La-rich Phase Research Articles

Microstructure Evolution and Mechanical Properties of Melt Spun Skutterudite-based Thermoelectric Materials.

The rapid solidification of melt spinning has been widely used in the fabrication of high-performance skutterudite thermoelectric materials. However, the microstructure formation mechanism of the spun ribbon and its effects on the mechanical properties are still unclear. Here, we report the microstructure evolution and mechanical properties of La–Fe–Co–Sb skutterudite alloys fabricated by both long-term annealing and melt-spinning, followed by sintering approaches. It was found that the skutterudite phase nucleated directly from the under-cooled melt and grew into submicron dendrites during the melt-spinning process. Upon heating, the spun ribbons started to form nanoscale La-rich and La-poor skutterudite phases through spinodal decomposition at temperatures as low as 473 K. The coexistence of the micron-scale grain size, the submicron-scale dendrite segregation and the nanoscale spinodal decomposition leads to high thermoelectric performance and mechanical strength. The maximum three-point bending strength of the melt spinning sample was about 195 MPa, which was 70% higher than that of the annealed sample.

Effects of minor La and Ce additions on microstructure and mechanical properties of A201 alloy

AbstractThis study investigates how minor La and Ce additions affect the microstructure and mechanical properties of A201 alloy. The results indicate that adding 0.2 wt.% La or Ce to A201 alloys could refine the grain size and led to a rise in the temperature of exothermic peak of Ω and θ' precipitates. In addition, the presence of La-rich phase or Ce-rich phase consumed the quantities of Cu atoms in the Al matrices during solidification. Neither of the phases could be dissolved during solution heat treatment, reducing the precipitation of Ω and θ' phases in A201 alloy after artificial aging heat treatment. The lower volume fraction of the Ω and θ' strengthening phases led to a decrease in hardness and tensile properties but an increase in ductility of the La-containing or Ce-containing A201 alloy.

Effect of excess lanthanum on corrosion and magnetocaloric property of LaFe11.5Si1.5 compounds

The corrosion and magnetocaloric property of LaFe11.5Si1.5 compound with excess La from 0 wt% to 15 wt% were investigated by a scanning electron microscope (SEM), electrochemical workstation and a vibrating sample magnetometer (VSM). With the increase of excess La from 0 wt%, 5 wt% to 10 wt% and 15 wt%, most of the La-rich phase changes from granular La2O3 phase to grain boundary LaFeSi phase. The La2O3 is more stable than the main 1:13 phase, while LaFeSi phase is easy to be corroded than the 1:13 phase. The maximum entropy change −ΔSmax is 18.60, 21.48, 19.66 and 16.28 J/(kg·K) for 0 wt%, 5 wt%, 10 wt% and 15 wt% excess of La, respectively. As adding 5 wt% excess La in LaFe11.5Si1.5 compound leads to nearly single 1:13 phase, the largest magnetic entropy change and better corrosion resistance are obtained.

Fabrication and optical properties of transparent LaErZr2O7 ceramic with high excess contents of La and Er

Abstract In this study, transparent LaErZr2O7 ceramic with high excess La and Er contents (nominally La1.28Er1.28Zr2O7.84) was successfully prepared by vacuum sintering at 1850 °C for 6 h using nanosized powder. The XRD, SEM, EDX and TEM results reveal that the single pyrochlore phase in the powder sample transforms to the coexistence of La-rich pyrochlore phase and Er-rich defect fluorite phase after high temperature sintering. The high excess amounts of La and Er favor the formation of pyrochlore structure. Despite the coexistence of two phases, the sample with 1 mm thickness shows excellent in-line transmittance in the visible to mid-infrared region (as high as 81% at 1100 nm). The upconversion and infrared emission under 980 nm exciting were measured and discussed as well.

Study of hydrogen storage and electrochemical properties of AB2-type Ti0.15Zr0.85La0.03Ni1.2Mn0.7V0.12Fe0.12 alloy

