Model of temperature field for the preparation process of melt-spun NdFeB powders

Melt-spun ribbons which are the important raw material for hot-deformed magnets can be prepared by single-roller melt-spinning. In order to prepare well-structured ribbons, the model of temperature field for single-roller melt-spinning process was constructed in this work. The heat conduction in this process was simplified as one dimensional heat conduction problem. It was shown by modeling that, the temperature field in the melt-spinning before solidification in this model could be described as this equation T(x,t)=Tmoexp[–k(x–x0)–k2αt]+T0. The temperature T(x,t) of the alloy melts decreased with increased position x and cooling time t exponentially from the wheel-free surface to the wheel-side surface. The constant k determined the decrease speed of alloy temperature T(x,t), which was proportional to the interfacial heat transfer coefficient h and the interfacial area of heat conduction A0, but inversely proportional to the thermal conductivity K. x0 was the thickness of the alloy melt. With increased x0, the temperature difference between wheel-free surface and the wheel-side surface became larger, which would lead to larger difference in grain size. In experiments, the influence of melt-spinning process parameters on the temperature field model was discussed, such as cooling roller materials, wheel speed, and so on. Melt-spun ribbons prepared by single-roller melt spinning at different wheel speed were investigated and magnetic properties of die-upset magnets from melt-spun ribbons on different cooling roller were analyzed. The variation of grain size in the depth direction consisted with temperature field model. This model provided directions for the preparation of melt-spun ribbons with uniformly distributed fine grains, which were very necessary for producing hot-deformed magnets with high magnetic performance.     

Magnetocaloric effect in ErNi_2 melt-spun ribbons

ErNi2 ribbons were produced by rapid solidification using the melt spinning technique.Their structural,magnetic and magnetocaloric properties in the as-solidified state were studied by X-ray diffraction,scanning electron microscopy,magnetization and specific heat measurements.Samples are single phase with the MgCu_2-type crystal structure,a Curie temperature T_C of 6.8 K and a saturation magnetization at2 K and 5 T of 124.0 A·m~2/kg.For a magnetic field change μ_0△H of 5 T(2 T) ribbons show a maximum magnetic entropy change |△S_M~(peak)| of 24.1(16.9) J/(kg·K),and an adiabatic temperature change △T_(ad)~(max) of8.1(4.4) K;this is similar to the previously reported literature for bulk alloys that were processed through conventional melting techniques followed by prolonged thermal annealing.In addition,the samples also show slightly wider △S_M(T) curves with respect to bulk alloys leading to a larger refrigerant capacity.     

Magnetic properties and magnetic entropy change of perovskite manganites La_(0.9-x)Eu_xSr_(0.1)MnO_3(x=0.000,0.075) by experimental method and numerical fitting

Polycrystalline samples La_(0.9-x)EuxSr_(0.1)MnO_3(x = 0.000, 0.075) were prepared by the standard solid-state reaction method. The results show that the samples preform a characteristic of clusters spin-glass state at low temperature. The samples show a characteristic of ferromagnetism(FM) characteristic in the temperature range of 15-125 K and 15-150 K respectively; the samples show preformed clusters in the temperature range of 125-343 K and 150-325 K, respectively, the samples show paramagnetism(PM)characteristic above 343 and 325 K, respectively. The second-order transitions are found at 118 and 135 K for undoped and doped sample, respectively. When the applied magnetic field is 7 T, the maximum magnetic entropy change |△S_M| value of the samples is near the Curie temperature(Tc), and the value of|△S_M| reaches 2.76 and 3.03 J/(K kg), respectively. In addition, the relative cooling power(RCP) is found to be 425.28 and 443.53 J/kg. The numerical fitting data fit well with experimental data. These results indicate that both the samples have the potential to realize magnetic refrigeration in the high temperature region(T 77 K).     

