Phase Thermal Stability Research Articles

IONIC LIQUIDS – NEW GAS CHROMATOGRAPHIC PHASES WITH UNIQUE PROPERTIES. A REVIEW

A necessary and the most difficult element in the study of many chemical processes is a reliable chemical analysis of both the initial reagents and the products of their transformation. One of the most common methods for such analysis is a capillary gas chromatography, which, unfortunately, has a number of limitations. The limitations include, for example, the low thermal stability of high polarity stationary liquid phases (SLF). A way to overcome this limitation is the use of ionic liquids (IL) as SLF. In this review, we focus on the use of liquid phases based on ILs with cations of various chemical classes. The properties of such phases are considered from the point of view of their possible chromatographic selectivity, and examples of the use of columns with ILs for solving specific analytical problems are given. The properties of currently available commercial columns where ILs are used as the phases are discussed.

Grain size effect on the temperature-dependence of elastic modulus of nanocrystalline NiTi

The effect of grain size on temperature-dependent elastic modulus (TDEM) of polycrystalline NiTi over a temperature span of 200 °C was investigated by dynamic thermomechanical analysis (DMA). It is found that the TDEM rapidly reduced with grain size down to nanoscale. In-situ neutron diffraction and molecular dynamics (MD) simulation reveal that the volume fraction and thermal stability of austenite phase at low temperature gradually increased with the reduction of grain size due to the mechanical constraint of grain boundary, which results in the two-phase coexistence during cooling. The compensation of the intrinsic opposite TDEM of austenite and martensite leads to the overall reduced TDEM in nanocrystalline NiTi. This study strongly implies that the TDEM of shape memory alloy can be tailored by grain size.

Emulsification and corrosivity study of bio-oil and vacuum gas oil mixtures with a novel surfactant system

Co-processing bio-oil (BO) with petroleum intermediates such as vacuum gas oil (VGO) in existing refining infrastructures provides a promising pathway to transform low-quality BO into value-added biofuels. Nevertheless, the immiscibility between BO and VGO is one of the major obstacles to BO co-processing, especially during the feeding stage of upgrading. To inject BO and VGO as a single-phase fluid and maximize their interactions, emulsification was applied to mix BO and VGO. In this study, a novel surfactant system, based on Span 80 (SP), Tween 80 (TW), and methanol (ME), was developed to suit the remedy to the immiscibility of BO and VGO. Thermogravimetric analysis (TGA), X-ray imaging, and optical microscopy were employed to examine the thermal behavior, phase stability, and microstructure of the as-developed emulsions. Immersion experiments were conducted with carbon steel (CS) strip coupons in BO/VGO emulsions at 50 °C for corrosivity assessment. Results showed that surfactant ST (a mixture of SP and TW) outranked SP and TW in stabilizing BO/VGO emulsions indicating that surfactants with a middle hydrophilic-lipophilic balance value (approximately 8) are more suitable for BO/VGO emulsions. Adding ME notably prolonged the stabilization time of BO/VGO emulsions. The addition of new mixed surfactant STM (e.g., BO:VGO:STM with a weight ratio of 1:4:0.1) resulted in the longest phase separation starting time (100 min, at 50 °C), one order of magnitude longer than that of BO:VGO as 1:4. The BO/VGO emulsion stayed single-phase for 80, 50, and 20 min when reduced the STM ratio to 0.05, 0.03, and 0.02, respectively. TGA results indicated that hydrogen transfers from VGO to BO reduced the coke formation of BO by 55 % at above 400 °C, highlighting the significance of mixing BO and VGO properly before co-processing. Immersion experiments concluded that the addition of studied surfactants did not show prominent effects on CS corrosion in BO/VGO emulsions.

