Low hysteresis and large room temperature magnetocaloric effect of Gd 5Si2.05-xGe1.95-xNi2x (2x 0.08, 0.1) alloys

Gd5Si2.05-xGe1.95-xNi2x (2x ¼ 0.08, 0.1) alloys were prepared by arc melting followed by annealing at 1273 K for 96 h. Mixed monoclinic Gd5Si2Ge2-type phase, orthorhombic Gd5Si4-type phase, and a small amount of Gd5Si3-type phase were obtained in these alloys. Gd5Si2.01Ge1.91Ni0.08 alloy undergoes a second-order transition (TC) around 300 K, whereas Gd5Si2Ge1.9Ni0.1 alloy exhibits two II I transitions including a first-order transition (TC ) at rv295 K and second-order transition (TC ) at rv301 K. Ni substitution can effectively reduce the thermal hysteresis and magnetic hysteresis while max maintaining large magnetic entropy change. The maximum magnetic entropy changes (|DSM |) of 1 -1 Gd5Si2.05-xGe1.95-xNi2x alloys with 2x ¼ 0.08 and 0.1 are 4.4 and 5.0 J kg- K, respectively, for 0–2 T, and are 8.0 and 9.1 J kg-1 K-1, respectively, for 0–5 T. Low hysteresis performance and relatively large magnetic entropy change make these alloys favorable for magnetic refrigeration applications

Magnetic Field Triggered Multicycle Damage Sensing and Self Healing

Multifunctional materials inspired by biological structures have attracted great interest, e.g. for wearable/ flexible “skin” and smart coatings. A current challenge in this area is to develop an artificial material which mimics biological skin by simultaneously displaying color change on damage as well as self healing of the damaged region. Here we report, for the first time, the development of a damage sensing and self healing magnet-polymer composite (Magpol), which actively responds to an external magnetic field. We incorporated reversible sensing using mechanochromic molecules in a shape memory thermoplastic matrix. Exposure to an alternating magnetic field (AMF) triggers shape recovery and facilitates damage repair. Magpol exhibited a linear strain response upto 150% strain and complete recovery after healing. We have demonstrated the use of this concept in a reusable biomedical device i.e., coated guidewires. Our findings offer a new synergistic method to bestow multifunctionality for applications ranging from medical device coatings to adaptive wing structures.Published versio

Iron and manganese based magnetocaloric materials for near room temperature thermal management

Thermal management technology based on the magnetocaloric effect offers several advantages over conventional gas compression cooling. The efficiency of magnetic cooling systems can be much higher than conventional gas based cooling technologies. Additionally, ozone layer depleting chemicals are not used and there is reduced noise and vibrations. Iron and manganese based magnetocaloric materials (MCM) are promising due to the challenges surrounding the use of conventional rare earth based MCM. We review the recent progress in the development of iron and manganese based MCM. The magnetic phase transitions, processing techniques, performance, as well as applications of these materials are discussed. Critical analysis to determine the critical exponents and phase transition behavior of these MCM, using modified Arrot plot, critical isotherm plots, the Kouvel-Fisher method, Landau theory and the Bean-Rodbell model, is also presented.NRF (Natl Research Foundation, S’pore)Accepted versio

Magnetocaloric effect in amorphous and partially crystallized Fe40Ni38Mo4B18 alloys

A study of magnetocaloric effect in amorphous and partially crystallized Fe40 Ni 38 Mo 4B18 alloys is reported. Amorphous Fe40 Ni 38 Mo 4B18, near its magnetic ordering temperature (600K) showed a magnetic entropy change ΔSM of 1.1 J/KgK and a relative cooling power of 36J/Kg in a field change of 10 kOe. Amorphous samples were partially crystallized by annealing at 700 K at different time intervals. Partially crystallized samples showed two distinct magnetic ordering temperature, one corresponding to the precipitated FeNi nanocrystals and the other one corresponding to the boron rich amorphous matrix. Magnetic ordering temperature of the residual amorphous matrix got shifted to the lower temperatures on increasing the annealing duration. Partially crystallised samples showed a magnetic entropy change of about 0.27J/kgK near the magnetic ordering temperature of the amorphous matrix (540K) in a field change of 10 kOe. The decrease in ΔSM on partial crystallisation is attributed to the biphasic magnetic nature of the sample.Published versio

Magnetocaloric effect in amorphous and partially crystallized Fe40Ni38Mo4B18 alloys

A study of magnetocaloric effect in amorphous and partially crystallized Fe40 Ni 38 Mo 4B18 alloys is reported. Amorphous Fe40 Ni 38 Mo 4B18, near its magnetic ordering temperature (600K) showed a magnetic entropy change ΔSM of 1.1 J/KgK and a relative cooling power of 36J/Kg in a field change of 10 kOe. Amorphous samples were partially crystallized by annealing at 700 K at different time intervals. Partially crystallized samples showed two distinct magnetic ordering temperature, one corresponding to the precipitated FeNi nanocrystals and the other one corresponding to the boron rich amorphous matrix. Magnetic ordering temperature of the residual amorphous matrix got shifted to the lower temperatures on increasing the annealing duration. Partially crystallised samples showed a magnetic entropy change of about 0.27J/kgK near the magnetic ordering temperature of the amorphous matrix (540K) in a field change of 10 kOe. The decrease in ΔSM on partial crystallisation is attributed to the biphasic magnetic nature of the sample.Published versio

