C54 Phase Research Articles

Binary Blends of Diblock Copolymers: An Efficient Route to Complex Spherical Packing Phases

AbstractThe phase behavior of binary blends composed of AB and AB diblock copolymers is systematically studied using the polymeric self‐consistent field theory, focusing on the formation and relative stability of various spherical packing phases. The results are summarized in a set of phase diagrams covering a large phase space of the system. Besides the commonly observed body‐centered‐cubic phase, complex spherical packing phases including the Frank–Kasper A15, and the Laves C14, C15 phases could be stabilized by the addition of longer AB‐copolymers to asymmetric AB‐copolymers. Stabilizing the complex spherical packing phases requires that the added AB‐copolymers have a longer A‐block and an overall chain length at least comparable to the host copolymer chains. A detailed analysis of the block distributions reveals the existence of inter‐ and intra‐domain segregation of different copolymers, which depends sensitively on the copolymer length ratio and composition. The predicted phase behaviours of the AB/AB diblock copolymer blends are in good agreement with previous experimental and theoretical results. The study demonstrates that binary blends of diblock copolymers provide an efficient route to regulate the emergence and stability of complex spherical packing phases.

Insights into structural and dynamical characteristics of III-V boron polytypes

III-V boron materials are a class of semiconductors with a wide range of outstanding properties and applications. In this paper, the density functional theory is used to explore the structural phase stability, atomic structure, electron band structures and dynamics properties of cubic (3C) and hexagonal (2H) polytypes of boron III-V compounds. We show that polytypism has generally a weak impact on electronic and lattice dynamics properties of BP, BAs, BSb, and BBi.We find small total energy difference between cubic and hexagonal boron-V compounds (~18.7meV/atom, ~15.8meV/atom, ~6.9meV/atom, and ~15.8meV/atom for BP, BAs, BSb and BBi respectively). Accurate quasiparticle (modified Becke-Johnson (mBJ) potential, and local density approximation-1/2) schemes predicted indirect bandgap for 3C and 2H phases of BP, BAs and BSb. Interestingly, BBi evidence narrower direct bandgap for both cubic (~0.38eV) and hexagonal (~0.19eV) crystal polytypes, which makes BBi compound a promising candidate for optoelectronic applications such as infrared sources and detectors. The dynamical stability of 2H boron compounds has also been studied to unravel the possible feasibility of experimental realizations of these compounds. Moreover, we find that the phonon properties of boron III-V compounds contrast considerably with other III-V materials, such the overbending of the transverse optical branches, the bunching of the acoustic branches, the weak LO-TO splitting, and the atypical negative Born effective charges.

Crystal structure and magnetic properties of ternary Al3CoNd2 compound

A ternary compound Al3CoNd2 was synthesized and its crystal structure parameters were determined by the Rietveld refinement method based on powder X-ray diffraction data. Results show that the compound crystallizes in the MgCu2-type structure (cubic Laves C15 phase, space group $Fd\bar{3}m$), with the lattice parameter of a = 7.8424(2) Ǻ, unit-cell volume of V = 482.33 Å3, and calculated density of Dcalc = 5.90 g.cm−3. The residual factors converge to Rp = 0.1024 and Rwp = 0.1287. The reference intensity ratio value obtained experimentally is 3.03. Magnetic susceptibility measurements indicate an agreement with the Curie–Weiss law in the temperature range of 385–450 K, and paramagnetic Curie temperature of θp = 379.9 K. Both rare-earth elements and cobalt ions contribute to the paramagnetic moment. The saturation magnetic moment and magnetic hysteresis loop were measured for the Al3CoNd2 compound at various temperatures. Results show that the saturation magnetic moment value decreases with an increase in temperature and the compound becomes a ferromagnet below the Curie temperature Tc.

Effects of Ti substitution for Zr on the electrochemical characteristics and structure of AB2-type Laves-phase alloys as metal hydride anodes

