitted]Nb Laves phase

Plastic deformation of the brittle C14-Fe2Nb Laves phase occurs mostly by basal slip due to the complex crystal structure. Here, we compare the barriers for basal slip for the known mechanisms of direct slip, synchroshear and undulating slip using density functional theory calculations. According to our calculated generalized stacking fault (SF) energies, the most favorable mechanisms are synchroshear and undulating slip. Both mechanisms lead to stable SF with a formation energy of 50 mJ/m2 through the same unstable SF configuration at the transition. The energy barrier of approximately 3 J/m2 indicates a low dislocation mobility as expected from the brittle character. We also determine the influence of vacancies and antisite defects on the formation energy of stable and unstable SF. Both kinds of point defects tend to lower the energy barrier on both sides of 2:1 stoichiometry. This explains the experimentally observed off-stoichiometric softening of C14-Fe2Nb. The small energy differences between the Fe2Nb Laves phase polytypes raises the question if there are further deformation mechanisms with low barrier. Therefore, we additionally consider the known transformations between C14, C15 and C36 Laves phases by successive synchroshear as further deformation mechanism. Our calculations for polytypic transformations by successive synchroshear steps show that the corresponding energy barriers are in fact very similar to the energy barrier for basal slip in C14. This suggests that the energy needed to create a stable SF in C14 by synchroshear is also sufficient to initiate polytypic transformations where existing SFs in C14 are further transformed to form C15 or C36 Laves phases.

LITHIUM IN THE SUBSOIL OF UKRAINE Part 4. Mineralogy of lithium-bearing objects: micas

Part 4 of the publication "Lithium in the subsoil of Ukraine" provides a description, mostly typomorphic, of layered silicates of lithium-bearing objects — muscovite, micas of the isomorphic series of annite — trilithionite (formerly known as annite (lepidomelane) — protolithionite — zinnwaldite — cryophyllite — lepidolite), and micas from the biotite group. The mineralogy of mica are characterized in the following order: Volyn chamber pegmatites — Perzhansky ore node — Kamyanomohyla (stone grave) complex of Azov region — rare metal pegmatites of the Azov region and the Inhul megablock. Muscovite is a secondary low-temperature mineral of the chamber pegmatites of Volyn which grew after the crystallization of micas from the isomorphic series of annite-trilithionite. It is represented by the polytypes 2M1 and 3T. Muscovite is common mica in the Perga ore district; in greisens and some metasomatites, it is the main mineral. It contains an elevated amount of iron and belongs to the 1M polytype modification: muscovite (Fe)-1M. Muscovite is also found in the Kamyanomohylskyi complex of the Azov region, primarily in altered granites and greisens formed on them. Its characteristic feature is the elevated iron content and the 2M1 structure. Fuchsite, fibrous muscovite and sericite-1Md are also found here. Muscovite in rare-metal pegmatites is mostly a secondary mineral. Rare pink muscovite, fuchsite and lithium muscovite have been found in the Azov pegmatites. Muscovite is present in relatively small amounts in the Shevchenkivske deposit and is represented by two generations. Muscovite (Li, Fe)-1M is rarely found. The X-ray luminescent properties of muscovite from spodumene pegmatites are of great importance as they are the main typomorphic features of the development of the pegmatitic process. Muscovite is rarely found in petalite pegmatites of the Inhul megablock and is represented by two generations. Its constitution is not fully understood and is controversial. Li-Fe мicas in granites and pegmatites of Volyn are important type-minerals indicating the crystallization of granites and pegmatites from magma with high content of rare elements and volatile components, the degree of differentiation of chamber pegmatites, and the extent of post-magmatic processes. The distribution of octahedral cations in the crystal structure of micas also has typomorphic significance, reflecting the activity of volatile components, the rate of crystallization, and the degree of differentiation of chamber pegmatites. The processes of forming fully differentiated pegmatites are accompanied by a polytypic transformation: 1M → 3T → 1M. Lithium-iron micas in greisens and metasomatites have similar but somewhat different typomorphic significance of the Perzhansky ore district (in which the polytype 2O was discovered) as well as in the Kamyanomohylskyi complex of the Azov region. The article concludes with a discussion of the results of research on rare alkalies in iron-magnesium micas from various types of metasomatic rocks surrounding pegmatites in Polohivske and Stankuvatsko-Lypnyazhske ore fields.