High-temperature strengthening mechanism and thermal stability of Laves phase in ferritic matrix

The high-temperature creep strengthening mechanism and thermal stability of Laves phase are the key factors that affect its strengthening property in ferritic matrix. Therefore, in this study, two types of C14 Laves phase alloys, namely, Fe2Nb Laves phase Alloy S1 (Fe–9Cr–4Al–2Nb) (wt.%) and Fe2W Laves phase Alloy S2 (Fe–9Cr–4Al–2W) (wt.%) were prepared to investigate the strengthening mechanism and strengthening effect of Laves phase with different compositions in ferritic matrix. The creep properties and microstructure characteristics of the two alloys, in particular, the interfacial structure characteristics between Laves phases (Fe2Nb and Fe2W) and ferritic matrix were analyzed. The true stress exponent n for both Alloys S1 and S2 is 5, and the true activation energies Q of Alloys S1 and S2 are 372.47 and 365.82 kJ mol−1, respectively. The creep mechanisms of Alloys S1 and Alloy S2 are both dislocation climbing-dominated processes reinforced with precipitated phases. Alloy S1 exhibits higher high-temperature creep resistance than that of Alloy S2. The grain size of Alloy S1 is smaller than that of Alloy S2. Both the precipitates Fe2Nb in Alloy S1 and Fe2W in Alloy S2 are mostly located at the grain boundary. The density of dislocation in Alloy S1 is higher than that in Alloy S2, and the bending degree of dislocation lines caused by precipitate hindrance is also higher than that in Alloy S2. The orientation relationships between the Laves phase Fe2Nb and ferritic matrix in Alloy S1 are [011]α-Fe//[5 1‾9‾4‾]Fe2Nb, (011‾)α-Fe//(03 3‾ 1)Fe2Nb, with the atomic interface mismatch of 1.6%; the orientation relationships between the Laves phase Fe2W and ferritic matrix in Alloy S2 are [001]α-Fe//[01 1‾ 2]Fe2W, (1 1‾ 0)α-Fe//(01 1‾1‾) Fe2W, with the atomic interface mismatch of 1.9%. The results indicate that the Fe2Nb-type Laves phase may be more suitable to candidate for the strengthener in the ferritic matrix compared with Fe2W-type.

A Lection in Humbleness: Crystallization of Chiral and Zwitterionic APIs Baclofen and Phenibut

Crystallization and multicomponent crystal formation of active pharmaceutical ingredients Baclofen and Phenibut with dicarboxylic acid co-formers are discussed. The crystallization process of several crystalline entities is elucidated via single crystal—as well as powder X-ray—diffraction, followed by thermal analysis and phase stability studies over time. Both APIs form increasingly complex crystalline phases with co-formers malic and tartaric acid, where phase purity of a desired compound is not necessarily a given. Therefore, the influence of different solution and milling environments during crystallization on the outcome is studied. Emphasis is laid on how molecular influences such as the chirality, propensity to form hydrates as well as low solubility of Baclofen and Phenibut impede attempts to gather high-quality single crystals. The results highlight that targeted crystallization of these compounds with dicarboxylic acids can be difficult and unreliable.

Room temperature imidazolium linked chalcone based ionic LCs: Role of terminal position on mesomorphism

A newly imidazolium linked chalcone ionic LCs (5a-5e, series-1) & (8a-8e, series-2) based on two phenyl rings have been synthesized by the quaternization of 1-methylimidazole and ortho & para-substituted hydroxy chalcone derivatives. The mesogenic behavior of synthesized ionic mesogens was perceived by POM, DSC and XRD measurements. The structure-property relationship of new class of chalcone linked imidazolium ILCs has been studied. All materials exhibited enantiotropical type mesogenic properties. Few selected ionic derivatives showed LC properties at room temperature with higher temperature range. Smectic C and nematic properties were detected in synthesized ionic compounds. Series-2 showed higher thermal stability of smectic and nematic phase as compared to series-1. The X-ray configurations revealed the coexistence of bilayer smectic C pattern over a well-defined temperature range. The molecular structures of target ILCs were identified by using IR and 1H NMR spectroscopy.