The structural composition and electrochemical capacity of four AB2 Laves-type intermetallic alloys with various Ti/Zr ratios (TixZr1−xLa0.03Ni1.2Mn0.7V0.12Fe0.12, x = 0.12, 0.15, 0.18, and 0.22) were investigated in this study. The alloys were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. These data revealed the coexistence of the main phase of the C15-type FCC Laves-type AB2 compounds with a secondary La–Ni intermetallic that was present in minor amounts. Increasing the Ti substitution for Zr caused the gradual shrinkage of the unit cells of the C15 phase. The neutron powder diffraction studies demonstrated that in the trihydride (Ti, Zr, V)(Ni, Mn, Fe, V)2D3.2, D atoms filled A2B2 tetrahedra, whereas V atoms occupied not only conventional 16d sites but also partially replaced Zr/Ti at 8a sites. All studied alloys showed similar activation behaviors, wherein four cycles were required to realize the highest electrochemical capacity of the anodes. The Ti12/Zr88 alloy demonstrated excellent full discharge capacity that reached 466 mAh/g. The Ti22/Zr78 alloy electrode exhibited good cycling stability (retention rate of ~71%) after 500 cycles and a superior high-rate discharge capability (retention rate of ~71%) at a discharge current density of 400 mA/g. The cycling of the Ti22/Zr78 alloy electrode was studied by electrochemical impedance spectroscopy (EIS) to understand the reasons for the deterioration of cycling capacity, which was related with the pulverization of the alloys and increase in irreversible capacity.

Revealing phase evolution mechanism for stabilizing formamidinium-based lead halide perovskites by a key intermediate phase

Revealing phase evolution mechanism for stabilizing formamidinium-based lead halide perovskites by a key intermediate phase

Experimental investigation of phase equilibria in the Ni-Ta-Si refractory alloy system

Three isothermal sections of the Ni-Ta-Si ternary system at 1300, 1100 and 900 °C were constructed in the whole composition range and analysed using electron probe microanalysis (EPMA), scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. In addition to the known ternary G, E, H and C14 phases, a new compound with the composition of Ni23–25Ta39–40Si35–38 was detected for the first time, herein designated as the F phase. Furthermore, the continuous (Ni, Si)Ta2 phase was found to be stable and the composition range of the ternary C14 Laves phase was revised to be Ni39–54Ta26–33Si13–27. The newly determined phase equilibria in the Ni-Ta-Si ternary system can be useful in guiding alloying strategies or composition design of Ni-based superalloys.

Investigation of the phase occurrence and H sorption properties in the Y33.33Ni66.67-xAlx (0 ≤ x ≤ 33.33) system

The Y33.33Ni66.67-xAlx system has been investigated in the region 0 ≤ x ≤ 33.3. The alloys were synthesized by induction melting. Phase occurrence and structural properties were studied by X-Ray powder Diffraction (XRD). The Al solubility in each phase has been investigated by XRD and Electron Probe Micro-Analysis (EPMA). The hydrogenation properties were characterized by pressure-composition isotherm measurements and kinetic curves at 473 K. For x = 0, the binary Y33.33Ni66.67 alloy crystallizes in the cubic superstructure with F4̅3m space group and ordered Y vacancies. For 1.67 ≤ x ≤ 8.33, the alloys contain mainly Y(Ni, Al)2 and Y(Ni, Al)3 pseudobinary phases; while for 16.67 ≤ x ≤ 33.33 they are mainly formed by ternary line compounds Y3Ni6Al2, Y2Ni2Al and YNiAl. Contrary to the binary Y33.33Ni66.67, Y(Ni, Al)2 pseudo-binary compounds crystalize in C15 phase (space group Fd3¯m) with disordered Y vacancies. The solubility limit of Al in the C15 YNi2-yAly phase is y = 0.11 (i.e., x = 3.67). The Y(Ni, Al)3 phase changes from rhombohedral (PuNi3-type, R3¯m) to hexagonal (CeNi3-type, P63/mmc) structure for x increasing from 5.00 to 6.67. Upon hydrogenation, the disproportion of C15 Y(Ni, Al)2 and losses of crystallinity of YNi and Y2Ni2Al are the main reasons causing capacity decay of Y33.33Ni66.67-xAlx (0 ≤ x ≤ 33.33) alloys upon cycling.

Second phase structure analysis and wear behavior of the ultra-high manganese steel

The high manganese steel is widely used in wear-resisting parts of the cone crusher, such as the grinding mill wall and crushing wall. In this work, the microstructure and wear resistance of an ultra-high manganese steel (Mn18Cr2Mo) were analyzed by electron diffraction and high-resolution transmission electron microscope (HRTEM) observation and impact abrasive wear test. The results show that there are spherical or block Mo2C and strip-like (Fe, Cr)3C phases in Mn18Cr2Mo manganese steel. The orientation of the Mo2C phase and the austenite matrix are [001]austenite// Mo2C, (020)austenite//(020)Mo2C, austenite//(202)Mo2C and austenite//(242)Mo2C. The that of the (Fe, Cr)3C phase and the austenite matrix are [011]austenite// (Fe, Cr)3C and austenite//(210)(Fe, Cr)3C. Besides, austenite is approximately parallel to (Fe, Cr)3C. The interface relation of (002)austenite is approximately parallel to (Fe, Cr)3C. Owing to high density dislocations, deformation bands, microcrystal and non-crystalline, the wear resistance of the Mn18Cr2Mo manganese steel is 1.83 times that of the Mn13 manganese steel under the impact abrasive wear test.

