13-based Compounds Research Articles

Magnetocaloric effect and applied refrigeration performance of La(Fe,Si)13-based compounds

As a solid-state refrigeration technology, room-temperature magnetic refrigeration has several significant advantages comparing with conventional gas compression refrigeration, such as environmental friendliness, high energy efficiency and dependable operation. In the present study, the magnetocaloric effect and applied refrigeration performance of La0.8Ce0.2Fe11.6-xMn0.15+xSi1.25Al0.1H1.8 (x = 0, 0.03, 0.06, 0.09, 0.12) compounds were studied systematically. All alloys crystallize mainly in NaZn13-type phase with trace α-Fe phase between 1.36 and 1.80 wt%. The necessary heat treatment improves the formation of main phase. As x increases, the Curie temperature (Tc) decreases gradually from 295.3 K to 273.0 K. All alloys undergo strong field-induced first-order itinerant-electron metamagnetic transition. Under 0–1.5 T, the maximum magnetic entropy changes (-ΔSM)max for La0.8Ce0.2Fe11.6-xMn0.15+xSi1.25Al0.1H1.8 (x = 0, 0.03, 0.06, 0.09, 0.12) compounds vary from 10.9 J/(kg K) to 13.5 J/(kg K). And the maximum adiabatic temperature changes (ΔTad)max vary from 2.7 K to 3.9 K. To study the applied refrigeration performance, a rotary magnetic refrigerator is designed successfully, whose refrigeration temperature and cooling power meet the requirement of cold storage. Successful large-quantity preparation, giant magnetocaloric effect and excellent refrigeration performance suggest that, La0.8Ce0.2Fe11.6-xMn0.15+xSi1.25Al0.1H1.8 (x = 0, 0.03, 0.06, 0.09, 0.12) compounds can be used as the promising candidates for magnetic refrigerants.

Magnetocaloric properties of phenolic resin bonded La(Fe,Si)13-based plates and its use in a hybrid magnetic refrigerator

We present a simple hot press-based method for processing La(Fe,Si)13-based compounds consisting of La–Fe–Co–Si–C particles and phenolic resin. The magnetic entropy change ΔS per unit mass for the LaFe10.87Co0.63Si1.5C0.2/phenolic resin compounds have nearly the same magnitude with the base materials. With the content of phenolic resin of 5.0 wt%, the compound conductivity is 3.13 W⋅m−1⋅K−1. In order to measure the cooling performance of La(Fe,Si)13-based compounds, the La(Fe11.6–x Co x )Si1.4C0.15 (x = 0.60, 0.65, 0.75, 0.80, 0.85)/phenolic resin compounds were pressed into thin plates and tested in a hybrid refrigerator that combines the active magnetic refrigeration effect with the Stirling cycle refrigeration effect. The test results showed that a maximum cooling power of 41 W was achieved over a temperature span of 30 K.

Tailoring Negative Thermal Expansion via Tunable Induced Strain in La-Fe-Si-Based Multifunctional Material.

Zero thermal expansion (ZTE) composites are typically designed by combining positive thermal expansion (PTE) with negative thermal expansion (NTE) materials acting as compensators and have many diverse applications, including in high-precision instrumentation and biomedical devices. La(Fe1-x,Six)13-based compounds display several remarkable properties, such as giant magnetocaloric effect and very large NTE at room temperature. Both are linked via strong magnetovolume coupling, which leads to sharp magnetic and volume changes occurring simultaneously across first-order phase transitions; the abrupt nature of these changes makes them unsuitable as thermal expansion compensators. To make these materials more useful practically, the mechanisms controlling the temperature over which this transition occurs and the magnitude of contraction need to be controlled. In this work, ball-milling was used to decrease particles and crystallite sizes and increase the strain in LaFe11.9Mn0.27Si1.29Hx alloys. Such size and strain tuning effectively broadened the temperature over which this transition occurs. The material's NTE operational temperature window was expanded, and its peak was suppressed by up to 85%. This work demonstrates that induced strain is the key mechanism controlling these materials' phase transitions. This allows the optimization of their thermal expansion toward room-temperature ZTE applications.

