Slow-cooled Samples Research Articles

Effect of prior microstructure on carbide precipitation in a Cr-Mo-V pressure vessel steel at 650 °C

Cr-Mo-V steels are key materials used for constructing thick reactor pressure vessels (RPV) owing to an impressive combination of their mechanical properties and weldability. Conventionally, these steels are subjected to a quality heat treatment that comprises austenitization, quenching and tempering. Owing to its large thickness, an RPV experiences widely different cooling rates across its thickness during quenching, leading to considerable variation in the microstructure from the surface to the centre of the vessel. In this study, a Cr-Mo-V steel was austenitized at 950 °C for 1 h and cooled at two rates (water quenching and 10 °C/min), representative of the cooling rates at the surface and centre of a typical RPV, to study the effect of prior microstructure on the carbide precipitation during tempering of the steel at 650 °C. Electron microscopy of the as-cooled samples revealed the presence of martensitic microstructure in water quenched sample and predominantly granular bainitic microstructure in slow cooled sample. Synchrotron studies revealed the presence of retained austenite containing ∼0.8% carbon in the slow-cooled sample. Tempering of the steel at 650 °C for 200 h precipitated M7C3 and MC (VC) carbides in the water-quenched sample, while the slow cooled samples precipitated M23C6 as an additional constituent. This difference in the carbide precipitation in the two samples has been attributed to the excess carbon in the martensite/austenite (M/A) regions in slow cooled samples and explained on the basis of carbide precipitation sequence predicted using Thermo-kinetic simulations. This analysis has shown that M23C6 and M7C3 carbides in the slow cooled sample precipitate simultaneously as critical radii of their nucleation as well as their incubation time become comparable when the matrix contains more than 0.25% C. The present study has demonstrated that the microstructure prior to tempering may significantly affect the precipitation sequence of carbides during tempering.

Synthetic Tailoring of Ionic Conductivity in Multicationic Substituted, High-Entropy Lithium Argyrodite Solid Electrolytes.

Superionic conductors are key components of solid-state batteries (SSBs). Multicomponent or high-entropy materials, offering a vast compositional space for tailoring properties, have recently attracted attention as novel solid electrolytes (SEs). However, the influence of synthetic parameters on ionic conductivity in compositionally complex SEs has not yet been investigated. Herein, the effect of cooling rate after high-temperature annealing on charge transport in the multicationic substituted lithium argyrodite Li6.5[P0.25Si0.25Ge0.25Sb0.25]S5I is reported. It is demonstrated that a room-temperature ionic conductivity of ∼12 mS cm-1 can be achieved upon cooling at a moderate rate, superior to that of fast- and slow-cooled samples. To rationalize the findings, the material is probed using powder diffraction, nuclear magnetic resonance and X-ray photoelectron spectroscopy combined with electrochemical methods. In the case of moderate cooling rate, favorable structural (bulk) and compositional (surface) characteristics for lithium diffusion evolve. Li6.5[P0.25Si0.25Ge0.25Sb0.25]S5I is also electrochemically tested in pellet-type SSBs with a layered Ni-rich oxide cathode. Although delivering larger specific capacities than Li6PS5Cl-based cells at high current rates, the lower (electro)chemical stability of the high-entropy Li-ion conductor led to pronounced capacity fading. The research data indicate that subtle changes in bulk structure and surface composition strongly affect the electrical conductivity of high-entropy lithium argyrodites.

Physical Aging of Polystyrene Confined in Anodic Aluminum Oxide Nanopores.

We investigated the glassy dynamics of polystyrene (PS) confined in anodic aluminum oxide (AAO) nanopores by differential scanning calorimetry. Based on the outcome of our experiments, we show that the cooling rate applied to process the 2D confined PS melt has a significant impact on both the glass transition and the structural relaxation in the glassy state. A single glass transition temperature (Tg) is observed in quenched samples, while slow-cooled PS chains show two Tgs corresponding to a core-shell structure. The former phenomenon resembles what is observed in freestanding structures, while the latter is imputed to the adsorption of PS onto AAO walls. A more complex picture was drawn for physical aging. In the case of quenched samples, we observed a non-monotonic trend of the apparent aging rate that in 400 nm pores, reaches a value almost twice as larger than what is measured in bulk and decreases upon further confinement in smaller nanopores. For slow-cooled samples, by adequately varying the aging conditions, we were able to control the equilibration kinetics and either separate the two aging processes or induce an intermediate aging regime. We propose a possible explanation of these findings in terms of distribution in free volume and the presence of different aging mechanisms.