Table-like magnetocaloric effect and large refrigerant capacity of composite magnetic refrigerants based on LaFe11.6Si1.4Hy alloys

Composite magnetic refrigerants were prepared by physical mixing LaFe_(11.6)Si_(1.4)H_y alloys with different Curie temperatures(Tc). The phase structures of these LaFe_(11.6)Si_(1.4)H_y alloys were analyzed by X-ray diffraction(XRD) and the magnetocaloric effect(MCE) and refrigerant capacity(RC) of these composite magnetic refrigerants were investigated by experiment and calculation in this paper. The magnetocaloric effect(MCE) and refrigerant capacity(RC) of these composite magnetic refrigerants were investigated by experiment and calculation in this paper. The results indicate the experimental magnetic entropy change(-△S_M)-Tcurve corresponds reasonably with the(-△S_M)-Tcurve calculated by the linear combination of(-△S_M)-T curves of the single material. An optimal mixing ratio can make the composite magnetic refrigerant possess a table-like(-△S_M)-Tcurve which is beneficial to magnetic Ericsson cycle. When three LaFe_(11.6)Si_(1.4)H_y alloys with different T_c are mixed, the full width at half maximum(△T_(FWHM)) of(-△S_M)-T curves is about 48.7 K and the RC is about 177.76 J/kg under a magnetic field change of 2 T. The composite magnetic refrigerants based on LaFe_(11.6)Si_(1.4)H_y alloys can be promising candidates for near room temperature magnetic refrigeration and the work will be helpful to develop novel composite magnetic refrigerants with table-like MCE and large RC.     

Experimental evidence for field-induced metamagnetic transition of EuCd2As2

Recent studies have shown that layered compound EuCd₂As₂ could exhibit diverse topological states depending on the different magnetic structures, such as Weyl semimetal, Dirac semimetal and topological insulator. In order to further study the interplay between magnetism and topology of EuCd₂As₂, it is necessary to figure out its magnetic structure. Here, by magnetization (M) measurements and negative magnetostriction (λ) along the [001] direction measured by scanning tunneling microscopy on EuCd₂As₂ single crystals, we observe field-induced metamagnetic phase transition from A-type antiferromagnetic (AFM) ground state to field-polarized state, with canted AFM (CAFM) state in between. Magnetization and magnetostriction are more sensitive to the in-plane field than the out-of-plane field, indicating the magnetic moments lying in the ab plane. The absence of abrupt jump on M-H and λ-H curves demonstrates that the phase transition is a second-order type. In CAFM state, M increases linearly with the field and λ is proportional to M². Tunneling conductance spectra show the field-induced evolution of the electronic density of states. Our results provide experimental evidence for understanding the magnetic structure of EuCd₂As₂.     

Influence of heat treatment on the residual magnetization of steel in partial magnetic hysteresis

A formula is derived for the residual magnetization M_r of steel after repeated magnetic hysteresis, on the basis of measurements of the saturation magnetization M_s, the coercive force H_cs, the residual magnetization M_rs for the limiting hysteresis loop, and the maximum magnetizing field strength H_m of the partial hysteresis loop. The influence of variation in the tempering temperature t_te of steel on M_r at different H_m is analyzed. The dependence M_r(t_te) is established for small and large H_m. It is established that the M_r results may effectively be used for nondestructive assessment of the tempering of moderate-carbon alloy steel.     

Phase transition and magnetocaloric effect of Ni55.2Mn18.6Ga26.2-xGdx (x=0, 0.05, 0.15) alloys

Phase transition process and magnetic entropy change -ΔS of Ni55.2Mn18.6Ga26.2-xGdx(x=0, 0.05, 0.15) alloys were studied.Ni55.2Mn18.6Ga26.2-xGdx(x=0, 0.05, 0.15) alloys still underwent simultaneous structural and magnetic transitions and transform from ferro-magnetic martensitic phase to paramagnetic austenitic phase during heating. Under a field of 2 T, the maximum magnetic entropy change -ΔSM of Ni55.2Mn18.6Ga26.15Gd0.05 alloy was 7.7 J/kg.K at 317 K during heating and 8.6 J/kg.K at 314 K during cooling while it was 11.8 J/kg.K at 317 K in Ni55.2Mn18.6Ga26.05Gd0.15alloy during heating.     

Large magnetocaloric effect in Er_2O_3 compounds at cryogenic temperature

We investigated the magnetocaloric effect in commercial Er_2 O_3 powders which presents almost no hysteresis losses at low temperature.At a magnetic field change of 5 T,it displays large magnetic entropy change(-ΔS_M)_(max) of 15.02 J/(kg·K) and a refrigerant capacity(RC) of 311 J/K at Neel temperature T_N=3.32 K.The magnetic transition was found to be of a second-order.The maximum values of adiabatic temperature change(ΔT_(ad))_(max) reach 0.70 K for a magnetic field change of 1 T.The large value,of(-AS_M)_(max) as well as no hysteresis loss,makes Er_2 O_3 a promising material as a magnetic refrigerant at low temperature.     