Tunable negative thermal expansion in La(Fe, Si)13/resin composites with high mechanical property and long-term cycle stability

Materials with tunable negative thermal expansion (NTE) are highly demanded in various functional devices. La(Fe, Si)13-based compounds are promising NTE materials due to their outstanding NTE properties. However, their poor mechanical properties and related short service life restrict their practical applications. In this work, epoxy resin with positive thermal expansion is used to synthesize La-Fe-Si/resin composites. The NTE of La-Fe-Si/resin composites can be manipulated by optimizing the La-Fe-Si particle size and resin content, and tailoring resin content could tune the NTE more effectively. The average linear coefficient of thermal expansion of the composites decreases from -275.0 × 10-6 K-1 to -4.9 × 10-6 K-1 over the magnetic transition temperature range as the resin content increases from 3 wt.% to 80 wt.%. In addition, zero thermal expansion is achieved in the La-Fe-Si/resin composite with 20 wt.% resin. The resin would reinforce the binding force by filling the pores between the particles. The La-Fe-Si/resin composite with 80 wt.% resin exhibits highly improved mechanical properties; for example, its compressive strength of 205 MPa is 75% higher than that of the La-Fe-Si/resin composite with 3 wt.% resin. The prepared La-Fe-Si/resin composites can be machined into different shapes for practical applications, such as thin plates, strips, and rods. Furthermore, the La-Fe-Si/resin composites can undergo 1000 thermal cycles without NTE performance degradation and mechanical integrity loss, indicating durable cycle stability. Hence, significantly tunable NTE with high mechanical properties and long-term cycle stability makes La-Fe-Si/resin composites present great application potential as NTE materials.

Reduction of hysteresis in (La1-xCex) y(MnzFe11.4-z)Si1.6 magnetocaloric compounds for cryogenic magnetic refrigeration

(La,Ce)(Mn,Fe,Si)13-based compounds which show a giant magnetocaloric effect are potential materials for the cryogenic magnetic refrigeration. However, large hysteresis originating from the first order magneto-elastic phase transition deteriorates cyclic performance of these materials, hindering their practical applications. In this work, Curie temperature of (La1-xCex)y(MnzFe11.4-z)Si1.6 compounds was tuned to the cryogenic temperatures below 77 K and hysteresis was successfully reduced to 1.5 K by tuning first order magneto-elastic transition to the critical point of second order magnetic phase transition. Based on detail microstructure characterizations, the reason for the reduction of hysteresis is ascribed to the change of a secondary phase from a paramagnetic LaFeSi phase to ferromagnetic Ce2Fe17 and α-Fe phases. Cryogenic Lorentz microscopy observations and micromagnetic simulations showed the α-Fe ferromagnetic phase produces a large stray field of ∼0.7 T at their interface. This causes the magnetic field assisted paramagnetic/ferromagnetic phase transition in the NaZn13-type phase. Cryogenic X-ray diffraction analysis indicated the energy barrier of magneto-elastic transition was reduced, resulting in an enhancement of their mechanical stability during the cyclic performance. This work has shown that the hysteresis in the magnetocaloric materials with first order magneto-elastic transition can be tuned by engineering the size, distribution, and magnetism of the secondary phases.

Effect of partial substitution of Ce for La on the structural, magnetic and abnormal thermal expansion properties of La1-Ce Fe11.2Al1.8 alloys

Abstract NaZn13-type La(Fe, M)13-based (M = Si, Al) intermetallic compounds find wide applications, ranging from microelectronics to space technology, because of their significant abnormal thermal expansion (ATE) associated with their huge magnetovolume effect. In this work, element La was partially substituted in LaFe11.2Al1.8 by element Ce to optimize the ATE characteristics of these alloys. The obtained average thermal expansion coefficient (CTE) of La0.8Ce0.2Fe11.2Al1.8 reaches as large as −21.7 × 10−6 K−1 in the temperature range of 100–200 K, which is nearly 25% larger than that of LaFe11.2Al1.8 (−17.31 × 10−6 K−1). Magnetic measurements confirmed that the improved negative thermal expansion (NTE) property is attributed to the enhanced magnetization originating from the magnetic coupling between Ce and Fe atoms. Additionally, smaller negative thermal expansion was observed in La0.4Ce0.6Fe11.2Al1.8 with an average CTE of −1.6 × 10−6 K−1 in a broad operating temperature window from 10 to 170 K. The enhanced NTE values of La(Fe, Al)13-based alloys with partial substitution of Ce for La demonstrated in this work show the potential of their practical applications.