Microstructural Activation of a Topochemical Reduction Reaction.

The progress of the topochemical reduction reaction that converts LaSrNiRuO6 into LaSrNiRuO4 depends on the synthesis conditions used to prepare the oxidized phase. Samples of LaSrNiRuO6 that have been quenched from high temperature can be readily and rapidly converted into LaSrNiRuO4. In contrast, samples that have been slow-cooled cannot be completely reduced. This reactivity difference is attributed to the differing microstructures of the quenched and slow-cooled samples, with the former having much smaller average crystalline domain sizes and larger lattice strains than the latter. A mechanism to explain this effect is presented, in which the greater "plasticity" of small crystalline domains helps lower the activation energy of the reduction reaction. In addition, we propose that the enhanced lattice strain in quenched samples also acts to destabilize the host phase, further enhancing reactivity. These observations suggest that the microstructure of a material can be used to "activate" topochemical reactions in the solid state, expanding the scope of phases that can be prepared by this type of reaction.

Bismuth kagome sublattice distortions by quenching and flux pinning in superconducting RbBi2

The properties of ${\mathrm{RbBi}}_{2}$, a 4.15 K superconductor, were investigated using magnetic field, pressure, and neutron diffraction. Under hydrostatic pressure, an almost 50% reduction of ${T}_{c}$ is observed, linked to a low Debye temperature estimated at 165 K. The resistivity and magnetic susceptibility were measured on quenched and slow-cooled polycrystalline samples. The resistivity follows a low temperature power-law dependence in both types of samples, while the diamagnetic susceptibility, $\ensuremath{\chi}$, is dependent on the sample cooling history. Slow-cooled samples have $\ensuremath{\chi}=\ensuremath{-}1$ while quenched samples have $\ensuremath{\chi}<\ensuremath{-}1$ due to grain size differences. Evidence of the effects of the cooling rate is also discerned from the local structure, obtained by neutron diffraction and the pair density function analysis. Slow-cooled samples have structurally symmetric Bi hexagons, in contrast to quenched samples in which disorder is manifested in periodic distortions of the Bi hexagonal rings of the kagome sublattice. Disorder may lead to flux pinning that reduces vortex mobility, but ${T}_{c}$ remains unaffected by the cooling rate.

Cationic disorder: A pathway for demonstrating inverse exchange bias in Gd2CoRuO6

We report here the observation of the inverse exchange bias phenomenon in the ferrimagnetic double-perovskite ${\mathrm{Gd}}_{2}{\mathrm{CoRuO}}_{6}$. The compound crystallizes in a monoclinic structure (space group $P{2}_{1}/n$), and it undergoes long-range magnetic ordering below ${T}_{C}=60$ K. The ferrimagnetic state of the sample is manifested through magnetization reversal at low temperature. In order to corroborate the role of local structure on the magnetic properties of ${\mathrm{Gd}}_{2}{\mathrm{CoRuO}}_{6}$, we have performed a combined magnetization and x-ray absorption spectroscopy study on two different samples prepared through different cooling protocols (slow cooling and furnace cooling). Extended x-ray absorption fine-structure analysis indicates the presence of strong antisite disorder between Co and Ru in both samples. However, the slow-cooled sample shows completely random antisite disorder, whereas, small Co- and Ru-rich patches appear in the furnace-cooled sample. In both samples, a significant amount of mixed valency in Co and Ru ions is evident from our x-ray absorption near-edge structure. The antiparallel ordering along with different temperature dependence of the three magnetic sublattices, Gd, Co, and Ru, can be responsible for negative magnetization, whereas the inverse exchange bias effect can be attributed to the mixed interfacial coupling between adjacent magnetic nanoclusters. The difference in local structure can nicely be accounted for by the observed variations of the magnetic properties between two samples of ${\mathrm{Gd}}_{2}{\mathrm{CoRuO}}_{6}$.

