Microscopic phase-field simulation of Cr atom substitution character during formation of Ll2 and DO22 phases in Ni-Cr-Al alloy

The simulations of Cr atom substitution character during the formation of Ll2 and DO22 phases in Ni-Cr-Al alloy were performed at 873 K based on microscopic phase-field model. It is found that the substitution of Cr is affected by Cr and Al contents and limits of occupation probabilities of Cr atom in Ll2 phase are present. The precipitate is single Ll2 phase when the component is less than the limit, Cr atoms substitute the Al sublattices in Ll2 phase, and both of atoms Al and Cr occupy the β-sites and complex phases Ni3(Al1-xCrx) are formed; Cr atoms enter Ni sites when Al and Cr contents exceed the limit, and substitute β-sites or both of α- and β-sites. The DO22 phase is formed at the boundary of Ll2 phase.     

Structure and migration characteristic of heterointerfaces during the phase transformation from L1<Subscript>2</Subscript> to DO<Subscript>22</Subscript> phase

Based on the microscopic phase-field model, the structure and migration characteristic of ordered domain interfaces formed between DO₂₂ and L1₂ phase are investigated, and the atomistic mechanism of phase transformation from L1₂ (Ni₃Al) to DO₂₂ (Ni₃V) in Ni₇₅Al _x V_25−x alloys are explored, using the simulated microstructure evolution pictures and the occupation probability evolution of alloy elements at the interface. The results show that five kinds of heterointerfaces are formed between DO₂₂ and L1₂ phase and four of them can migrate during the phase transformation from L1₂ to DO₂₂ except the interface (002)_D//(001)_L. The structure of interface (100)_D//(200)_L and interface (100)_D//(200)_L·1/2[001] remain the same before and after migration, while the interface (002)_D//(002)_L is formed after the migration of interface (002)_D//(002)_L·1/2[100] and vice versa. These two kinds of interface appear alternatively. The jump and substitute of atoms selects the optimization way to induce the migration of interface during the phase transformation, and the number of atoms needing to jump during the migration is the least among all of the possible atom jump modes.     

Influence of Tb substitution on low-field magnetocaloric effect in Gd5Si1.72Ge2.28 alloy

The lattice parameters, magnetic phase transition, Curie temperature and magnetocaloric properties for (Gd_1−x Tb _x )₅Si_1.72-Ge_2.28 alloys with x = 0, 0.15, 0.20 and 0.25 were investigated by X-ray powder diffractometry and magnetization measurements. The results show that suitable partial substitution of Tb in Gd₅Si_1.72Ge_2.28 compound remains the first-order magnetic-crystallographic transition and enhances the magnetic entropy change, although Tb substitution decreases the Curie temperature (T _C) of the compounds. The magnetic entropy change of (Gd_1−x Tb _x )₅Si_1.72Ge_2.28 alloys retains a large value in the low magnetic field of 1.0 T. The maximum magnetic entropy change for (Gd_0.80Tb_0.20)₅Si_1.72Ge_2.28 alloy in the magnetic field from 0 to 1.0 T reaches 8.7 J/(kg·K), which is nearly 4 times as large as that of (Gd_0.3Dy_0.7)₅Si₄ compound (|ΔS _max| = 2.24 J/(kg·K), T _C = 198 K). Foundation item: Project (50371058) supported by the National Natural Science Foundation of China     

Magnetostrictive distortion in Pr0.15Tbx Dy0.85-xFe2 polycrystals by X-ray diffraction

The ingots of Pr0.15 Tbx Dy0.85-x Fe2 (x=0.10-0.85) series compounds with a single phase were prepared by a arc melting method. The X-ray diffraction patterns were measured using a Philips X‘pert MPD X-ray diffractometer with a non-ambient sample stage at different temperatures, the magnetostfictive distortion in Pr0.15 Tbx Dy0.85-x Fe2 polycrystals was investigated by X-ray diffraction patterns and the magnetostfiction coefficient λ111 was calculated. The results show when the temperature is raised above the spin reorientation temperature region, a splitting appears in the reflection (440); the λ111 increase with the increasing of Tb content for Pr0.15 Tbx Dy0.85-x Fe2 polycrystals and thefull width at half maximum (FWHM) of the reflection (440) increases gradually with the increasing of Tb content.Moreover, as the FWHM of the reflection (440) decreases gradually with the increasing of temperature, the λ111 decreases slightly with the increasing of temperature at the temperature region of 223 -373 K for Pr0.15 Tb0.3 Dy0.55 Fe2 alloy.     