Effect of Deposition Flux on Polytypic Competitive Growth of SiC Crystal by Kinetic Monte Carlo Method

A competitive kinetic Monte Carlo (KMC) model for gas‐phase epitaxy growth of 4H‐SiC and 6H‐SiC crystals is established based on polytype transformation and the Monte Carlo algorithm. The competitive growth processes of the two polytypes are simulated, and the effect of deposition flux on the polytype competition is analyzed. The 4H‐SiC competitive growth is promoted by the increase in deposition rate, and the effect of the Si/C ratio on polytypic competition is suppressed by the high deposition rate. Moreover, competitive growth is affected by deposition flux by changing the ratio of diffusion rate to deposition rate.

Formation of polytypes structures in Mg single crystals

We report studies of polytypes structures (PSs) formed in the microstructure of deformed Mg single crystals. PSs emerge as co-products of lattice distortion developed in dislocations’ tangles due to reactions and rearrangements of dislocations to the lower energy configurations. TEM observations indicate that entanglements containing an excess content of 〈c〉-component dislocations that maintain dislocation interactions and the evolving pattern in a confined space nucleate PSs. The distortion and rotation of the hcp lattice induce the transition to the triclinic lattice originating PSs. The overlapping strain fields from mutually interlocked dislocations and stacking faults promote local shears and shuffles that develop polytypes domains within the PSs. Ab initio and molecular statics simulations permit understanding the processes governing triclinic and polytypic transformations in Mg and the role of dislocations in PSs’ self-assembly.

Про механізм формування зеренної структури і границь розділу в самоармованому нітриді алюмінію

Results of an investigation by scanning electron microscopy methods of the microstructure of self-reinforced aluminum nitride obtained on the basis of aluminum nitride powder and containing 3 mass.% oxygen by plasma-chemical synthesis in the temperature range 1700-2000 °C are presented. Initial aluminum nitride was represented by the wurtzite (2H) phase. Samples were obtained by free sintering in a nitrogen atmosphere. It was established that, during sintering of AlN in the indicated temperature range, three microstructural types of the material and six types of interfaces (three types of intergranular and three types of intragranular ones) formed. The features of the microstructure of the materials are fully determined by the development of intergranular crystal-oriented polytype transitions of 2H AlNmultilayer polytypes (MP) in sintering. The sequence of successive structural transformations that determine the development of polytype transitions was established. 1. Formation of initial 2H AlN grains of the solid solution 2H AlN-O. The substitution of nitrogen by oxygen takes place. 2. The development of isomorphous delamination of the solid solution in every grain with the precipitation of an interlayer enriched in oxygen. 3. In interlayers, polytypes consisting of a series of polytypes with different number of layers (MP) form. Such an interlayer has developed base surfaces and propagates from one boundary to another in the grain, which determines the formation of a special structural state of fragments of boundaries, that border interlayers in the direction <hk0>. 4. The high mobility of the indicated fragments of boundaries determines their break-off from the common boundary and formation of a grain nucleus of anisometric (plate-like) shape. This process begins already at a sintering temperature Tsint. = 1800 °C, and, at Tsint. = 2000 °C, polycrystals practically entirely consist of grains of plate-like shape. In this case, with increase in the sintering temperature, the aspect ratio (the length-to-width ratio) of such grains rises. Keywords: aluminum nitride, polytype transformations, grains, microstructure, boundaries, self-reinforcement.

