V. Chaudhary

Fe–Ni–Mn Nanoparticles for Magnetic Cooling Near Room Temperature

We have studied the magnetocaloric effect in high-energy ball-milled (Fe₇₀Ni₃₀)₉₅Mn₅ alloy nanoparticles. The partial substitution of Fe and Ni by Mn decreases the Curie temperature (T_C) of the alloy to 338 K from 443 K. The change in entropy (Δ S_M) occurs over a broad range of temperatures, which results in high relative cooling power (RCP). RCP increases from 26 to 470 J · kg^-1 for a field change of 0.5 T and 5 T, respectively; these values are comparable to the benchmark magnetocaloric material, gadolinium. The RCP is proportional to field H to the power 1+1/δ, with a critical exponent δ of 4.34.

High Relative Cooling Power in a Multiphase Magnetocaloric FeNiB Alloy

Low-cost magnetic cooling based on the magnetocaloric effect is an energy efficient, environmentally friendly, thermal management technology. However, inadequate temperature span is often a challenge in developing a magnetic cooling system. We report the novel use of multiphase materials to enhance the working temperature span (δT_FWHM) of the magnetic entropy change and the relative cooling power of a Fe-Ni-B bulk alloy. The coexistence of bcc, fcc, and spinel phases results in large working temperature spans of 322.3 and 439.0 K for magnetic field change of 1 and 5 T, respectively. δT_FWHM for this multiphase (Fe₇₀Ni₃₀)₈₉B₁₁ alloy is about 86% higher than the corresponding value for single-phase γ-(Fe₇₀Ni₃₀)₈₉B₁₁ alloy for ΔH = 1 T. These values are the largest for any bulk magnetocaloric material and even higher than most magnetocaloric nanoparticles. The relative cooling power is also higher than comparable materials, including the benchmark magnetocaloric material, gadolinium.

Hot exciton cooling and multiple exciton generation in PbSe quantum dots

Multiple exciton generation (MEG) is a promising process to improve the power conversion efficiency of solar cells. PbSe quantum dots (QDs) have shown reasonably high MEG quantum yield (QY), although the photon energy threshold for this process is still under debate. One of the reasons for this inconsistency is the complicated competition of MEG and hot exciton cooling, especially at higher excited states. Here, we investigate MEG QY and the origin of the photon energy threshold for MEG in PbSe QDs of three different sizes by studying the transient absorption (TA) spectra, both at the band gap (near infrared, NIR) and far from the band gap energy (visible range). The comparison of visible TA spectra and dynamics for different pump wavelengths, below, around and above the MEG threshold, provides evidence of the role of the Σ transition in slowing down the exciton cooling process that can help MEG to take over the phonon relaxation process. The universality of this behavior is confirmed by studying QDs of three different sizes. Moreover, our results suggest that MEG QY can be determined by pump-probe experiments probed above the band gap.

Influence of Cr Substitution and Temperature on Hierarchical Phase Decomposition in the AlCoFeNi High Entropy Alloy

While the AlCoFeNi high entropy alloy exhibits a single ordered B2 phase at high temperature, both the substitution of ferromagnetic Co with antiferromagnetic Cr, and lower annealing temperatures lead to a tendency for this system to decompose into a two-phase mixture of ordered B2 and disordered BCC solid solution. The length scale of this decomposition is determined by the combination of composition and annealing temperature, as demonstrated in this investigation by comparing and contrasting AlCoFeNi with the AlCo0.5Cr0.5FeNi alloy. The resulting phase stability has been rationalized based on solution thermodynamic predictions. Additionally, it is shown that replacement of Co by Cr in the AlCoFeNi alloy resulted in a substantial reduction in saturation magnetization and increase in coercivity. The microhardness is also strongly influenced by the composition and the length scale of B2 + BCC decomposition in these high entropy alloys.