Block Copolymer and Cellulose Templated Mesoporous TiO2-SiO2 Nanocomposite as Superior Photocatalyst

A dual soft-templating method was developed to produce highly crystalline and mesoporous TiO2-SiO2 nanocomposites. Pluronic F127 as the structure-directing agent and pure cellulose as the surface area modifier were used as the templating media. While Pluronic F127 served as the sacrificing media for generating a mesoporous structure in an acidic pH, cellulose templating helped to increase the specific surface area without affecting the mesoporosity of the TiO2-SiO2 nanostructures. Calcination at elevated temperature removed all the organics and formed pure inorganic TiO2-SiO2 composites as revealed by TGA and FTIR analyses. An optimum amount of SiO2 insertion in the TiO2 matrix increased the thermal stability of the crystalline anatase phase. BET surface area measurement along with low angle XRD revealed the formation of a mesoporous structure in the composites. The photocatalytic activity was evaluated by the degradation of Rhodamine B, Methylene Blue, and 4-Nitrophenol as the model pollutants under solar light irradiation, where the superior photo-degradation activity of Pluronic F127/cellulose templated TiO2-SiO2 was observed compared to pure Pluronic templated composite and commercial Evonik P25 TiO2. The higher photocatalytic activity was achieved due to the higher thermal stability of the nanocrystalline anatase phase, the mesoporosity, and the higher specific surface area.

Development of MoNbVTax refractory high entropy alloy with high strength at elevated temperature

Due to a monolithic body-centered cubic solid solution structure, refractory high-entropy alloys (HEA) always exhibit high strength and good softening resistance at both ambient and elevated temperatures. However, high density and low plasticity at ambient temperature limit their widespread applications as one kind of potential structural materials. The present study investigated the microstructure and mechanical properties of MoNbVTax HEA with various Ta conent at both ambient and elevated temperatures. The sample of MoNbVTax (x = 0, 0.3, 0.5, 0.7, 1) all shows a monolithic BCC solid solution phase with a dendritic microstructure. The micro-segregation of constituent elements was caused by the non-equilibrium solidification. The alloys exhibit superior high temperature performance at 1000 °C and 1200 °C, with a certain degree of plasticity at ambient temperature. The MoNbVTa0.5 alloy possesses both excellent high temperature performance (σ0.2 =697 MPa, εp = 35.4% at 1200 °C) and relatively low density (∼9.65 g/cm3), which is attributed to the solid solution strengthening effects and excellent thermal stability of constituent phase. Compared with commercial superalloys, the present MoNbVTa HEAs possess the advantages of high temperature performance, making it a potential engineering material for structural application.

The effect of oxidation state and tert-butyl substituents on the thermal behavior and thin-film morphology of cobalt-complexes (FK 102 and FK 209)

This work reports the thermodynamic and morphological study and characterization of four salts consisting of a divalent/trivalent cobalt complex with pyrazole-pyridine ligands (FK 102 and FK 209 samples) and bis(trifluoromethylsulfonyl)imide (TFSI) moieties as counter anions. The oxidation state of the central metal (Co(II) or Co(III)) and the presence of tert-butyl (t-Bu) groups in the ligand structure were found to have a strong impact on the thermal behavior, phase stability, heat capacities, and thin-film morphology of each salt. The Co(II) complexes exhibited good thermal stability up to 600 K. Lower thermal stability was observed for the Co(III) congeners. The FK209 Co(III) displayed a higher melting temperature but a partial decomposition during or above melting was detected. The higher melting temperatures observed for the Co(III) complexes were found to be entropically driven. However, the addition of t-Bu in the ligand (FK 209) leads to an increase in the melting temperature, which is driven by the enthalpy of fusion. The four compounds studied evidenced a large glass-forming ability. Moreover, the thermal stability of the glassy state was clearly increased when the ligands comprised t-Bu groups. The contribution of the t-Bu group for the molar heat capacity in the solid phase, at T = 298.15 K, was found to be (110 ± 3) J·K−1·mol−1 and (98 ± 4) J·K−1·mol−1 for the Co(II) and Co(III) complexes, respectively. These results are in good agreement with the contribution of the t-Bu group observed for both solid and liquid phases in other materials, indicating that the t-Bu groups are relatively unhindered in the crystalline phase of the salts. The morphological behavior of the thin films of FK 102 samples was found to be quite similar to the observed for typical ionic liquids, with the formation of micro- and nanodroplets onto different substrates. The introduction of t-Bu substituents in the ligand structure was found to have a strong impact on the formation of homogeneous and compact nanofilms for the FK 209 salts.