Defect characterization of slow-cooled and quenched samples of calcium-copper-titanate through positron annihilation spectroscopy

Positron annihilation lifetime and Doppler-broadened gamma-ray spectra have been analyzed for slow-cooled and thermally quenched polycrystalline samples of calcium-copper-titanate. Two positron lifetimes revealing the characteristic defects in the respective samples were carefully analyzed to compare and contrast the significance of their origin and implication. A third component arising from positronium formation at the powdered particle surfaces has been considered in the analysis although its significance is lost in its very small intensity (∼ 1.1–1.2%). In the quenched sample, the defect-specific long positron lifetime (τ2) is found to larger and the mean lifetime smaller while its intensity I2 is found drastically smaller and the concentration of defects less by an order of magnitude as compared to the slow-cooled sample. The observed changes in electrical parameters of slow-cooled and quenched samples were found to have correlations with the positron annihilation lifetime and Doppler-broadened lineshape parameters.

Effects of quenching on phase transformations and ferroelectric properties of 0.35BCZT-0.65KBT ceramics

Solid solutions of 0.35(Ba,Ca)(Zr,Ti)O3-0.65(K0.5Bi0.5)TiO3 (BCZT-KBT) having various Ca and Zr contents were synthesized by solid state reaction. The sintered ceramics exhibited interesting features comprising core-shell type microstructures and relaxor ferroelectric behaviour. The influence of air-quenching on structure and electrical properties has been systematically investigated. The results indicate that the compositional heterogeneity in the shell regions, for the slow-cooled state, was reduced by air quenching. Improvements are evident in ferroelectric tetragonal phase content, accompanied by increased polarisation values and depolarisation temperatures. Comparing the results obtained for two BCZT compositions, it was demonstrated that the stability of the ferroelectric tetragonal phase in slow-cooled BCZT-KBT samples was improved for the ceramic with lower Ca and Zr concentrations, denoted x = 0.06, comparing with that for higher levels, denoted x = 0.15. Furthermore, the electric field-induced ferroelectric state in the quenched ceramic with x = 0.06 was found to be more stable during heating, yielding an enhanced depolarisation temperature.

Numerical modeling of wear behavior of solid fats

The objective of this study was to develop a numerical model to characterize wear behavior of solid fats with different crystalline networks. Wear behaviors of three different fats (cocoa butter [CB], anhydrous milk fat [AMF], and palm oil [PO]) crystallized at three temperatures (dependent on fat type) and two cooling rates (0.1 and 15 °C min−1) were evaluated under several normal forces (0.3–2.0 N) at room temperature (20 °C). A numerical model for penetration depth that accounted for both deformation and removal of material due to wear was developed. All models had mean absolute error <267 μm, indicating good fit. All fats crystallized at high temperature, and slow-cooled samples had higher wear rates and were easier to deform. Fats that had two distinct melting peaks deformed easier compared to those with single peaks or peaks with undifferentiated shoulders. CB showed β′IV and βV polymorphs, while AMF and PO showed β′ polymorphs.

Meso-Structure Controlled Synthesis of Sodium Iron-Manganese Oxides Cathode for Low-Cost Na-Ion Batteries

A modified co-precipitation method is introduced to synthesize P2-type Na0.67Fe1/4Mn3/4O2 cathode for low-cost Na-ion batteries. The meso-structure of the obtained material is well controlled through regulated cooling after a high temperature calcination process. The material via slow-cooling consists of sphere-like secondary particles with a uniform dispersion while the quenched material exhibits a hexagonal plate-like primary particle without meso-structure. Meso-structure is found to have a distinct effect upon the electrochemical performance of P2-type layered cathode. The slow-cooled sample exhibits a larger capacity and improved cyclability compared with the quenched sample, which is attributed to the larger surface area, reduced surface contamination, and surprisingly higher transition metal redox activity. This work demonstrates that cooling rate plays the key role in controlling the formation of spherical meso-structure for sodium iron-manganese oxides with enhanced electrochemical performance.

Effect of the Processing Route on the Thermoelectric Performance of Nanostructured CuPb18SbTe20.

