Mohammadreza Ghahremani

Design and Instrumentation of an Advanced Magnetocaloric Direct Temperature Measurement System

The magnetocaloric temperature change (ΔTTemp) is an important factor in the performance evaluation of the magnetocaloric materials magnetocaloric effect (MCE), which has been attracting significant interest due to its application in energy efficient near-room temperature magnetic refrigeration technology with environmentally desirable characteristics. A novel magnetocaloric temperature change test system with fully controlled field, temperature, and time capabilities is designed and analyzed. This test system allows the detailed observations of the applied field effect, sample temperature effect, and time effect on the testing magnetocaloric material. The effectiveness of this test system was evaluated by testing the MCE within sample of Gd turnings. The magnetocaloric temperature change measurements, TTemp, at various H and Temp was measured and showed numerical and characteristic agreements with literature, thus provided evidence that this test is capable of making conventional measurement as well as new measurements on the materials dynamic time effects.

Adiabatic magnetocaloric temperature change in polycrystalline gadolinium - A new approach highlighting reversibility

The adiabatic temperature change (ΔT) during the magnetization and demagnetization processes of bulk gadolinium is directly measured for several applied magnetic fields in the temperature range 285 K to 305 K. During the magnetization process, ΔT measurements display the same maximum for each applied field when plotted against the initial temperature (Ti). However, during the demagnetization process, the maximum ΔT varies for each applied field. This discrepancy between the magnetization and demagnetization measurements appears inconsistent with the reversibility of the magnetocaloric effect. A new approach is undertaken to highlight the reversibility of the magnetocaloric effect by plotting ΔT against the average temperature change (Tavg) instead of Ti. The value of Tavg which corresponds to the maximum ΔT is found to increase linearly with the applied magnetic field, consistently for both the magnetization and demagnetization measurements. Solving the linear-fitting equations of these measurements gives...

Implicit measurement of the latent heat in a magnetocaloric NiMnIn Heusler alloy

The latent heat linked with the first-order transformation of a NiMnIn Heusler alloy has been studied through direct measurements of the adiabatic temperature change, ΔTad, during magnetization process. The experimental procedure used guarantees independent data points and negates any contribution of hysteretic losses to the magnetocaloric effect. Thus, the differences between the magnitudes of ΔTad measurements during the magnetization with the initial temperature change directions from low-to-high and high-to-low are solely attributed to the latent heat exchange, which accompanies the irreversible structural first-order transformation. An estimate of the latent heat inducing such differences is about 0.292 J/g.

Metastability in the Magnetic Structure of Ni 51 Mn 33.4 In 15.6 Heusler Alloy

The metastability of the nonstoichiometric Ni₅₁Mn_33.4In_15.6 Heusler alloy in the first-order transition region is studied using direct adiabatic temperature change (ΔT_ad) measurements for heating and cooling magnetization processes. During the heating process, ΔT_ad shows an unusual pronounced peak at 294 K in low applied magnetic fields. The height of this peak diminishes and eventually vanishes at higher fields. Kinks are as well observed in the 290-295 K temperature interval for both processes. The existence of the peak and kinks is indicative of the complex magnetic dynamics in Ni₅₁Mn_33.4In_15.6 and is attributed to a spin-reorientation transition near the first-order phase transition temperature.

Surface Acoustic Wave devices for ocular drug delivery

In this work we are reporting on the development of a novel Surface Acoustic Wave (SAW) device based on MEMS technology for drug delivery in the treatment of ocular diseases. The miniaturized SAW drug delivery device will be placed on the eye surface to allow non-invasive long-term drug application based on the programmed timeline and electronic control of a drug regiment. This novel drug delivery method is expected to lead to better clinical outcomes in the treatment of various eye diseases, and improved patient compliance with therapy. The device can be programmed for delivery of precise amounts of drugs at predetermined times over several months, and will use acoustic streaming to push the drug outside of the reservoir. Our modeling results were obtained using COMSOL multiphysics program and Coventor microfluidic simulator. Different force values were applied to investigate the force necessary to push the drug through the outlet.

Enhanced magnetic properties of yttrium-iron nanoparticles

A systematic study of the size effect on the magnetic and structural properties of Y2Fe17 nanoparticles has been performed. We present new data to explain the enhanced magnetic properties of nanostructured yttrium-iron alloy synthesized through alkalide reduction chemical synthesis. The properties of the particles were characterized by x-ray diffraction, electron microscopy, and magnetometer techniques. As the size of the nanoparticles is reduced, there is an increase in magnetization per unit of applied magnetic field, a decrease in the coercivity and a substantial reduction in hysteresis.

Tuning the heat transfer medium and operating conditions in magnetic refrigeration

A new experimental test bed has been designed, built, and tested to evaluate the effect of the system’s parameters on a reciprocating Active Magnetic Regenerator (AMR) near room temperature. Bulk gadolinium was used as the refrigerant, silicon oil as the heat transfer medium, and a magnetic field of 1.3 T was cycled. This study focuses on the methodology of single stage AMR operation conditions to get a high temperature span near room temperature. Herein, the main objective is not to report the absolute maximum attainable temperature span seen in an AMR system, but rather to find the system’s optimal operating conditions to reach that maximum span. The results of this research show that there is a optimal operating frequency, heat transfer fluid flow rate, flow duration, and displaced volume ratio in any AMR system. By optimizing these parameters in our AMR apparatus the temperature span between the hot and cold ends increased by 24%. The optimized values are system dependent and need to be determined and...

Ferri-to-ferro-magnetic and ferro-to-para-magnetic transitions in Ni48Co2Mn35In13Ga2 Heusler alloy

Heusler alloys feature both conventional and inverse magnetocaloric effects near room temperature as they undergo two different transitions. In this paper, new data are presented and analyzed and a new mechanism to explain the complex hysteretic behavior of a Ni48Co2Mn35In13Ga2 Heusler alloy is developed. This mechanism explains isothermal loops near room temperature. The various descriptions and classifications of these transitions, however, is not critical to this analysis.

Magnetocaloric effect in NiMnInSi Heusler alloys

The NiMnInSi Heusler alloy family is analyzed, and a self-similarity based method is used to analyze the first-order transition of Ni-Mn Heusler alloys. This method is appropriate to determine magnetic characteristics of the Magnetocaloric Effect providing that there is sufficient separation in their phase transition temperatures. A temperature scaling methodology is used to model the cluster compositions in the mixed-state regions where two stable magnetic states co-exist. The various descriptions and classifications of these transitions, however, are not critical to this analysis.

A Preisach-Type Magnetostriction Model for Materials Exhibiting Villari Reversal

A model for the magnetostriction of high-strength steels under uniaxial compressive stresses is presented to simulate experimental measurements. A previous Preisach-type magnetostriction model is modified to simulate the Villari reversal phenomenon. The model can also account for the crystallographic anisotropy which arises from the compressive stress magnitude as well as the machining process of the sample. The net axial magnetostriction of the sample is modeled by a Preisach-type, two-component model. The first component corresponds to the magnetostriction steep rise in the initial stage followed by rotation of magnetic domains to the direction of the external magnetic field while the second component corresponds to the counteracting restoring torques.