Surface decorations to abrupt change performance, robustness, and stability of electrodes for solid oxide cells

AbstractSolid oxide cell electrodes need to display a wide range of combined properties, for example, electronic and ionic conductivities, catalytic activity, chemical compatibility with other parts, thermal expansion match, phase stability, and tolerance to foreign species/contaminants. It is extremely challenging for a single material to meet all these features required. Surface decoration is a remarkably adaptive strategy to provide needed features to the electrodes by introducing functioning materials through various target‐guided methods. This review provides an overview of typical and novel techniques to create surface decoration on the electrodes to abruptly improve the drawbacks of the electrodes of solid oxide cells, including performance enhancement, stability of long‐term operation, and tolerance to poisoning foreign species. It starts with the introduction of surface decoration experimental procedure, proceeds to crucial characterization methods that are particularly effective in identifying the properties of the surface coating, continues with target‐oriented practice in terms of surface decoration materials choice and microstructure manipulation, and ends with the explorations of functioning mechanisms for the resultant behavior. With hope, the review will spark more novel utilizations of surface decoration practice to promote the technical status of the solid oxide cells for widespread commercialization in the future.

Scaling effects in the alternating-current poling of thin PIN-PMN-PT single crystals

AC-poling of Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) single crystals with a thickness of 0.06–0.16 mm was studied in this paper. Compared with DC-poled samples, enhancements in piezoelectric and dielectric properties can be obtained when the thickness is above 0.1 mm. However, inconsistency in poling effects was found in the crystals with thickness below 0.1 mm. To elucidate why such scaling effect arises, surface roughness was measured by an atomic force microscopy to correlate surface morphology and poling effects. It was found that non-uniform surface roughness led to inconsistent and decreased properties. Furthermore, temperature-dependent dielectric permittivity spectra were measured to explore how crystal thickness affects the thermal stability of ferroelectric phases. It is noted that complex changes in crystallographic symmetries emanate by decreasing thickness. Such phenomena can be attributed to more influential effects of surface morphology when thickness is reduced. We hope this work suggests a clue for solving the scaling effects of AC-poling on relaxor-PbTiO3 single crystals.

Aluminous hydrous magnesium silicate as a lower-mantle hydrogen reservoir: a role as an agent for material transport

The potential for storage of a large quantity of water/hydrogen in the lower mantle has important implications for the dynamics and evolution of the Earth. A dense hydrous magnesium silicate called phase D is a potential candidate for such a hydrogen reservoir. Its MgO–SiO2–H2O form has been believed to be stable at lower-mantle pressures but only in low-temperature regimes such as subducting slabs because of decomposition below mantle geotherm. Meanwhile, the presence of Al was reported to be a key to enhancing the thermal stability of phase D; however, the detailed Al-incorporation effect on its stability remains unclear. Here we report on Al-bearing phase D (Al-phase D) synthesized from a bridgmanite composition, with Al content expected in bridgmanite formed from a representative mantle composition, under over-saturation of water. We find that the incorporation of Al, despite smaller amounts, into phase D increases its hydrogen content and moreover extends its stability field not only to higher temperatures but also presumably to higher pressures. This leads to that Al-phase D can be one of the most potential reservoirs for a large quantity of hydrogen in the lower mantle. Further, Al-phase D formed by reaction between bridgmanite and water could play an important role in material transport in the lower mantle.

Computational Thermodynamics and Microstructure Simulations to Understand the Role of Grain Boundary Phase in Nd-Fe-B Hard Magnets

To control the coercivity of Nd hard magnets efficiently, the thermal stability of constituent phases and the microstructure changes observed in hard magnets during thermal processes should be understood. Recently, the CALPHAD method and phase-field method have been recognized as promising approaches to realize phase stability and microstructure developments in engineering materials. Thus, we applied these methods to understand the thermodynamic feature of grain boundary phase and the microstructural developments in Nd-Fe-B hard magnets. The results are as follows. 1) The liquid phase is a promising phase for covering the Nd2Fe14B grains uniformly. 2) The metastable phase diagram of the Fe-Nd-B ternary system suggests that the tie line end of the liquid phase changes drastically depending on the average composition of Nd. 3) The Nd concentration in the grain boundary phase can reach 100 at% if the volume fraction of the grain boundary phase is constrained. 4) The effect of Cu addition to the Nd-Fe-B system on the microstructural morphology is reasonably modeled based on the phase-field method. 5) The morphology of the liquid phase can be controlled using phase separation in the liquid phase and the grain size of Nd2Fe14B phase.