The sign of lattice and spin entropy change in the giant magnetocaloric materials with negative lattice expansions

Solid-state refrigeration based on the magnetocaloric effect (MCE) has garnered worldwide attention because of its superior energy conservation and its environmentally friendly impact. Many materials exhibiting magnetostructural/magnetoelastic transitions have been identified by their giant MCE as promising refrigerants. The common feature of these materials is their simultaneous magnetic and lattice transitions; some also undergo negative expansions, i.e., lattice contractions, along with a ferromagnetic (FM) to paramagnetic (PM) transition. For these materials, whether the signs of lattice and spin entropy change are the same or opposite has become a controversial issue, noting that a larger unit cell volume usually indicates softer phonons and therefore a bigger phonon entropy. On the basis of our experiments and published data, we demonstrate that the lattice and spin entropy changes retain the same sign at least for La(Fe,Si)13-based compounds and MM’X alloys with giant MCE, for which the lattice undergoes negative expansion along with a FM to PM transition on heating. The clarification of the sign of lattice entropy change in the total entropy change is of particular importance for a comprehensive understanding of new materials with giant magnetocaloric effect and their design.

Effect of annealing pressure on surface oxidation in annealing process for LaFe11.5Si1.5C0.13 strips

La(Fe,Si)13-based compounds are promising refrigerants for new magnetic refrigeration technology. Cumulatively, rapid solidification is one of the most effective synthetic methods for these compounds because it requires a short annealing time. However, the strips were found to become more susceptible to external factors due to the reduced dimensionality. In this work, the effect of annealing pressure on the surface oxidation of rapidly solidified LaFe11.5Si1.5C0.13 strips during heat treatment is explored. The results show that a lower annealing pressure (5 × 10−5 Pa) leads to obvious oxidation on the surface of the strip during the heat treatment. As a result, the chemical composition of the 1:13 phase becomes nonuniform on the strips after heat treatment. By annealing the strip composed of almost the single 1:13 phase under lower annealing pressure, the 1:13 phase is also decomposed because of oxidation. On the contrary, no obvious oxide layer is observed on the surface of the strip with the increase in the annealing pressure to 1 atm during the heat treatment. Additionally, the magnetic measurements show that the maximum magnetic entropy change (|ΔS|max) becomes smaller due to surface oxidation. The strips annealed under a higher annealing pressure exhibit a higher |ΔS|max of 15.97 J/(kg K). Therefore, the higher annealing pressure (1 atm) can be helpful to fully exploit the magnetocaloric potential of La(Fe,Si)13-based strips.

Influence of electronic and metallographic structures on hydrogen redistribution in La(Fe,Si)13-based magnetocaloric compounds

Hydrogen redistribution between the two magnetic states that coexist at transition temperature, which is called the splitting phenomenon, was investigated in La(Fe,Si)13-based magnetocaloric compounds. Asymmetric splitting into two portions with different volumes and H concentrations was observed on analysis of thermomagnetization curves, and this asymmetric growth was determined, using first-principle calculations, to be correlated with the electronic band structure. Excellent entropy change was observed in (Ce,La)(Fe,Mn,Si)13H specimens, on which full hydrogenation was performed to inhibit the splitting phenomenon. The splitting phenomenon was found to be incompletely suppressed in these specimens after a short duration of homogenization annealing, and it gradually disappeared with increasing annealing duration. Magneto-optical observation with a magnetic transfer film shows that first heterogeneous nucleation appears at the spot-like sites; this is followed by the instant growth of a cloud-shaped droplet. The pinning of the droplet boundary by the spot-like nucleation site was also confirmed. Comparison of the metallographic structure revealed that the spot-like heterogeneous nucleation sites correspond to the grain-boundary triple points formed by imperfect grains, which would be swallowed by other stable grains during a ripening action. The droplet boundary, which is central to the hydrogen diffusion process, runs inside of crystalline grains, and in consequence, the main diffusion path of hydrogen was considered to be the body diffusion.

Adjustable thermal expansion in La(Fe, Si)13-based conductive composites by high-pressure synthesis

Providing a suitable contact interface, where a high conductivity material with a desirable coefficient of thermal expansion (CTE) adjoins the target micro-electric devices, is very crucial to optimize the properties and service life of the relevant instruments. Regrettably, a high conductivity, low thermal expansion and relatively inexpensive material is very rare. Composites, fortunately, can offer a method to design materials with adjustable properties by mixing two or more diverse constituents. In this paper, high conductivity composites with adjustable thermal expansion were successfully prepared by a high-pressure synthesis. The composites are based on combining La(Fe,Si)13-based compounds, the materials showing a giant, isotropic negative thermal expansion (NTE) properties, within Cu matrix. The La(Fe,Si)13-based compounds were used to adjust the CTE of the composites, while the Cu phase is in charge of tuning the thermal conductivity properties. Thus, by changing the relative amount of the two components, the composites with high conductivities and adjustable CTE were achieved. Furthermore, the thermal expansion and magnetic properties of the composites were investigated by a physical property measurement system. The present results highlight the potential applications of the Cu-based high conductivity composites with room-temperature NTE properties in the thermal contacts to various semiconductor and microelectronic devices.