The quaternary AgPb18SbTe20 compound (abbreviated as LAST) is a prominent thermoelectric material with good performance. Endotaxially embedded nanoscale Ag-rich precipitates contribute significantly to decreased lattice thermal conductivity (κlatt) in LAST alloys. In this work, Ag in LAST alloys was completely replaced by the more economically available Cu. Herein, we conscientiously investigated the different routes of synthesizing CuPb18SbTe20 after vacuum-sealed-tube melt processing, including (i) slow cooling of the melt, (ii) quenching and annealing, and consolidation by (iii) spark plasma sintering (SPS) and also (iv) by the state-of-the-art flash SPS. Irrespective of the method of synthesis, the electrical (σ) and thermal (κtot) conductivities of the CuPb18SbTe20 samples were akin to those of LAST alloys. Both the flash-SPSed and slow-cooled CuPb18SbTe20 samples with nanoscale dislocations and Cu-rich nanoprecipitates exhibited an ultralow κlatt ∼ 0.58 W/m·K at 723 K, comparable with that of its Ag counterpart, regardless of the differences in the size of the precipitates, type of precipitate-matrix interfaces, and other nanoscopic architectures. The sample processed by flash SPS manifested higher figure of merit ( zT ∼ 0.9 at 723 K) because of better optimization and a trade-off between the transport properties by decreasing the carrier concentration and κlatt without degrading the carrier mobility. In spite of their comparable σ and κtot, zT of the Cu samples is low compared to that of the Ag samples because of their contrasting thermopower values. First-principles calculations attribute this variation in the Seebeck coefficient to dwindling of the energy gap (from 0.1 to 0.02 eV) between the valence and conduction bands in MPb18SbTe20 (M = Cu or Ag) when Cu replaces Ag.

Structural and electrical studies of olivine LiNi1 − x(Co0.5Mn0.5)xPO4 (0 ≤ x ≤ 1) at high temperature

A series of olivine-structured of LiNi1 − x(Co0.5Mn0.5)xPO4 (0 ≤ x ≤ 1) compositions have been synthesized by two-step conventional solid-state method at high temperature of 1000 °C followed by slow cooling or quenching. The XRD patterns of the obtained samples confirmed the formation of LiNi1 − x(Co0.5Mn0.5)xPO4 (0 ≤ x ≤ 1) phase with orthorhombic structure, Pmna space group. The lattice parameters determined by Rietveld refinements increase significantly as Co and Mn substitute for Ni. The SEM images show that the sample x = 0.8 started to partially melt while x = 1 has fully melted at temperature 1000 °C. The AC impedance spectroscopy measurements have revealed that the conductivity measured at 300 °C of all the slow-cooled samples can be enhanced by quenching. The quenched sample with x = 0.8 showed the largest increment of conductivities from 3.70 × 10−8 S cm−1 to 1.05 × 10−6 S cm−1 as a function of the cooling condition.

The effect of matrix morphology on dynamic-mechanical properties of polypropylene/layered silicate nanocomposites

In this work, the influence on the morphology and viscoelastic behavior of polypropylene/clay nanocomposites of clay, in combination with different crystallization rates applied in compression molding, is reported. By deconvolution of differential scanning calorimetry (DSC) melting endotherms, it was found that the slowly cooled samples had slightly higher melting temperatures, and the crystal dimensions decreased progressively with the clay content; while, in contrast, the presence of clay particles had no influence on the crystal dimensions in fast-cooled samples. Dynamic mechanical thermal analysis (DMTA) has shown that above the glass transition temperature, nanocomposites obtained by slow cooling exhibited better mechanical response compared to the fast-cooled samples. The value of dynamic modulus E’ of slow-cooled samples increased by ~55 % with addition of only 1 wt% clay, which was attributed to the better reinforcing effect achieved during prolonged time of crystallization.