Resonant phonon modes induced by molecular rotations in α-pentaerythritol crystals

Resonant phonon modes induced by molecular rotations in α-pentaerythritol crystals play a critical role in thermal transport and phase stability.

Morphology, thermal stability, electronic structure and mechanical properties of α-AlFeMnSi phases with varying Mn/Fe atomic ratios: Experimental studies and DFT calculations

Separation of Fe-containing intermetallics is an effective method to decrease Fe content in Al-Si alloys. The three-dimensional (3D) morphology and stability of Fe-containing intermetallics are important parameters affecting their separation efficiency. Combined with experiments and first-principles calculations, the variation of 3D morphology, stability, electronic structure and mechanical properties of α-AlFeMnSi are studied with the evolution of Fe/Mn atomic ratio. The experimental results show that the 3D morphology of α-AlFeMnSi transforms from Chinese character to regular polyhedron with the increment of Mn/Fe atomic ratio and all α-AlFeMnSi phases have cubic crystal structure. The calculated mixing enthalpies indicate that all α-AlFeMnSi phases are stable in thermodynamics due to the negative values. The electronic structure and Mulliken population calculations show that the variation of Mn/Fe ratio results in the overall change of chemical bondings of Si-Mn, Si-Fe, Al-Mn and Al-Fe, which leading to the evolution of stability and mechanical properties of α-AlFeMnSi phases. The calculated results of mechanical properties demonstrate that the Young’s modulus and hardness of α-AlFeMnSi phases are improved with the increment of Mn/Fe atomic ratio, which is also validated by nano-indentation measurements. These results provide the componential guidance about designing easy-separable Fe-containing intermetallics and try to explain physical mechanism of property evolution of α-AlFeMnSi phase.

Effect of Fe addition on glass-forming ability, thermal stability of B2 CuZr phase and crystallization kinetics for CuZr-based amorphous alloys

The glass-forming ability (GFA), thermal stability and crystallization kinetics of (Cu0.5Zr0.5)100-xFex (x = 2.5, 5, 7.5, 10 and 12.5 at.%) amorphous alloys were studied in detail. As Fe concentration increases, the GFA and thermal stability decrease first and then increase. The excessive Fe addition significantly deteriorates the GFA due to the precipitation of the FeZr2 phase. Interestingly, with increasing Fe concentration, the crystallization products gradually transit from Cu10Zr7 and CuZr2 to B2 CuZr phase, indicating that Fe addition can effectively enhance the thermal stability of B2 phase. Simulation results show that Fe atoms may be dissolved into the B2 lattice substituting Cu sites and decrease the formation energy, which is similar to Co–or Zn-added CuZr-based alloys. Non-isothermal crystallization behaviors of Cu–Zr–Fe amorphous alloys are well described by the Šesták-Berggren (SB) model instead of the John-Mehl-Avrami (JMA) model due to the multi-phase formation (low Fe concentration) or the nucleation and growth of B2 phase (high Fe concentration). These results could help with the understanding of further stabilizing B2 CuZr phase and the fabrication of high-performance CuZr-based composites.

Formation mechanism and thermal stability of C15-Laves phase in a Hf-containing Co-based superalloy

Topologically close-packed (TCP) phases such as μ and Laves phases formed during solidification has an important influence on the mechanical properties of superalloys. Understanding the formation and elimination mechanism of TCP phases is the key to promote the development of superalloys. In this study, C15-Laves phase was first found in a Hf-containing Co-based superalloy, meanwhile, the formation mechanism and thermal stability of the C15-Laves phase were clarified. The results showed that the formation of C15-Laves phase was associated with the segregation behavior of alloying elements during solidification: Co, W and Cr segregated to the dendrite core, and Hf, Ta, Ti, Mo, Ni, Al, Si segregated to the interdendritic region. In the final stage of solidification, the C15-Laves phase enriched in Hf, Ta and Ti formed, and it had similar lattice parameters to the C15-Co2Hf phase. The incipient melting point of C15-Laves phase was between 1170 ℃ and 1180 ℃, and it gradually decomposed to form the Hf-rich MC carbide during solution treatment. The large formation energy differences between the MC carbide and C15-Laves phase promoted a rapid elimination of the C15-Laves phase, which reduced solution treatment time of the alloy. The results can provide a theoretical basis and experimental data support for the composition design and heat treatment process optimization of multicomponent novel Co-based superalloys.