Deformation behavior and microstructure evolution of as-cast Ti2ZrMo0.5Nb0.5 high entropy alloy

Abstract A high entropy alloy (HEA) with a composition of Ti2ZrMo0.5Nb0.5 was prepared by vacuum induction melting technology. The mechanical properties at room temperature and elevated temperature (900 °C–1150 °C), microstructure evolution and hot deformation behavior during hot deformation are studied. The as-cast alloy is composed of equiaxed grains with the yield strength, the apparent plastic strain of 1306 MPa and 44%, respectively. By TEM analysis, the ZrTi2 Laves C15 phase is found corresponding the precipitation strengthening effect accompanied with the solid solution strengthening as a ramification of the high-entropy alloy. Then the hot compression tests were carried out. The deformed alloy still maintains the same single BCC structure as the as-cast state. It is noted that when the temperature is 900 °C, the structure of the alloy mainly exhibits dynamic recovery (DRV) characteristics, and when the temperature reaches 1000 °C, there are clear dynamic recrystallization (DRX) characteristics in the structure, which are analyzed as the features of continuous dynamic recrystallization (CDRX). Keeping the temperature at 1150 °C, when the strain rate is 0.1 s−1, the characteristics of discontinuous dynamic recrystallization (DDRX) appear, and as the strain rate decreases, CDRX becomes the main recrystallization mechanism again.

Microstructure and properties of NbVZr refractory complex concentrated alloys

The microstructure and mechanical properties of the equiatomic NbVZr alloy are reported. The laser-processed and as-cast samples both exhibit a dendritic BCC solid solution with hexagonal C14 and cubic C15 Laves phases in the interdendritic regions. The stability of the Laves phases is discussed based on first-principles total energy calculations. Nb is found to increase the stability of C14 and C15 phases. The Laves phases strengthen the material by acting as obstacles to dislocation motion, as evidenced by nanoindentation and TEM observations of deformed samples. A high concentration of stacking faults was observed in the Laves region, which confirms the low stacking fault energy predicted by first-principles calculations, and are expected to result in additional plasticity. This work serves as an example of a systematic study of the stability of intermetallic phases in refractory complex concentrated alloys using a high-throughput synthesis technique. Such phases play an important role in the overall mechanical properties of the alloy.

Crystal Structure of BaCa(CO3)2 Alstonite Carbonate and Its Phase Stability upon Compression.

New single-crystal X-ray diffraction experiments and density functional theory (DFT) calculations reveal that the crystal chemistry of the CaO–BaO–CO2 system is more complex than previously thought. We characterized the BaCa(CO3)2 alstonite structure at ambient conditions, which differs from the recently reported crystal structure of this mineral in the stacking of the carbonate groups. This structural change entails the existence of different cation coordination environments. The structural behavior of alstonite at high pressures was studied using synchrotron powder X-ray diffraction data and ab initio calculations up to 19 and 50 GPa, respectively. According to the experiments, above 9 GPa, the alstonite structure distorts into a monoclinic C2 phase derived from the initial trigonal structure. This is consistent with the appearance of imaginary frequencies and geometry relaxation in DFT calculations. Moreover, calculations predict a second phase transition at 24 GPa, which would cause the increase in the coordination number of Ba atoms from 10 to 11 and 12. We determined the equation of state of alstonite (V0 = 1608(2) Å3, B0 = 60(3) GPa, B′0 = 4.4(8) from experimental data) and analyzed the evolution of the polyhedral units under compression. The crystal chemistry of alstonite was compared to that of other carbonates and the relative stability of all known BaCa(CO3)2 polymorphs was investigated.

Epitaxial Stabilization of Metastable 3C BaRuO3 Thin Film with Ferromagnetic Non‐Fermi Liquid Phase

AbstractThin films of perovskite ruthenates of the general formula ARuO3 (A = Ca and Sr) are versatile electrical conductors for viable oxide electronics. They are also scientifically intriguing, as they exhibit nontrivial electromagnetic ground states depending on the A‐site element. Among them, realization of the cubic perovskite (3C) BaRuO3 in thin film form has been a challenge so far, because the 3C phase is metastable with the largest formation energy among the various polymorph phases of BaRuO3. In this study, 3C BaRuO3 thin films are successfully prepared employing epitaxial stabilization. The 3C BaRuO3 thin films show itinerant ferromagnetism with a transition temperature of ≈48 K and a non‐Fermi liquid phase. The epitaxial stabilization of the 3C BaRuO3 further enables to make a standard comparison of perovskite ruthenates, thereby establishing the importance of the RuO orbital hybridization in understanding the itinerant magnetic system.