Modification of defects and potential fluctuations in slow-cooled and quenched Cu2ZnSnSe4 single crystals

Recent literature reports have shown the ability to manipulate Cu-Zn cation ordering for Cu2ZnSnSe4 (CZTSe) via low temperature treatments. Theoretical arguments suggest that one of the major roadblocks to higher VOC—significant band tailing—could be improved with increased cation order; however, few direct measurements have been reported and significant device improvements have not yet been realized. This report investigates electrical properties, defects, and devices from quenched and slow-cooled single crystals of CZTSe. The extent of disorder was characterized by Raman spectroscopy as well as x-ray diffraction, where the change in Cu-Zn order can be detected by a changing c/a ratio. Quenched samples show higher acceptor concentrations, lower hole mobilities, and a lower-energy photoluminescence (PL) peak than crystals cooled at slower rates, consistent with a reduction in the bandgap. In addition, samples quenched at the highest temperatures showed lower PL yield consistent with higher quantities of deep defects. Devices fabricated using slow-cooled CZTSe single crystals showed improved efficiencies, most notably with increased VOC; however, low temperature intensity-dependent photoluminescence measurements continue to indicate the existence of potential fluctuations. We discuss the possibility that potential fluctuations in slow-cooled samples may be related to the inability to achieve a long range order of the Cu-Zn sub-lattice resulting in local regions of high and low levels of cation order, and consequent local variations in the bandgap. The presence of significant potential fluctuations, even after the slow-cooling step, suggests the difficulty in eliminating band-tailing in this system, and thus, additional approaches may be needed for significant reduction of the VOC deficit.

Neutron diffraction measurement of residual stresses in Al-clad U–10Mo fuel plates

Neutron diffraction was used to determine residual stress in monolithic two Al-clad U 10 weight percent Mo mini-fuel plates and a full sized fuel plate. One mini-plate was cooled following hot isostatic pressing at a rate of 6.75 °C/min, the second at 0.675 °C/min. A non-traditional method of calibrating the neutron diffractometer at each measurement point was necessitated by the thin nature of the sample. The in-plane stresses in the U–10Mo foils are relatively large, −250 MPa in the U–10Mo foil of the fast cooled mini-plate,-150 MPa in the slow cooled mini-plate and −275 MPa in the full-sized plate. Likewise, the in-plane stresses in the Al-cladding of the fast-cooled mini-plate and full-sized plate were determined to reach ∼50 MPa, while in the slow-cooled sample the stresses in the Al cladding were on the level of the measurement uncertainty. The in-plane stresses in the Zr diffusion barrier were estimated to be as large as −300 MPa.

Structural inhomogeneities in [formula omitted]: Relation to superconductivity

Chemical and structural phase compositions of two single-crystalline samples prepared with different cooling rates from stoichiometric FeTe0.6Se0.4 melts were studied. Both types of samples were investigated in a very comprehensive way using magnetic and electrical transport measurements combined with X-ray, neutron and electron backscatter diffraction. We show that slowly cooled samples are homogeneous on a microscopic scale with only a small excess of iron. Those slowly cooled samples do not exhibit bulk superconductivity down to 1.8K. In contrast, fast-cooled samples are superconducting below about 14K but are composed of several chemical phases: they consist of a matrix preserving the crystal structure of slow-cooled samples, and of core-shell structured dendritic inclusions (about 20–30vol%). These have different crystal structures and chemical compositions and order magnetically at temperatures far above the superconducting transition temperature of the inhomogeneous samples. These structural and chemical inhomogeneities seem to play a vital role in the superconducting properties of this and similar iron-based systems as they lead to internal stress and act in a similar way as the application of the external pressure that reportedly increase the superconducting transition temperature in many iron pnictides and chalcogenides. We argue that a phase pure, homogeneous and stress-free FeTe0.6Se0.4 is non-superconducting.

Isothermal Treatment Effects on the Carbon Vacancy in 4H Silicon Carbide

The carbon vacancy (VC) is a minority carrier lifetime controlling defect in 4H-SiC and it is formed during high temperature treatment. In this study, we have performed heat treatment on two sets of n-type 4H-SiC epitaxial samples. The first set was isothermally treated at 1850 °C to follow the evolution of VC as a function of time. The VC concentration is not affected by changing the duration. Samples of the other set were treated at 1950 °C for 10 min, but with different cooling rates and a reduction of the VC concentration was indeed demonstrated by lowering the cooling rate. The VC concentration in the slow-cooled sample is about 2 times less than in the fast-cooled one, reflecting a competition between equilibrium conditions and the cooling rate.