Tunable near-zero thermal expansion in the C-doped La(Fe, Si)13 compounds at cryogenic temperatures

The controllable thermal expansion property gives us a method to design materials with desirable coefficient of thermal expansion. In this paper, we report a tunable zero thermal expansion (ZTE) in C-doped La(Fe, Si)13 compounds over a wide operation-temperature window (77–260 K). The effects of the interstitial C atoms on the thermal expansion and magnetic properties of the LaFe10.0Si3.0Cx (x = 0, 0.2, 0.3, 0.4) compounds were respectively investigated. The further magnetic analysis indicates that the ZTE behaviors of this La(Fe, Si)13-based compounds are attributed to the changes of magnetic exchange couplings because of the introduction of interstitial C atoms.

Influence of particle size on the mechanical properties and magnetocaloric effect of La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2/Sn composites

The particle size dependence of the mechanical properties and the magnetocaloric effect (MCE) in La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2/Sn composites were studied. The compressive strength (σbc) was in the range of 180–200 MPa for composites with particle sizes less than 180 μm, which is much higher than the compressive strength of larger size powders (136 MPa). When the particles were larger than 45 μm, the observed maximum magnetic entropy change (−ΔSM)max of 7.66–7.99 J/(kg⋅K) shows that surface/interface anisotropy effects have a negligible impact on MCE. The adiabatic temperature change (ΔTad) increased from 1.74 K@1.4 T, for particles in the size range of 0–45 μm, to 1.91 K@1.4 T for particles in the size range of 45–100 μm. The ΔTad was in the range of ∼2.0 K@1.4 T when the particle size increased from 100 to 250 μm. Magnetic hysteresis in these second-order phase transition alloys showed negligible change in the particle size range of 0–250 μm. These results are useful of La(Fe,Si)13-based compounds for magnetocaloric applications.

Nucleation and dynamics of the metamagnetic transition in magnetocaloric La(Fe,Mn,Si)13

Refrigeration cycle rates of the order of 15 Hz are desirable for efficient solid state based magnetocaloric cooling, placing an upper bound on the combined magnetic transition and the heat transfer times of the order of tens of msecs. We use microcalorimetry and magnetometry to probe the transition dynamics as a function of magnetic field sweep-rate, sample size, thermal environment, temperature and hydrostatic pressure in LaFe11.74Mn0.06Si1.20. Although second order caloric materials follow the magnetisation or demagnetisation driving field without lag, here we show that the field driven evolution of the first-order phase transition in La(Fe,Si)13-based compounds show temporal dynamics on timescales that are significantly longer than tens of msecs, associated with the thermal linkage within the sample and the linkage to the external bath. We observe that features associated with the first nucleation of the transition are field sweep rate independent, and from measurements of the latent heat we infer that the barriers to magnetisation and demagnetisation are of different magnitude. Increasing the temperature or applying hydrostatic pressure reduces the dynamic effects, suggestive of diminishing first-order character of the transition under these conditions.

Giant isotropic magnetostriction in NaZn13-type LaFe13−xAlx compounds

The unusual low-temperature magnetostrictive property is of fundamental interest due to significant applications in rapidly developing cryogenic engineering. Here, we report a giant isotropic magnetostriction (λ = 1500 ppm, ω = 4500 ppm) over a wide temperature range (∼210 K), and the saturated volume magnetostriction can be up to 8400 ppm in cubic NaZn13-type LaFe13−xAlx compounds by optimizing the chemical composition. The large magnetostrictive effect often occurs in ferromagnetic materials. However, we discovered that the magnetic-field-induced volume expansion originates from the change of lattice parameters across the first-order transition from the low-volume antiferromagnetic ground state to the high-volume ferromagnetic state. Moreover, the magnetostrictive materials show an excellent zero thermal expansion (ZTE) property, which guarantees their reliability and stability operating at various temperatures. The present study suggests potential applications of La(Fe, Al)13-based compounds as ZTE and magnetostrictive dual-function materials in cryogenic engineering.

Local Structural Distortion Induced Uniaxial Negative Thermal Expansion in Nanosized Semimetal Bismuth.