Devitrification behaviour of rapidly solidified Al87Ni7Cu3Nd3 amorphous alloy prepared by melt spinning method

Rapidly solidified Al87Ni7Cu3Nd3 amorphous alloy was prepared by using melt spinning. Its calorimetric behavior was characterized by using differential scanning calorimeter in a continuous or isothermal heating mode. phase transformation was investigated, with a special interest in primary crystallization, by using an in-situ examination of X-ray diffractometry （XRD） and high resolution transmission electron microscopy （HRTEM）. The results show that, the whole devitrification of rapidly solidified Al87NiyCu3Nd3 amorphous alloy involves two main processes of primary crystallization and secondary crystallization that consist mainly of two reactions. For primary crystallization, the apparent activation energies, EIso and EKis and growth activation energies Eg are about 153, 166 and 288 kJ/mol, respectively. The interdiffusion of Al atoms is a rate-controlled step of formation of the a（Al） particles, but slow diffusion of Ni and Nd atoms plays a significant role in retarding growth of the α （Al） particles. For secondary crystallization, EIso, EKis and Eg of the first reaction are about 291,208 and 290 kJ/mol, and those of the second reaction are about 367, 269 and 372 kJ/mol. The two reactions of secondary crystallization are controlled mainly in an interface-controlled three-dimensional mode, depending mainly on slow diffusion of Ni and Nd atoms.     

Microscopic phase-field simulation for coarsening behavior of L12 and DO22 phases of Ni75Cr x Al25? x alloy

Based on the microscopic phase-field dynamic model and the microelasticity theory, the coarsening behavior of L1₂ and DO₂₂ phases in Ni₇₅Cr _x Al_25−x alloy was simulated. The results show that the initial irregular shaped, randomly distributed L1₂ and DO₂₂ phases are gradually transformed into cuboidal shape with round corner, regularly aligned along directions [100] and [001], and highly preferential selected microstructure is formed during the later stage of precipitation. The elastic field produced by the lattice mismatch between the coherent precipitates and the matrix has a strong influence on the coarsening kinetics, and there is no linear relationship between the cube of the average size of precipitates and the aging time, which does not agree with the results predicted by the classical Lifshitz-Slyozov-Wagner. The coarsening processes of L1₂ and DO₂₂ phases are retarded in elastically constrained system. In the concurrent system of L1₂ and DO₂₂ phases, there are two types of coarsening modes: the migration of antiphase domain boundaries and the interphase Ostwald ripening. Foundation item: Project(50671084) supported by the National Natural Science Foundation of China; Project(20070420218) supported by China Postdoctoral Science Foundation     

Electrolysis expansion performance of semigraphitic cathode in [K3AlF6/Na3AlF6]-AlF3-Al2O3 bath system

The electrolysis expansion of semigraphitic cathode in [K₃AlF₆/Na₃AlF₆]-AlF₃-Al₂O₃ bath system was tested by self-made modified Rapoport apparatus. A mathematical model was introduced to discuss the effects of α _CR (cryolite ratio) and β _KR (elpasolite content divided by the total amount of elpasolite and sodium cryolite) on performance of cathode electrolysis expansion. The results show that K and Na (potassium and sodium) penetrate into the cathode together and have an obvious influence on the performance of cathode electrolysis expansion. The electrolysis expansion and K/Na penetration rate increase with the increase of α _CR. When α _CR=1.9 and β _KR=0.5, the electrolysis expansion is the highest, which is 3.95%; and when α _CR=1.4 and β _KR=0.1, the electrolysis expansion is the lowest, which is 1.28%. But the effect of β _KR is correlative with α _CR. When α _CR=1.6 and 1.9, with the increase of β _KR, the electrolysis expansion and K/Na penetration rate increase. However, when α _CR=1.4, the electrolysis expansion and K/Na penetration rate firstly increase and then decrease with the increase of β _KR. Foundation item: Project (2005CB623703) supported by the Major State Basic Research and Development Program of China; Project (2008AA030502) supported by the National High-Tech Research and Development Program of China     