Synthesis and Reaction Mechanism of High (BH)max Exchange Coupled Nd2(Fe,Co)14B/α-Fe Nanoparticles by Novel One-Pot Microwave Technique

Nd–Fe–B based magnets, exhibiting the highest energy product, have a wide range of applications in industry. We developed a novel one-pot microwave synthesis technique to produce hard magnetic exchange coupled Nd2(Fe,Co)14B/α-Fe nanoparticles. Nd–Fe–Co–B mixed oxides were synthesized from metal nitrates via microwave combustion followed by microwave reduction of these oxides to hard magnetic Nd2(Fe,Co)14B powders in the same microwave chamber. The conventional use of a furnace for oxide reduction is eliminated. This method is cost effective, facile, energy efficient and widely applicable to a variety of materials. The detailed reaction mechanisms in both microwave combustion and microwave reduction were studied for the first time. During mixed oxide formation, the formation sequence is: boron oxide, iron oxide, cobalt oxide and finally, neodymium-iron mixed oxide. In the microwave reduction process, iron, cobalt and boron oxides were reduced, followed by reduction of neodymium oxide, resulting finally in the desired Nd2(Fe,Co)14B and α-Fe exchange coupled nanoparticles. The synthesized Nd2(Fe,Co)14B/α-Fe nanoparticles have a narrow size distribution with 60% in the size range of 35–45 nm. With a high remanence of 99 emu g−1, the maximum energy product of these hard-magnetic nanoparticles reached a value of 11.4 MGOe, which is the highest among the values reported for Nd2Fe14B/α-Fe nanoparticles synthesized by chemical approaches.

Mechanochemical synthesis of iron and cobalt magnetic metal nanoparticles and iron/calcium oxide and cobalt/calcium oxide nanocomposites

Abstract We report an environmentally benign and cost‐effective method to produce Fe and Co magnetic metal nanoparticles as well as the Fe/Cao and Co/CaO nanocomposites by using a novel, dry mechanochemical process. Mechanochemical milling of metal oxides with a suitable reducing agent resulted in the production of magnetic metal nanoparticles. The process involved grinding and consequent reduction of low‐costing oxide powders, unlike conventional processing techniques involving metal salts or metal complexes. Calcium granules were used as the reducing agent. Magnetometry measurements were performed over a large range of temperatures, from 10 to 1273 K, to evaluate the Curie temperature, blocking temperature, irreversibility temperature, saturation magnetization, and coercivity. The saturation magnetizations of the iron and cobalt nanoparticles were found to be 191 and 102 emu g−1, respectively. The heating abilities of these nanoparticles suspended in several liquids under alternating magnetic fields were measured and the specific loss power was determined. Our results suggest that the dry mechanochemical process is a robust method to produce metallic nanoparticles and nanocomposites.

Mechanochemically Processed Nd−Fe−Co−Cr−B Nanoparticles with High Coercivity and Reduced Spin Reorientation Transition Temperature

Nd-Fe-B magnets, possessing the highest energy product, are extensively used in cutting-edge applications, including electrical machines and electrical vehicles. An environmentally benign and cost effective synthesis method of Cr alloyed Nd2 (Fe,Co)14 B magnetic nanoparticles using a dry mechanochemical process is reported. The method is solvent free, facile, energy efficient and scalable. The reduction of mixed oxides of Nd, Fe, Co, B and Cr is performed by using Ca. The coercivity (HC ) of the nanoparticles is found to depend on the dispersant content, with the highest value obtained for Nd2 (Fe11.25 Co2 Cr0.75 )B with 40 % CaO dispersant. The HC of isolated Nd2 (Fe11.25 Co2 Cr0.75 )B nanoparticles and nanoparticles embedded in a CaO matrix is found to be 11.5 kOe and 14.4 kOe, respectively, largest values for heavy rare earth free Nd-Fe-B nanoparticles with reasonable saturation and remanent magnetization, regardless of synthesis route. Considering the density of Nd2 Fe14 B, an energy product of 14.2 MGOe is obtained for the nanoparticles. The thermal coefficient of remanence and thermal coefficient of coercivity for aligned samples are -0.06 % and -0.29 %, respectively, in the temperature range between 100 K and 400 K. The spin reorientation temperature is found to be ∼30 K less than that of bulk Nd2 Fe14 B magnets.

Magnetocaloric behavior of Fe-Ni-Mn nanoparticies

We have studied the magnetocaloric effect in high energy ball milled (Fe₇₀Ni₃₀)₉₅Mn₅ alloy nanoparticles. The partial substitution of Fe and Ni by Mn decreases the Curie temperature of the alloy to near room temperature (338 K). Relative cooling power (RCP) increases from 26 to 470 J-kg^-1 for a field change of 0.5 and 5 T, respectively, these values are comparable to the benchmark magnetocaloric material, gadolinium.