T-S Fuzzy System Controller for Stabilizing the Double Inverted Pendulum

This article provides a representation of the double inverted pendulum system that is shaped and regulated in response to torque application at the top rather than the bottom of the pendulum, given that most researchers have controlled the double inverted pendulum based on the lower part or the base. To achieve this objective, we designed a dynamic Lagrangian conceptualization of the double inverted pendulum and a state feedback representation based on the simple convex polytypic transformation. Finally, we used the fuzzy state feedback approach to linearize the mathematical nonlinear model and to develop a fuzzy controller H ∞ , given its great ability to simplify nonlinear systems in order to reduce the error rate and to increase precision. In our virtual conceptualization of the inverted pendulum, we used MATLAB software to simulate the movement of the system before applying a command on the upper part of the system to check its stability. Concerning the nonlinearities of the system, we have found a state feedback fuzzy control approach. Overall, the simulation results have shown that the fuzzy state feedback model is very efficient and flexible as it can be modified in different positions.

Intrinsic microstructures of silica‐bonded porous nano‐SiC ceramics

AbstractSilica‐bonded porous SiC ceramics were fabricated using nano‐β‐SiC powder‐carbon black template compacts by sintering in air at 600°C‐1200°C. The intrinsic microstructures of the porous ceramics were characterized by high‐resolution transmission electron microscopy, which led to the following observations: (a) a core (SiC)‐shell (SiO2) structure was formed, owing to the partial oxidation of nano‐SiC particles during sintering; (b) a low‐temperature (800°C) β‐to‐α polytypic phase transformation was observed, owing to the oxidation‐induced residual thermal stresses; and (c) non‐graphitic carbons were precipitated inside the SiC core, owing to the segregation of C atoms emitted at the strained SiC‐SiO2 interface.

Effect of additive content on the mechanical and thermal properties of pressureless liquid-phase sintered SiC

ABSTRACT Pressureless liquid-phase sintered SiC (LPS-SiC) ceramics fabricated from β-SiC and 1 vol% α-SiC seeds with 3–15 vol% Al2O3-Y2O3-CaO additives were prepared to investigate the effects of additive content on the mechanical and thermal properties. All the specimens consisted primarily of elongated α-SiC grains that were predominantly formed via β to α phase transformation and via the growth of the initially added α-SiC seeds during the sintering. The grain growth and polytypic transformation exhibited diffusion-controlled kinetics. The highest value for fracture toughness (5.4 MPa·m1/2) was exhibited at an additive content level of 7 vol%; this was attributable to the optimal interfacial strength and well-interlocked elongated grains with active crack–microstructure interaction. The specimen with 7 vol% additive content exhibited the highest flexural strength (686 MPa) among the reported pressureless LPS-SiC ceramics, and this was attributable to its high density and the optimal content of soft intergranular phase (IGP). The hardness and thermal conductivity decreased gradually with the increase in additive content, owing to the increased volume fraction of the soft IGP with low thermal conductivity.

Techniques for Polytypic Transformations in Silicon Carbide

Two main polytype transformations in silicon carbide, namely, 2H → 6H and 3C → 6H, have been studied by ab initio methods. It has been shown that the intermediate phases with trigonal symmetry P3m1 and monoclinic symmetry Cm make it much easier to move the close-packed layers in such transitions by breaking them up into separate stages. It has been found that these two polytype transformations proceed in completely different ways. The links being relocated noticeably tilted compared to their initial position at the transition 2H → 6H, which allows the compression of the SiC links in the plane ( $$11\bar {2}0$$ ). The transition 3C → 6H is carried out through the formation of Si–Si and C–C auxiliary links, living for a short time and helping densely packed layers to swap places. As a result, the activation barrier of the transformation 2H → 6H (1.7 eV/atom) is significantly less than the activation barrier of the transformation 3C → 6H (3.6 eV/atom), which means that the second transition should occur at the temperatures by 750–800°C higher than the first one. The energy profiles of this polytypic transformations, as well as the geometry of all intermediate and transition phases have been calculated. It has been shown that all the transition states have monoclinic symmetry.

Strain-Induced Metastable Topological Networks in Laser-Fabricated TaS2 Polytype Heterostructures for Nanoscale Devices.