Effect of doped Mn on piezoelectric properties of （Na0.5Bi0.5 ）0.92 Ba0.08 TiO3 lead-free ceramics

Piezoelectric ceramics (Na0.5Bi0.5)0.92Ba0.05TiO3 x%MnCO3(BNBT-Mn, x=0-1.6,mass fraction)were synthesized by conventional solid state reaction. The results show that when the addition of MnCO3 is 0-1.4%, BNBT-Mn ceramics exhibit a single-phase perovskite structure. With the increase of content of MnCO3, piezoelectric constant and electromechanical coupling factor increase rapidly when x is lower than 0.3. Then they both decrease when x is in the range of 0.3 and 1.6. When x=0.3, piezoelectric constant and electromechanical coupling factor reach the maximum value of 160 pC/N and 58.5% respectively, which can improve the temperature stability of BNBT-Mn.     

First-principle investigation on stability of Co-doped spinel λ-Mn4-xCoxO8

The mechanism of stability of Co-doped spinel λ-MnO2 that is referred to as spinel LixMn2O4 （x=0） was studied by using the first-principle calculation method. The total energy and formation enthalpy can be decreased remarkably due to the Co substation, resulting in a more stable structure ofλ-MnxCr2-xO4. The bond order and DOS analysis were given in detail to explain the nature of stability improvement. The calculated results show that as the content of Co dopant increases, the bond order of Mn-O becomes larger and the peak of density of states around Fermi level shifts toward lower energy. The charge density distribution illustrates that the Mn-O bonding is ionic and partially covalent, and the covalent Mn-O bonding becomes stronger with the increase of Co dopant content. The results confirm that the Co-doping will enhance the stability of λ-MnO2 and hence improve the electrochemistry performance of LixMn2O4.     

Effects of heat treatment temperature on crystallization and thermal expansion coefficient of Li2O-Al2O3-SiO2

The basic glass of Li₂O-Al₂O₃-SiO₂ system using P₂O₅ as nucleator was prepared by means of conventional melt quenching technology, and the heat-treatment process was determined by using differential thermal analysis. The crystalline phases and the microstructure of the glass-ceramics were investigated by using X-ray diffraction and scanning electron microscopy. The results show that the glass based on Li₂O-Al₂O₃-SiO₂ oxides using P₂O₅ as nucleator can be prepared at lower melt temperature of 1450°C and the glass-ceramics with lower thermal expansion coefficient of 21.6 × 10⁻⁷°C⁻¹ can also be obtained at 750°C. The glass-ceramics contain a few crystal phases in which the main crystal phase is β-quartz solid solution and the second crystal phase is β-spodumene solid solution. When the heat treatment temperature is not higher than 650°C, the transparent glass-ceramics containing β-quartz solid solution can be prepared. β-quartz solid solution changes into β-spodumene solid solution at about 750°C. And the appearance of the glass-ceramics changes from translucent, part opaque to complete opaque with increasing temperature.     

The Effect of Bi2O3 on the Structure and Microwave Dielectric Properties of Ba6-3xNd8＋2xTi18O54 System（x=2/3）

The structures and dielectric properties of Ba6-3xNd8＋2xTi18O54 system（x=2/3） doped with different contents of Bi2O3, whose final molecular formula is Ba6-3x（Nd1-yBiy）8＋2xTi18O54 were investigated. It is indicated that the dielectric constant increases greatly whereas Q value（f0=4 GHz） decreases with the increase of Bi2O3 content. However, the temperature coefficient could be controlled below 0±30×10^-6/℃ in the experiment. These phenomena are related to the appearance of a new phase, Bi4Ti3O12, which has high dielectric constant. Also, that Bi^3＋（0.13 nm） substitutes for Nd^3＋（0.099 5 nm） will increase the unit cell volume, which will lead to the enlargement of the octahedron B site occupied by Ti^4＋. So the spontaneous polarization of Ti^4＋ ions will be strengthened. Besides, Bi^3＋ will fill up some vacancies which Ba^2＋ or Nd^3＋ ions leave in two A1 sites and four A2 sites. More positive ions polarize, which also contributes to higher dielectric constant. The samples got with the optimium properties are sintered at 1 200 ℃ for 4 h, when y=0.25, ε≈110, Q≈5 400（f0=4 GHz）, TCC=-4.7×10^-6/℃; When y=0.3, ε≈120, Q≈5 000（f0=4 GHz）, TCC=-24×10^-6/℃.     