Machine learning discovery of a new cobalt free multi-principal-element alloy with excellent mechanical properties

In the present study, the machine learning (ML) method was utilized to construct a composition–structure–property model incorporating physical features. To enhance the predictive accuracy, the volume fraction of the two phase microstructure was merged into the dataset serving as the physical constraint for the input variables. The physical features, the chemical composition and the temperature difference between the initial and final melting temperatures were selected as the input and output variables, respectively. To deal with the small sample data, the generalized regression neural network (GRNN) was selected and applied with optimization algorithms e.g., fruit fly optimization algorithm (FOA) and particle swarm optimization (PSO). The performance of the GRNN, FOA-GRNN and PSO-GRNN models were compared. As a result, the PSO-GRNN model was the most promising model and could be utilized to search for new multi-principal elements alloy (MPEAs) with targeted properties. Based on the ML results, a novel Fe2.5Ni2.5CrAl MPEA was designed and synthesized for experimental characterization. The DSC analysis shows that the developed alloy possesses narrower melting range and the predicted value is in excellent agreement with experiments with a relative error below 10%. The designed alloy possesses a typical dual-phase structure (FCC+BCC/B2) and exhibits exceptional mechanical properties with superior plasticity at the cast condition. This property improvement is due to solid solution strengthening and nanoparticles strengthening effects. Our proposed alloy can be a promising choice for selected high performance applications.This work is supported by AME Programmatic Fund by the Agency for Science, Technology and Research, Singapore under Grants No. A1898b0043 and A18B1b0061 and the China Scholarship Council

Cyclic structural ordering induced by high energy ball milling in a Fe2.1Cr0.9Al magnetocaloric alloy

Current research on magnetocaloric materials (MCM) aims at developing low cost, environmentally friendly materials which can be manufactured by simple processes. We report the effect of high energy ball milling on the structural, magnetic and magnetocaloric properties of low cost Fe2.1Cr0.9Al alloys. The degree of structural order in the Fe-Cr sublattice was found to vary with milling time in a cyclic fashion at a milling speed of 600 rpm. This phenomenon was analyzed by a reaction rate model based on the stored energy during high energy milling. The sample possessed a B2 crystal structure before milling. Milling for 15 min. at 600 rpm induced structural changes to produce a L21 structure. Interestingly, the 15 min. milled sample exhibited ∼50% higher saturation magnetization (MS) compared to the value before milling. The relative cooling power (RCP) also increased to 300 Jkg−1 at 5 T, compared to a value of 244 Jkg−1 before milling. Extended X-ray absorption fine structure (EXAFS) studies revealed the emergence of short range order in the milled samples.NRF (Natl Research Foundation, S’pore

Morphing soft magnetic composites

Magnet filler–polymer matrix composites (Magpol) are an emerging class of morphing materials. Applications of Magpol can include artificial muscles, drug delivery, adaptive optics and self healing structures. Advantages of Magpol include remote contactless actuation, several actuation modes, high actuation strain and strain rate, self-sensing and quick response. The actuation modes of Magpol, its dynamic properties, work output and transduction characteristics are described. Analogies between Magpol actuation and phase transformations are presented. As an illustration of Magpol actuation, a proof of concept artificial muscle is presented. Current applications and future prospects are described

Magnetocaloric properties and magnetic cooling performance of low-cost Fe75 − xCrxAl25 alloys

Low-cost, earth-abundant magnetocaloric materials (MCMs) are required for energy-efficient, green, and affordable magnetic cooling technology. We investigated the magnetic and magnetocaloric properties of rare-earth-free Fe75−xCrxAl25 (19≤x≤25) arc-melted alloys. The Curie temperature (Tc) of these alloys could be tuned from 220 K up to room temperature by Cr additions. The relative cooling power/US$ was found to be superior to other promising MCMs. Fe50Cr25Al25 ball-milled powders, with an average particle size of ~25 nm, were used to prepare magnetic fluid. Maximum cooling (ΔT) of 5.4°C was observed for Fe50Cr25Al25-based fluids.NRF (Natl Research Foundation, S’pore

Hybrid thermomagnetic oscillator for cooling and direct waste heat conversion to electricity

Waste heat is an unavoidable and undesirable product of a huge number of industrially important processes. Cooling of such a heat load is of high interest. We developed a novel hybrid thermomagnetic oscillator (TMO) for cooling of the heat load as well as electricity harvesting. A bulk alloy, with a composition of (MnNiSi)0.7(Fe2Ge)0.3 and Curie temperature of 144 °C, was used as the thermomagnetic material. Heat load cooling by mechanical oscillation between the load and the sink by up to 70 °C was achieved. Voltage of up to 10 V/cycle and a current of 15 mA was generated by the mechanical oscillation of this alloy and a coupled permanent magnet through solenoid type Cu coils. This energy was stored in a capacitor and used to light up a LED. The thermomagnetic material transferred heat from the heat load to the heat sink. A moving mesh based numerical model was developed to determine the role of various parameters on the performance. Our simulations are in good agreement with our experimental findings. Superior device performance can be achieved by higher magnetic field strength, sample mass, thermal conductivity of the sample, and optimum device height.NRF (Natl Research Foundation, S’pore)Accepted versio