Formation of L10-FeNi hard magnetic material from FeNi-based amorphous alloys

L10-FeNi hard magnetic alloy with coercivity reaching 861 Oe was synthesized through annealing Fe42Ni41.3Si8B4P4Cu0.7 amorphous alloy, and the L10-FeNi formation mechanism has been studied. It is found the L10-FeNi in annealed samples at 400 °C mainly originated from the residual amorphous phase during the second stage of crystallization which could take place over 60 °C lower than the measured onset temperature of the second stage with a 5 °C/min heating rate. Annealing at 400 °C after fully crystallization still caused a slight increase of coercivity, which was probably contributed by the limited transformation from other high temperature crystalline phases towards L10 phase, or the removal of B from L10 lattice and improvement of the ordering quality of L10 phase due to the reduced temperature from 520 °C to 400 °C. The first stage of crystallization has hardly direct contribution to L10-FeNi formation. Ab initio simulations show that the addition of Si or Co in L10-FeNi has the effect of enhancing the thermal stability of L10 phase without seriously deteriorating its magnetic hardness. The non-monotonic feature of direction dependent coercivity in ribbon segments resulted from the combination of domain wall pinning and demagnetization effects. The approaches of synthesizing L10-FeNi magnets by adding Si or Co and decreasing the onset crystallization temperature have been discussed in detail.

Twist-bend nematics and heliconical cholesterics: a physico-chemical analysis of phase transitions and related specific properties

ABSTRACT Certain nematic liquid crystals, for example, those formed by banana-shaped molecules, can exhibit a low-temperature twist-bend nematic (Ntb) phase. Upon addition of chiral dopants, a chiral version of the twist-bend phase (N*tb) can be observed below the conventional chiral nematic (N*) phase, while under electric field the N* phase is transformed into a specific state known as ‘oblique helicoidal cholesteric’. In this work, we have studied the well-known Ntb-forming system CB7CB:CB6OCB with addition of 5CB and chiral dopants (ChDs) using optical microscopy, differential scanning calorimetry (DSC), and measurements of temperature-dependent optical transmission using optical microscopy. Effects of 5CB and different chiral dopants (of steroid or non-steroid nature) upon thermal stability of Ntb phase were analysed, and selective reflection in the visible range could be observed in the N* phase with a sufficiently high ChD content. As a peculiar feature, sharp unwinding of the cholesteric helix was observed at lower temperatures when approaching the N*tb phase, which appears to be similar to the helix unwinding close to the smectic-A transition.

Microstructure and Mechanical Properties of Mg–Er–Cu/Ni/Zn Alloys with Long Period Stacking Ordered Phases

The microstructure and mechanical properties of as‐cast Mg96Er3Cu1, Mg96Er3Ni1, and Mg96Er3Zn1 alloys prepared by conventional ingot metallurgy are investigated. Results show that the bulk 18R long period stacking ordered (LPSO) phases in all three as‐cast alloys solidify directly from the melt. The grain size of the Mg96Er3Zn1 alloy is the smallest; the volume fraction of the LPSO phase in the Mg96Er3Ni1 alloy is the highest in all three alloys due to negligible Ni solid solubility in α‐Mg. The thermal stability of the 18R LPSO phase in the as‐cast Mg96Er3Ni1 alloy is much higher than in the other two alloys. Tensile tests indicate that the as‐cast Mg96Er3Zn1 alloy shows the best ductility; the as‐cast Mg96Er3Ni1 alloy possesses the highest tensile strength, and the ultimate tensile strength and yield tensile strength are 245 and 149 MPa, respectively, accompanied by a good elongation of 7.1%.