Influence on performance and structure of spinel LiMn2O4 for lithium-ion batteries by doping rare-earth Sm

The spinel LiMn₂O₄ used as cathode materials for lithium-ion batteries was synthesized by mechano-chemistry fluid activation process, and modified by doping rare-earth Sm. Thesting of X-ray diffraction, cyclic voltammograms, charge-discharge and SEM was carried out for LiMn₂O₄ cathode materials and the modified materials. The results show that the cathode materials doped rare earth Li _x Mn_2−y Sm _z O₄ (0.95⩽x⩽1.2, 0⩽y⩽0.3, 0⩽z⩽0.2) exhibit standard spinel structure, high reversibility of electrochemistry and excellent properties of charge-discharge. In EC: DMC(1 : 1)+1 mol/L LiPF₆ electrolyte with discharge capacity more than 130 mA · h/g, and its capacity is deteriorated less than 15% after 300 cycles at room temperature and less than 20% after 200 cycles at 55°C. At the same time, Crystal Field Theory was applied to explain the function and mechanism of doped rare earth element. Foundation item: Project (02JJY2081) supported by the Natural Science Foundation of Hunan Province     

Thermodynamic assessment of the ZrO2-AlO1.5 system

The ZrO₂-AlO_1.5 quasibinary system has been assessed by means of the CALPHAD (CALculation of PHAse Diagram) technique. The liquid phase, cubic (fluorite-type) zirconia solid solution and tetragonal zirconia solid solution are described by a regular solution model. The monoclinic zirconia and α-AlO_1.5 are treated as stoichiometric phase. A consistent set of optimized parameters describing the system have been obtained to agree with almost all of the avaiable experimental data. Comparisons between the assessed and experimental data are presented. It is shown that further studies are needed for equilibrium solubilities of AlO_1.5 in ZrO₂ phases. Project supported by the National Natural Science Foundation of China and the National Key Laboratory of Powder Metallurgy Synopsis of the first author Wang Tao, born in 1972, doctoral student, majoring in phase diagram and materials design.     

Preparation of LiFePO4 for lithium ion battery using Fe2P2O7 as precursor

In order to obtain a new precursor for LiFePO4, Fe2P2O7 with high purity was prepared through solid phase reaction at 650 ℃ using starting materials of FeC2O4 and NH4H2PO4 in an argon atmosphere. Using the as-prepared Fe2P2O7, Li2CO3 and glucose as raw materials, pure LiFePO4 and LiFePO4/C composite materials were respectively synthesized by solid state reaction at 700 ℃ in an argon atmosphere. X-ray diffractometry and scanning electron microscopy（SEM） were employed to characterize the as-prepared Fe2P2O7, LiFePO4 and LiFePO4/C. The as-prepared Fe2P2O7 crystallizes in the Cl space group and belongs to β-Fe2P2O7 for crystal phase. The particle size distribution of Fe2P2O7 observed by SEM is 0.4-3.0 μm. During the Li^＋ ion chemical intercalation, radical P2O7^4- is disrupted into two PO4^3- ions in the presence of O^2-, thus providing a feasible technique to dispose this poor dissolvable pyrophosphate. LiFePO4/C composite exhibits initial charge and discharge capacities of 154 and 132 mA·h/g, respectively.     

Synthesis of γ-Al2O3 nanoparticles by chemical precipitation method

Highly pure active γ-Al₂O₃ nanoparticles were synthesized from aluminum nitrate and ammonium carbonate with a little surfactant by chemical precipitation method. The factors affecting the synthesis process were studied. The properties of γ-Al₂O₃ nanoparticles were characterized by DTA, XRD, BET, TEM, laser granularity analysis and impurity content analysis. The results show that the amorphous precursor Al(OH)₃ sols are produced by using 0.1 mol/L Al(NO₃)₃ · 9H₂O and 0.16 mol/L (NH₄)₂CO₃ · H₂O reaction solutions, according to the volume ratio 1.33, adding 0.024% (volume fraction) surfactant PEG600, and reacting at 40 °C, 1 000 r/min stirring rate for 15 min. Then, after stabilizing for 24 h, the precursors were extracted and filtrated by vacuum, washed thoroughly with deionized water and dehydrated ethanol, dried in vacuum at 80°C for 8 h, final calcined at 800 °C for 1 h in the air, and high purity active γ-Al₂O₃ nanoparticles can be prepared with cubic in crystal system, O _H ⁷ -FD3M in space group, about 9 nm in crystal grain size, about 20 nm in particle size and uniform size distribution, 131. 35 m²/g in BET specific surface area, 7 – 11 nm in pore diameter, and not lower than 99.93% in purity. Foundation item: Project(03JJY3015) supported by the Natural Science Foundation of Hunan Province     