The microstructural path of grain-boundary-nucleated phase transformations

Microstructural path functions, S_v(t) = f{X_v(t)}, where X_v is the volume fraction transformed and S_v is the interfacial area per unit volume (separating transformed from untransformed material), are derived for transformations in which the product phase nucleates either at parent phase grain boundaries, grain edges or grain corners and grows at a constant rate. The treatment considers that either there is a fixed number of pre-existing nuclei, or nucleation is at a constant rate, or some combination of these two means of nucleation are operative. The rate equations for X_v are deduced using existing formalisms and by using the same basic premises to treat the problem of impingement, the S_v rate equations are derived for the first time. The microstructural path functions are obtained by eliminating time from these rate equations. An example of the usefulness of the microstructural path function to model recrystallization in an aluminum-gold alloy is presented.     

Reordering kinetics and magnetic properties of mechanically disordered nanocrystalline Ll2-type Ni3Al + Fe alloys

The paramagnetic disordered high temperature states of Ll₂ or γ′-Type Ni₃AlFe alloys become ferromagnetic at low temperatures if retained in metastable disordered (γ) state. Using mechanical attrition (milling) which also transforms the material into nanocrystalline form, various Ni₃AlFe alloys have been prepared in metaastable-γ form and their magnetisations M(γ) and M(γ′) and Curie temperatures T_c(γ) and T_c(γ′) measured. Fe atoms possess a giant magnetic moment in these alloys and polarise neighbouring Ni atoms but this polarisation effect is lower in the disordered state due to a lower number of FeNi nearest neighbours resulting in M(γ) < M(γ′) and T_c(γ) < T_c(γ′). Currently available expressions for magnetic ordering energies based on Bragg-Williams nearest neighbour interactions are found to be inadequate for these giant-moment-type alloys. Isothermal ordering kinetics of the metastable-γ-state which is also nanocrystalline indicate that the ordering reaction cannot propagate across the disordered nanograin boundaries and each grain must obtain its own γ′ nucleus. This results in a Johnson-Mehl-Avrami exponent n ≈ 1.     

Internal friction in diamagnetic copper and lead polycrystals in a DC magnetic field

The amplitude-dependent internal friction of diamagnetic copper and lead polycrystals 99.9% pure has been studied experimentally in dc magnetic fields with an induction 0 ≤ B ≤ 0.75 T. In a dc magnetic field B = 0.1 T at room temperature, the sign inversion (from plus to minus) of internal-friction increments ΔQ ^?1(B) = Q ^?1(B) ? Q ^?1(0) is revealed for two copper samples. The copper contained impurity-pinned dislocations at ΔQ ^?1(B) > 0 and impurity-free dislocations at ΔQ ^?1(B) < 0. The positive sign of ΔQ ^?1(B) for the internal friction in copper is due to a decrease in the potential-barrier height U _b for dislocation motion in impurity-containing metals. The negative sign of ΔQ ^?1(B) for the internal friction in copper is due to a simultaneous decrease in the barrier height U _b and width (activation volume Θ) during dislocation motion in impurity-containing metals. For a magnetic field B = 0.1 T, the sign of ΔQ ^?1(B) in lead is positive at temperatures below 70°C and the sign of ΔQ ^?1(B) is negative at temperatures above 70°C. For lead, the sign inversion of ΔQ ^?1(B) is explained by the dissolution of dislocation impurity atmospheres at temperatures higher than 70°C.     

Magnetic properties and magnetocaloric effects of manganite (La0.8Ho0.2)2/3Ca1/3MnO3 and (La0.5Ho0.5)2/3Ca1/3MnO3

We reported the magnetic properties and magnetocaloric effects (MCE) of (La_0.8Ho_0.2)_2/3Ca_1/3MnO₃ and (La_0.5Ho_0.5)_2/3Ca_1/3MnO₃ nanoparticles by sol-gel technique. With this method, we were able to obtain the samples with particle diameters ranging from 50 to 200 nm. In the (La_1-xHo_x)_2/3Ca_1/3MnO₃ compound, an external magnetic field induced a magnetic transition from an paramagnetic phase to a ferromagnetic phase above T_s=105-135 K, leading to magnetocaloric effects. The maximum value of ΔS_M was 1.19 J/(kg·K) at 100 K and 2.03 J/(kg·K) at 152 K for a magnetic field change of 5 T. Because both samples had large relative cooling power (RCP) and wide δT_FWHM, the study on systems with the (La_1-xHo_x)_2/3Ca_1/3MnO₃-related magnetic transitions may open an important field in searching good magnetic materials.     

