The Regulation Of Magnetic Transfomation And Magnetocaloric Effect In MnNiGe-based Alloy Bulks And Ribbons By Different Preparation Methods

Magnetic refrigeration technology based on magnetocaloric effect?MCE? is honored as the most likely to substitute gas compression technology, because of energy efficiency and the environment friendliness. The first-order magnetostructural transformation?MST? materials have attracted much attention due to large change of resistance and magnetization. In many types of magnetic phase transformation alloys, Mn-based alloys have been investigated widely because of large MCE, low-cost raw and simple preparation means. However, most research mainly concentrated on the bulks and ribbons, rarely on rods so far. Similar to ribbons, rods just need a short annealing time to form stable phase and own excellent magnetic properties because of a rapid cooling process. In this paper, we tried to obtain the MST from ferromagnetic TiNiSi-type structure to paramagnetic Ni2In-type structure by doping nonferromagnetic elements with large radius or ferromagnetic elements, converting the TiNiSi-type state from antiferromagnetic to ferromagnetic, and tuned the magnetic transformation by substituting In or changing the diameter, annealing time,speeds of spinning, and studied MCE. The main results are as follows: 1. The magnetic transformation and magnetocaloric effect in MnNiGe_1-xIn_x bulksIn this paper, MnNiGe_1-xIn_x?x=0.05, 0.055.0.065? alloy bulks were prepared by arc-melting under high-purity argon atmosphere. The stoichiometric MnNiGe alloy is a Spiral antiferromagnetic below Neer temperature?TN=346 K?. A structure transformation from low-temperature orthorhombic TiNiSi-type structure to high-temperature hexagonal Ni2In-type structure happens at Tt=470 K. The samples reveal the hexagonal Ni2In-type structure with a small amount of orthorhombic TiNiSi-type structure at room temperature. It is found that the substitution of main group elements In with large radius can reduce the MST temperature, obtaining the couple of magnetic and structure transformation. A first-order magnetostructural transformation between TiNiSi-type antiferromagnetic and Ni2In-type paramagnetic is achieved. For the MnNiGe0.95In0.05 alloy bulk, the largest magnetic entropy change??SM? were 4.82 Jkg^-1K^-1 under the field change of 50 kOe at the temperature of 301 K with no clear magnetic hysteresis. 2. The magnetic transformation and magnetocaloric effect in MnNi_0.85Fe_0.15Ge rodsIn this paper, MnNi_0.85Fe_0.15 Ge alloy rods were prepared by arc-melting and injection casting. We obtain the second-type MST in these alloy rods, that is MST from ferromagnetic to paramagnetic state. The study shows that the kind alloy rods crystallize mainly Ni2In-type structure. The lath martensite-like microstructure were observed from the SEM observation and TEM in rod 4.8 mm in diameter, this organization rarely reported in other MnNiGe-based alloy. The effective refrigerate capacity?RCeff? of 73.04 Jkg^-1K^-1, 87.12 Jkg^-1K^-1 and 80.51 Jkg^-1K^-1 are obtained for rods 2.0 mm in diameter annealed for 24 h, 4.8 mm in diameter annealed for 24 h, 2.0 mm in diameter annealed for 100 h, respectively, under the field change of 50 kOe. Changing the diameter has much impact on magnetic transition and magnetic entropy change??SM?, but it will reduce the annealing time, 24 h is enough, greatly saving energy compared to the bulks. 3. The regulation of magnetic transition and related magnetocaloric effect in Mn_0.45Fe_0.55NiGe_0.25Si_0.75 ribbonsThe Mn_0.45Fe_0.55NiGe_0.25Si_0.75 alloy ribbons were prepared successfully by arc-mel ting and melt-spinning techniques. The magneto-calorific properties of alloy ribbons w ith speeds at 10m/s and 20m/s have been studied. The study shows that this kind alloy r ibbons mainly crystallize TiNiSi-type structure at room temperature with a little Ni2In-type phase. Mn_0.45Fe_0.55NiGe_0.25Si_0.75 alloy ribbons realized the first order magneto-structure phase transition with giant saltation of magnetization of magnetization which belongs to the second kind MST. Compared with the ribbon with spinning speed at 10 m/s, the transformation temperature of ribbon with spinning speed at 20 m/s is relatively lower, spinning speed changes the magnetic transition. The refrigerating capacity of the two ribbons speed at 10 m/s and 20 m/s are 103.9J kg^-1 and 125.3J kg^-1 respectively in the field change of 0-5 T. and their magnetic hysteresis losses are very small. These indicate that this kind of alloy ribbon is promising in the application of magnetic cooling.