Effect of acetic acid on electrochemical deposition of carbon-nitride thin film

Electrochemical deposition method was employed to prepare CNx thin film from methanol-urea solution,and it was shown that adding a little acetic acid in the solution significantly affected the deposition process.After optimizing the experiment conditions,we obtained polycrystalline grains with sizes of about 3―7μm on the faces of single crystal silicon.X-ray diffraction spectrua indicate that the grains are mainly composed of cubic phase mixed with a small amount of β and α phases.     

Crystallization kinetics of amorphous Nd3.6 Pr5.4 Fe83Co3B5 and preparation of α-Fe/Nd2Fe14B nanocomposite magnets by controlled melt-solidification technique

The crystallization kinetics of amorphous Nd3. 6 Pr5.4 Fe83 Co3 B5 and the preparation of α-Fe/Nd2 Fe14 B nanocomposite magnets by controlled melt-solidification of Nd3.6Pr5.4Fe83Co3B5 was investigated by employing DTA, XRD, and TEM. The results show that a metastable intermediate phase Nd8Fe27B24 prior to α-Fe and Nd2 Fe14 B phases is crystallized as the amorphous Nd3.6 Pr5.4 Fe83 Co3 B5 is heated to 1 223 K. The crystallization activation energy of α-Fe and Nd8 Fe27324 phases is larger at the beginning stage of crystallization, and then it decreases with crystallized fraction x for the former and has little change when x is below 70% for the latter, which essentially results in an α-Fe/Nd2 Fe14 B microstructure with a relatively coarse grain size about 20-60 nm and a non-uniform distribution of grain size in the annealed alloy. The a-Fe/Nd2 Fe14 B nanocomposite magnets with a small average grain size about 14 nm and a quite uniform grain size distribution were prepared by controlled melt-solidification of nealing the amorphous Nd3. 6 Pr5. 4 Fe83 Co3 B5 precursor alloy.     

Effects of temperature and initial molar ratio of Na2O to Al2O3 on agglomeration of fine Al（OH）3 seed in synthetic Bayer solution

Fine Al（OH）3 crystals were aggregated from supersaturated aluminate solution in the batch reaction tanks. By means of laser particle size analyzer and scanning electron microscopy, the influences of temperature and initial molar ratio of Na2O to Al2O3 （aK） on agglomeration of fine seed in Bayer process were investigated. The results show that agglomeration is almost finished in 8 h, and seeds with size less than 2 μm are easily aggregated together, and almost disappear in 8 h under the optimal process conditions. In the aluminate solution with the same moderate initial aK, when the reaction temperature reaches 75 ℃, the secondary nucleation does not occur, and the effect of agglomeration is better. And at the same reaction temperature, when the initial aK is 1.62, the initial supersaturation of aluminate solution is moderate, the binders on the surfaces of the seed are enough to maintain the agglomeration process, and the agglomeration degree is better. From SEM images, agglomeration mainly occurs in the fine particles, the combinations among the fine particles are loose and the new formed coarse crystal shapes are irregular.     

Transport Properties of Polycrystalline La2/3Ca1/3Mn1-xCuxO3

The infuence of Cu dopant (x) and sintering temperature(Ts) on the troansport propeties of La2/3Ca1/3Mn1-xCuxO3 series samples prepared by Sol-Gel technique was investigated.X-ray diffraction patterns show that all the samples with different Cu dopant and sintering temperatures (Ts) are of single phase without obvious latice distortion.Experimental results indicate that the insulator-metal transition temperature is diectly related to the sintering temperature and Cu dopant x.It is interesting to observe that a proper amount of Cu dompant can substantially improve magnetoresistance effects.     

Electronic structure and FIM imaging mechanism of Fe-Al intermetallic compounds

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