Effect of chromium on magnetic properties and corrosion resistance of LaFe11.5Si1.5 compound

Effects of Cr addition on magnetic and corrosion properties of LaFe11.5-xCrxSi1.5 (x=0.1, 0.3, 0.4) magnetic refrigerants were investigated. It was found that the addition of Cr slightly decreased the magnetic entropy change, but it was effective in increasing corrosion resistance of La-Fe-Si compounds within the composition range examined. Corrosion measurements showed that corrosion current density of these La-Fe-Si compounds in distilled water decreased from 2.6×10-6 to 8.7×10-7 A/cm2, with x increasing from 0 to 0.3, while magnetic entropy change in LaFe11.5-xCrxSi1.5 compounds did not change significantly (-ΔSmax was 18.0, 17.0, 13.6, 11.1 J/(kg·K)) under 2 T for x=0, 0.1, 0.3, 0.4, respectively). Surface optical microstructures of compounds after corroded showed that corrosion took place at grains and grain boundaries simultaneously and surface concentration of corroded points on LaFe11.5-xCrxSi1.5 (x=0.1, 0.3, 0.4) compounds reduced obviously.     

Modelling effects of additives on viscosity of molten blast furnace slag

Abstract

A simple model was established to calculate and predict the effects of network modifiers on the viscosity of molten blast furnace slag by relating the effects to the ionic bond force parameter. It was found that the model performs very well in calculating the viscosity of blast furnace slags containing FeO, MnO, BaO, Na₂O, K₂O and CaF₂, with relative error Δ of 0·219 (±0·196) and overall absolute error (root mean square error) of 0·402 Pa s. The viscosity decreasing abilities are in the order of K₂O<(Na₂O, BaO)<(CaO, MnO, FeO)<MgO<CaF₂. The effects of other network modifiers on the viscosity can also be predicted. As alkali metal oxides have much lower bond strength and smaller molar weight, their viscosity decreasing abilities evolve differently from divalent metal network modifiers, whereas the viscosity decreasing abilities of alkali and divalent metal network modifiers increase linearly with increasing I/M, where I is ionic bond strength parameter, and M is molar weight.     

Tunable Curie temperature and magnetocaloric effect of FeCrMoCBYNi bulk metallic glass with different crystallized phases

The investigation on Curie temperature and magnetocaloric effect of the FeCrMoCBYNi bulk metallic glass(BMG) with different crystallized phases was carried out by XRD,TEM and PPMS. The experimental results show that the Curie temperature(T_c) of Fe_(45)Cr_(15)Mo_(14)C_(15)B_6 Y_2 Ni_3 BMG with different annealing condition reaches a highest value of 95 K. The value of magnetic entropy change △S_M(T) of Sample 3 reaches a maxima of 0.48 J/(kg·K) at Tc temperature, which result from the interaction among the precipitated phases of(Fe,Cr)_(23)(C,B)_6, Fe_3 Mo_3 C and residual amorphous phase. Based on the experiment results, it can be obtained that the Curie temperature, magnetocaloric effect can reach their optimal value at low temperature, when the content of amorphous phase and precipitated phases type run up to certain value. The magnetic properties of Sample 1 with full amorphous phase and Sample 4 with full crystalline phase will both decrease.     

Anisotropic magnetocaloric effect in a dysprosium(III) single-molecule magnet — Commemorating the 100th anniversary of the birth of Academician Guangxian Xu

Dysprosium compounds with high magnetic anisotropy are widely studied as single molecule magnets (SMMs). Here the anisotropic magnetocaloric effect in a Dy(III) SMM, {[Dy(OSiMe₃)₂(4-Mepy)₅(BPh₄)] 0.5Toluene}, was studied by single crystals. Angular dependent magnetization can be observed at 300 K because of its high magnetic anisotropy. SMM behavior measured along the easy axis direction is identical to that of the polycrystalline sample. Rotating magnetization from the easy axis to the hard plane gives a maximum magnetic entropy change (–ΔS_R) of 3.05 J/(kg∙K) at 19 K at ΔB = 5 T, which enables the Dy(III) SMM to be used as a low-temperature rotating magnetic refrigerant.     