A C15 AB2 Laves-type Ti0.15Zr0.85La0.03Ni1.2Mn0.7V0.12Fe0.12 alloy was prepared by arc melting and annealing. Phase-structural composition, microstructure, hydrogen absorption-desorption properties, thermodynamic and electrochemical performances were characterized by X-ray diffraction, scanning electron microscopy, hydrogen absorption-desorption measurements and electrochemical characterization and were related to the use of the alloys as metal hydride battery anodes.The alloy contains a C15 FCC intermetallic compound as the main phase and a LaNi secondary phase as the minor constituent (∼1 wt%).During the electrochemical tests, the anode electrodes quickly, after just a few activation cycles, reached a maximum discharge capacity. This was related to the catalytic effect of the La-rich secondary phase which acted as a catalyst of hydrogen absorption-desorption.Annealing resulted in increase of the maximum discharge capacity from 345 mAh/g for the as cast alloy to 370 mAh/g. Furthermore, the annealed alloy showed a better high rate dischargeability and a higher cyclic stability. After 100 cycles with 100% DOD at discharge current density of 1C, the discharge capacity of the annealed alloy was very high, at a level of 90% of the initial capacity.The rates of hydrogen diffusion have been characterized by Potentiostatic Intermittent Titration Technique and Electrochemical Impedance Spectroscopy. With increasing an extent of transformation into the hydride, the H diffusion rate in the bulk of the alloy particles decreased. The maximum value of DH measured by PITT for the annealed alloy was observed for the nearly fully discharged electrode, (SOC 2%).

Magnetic properties and microstructure of nanocomposite (La, Pr)3Fe14B ribbons by doping La element

The research on the mixed rare earth-iron based permanent magnet compounds by doping rare earth elements in Nd-Fe-B or Pr-Fe-B permanent magnet has been a hot topic in recent years. In this paper, La doped Pr-Fe-B ((LaxPr1-x)3Fe14B, x=0, 0.3, 0.6, (La, Pr)-Fe-B) ribbons were prepared by vacuum arc melting technique and subsequently melt-spinning method at the optimal quenching speed. The microstructure, phase distribution and magnetic properties of the nanocomposite magnets were investigated. It is found that the samples present 2:14:1 main phase, La2O3 phase and minor α-Fe phase. No excess amorphous phase is observed, indicating that 20 m/s is an optimization of preparation condition to fabricate the (La, Pr)-Fe-B ribbons in this work. The EBSD results reveal that La-rich phases distribute mostly over the grain boundary. The substitution of La for Pr in Pr-Fe-B has some influence on the coercivity, but little on the saturation magnetization. This is thought to be related with the agglomeration of rare earth-rich intermetallic phase. We hope that this work can provide new ideas in the exploration of mixed rare earth-iron based permanent magnet compounds, which possess a cost advantage and a promising potential for practical applications.

Stability and mechanical properties of high-La content La–Ni phases by first-principles study

The structural stability, mechanical properties and Debye temperatures of La-rich La–Ni (LaNi, La2Ni3 and LaNi2) phases were studied by a first-principles method. The formation enthalpy indicates that the La-rich phase is stable. As the La content increases, the enthalpy of formation decreases. The bonding of La–Ni is covalent and metallic. The La-d and Ni-d orbitals contribute mainly at the Fermi level. The mechanical properties indicate that the La-rich La–Ni phases are brittle. La2Ni3 has the most prominent anisotropy. The hardness of LaNi, La2Ni3 and LaNi2 are 37.62 GPa, 41.61 GPa and 65.06 GPa, respectively. Those phases have the potential to form a superhard material. LaNi2 has the highest Debye temperature (232.88 K). LaNi2 is the most stable among these phases.

The effect of cooling rate on the phase formation and magnetocaloric properties in La0.6Ce0.4Fe11.0Si2.0 alloys

In this work, the microstructure, phase formation behavior of the NaZn13-type 1:13 phase and related magnetocaloric effect have been investigated in La0.6Ce0.4Fe11.0Si2.0 as-cast bulk and melt-spun ribbons with different cooling rates. A multi-phase structure consisting of 1:13, α-Fe and La-rich phases is observed in the induction-melted sample with slow cooling. By fast cooling in the melt spinning processing, the La-rich phase can be almost eliminated and thus 1:13 phases with volume fraction as high as 74.4% directly form in the absence of further heat treatment. The resulting maximum magnetic entropy change of 3.1 J/kg K in 2 T field appears at its Curie temperature of 210 K for the La0.6Ce0.4Fe11.0Si2.0 ribbon prepared in 25 m/s.