New wide-pore superficially porous stationary phases with low hydrophobicity applied for the analysis of monoclonal antibodies

The article describes the development of new stationary phases for the analysis of proteins in reversed phase liquid chromatography (RPLC). The goal was to have columns offering high recovery at low temperature, low hydrophobicity and novel selectivity. For this purpose, three different ligands bound onto the surface of superficially porous silica-based particles were compared, including trimethyl-silane (C1), ethyl-dimethyl-silane (C2) and N-(trifluoroacetomidyl)-propyl-diisopropylsilane (ES-LH). These three phases were compared with two commercial RPLC phases.In terms of protein recovery, the new ES-LH stationary phase clearly outperforms the other phases for any type of biopharmaceutical sample, and can already be successfully used at a temperature of only 60°C. In terms of retention, the new ES-LH and C1 materials were the less retentive ones, requiring lower organic solvent in the mobile phase. However, it is important to mention that the stability of C1 phase was critical under acidic, high temperature conditions. Finally, some differences were observed in terms of selectivity, particularly for the ES-LH column. Besides the chemical nature of the stationary phase, it was found that the nature of organic modifier also plays a key role in selectivity.

Lattice stability and point defect energetics of TiSi2 and TiGe2 allotropes from first-principles calculations

This work determines the phase stabilities and point defect energetics of TiSi2 and TiGe2 allotropes using density functional theory. The primary focus is on the C49 and C54 allotropes, which compete during TiSi2 phase formation. It is found that the ground state structure for TiGe2 is the C54 allotrope, desirable for its low sheet resistance, while the less desirable, higher resistance C49 allotrope forms the ground state structure of TiSi2. A first attempt to understand the Ge atom's role in lowering the enthalpy of formation for the C54 structure is made from the perspective of the extended Born model. Charge density differences, the density of states, and Bader charge analysis show that these systems are predominantly ionically bonded, with the Ge atoms introducing additional covalent bond stability for the C54 allotrope. It is known that higher temperatures favor C54 formation in TiSi2. Helmholtz free energy calculations for TiSi2 suggest that the vibrational free energy does not drive the system to the C54 phase. The formation energies of certain point defects within the C49 structure of TiSi2 are less than 1 eV, which is consistent with experiments that show high defect concentrations. Thus, the driving force for C54 formation at higher temperatures may be related to the high defect concentration in the C49 allotrope.

Phase Transformations During Homogenization of Inconel 718 Alloy Fabricated by Suction Casting and Laser Powder Bed Fusion: A CALPHAD Case Study Evaluating Different Homogenization Models

Estimating the required homogenization time during post-heat treatment after alloy fabrication is critical for both casting and additive manufacturing (AM). In this work, three CALPHAD-based modeling approaches to estimate the homogenization time in order to dissolve the Laves_C14 phase into γ matrix are evaluated for Inconel 718 alloys made by suction casting and laser powder bed fusion. These values are compared with the homogenization at 1180 °C for different durations. The compositions of the γ matrix obtained from experiments are used as inputs for the first model. The first model involves single-phase diffusion simulations using the diffusion module (DICTRA) implemented in the Thermo-Calc software package with composition profile for the Laves_C14 phase either determined from experiments or calculated by the lever rule. The second model uses Scheil simulations for predicting the segregation profiles, which are used as inputs for single-phase simulations. In the third model, moving boundary simulations using DICTRA are performed using the composition of Laves_C14 phase from the lever rule. The homogenization time determined using the first model matches reasonably well with the experimental observation for the AM alloy. The second model is imprecise as the segregation from Scheil calculation is not reliable for AM alloy. The last model is inaccurate due to lack of mobility data for atomic diffusion in the Laves_C14 phase. The predicted homogenization times for the cast alloys using these models do not match with the experimental values. This necessitates the need for determining the mobilities for Laves_C14 phase for improving the accuracy of DICTRA simulations to estimate the homogenization time.