Role of boron in the formation of the magnetic properties of sintered Nd-Dy-Fe-Co-B materials with a high cobalt content

Sintered materials having the compositions (Nd_{1 ? x} Dy _x ) _S (Fe_{1 ? y} Co _y )_balB_6–8 (atomic fractions x = 0.09–0.71, y = 0.19–0.34, S = 13.3–15 at %) have been studied. It is shown that, as the content of the boron-containing (Nd, Dy)(Fe, Co)₄B phase increases, the coercive force decreases abruptly. This effect is explained by the transition of boron atoms from the principal magnetic boron-containing (Nd, Dy)₂(Fe, Co)₁₄B phase to the (Nd, Dy)(Fe, Co)₄B phase during sintering.     

Magnetic and magnetocaloric properties in GdZn2 synthesized by ball milling

In this study, we prepared GdZn₂ by mechanical alloying method and measured its magnetization to reveal the magnetic and magnetocaloric properties. For a milling time (MT) ≥ 10 and 20 h, the GdZn₂ phase was confirmed by X-ray diffraction measurements. For MT = 40 and 60 h, amorphous like patterns are observed. For MT = 10 and 20 h, magnetization (M) as a function of temperature below 80 K shows ferromagnetic-like increase. The observed values of M are 4.5 μ_B at 5 K for MT = 10 and 20 h, which is smaller than that of the theoretical expected value of 7 μ_B. For MT = 40 and 60 h, the long-range ferromagnetic order collapses because there is no spontaneous magnetization. The value of ΔT, where ΔT is the full width at half maximum of the magnetic entropy change (ΔS_m (T)), increases significantly for MT ≤ 20 h because of the gradual release of magnetic entropy by the milling effect. Since M decreases with increasing MT, the maximum values of |ΔS_m|, the relative cooling power, and the refrigerant capacity also decreases.     

Magnetic properties and large magnetocaloric effects of GdPd intermetallic compound

With the intention to explore excellent magnetocaloric materials, the intermetallic compound GdPd was synthesized by arc melting and heat treatment. The microstructure, magnetic and magnetocaloric properties of the intermetallic compound of GdPd were investigated by X-ray diffraction(XRD), scanning electron microscopy(SEM) and the physical property measurement system(PPMS). A large reversible magnetocaloric effect is observed in GdPd accompanied by a second order magnetic phase transition from paramagnetism to ferromagnetism at ～39 K. The paramagnetic Curie temperature(θp) and the effective magnetic moment(μ_(eff))are determined to be 34.7 K and 8.12 μ_B/Gd,respectively. The maximum entropy change(|△S_M~(Max)|) and the relative cooling power(RCP) under a field change of 5 T are estimated to be 20.14 J/(kg·K) and 433 J/kg, respectively. The giant reversible magnetocaloric effects(both the large△S_M and the high RCP) together with the absence of thermal and field hysteresis make the GdPd compound an attractive candidate for low-temperature magnetic refrigeration.     

Effects of adding different types of carbon on the structure and magnetic properties of SmCo6.9Hf0.1 alloy

In this paper,SmCo6.9 Hf0.1 as-cast alloys and ribbons with the addition of either graphite（C） or carbon nanotubes（CNTs） were prepared by arc melting and melt-spinning,respectively.The effects of adding carbon on the structure and magnetic properties SmCo6.9 Hf0.1 were investigated by means of X-ray diffraction（XRD）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,magnetic force microscopy（MFM） and vibrating sample magnetometer（VSM）.It was found that the microstructure and magnetic structure of SmCo6.9 Hf0.1 ribbons were changed obviously due to the introduction of C or CNTs,although their crystal structure was characterized as the same Sm（Co,Hf）7 single phase,no matter carbon was added or not.As a result,the magnetic properties of carbon-contained ribbons were enhanced in a certain degree.This was considered to be related to the refined equiaxed grains,small domain size and the pinning effect of C or CNTs-rich regions.The magnetic properties of SmCo6.9 Hf0.1（CNTs）0.05 ribbons reached Hc =12.5 kOe,Mr =57.0 emu/g and Mr/M2 T =0.788.