Effect of co-doping Y2O3–La2O3 on microstructure and electrical properties of ZnO-based varistor ceramics prepared from nanosize ZnO powder

ZnO-based varistor ceramics doped with fixed La2O3 and different Y2O3 have been fabricated by the conventional solid-state reaction route. Phase composition, microstructure, and electrical properties were studied by X-ray diffractometry (XRD), scanning electron microscopy (SEM), and direct current electrical measurement. The Y2O3 phase, Y-rich phase and La-rich phase were found to distribute along the grain boundaries of predominant ZnO grains from XRD and SEM analyses. The average grain size of the primary phases was calculated to be in the range of 3.80 to 4.17 μm. The varistor ceramic doped with 0.1 mol% La2O3 and 0.5 mol% Y2O3 sintered at 1100 °C for 2 h exhibited high comprehensive electrical properties with the breakdown field of 1055 V/mm, the nonlinear coefficient of 65.5 and the leakage current of 0.6 μA. The results illustrated that co-doping La2O3 and Y2O3 and employing the ZnO powder in nanometer size may be a promising route to obtain varistor ceramics with excellent electrical properties.

Thickness dependence of microstructure in thin La0.7Sr0.3MnO3 films grown on (1 0 0) SrTiO3 substrate

This study investigates the nanostructure of perovskite thin films, and its influence on magnetic properties. Epitaxial thin films of perovskite MnO3 with thickness ranging from nm to 320 nm were deposited on SrTiO3 (1 0 0) substrates using pulsed laser deposition techniques. X-ray diffraction, along with high-resolution transmission electron microscopy (HRTEM) investigations were carried out, and showed the presence of a second, La-rich phase in thick film. Scanning electron microscopy analysis showed thinner films to have smooth surfaces, while thicker films presented large triangular shapes emerging out of the film surface. Also HRTEM images revealed epitaxial growth for thinner films. Thicker films grow with numerous misfit dislocations leading to columnar structure, where the thickest film presents branched structure with an intermediate La-rich layer. Lattice mismatch, diffusion of Mn ions into the substrate and substituting Ti ions are known as sources of strain in films near interfaces. Magnetic properties in the structure mentioned are described by double-exchange interaction, which is very sensitive to local structure. Hence, the nanostructure of films reported here has a remarkable effect on their magnetic properties.

Phase precipitation behavior of rapidly quenched ternary La–Fe–B alloy and the effects of Nd substitution

To understand the structure and phase precipitation behavior of the ternary La–Fe–B alloy, the effects of wheel speed and heat treatment on the melt spun La2Fe14B alloy were investigated. The results show that the hard magnetic La2Fe14B (2:14:1) phase is difficult to obtain in the ternary alloy. The α-Fe and La phases mainly precipitated as spun ribbons prepared at relatively low wheel speeds, and a more amorphous phase formed at higher wheel speeds. With the annealing temperature increasing from 400 °C to 500 °C and 800 °C, the α-Fe and La phases also gradually precipitated from the amorphous matrix. A liquid La-rich phase and α-Fe phase could coexist in the alloy at high temperature. However, the substitution of Nd for La can greatly enhance the formation of a hard 2:14:1 phase. The effects of Nd substitution on the structure and phase precipitation during annealing for the (La1−xNdx)2Fe14B (x = 0.1. 0.2, 0.3 and 0.4) alloys have been also investigated. The present results are important for exploring high-performance low-cost La-based Nd–Fe–B magnets.

Filling-Fraction Fluctuation Leading to Glasslike Ultralow Thermal Conductivity in Caged Skutterudites.

It is generally believed that filling atoms randomly and uniformly distribute in caged crystals, such as skutterudite compounds. Here, we report first-principles and experimental discovery of a multiscale filling-fraction fluctuation in the RFe_{4}Sb_{12} system. La_{0.8}Ti_{0.1}Ga_{0.1}Fe_{4}Sb_{12} spontaneously separates into La-rich and La-poor skutterudite phases, leading to multiscale strain field fluctuations. As a result, glasslike ultralow lattice thermal conductivity approaching the theoretical minimum is achieved, mainly due to strain field scattering of high-energy phonons. These findings reveal that an uneven distribution of filling atoms is efficient to further reduce the lattice thermal conductivity of caged crystals.

Hard magnetic properties and coercivity mechanism of melt-spun Misch Metal-Fe-B alloy

Magnetic and structural properties of Misch Metal (MM)-Fe-B alloys, were examined in the melt-spun ribbons. Melt-spun MM-Fe-B samples were prepared at the surface velocities of 18–30m/s. Crystalline structure and their room-temperature magnetization characteristics were analyzed, and the optimum surface velocity of 20m/s and nominal composition of MM13Fe81B6 were obtained. Microstructural analyses indicate that the grain size is approximately 30–50nm in the alloys with the optimum characteristics. In the MM16Fe78B6 alloys, Pr-rich and La-rich phases concentrated on grain boundaries, which resulted in the coercivity augment with the increase of MMFe2 content. Dependence of coercivity on applied magnetic field suggested that the mechanism of coercivity in moderate MM-content samples was inhomogeneous domain wall pinning type. The melt-spun ribbons in the optimum condition exhibit a coercive force of 6.9kOe and an energy product of 8.5MGOe, which can be used as the potential isotropy magnetic powders in the bonded magnets.