Dynamic Wetting of CaO-Al2O3-SiO2-MgO Liquid Oxide on MgAl2O4 Spinel

Inclusion type and content in steel is critical in steelmaking, affecting both productivity through clogging, and downstream physical properties of the steel. They are normally removed from steel by reacting with a slag (liquid oxide) phase. For efficient inclusion removal, the inclusions must attach/bond with this liquid phase. The strength of the attachment can be in part characterized by the wettability of the liquid oxide on the inclusions. In this study, the dynamic wetting of liquid oxides of the CaO-Al2O3-SiO2-MgO system on a solid spinel (MgAl2O4) substrate with low porosity of 1.9 pct was measured at 1773 K (1500 °C) using a modified sessile drop technique. The dynamic contact angle between the liquid and solid spinel was determined for different CaO/Al2O3 mass percent ratios ranging from 0.98 to 1.55. Characteristic curves of wettability (θ) vs time showed a rapid decrease in wetting in the first 10 seconds tending to a plateau value at extended times. A mathematical model for spreading behavior of liquid oxides by Choi and Lee was adopted and shown to provide a reasonable representation of the spreading behavior with time. The chemical interaction at the interface between spinel (MgAl2O4) and slag was analyzed by carrying out detailed thermodynamic evaluation and characterization using scanning electron microscopy/energy dispersive spectroscopy. There is evidence of liquid penetrating the substrate via pores and along grain boundaries, forming a penetration layer in the substrate. The depth of the penetration layer was found to be a function of substrate porosity and sample cooling rate. It decreased from ~350 µm for 6.7 pct-porous substrate to ~190 µm for substrate with porosity of 1.9 pct and from ~190 µm to ~50 µm for a slow-cooled liquid oxide-spinel substrate sample in the furnace to a rapidly cooled liquid cooled-spinel substrate sample, respectively.

B-Cation Order Control of Magnetism in the 1322 Perovskite CaCu3Fe2Nb2O12

Cation ordering in the 1322 perovskite CaCu3Fe2Nb2O12 has been investigated by synchrotron X-ray diffraction structure analysis, high-resolution electron microscopy, and magnetic measurements. All of the samples have full 1:3 ordering of Ca and Cu cations at the perovskite A-sites, but the Fe and Nb cations at the B sites show different degrees of ordering depending on the synthesis conditions. A slow-cooled sample has almost complete rock-salt-type 1:1 cation ordering at the B sites, but a rapidly cooled sample has no long-range B-cation arrangement. The ordered compound shows a large saturated magnetization (5.7 μB/formula unit (fu)) below 170 K due to the ferrimagnetism of the A′-site Cu2+ and B-site Fe3+ spins, whereas the B-site disordered form has a small magnetization (<1 μB/fu at 5 K). The degree of B-site cation order thus controls the magnetic behavior of 1322 CaCu3Fe2Nb2O12 perovskites.

Influence of Jahn–Teller Cu2+ (3d9) ion on structural and magnetic properties of Al–Cr co-substituted CuFe2O4

Four specimens of two compositions of the spinel oxide system CuAlxCrxFe2−2xO4 with x = 0.2 and 0.6 have been synthesized by double sintering ceramic technique with different thermal history i.e. quenched and slow-cooled. X-ray diffractometry has confirmed that tetragonal distortion is present in slow-cooled sample, while it is absent in quenched sample. The lower value of saturation moment compared to Neel’s moment for each composition is explained by invoking random canting of spins model. Hyperfine interaction parameters: nuclear hyperfine field, quadrupole splitting (shifts), isomer shift and Fe3+ ion distribution are deduced through computer simulation of Mossbauer spectra. 57Fe Mossbauer spectra recorded at room temperature show the effect of magnetic dilution of inter-sublattice (A–B) interaction (frustration) more in case of quenched-sample compared to slow-cooled one. FTIR spectra also corroborate the influence of Jahn–Teller (J–T) distortion of octahedral void arising due to larger Cu2+ ion occupancy at octahedral sites. The force constants are also estimated from FTIR spectra.