The corrugated layer structure bismuth has been successfully tailored into negative thermal expansion along c axis by size effect. Pair distribution function and extended X-ray absorption fine structure are combined to reveal the local structural distortion for nanosized bismuth. The comprehensive method to identify the local structure of nanomaterials can benefit the regulating and controlling of thermal expansion in nanodivices.

Cooling Properties of Gd Alloys and La(Fe,Si)13-Based Compounds in Active Magnetic Refrigeration for Environmentally-Friendly Cooling Systems

Cooling Properties of Gd Alloys and La(Fe,Si)13-Based Compounds in Active Magnetic Refrigeration for Environmentally-Friendly Cooling Systems

Broad Negative Thermal Expansion Operation-Temperature Window Achieved by Adjusting Fe-Fe Magnetic Exchange Coupling in La(Fe,Si)13 Compounds.

Cubic La(Fe,Si)13-based compounds have been recently developed as promising negative thermal expansion(NTE) materials, but the narrow NTE operation-temperature window(∼110 K) restricts their actual applications. In this work, we demonstrate that the NTE operation-temperature window of LaFe(13-x)Si(x) can be significantly broadened by adjusting Fe-Fe magnetic exchange coupling as x ranges from 2.8 to 3.1. In particular, the NTE operation-temperature window of LaFe10.1Si2.9 is extended to 220 K. More attractively, the coefficients of thermal expansion of LaFe10.0Si3.0 and LaFe9.9Si3.1 are homogeneous in the NTE operation-temperature range of about 200 K, which is much valuable for the stability of fabricating devices. The further experimental characterizations combined with first-principles studies reveal that the tetragonal phase is gradually introduced into the cubic phase as the Si content increases, hence modifies the Fe-Fe interatomic distance. The reduction of the overall Fe-Fe magnetic exchange interactions contributes to the broadness of NTE operation-temperature window for LaFe(13-x)Si(x).

Low-temperature negative thermal expansion behavior of LaFe11.2Al1.8−xSix compounds

The cubic NaZn13-type LaFe11.2Al1.8−xSix(x = 0.2, 0.3, 0.4 and 0.5) compounds with different Si content were fabricated by conventional arc-melting method, the structures of which were confirmed by powder X-ray diffraction (XRD) measurement at ambient temperature. Besides, the thermal expansion and magnetic properties of these samples were also researched by means of a strain gage and a physical property measurement system (PPMS). Significantly, it was found that the negative thermal expansion (NTE) behavior have been remarkably enhanced with substituting Al with Si atoms. Furthermore, the NTE operation-temperature window concurrently shifts toward a higher temperature region. The variable temperature XRD results indicate that LaFe11.2Al1.8−xSix retain cubic NaZn13-type structure when temperature varies from 20 K to 270 K, including the temperature region where NTE occurs. The further theoretical analysis combined with magnetic characterization reveal that the improvement of NTE behavior is attributed to the enhancement of Fe–Fe magnetic exchange interactions with doping Si atoms. It is noteworthy that this study displays a new pathway to improve the NTE property of La(Fe,Al)13-based compounds at low temperature region, which highlights the potential applications of NTE materials in cryogenic engineering.

Cryogenic abnormal thermal expansion properties of carbon-doped La(Fe,Si)13 compounds.

Recently, La(Fe,Si)13-based compounds have attracted much attention due to their isotropic and tunable abnormal thermal expansion (ATE) properties as well as bright prospects for practical applications. In this research, we have prepared cubic NaZn13-type carbon-doped La(Fe,Si)13 compounds by the arc-melting method, and their ATE and magnetic properties were investigated by means of variable-temperature X-ray diffraction, strain gauge and the physical property measurement system (PPMS). The experimental results indicate that both micro and macro negative thermal expansion (NTE) behaviors gradually weaken with the increase of interstitial carbon atoms. Moreover, the temperature region with the most remarkable NTE properties has been broadened and near zero thermal expansion (NZTE) behavior occurs in the bulk carbon-doped La(Fe,Si)13 compounds.

Zero thermal expansion in NaZn13-type La(Fe,Si)13 compounds.

A zero thermal expansion material in a pure form of NaZn13-type La(Fe,Si)13 was fabricated. Through optimizing the chemical composition, an isotropic zero thermal expansion material is achieved. The obtained materials exhibit a low expansion of |α| < 1.0 × 10(-6) K(-1) (α is the coefficient of linear thermal expansion) over a broad temperature range (15-150 K). The present study indicates that the thermal expansion behavior of the NaZn13-type La(Fe,Si)13 compounds depends mainly on the content of Si element. This new material is desirable in many fields of industry as a reliable and low-cost zero thermal expansion material.