Strain development during the phase transition of La(Fe,Mn,Si)13Hz

We use powder X-ray diffraction to evaluate the temperature dependence of the crystalline properties during the magnetic phase transition of La(Fe,Mn,Si)13Hz as a function of the Fe/Mn/Si ratio. Both the paramagnetic and ferromagnetic phases were observed as peak overlaps in the patterns around the Curie temperature (TC) occurring continuously in a temperature range of about 5 K around TC. Using the Williamson-Hall method, we evaluate the strain developing in the crystallites during the transition and find that it is associated with the growth of the paramagnetic phase as the transition occurs. Based on our measurements and microstructure analyses, we propose that cracking during the phase transition is due to or aggravated by the small content of a La-rich phase.

Lanthanum-doped mesostructured strontium titanates synthesized via sol–gel combustion route using citric acid as complexing agent

In the present work, a series of lanthanum-doped mesostructured strontium titanate (LMST) materials with different La/Sr ratios were synthesized via a sol–gel combustion method in the presence of citric acid as a complexing agent and Pluronic P123 as a templating agent. The effects of the amount of doped La and calcination temperature on the physicochemical properties of the LMSTs were examined using various techniques. Powder X-ray diffraction confirmed the substitution of La3+ into the SrTiO3 lattice, generating cubic perovskite LaxSr1−xTiO3, for the LMST materials calcined at 600 °C. The purity and crystallinity of the desired perovskite phase were enhanced by citric acid addition. The solubility limit of La3+ substitution at an La/Sr ratio of 0.43 was determined by structural and morphological studies. Increasing the La doping amount decreased the crystallinity and compositional homogeneity, because an La-rich amorphous phase segregated on the surface, but improved the mesoporosity. N2 physisorption measurements indicated that the LMSTs had a bimodal pore size distribution, of which the larger one was characterized by the crystallite size of mixed oxides, and the specific surface area of 24.9–37.3 m2 g−1. The formation of mesopores in the LMST materials synthesized via sol–gel combustion was explained based on a combination of soft- and hard-templating chemistries.

Structure and magnetocaloric effect of La0.7Ce0.3(Fe0.92Co0.08)11.4Si1.6 bulk alloy prepared by powder metallurgy

La0.7Ce0.3(Fe0.92Co0.08)11.4Si1.6 ingot was prepared by arc melting furnace in argon atmosphere, and the starting powders was prepared from the stoichiometric melt spun ribbons by manually grinding. Bulk samples have been prepared by powder metallurgy (PM) without sintering aid from the starting powders. After sintering at 1000 °C for 3 h under argon atmosphere, the samples achieve densities of about 7.05 g/cm3. The structure and magnetocaloric effect of the bulk sample were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and physical property measurement system (PPMS-9). The results showed that the La0.7Ce0.3(Fe0.92Co0.08)11.4Si1.6 bulk alloy consists of a dominant NaZn13-type 1:13 phase, some α-Fe(Si) and La-rich (LaFeSi) impurity phases, which exhibits a second-order magnetic phase transition from ferromagnetism to paramagnetism in the vicinity of Curie temperature (TC). The maximum magnetic entropy changes occurring near its TC are 4.1 and 8.3 J/kg K under the applied field change of 2T and 5T, respectively, which is comparable to that of Gd, suggesting that the La0.7Ce0.3(Fe0.92Co0.08)11.4Si1.6 bulk alloy is promising to be a low-cost, high efficiency magnetic refrigeration material at room temperature.