Preceramic paper-derived Ti3Al(Si)C2-based composites obtained by spark plasma sintering

The paper describes the structure and properties of preceramic paper-derived Ti3Al(Si)C2-based composites fabricated by spark plasma sintering. The effect of sintering temperature and pressure on microstructure and mechanical properties of the composites was studied. The microstructure and phase composition were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. It was found that at 1150 °C the sintering of materials with the MAX-phase content above 84 vol% leads to nearly dense composites. The partial decomposition of the Ti3Al(Si)C2 phase becomes stronger with the temperature increase from 1150 to 1350 °C. In this case, composite materials with more than 20 vol% of TiC were obtained. The paper-derived Ti3Al(Si)C2-based composites with the flexural strength > 900 MPa and fracture toughness of >5 MPa m1/2 were sintered at 1150 °C. The high values of flexural strength were attributed to fine microstructure and strengthening effect by secondary TiC and Al2O3 phases. The flexural strength and fracture toughness decrease with increase of the sintering temperature that is caused by phase composition and porosity of the composites. The hardness of composites increases from ~9.7 GPa (at 1150 °C) to ~11.2 GPa (at 1350 °C) due to higher content of TiC and Al2O3 phases.

Experimental investigation of phase equilibria in the Fe-Hf-Zr system at 1173 K and 1373 K

A series of Fe-Hf-Zr alloys were designed to determine the phase equilibria for the entire composition range by annealing at 1173 K and 1373 K for 440–720 h and 12–360 h respectively, followed by X-ray diffraction (XRD) measurements and electron probe microanalysis (EPMA). At the Fe-rich corner, it was argued that the Fe23Zr6 phase is a stable phase without oxygen contamination, of which the formation mechanism was discussed. The Fe2(Hf,Zr)_C15 phase forms a continuous solid solution. The two-phase region of β(Hf,Zr) + Fe(Hf,Zr)3 was detected at 1173 K. The Fe(Hf,Zr)3 phase was observed in this ternary system at 1173 K, which is attributed to the increasing thermodynamic stability by dissolving Hf. The solubility of Fe in α(Hf,Zr) was determined to be extremely low. The liquid phase appears at 1373 K in a large composition range. No ternary compound was identified at both temperatures in this system. Based on the experimental results and reasonable extrapolations, the isothermal sections of 1173 K and 1373 K were constructed, which respectively consist of seven and five three-phase regions.

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The C36 Laves phase in diblock polymer melts.

The C14 and C15 Laves phases form as micelle packing structures in many types of soft matter, but the related C36 phase, which consists of alternating C14-type and C15-type layers, has not been observed in any such system. To understand this absence in the context of diblock polymers, we used self-consistent field theory to relate the morphology and energetics of C36 to other known mesophases. Two case studies were conducted: blends of AB diblock polymers with A homopolymers (where A forms the micelle core), in which C14 and C15 have stability windows, and neat AB diblock melts, in which Laves phases are metastable. Laves phases exhibit nearly identical micelle morphologies and nearly degenerate free energies, with the free energy of C36 being a near-perfect bisector of the C14 and C15 free energies in all cases, revealing an intrinsic symmetry in free energy that is attributed solely to the structural relationship between the phases in which the packing of C36 is intermediate between C14 and C15. Based on this connection between structure and free energy, C36 is thus not expected to form in flexible diblock polymers, since C14 and C15 can always form instead via facile mass transfer.

Formation of complex spherical packing phases in diblock copolymer/homopolymer blends

The formation and relative stability of spherical packing phases in binary blends composed of AB-diblock copolymers and A-homopolymers are systematically studied using the self-consistent field theory applied to the freely-jointed chain model of polymers. Phase diagrams with different model parameters of the blends are constructed, revealing that the emergence of various complex spherical packing phases, including the Frank-Kasper σ phase and the Laves C14 and C15 phases, could be induced by the addition of the homopolymers. For BCC-forming diblock copolymers, a phase transition sequence of BCC →σ→ C14 → C15 is predicted when the homopolymer concentration is increased. An analysis of the properties of A-domains reveals that their sizes are regulated by the differential localization of A-homopolymers. The resultant spherical domains of different sizes is a key factor to stabilize complex spherical packing phases, especially the Laves C14 and C15 phases. Furthermore, the phase behaviour is strongly affected by the chain length of the homopolymers. In particular, the addition of short homopolymers results in an expanded region of the cylindrical phase, whereas the addition of long homopolymers stabilizes the complex spherical packing phases. The theoretical results from current study are in good agreement with recent experiments and theory, and shed light to the formation of complex spherical packing phases in other self-assembling soft matter systems.