The stacking of layered materials into heterostructures offers diverse possibilities for generating deformed moiré states arising from their mutual interaction. Here we report self-assembled two-dimensional nanoscale strain networks formed within a single prismatic (H) polytype monolayer of TaS2 created in situ on the surface of an orthorhombic 1T-TaS2 single crystal by a low-temperature laser-induced polytype transformation. The networks revealed by scanning tunneling microscopy (STM) take on diverse configurations at different temperatures, including extensive double stripes and a twisted 3-gonal mesh of connected 6-pronged vertices. The resulting phase diagram can be understood to be a consequence of thermally driven minimization of discommensurations between the H and 1T layers. Nontrivial dislocation defects of embedded 2- and 4-gonal structures are shown to be associated with local inhomogeneous strains. The creation of metastable heterostructures by laser quench at cryogenic temperatures in combination with STM manipulation of local strain demonstrates nanoscale control of topological defects in transition metal dichalcogenide heterostructures may be utilized in the fabrication of nanoscale electronic devices and neural networks.

High-speed grinding of HIP-SiC ceramics on transformation of microscopic features

Grinding has become one of the most efficient precision machining methods to treat with undesired machining defects and improve the surface integrity for the hard and brittle engineering ceramics. However, it is inevitable to cause micro-damages and related transformation of microscopic features, which will eventually affect the grinding quality. This paper is devoted to investigate the high-speed grinding microscopic features of silicon carbide ceramics to reveal the application of high-speed grinding technique in precision machining of ceramics. A comparative study of high-speed and conventional speed grinding of silicon carbide ceramics is discussed in terms of phase transformation, residual stresses, micro-damages, grinding chips, and surface topography. The results show that the high-speed grinding (HSG) process could help substantially improve the workpiece integrity in terms of better surface finish, smaller damages, and controlled residual stresses with a higher material removal rate. Moreover, it has also been proved that a polytypic phase transformation could be induced in HSG process.

Пути политипных превращений в карбиде кремния

In this work, two main polytypic transformations in silicon carbide, namely, 2H→6H and 3C→6H, were studied using ab initio methods. It is shown that intermediate states with trigonal symmetry P3m1 and monoclinic symmetry Cm greatly simplify the movement of close-packed layers during such transitions, breaking them up into separate stages. It was found that the two polytypic transformations occur quite differently. During the 2H→6H transition, the displaceable bonds are noticeably tilted compared to the initial position, which makes it possible to reduce the compression of SiC bonds in the (112 ̅0) plane. The 3C→6H transition takes place through the formation of auxiliary Si – Si and C – C bonds that live for a short time and help the tightly packed layers exchange places. As a result, the activation barrier for the conversion of 2H→6H (1.7 eV/atom) is significantly less than the activation barrier for the conversion of 3C→6H (3.6 eV/atom), which means that the second transition must occur at temperatures of 750-800 ℃ higher than the first one. The energy profiles of these polytypic transformations, as well as the geometry of all intermediate and transitional phases, are calculated. It is shown that all transition states have monoclinic symmetry.

Suppression of Polytype Transformation with Extremely Low-Dislocation-Density 4H-SiC Crystal in Two-Step Solution Method

We achieved the growth of extremely-high quality SiC crystal with two-step solution method with specially-designed seed crystals. The two-step growth consists of 1st step growth on Si-face for the reduction of threading dislocations and 2nd step growth on C-face for the reduction of basal plane dislocations and thickening. In this method, we can make the dislocation density extremely low, while the polytype easily changes during growth due to the absence of spiral hillocks originating from threading screw dislocation (TSD). In this study, we prepared specially designed seed crystals for both 1st and 2nd growth steps to provide steps continuously. In the seeds, a few TSDs exist at the upper-side of the step structure. Consequently, we demonstrated the suppression of the polytype transformation during the C-face growth with extremely low-dislocation-density crystal. Accordingly, we successfully obtained extremely low-dislocation density 4H-SiC with TSD, TED and BPD density of 11, 385 and 28 cm-2.

Genesis of Rhombohedral Structures and Mode of Polytype Transformations in Close Packing of Identical Atoms

Genesis of Rhombohedral Structures and Mode of Polytype Transformations in Close Packing of Identical Atoms