Exploring corrosion protection of La-Fe-Si magnetocaloric alloys by passivation

Magnetocaloric La(Fe,Si)13-based alloys are promising materials for magnetic cooling systems but their limited corrosion resistance in water-based heat transfer fluids is critical. The corrosion behavior of as-cast and annealed La-Fe-Si alloy samples was analyzed in comparison to that of La and Fe for evaluation of the impact of alloy chemistry and microstructure. Electrochemical studies were conducted in defined electrolytes starting with aerated distilled water (pH = 6) for assessing the influence of pH value changes and anion contaminations. Specifically, forced flow electrolyte conditions were applied which are closer to operation conditions of real magnetocaloric regenerator beds than stagnant ones. Corroded sample surfaces were analyzed with SEM to assess damage mechanisms. The reactive nature of the alloy constituents determines the high corrosion activity and limited passivation ability of La-Fe-Si alloys. Their exposure to distilled water is particularly detrimental under stagnant conditions as local fluid acidification enhances corrosion processes. These are based on galvanic coupling between the phases with different corrosion activities: La-rich phases > La(Fe,Si)13-based matrix > alpha-Fe(Si). Laminar fluid flow is beneficial for alloy surface passivation. But anion contaminants like sulfate or hydrogen phosphate ions counteract the weak passivity in flowing distilled water. While acidic conditions lead to instable corrosive states, a pH value control of the heat transfer fluid at alkaline conditions is effective for stable passivity of the alloy surface. Also, the applicability of a phosphate conversion coating treatment in 0.15 M NaH2PO4 (pH = 4) was evaluated and prospects of this approach are discussed.

Influence of Lanthanum on Solidification, Microstructure, and Mechanical Properties of Eutectic Al-Si Piston Alloy

The effects of Lanthanum (La) concentration on the solidification parameters of the α-Al, Al-Si, and Al-Cu phases and on the microstructure, tensile, and hardness properties of eutectic Al-Si-Cu-Mg alloy were systematically investigated. The solidification parameters were examined using computer-aided cooling curve thermal analysis (CA-CCTA). The cooling curve and microstructure analysis showed that La altered the Si structure. The nucleation and growth temperatures of eutectic Si decreased when 0.3 wt.% La was added, and a high depression temperature was obtained with 1.0 wt.% La. High amounts of La considerably modified the Si structure and decreased the area and aspect ratio by 69.9 and 51%, respectively. The thermal analysis result recorded a faster freezing time with the La addition and a 36% alteration in the secondary dendrite arm spacing. Two secondary or ternary La-rich intermetallic phases were formed with needle- and plate-like structures. Furthermore, the mechanical properties were investigated by hardness and tensile tests with different La concentrations. The addition of small amounts of La (0.1 wt.%) significantly improved the ultimate tensile strength and quality index of the Al-Si-Cu-Mg alloy. In addition, the hardness value of Al-11Si-Cu increased by 7-8% with the increasing amount of La added.

Solid solubility in 1:13 phase of doping element for La(Fe,Si)13 alloys

The influences of Ni, Cr and Nb as substitution elements for Fe were investigated. The change in microstructure and the magnetic properties have been discussed in detail. Substitution elements Ni, Cr and Nb not only have limited solubility in NaZn13-type (1:13) phase, but also hinder the peritectoid reaction. Ni element mainly enters into La-rich phase while Cr element mainly concentrates in α-Fe phase, which both have detriment effect on the peritectoid reaction, leading to a large residual of impurity phases after annealing and a decrease of magnetic entropy change. Besides, Ni and Cr participated in peritectoid reaction by entering parent phases but slightly entering 1:13 phase, which would cause the disappearance of first order magnetic phase transition. A new phase (Fe,Si)2Nb was found when Nb element substitutes Fe in La(Fe,Si)13, suggesting that Nb does not participate in peritectoid reaction and only exists in (Fe,Si)2Nb phase after annealing. The alloy with Nb substitution maintains the first order magnetic phase transition character.

Effect of impurity phase on corrosion resistance and magnetic entropy change for LaFe11.3Co0.4Si1.3C0.15 magnetocaloric compound

AbstractEffect of impurity phase (α-Fe phase and La-rich phase) on corrosion resistance and magnetic entropy change of LaFe11.3Co0.4Si1.3C0.15 compound was studied using scanning electron microscopy, potentiodynamic polarization, electrochemical impedance spectroscopy techniques and magnetism testing. With the decrease of impurity phase, the corrosion resistance of LaFe11.3Co0.4Si1.3C0.15 compound was first enhanced and then slightly impaired. Corrosion resistance could be significantly improved by the decrease of α-Fe phase. However, the matrix phase was corroded if the La-rich phase as anode was too few. This caused the corrosion resistance to decrease slightly. After immersing the sample in distilled water for 15 d, –ΔSmax of the samples annealed for 3, 12 h, 3 and 7 d decreased about 50%, 41%, 16% and 17%, respectively.