CN103205590B - Preparation process of magnetic refrigeration material - Google Patents

Abstract

The invention provides a preparation process of the magnetic refrigeration material, the chemical general formula of the magnetic refrigeration material is: Mn _(2‐x) Fe _(x) P _(1‐y) Ge _(y) , the range of x is: 0.8 ～0.9, the range of y is: 0.2～0.27; it is characterized in that it comprises the following steps: (1) The raw materials used are manganese, iron, phosphorus powder, germanium fragments, the purity is 99.9～99.9999wt%, and the raw materials are continuously ball milled for 0.5～4 (2) The ball mill powder is pre-annealed at 400-600°C under vacuum or protective atmosphere for 2-30 minutes, and the powder is sintered by spark plasma sintering technology. The sintering vacuum degree is higher than 6Pa, and the heating rate is 60-120°C/min , heat up to the sintering temperature and keep it warm. The sintering temperature is 880-950°C, the sintering pressure is 10-40MPa, and the holding time is 2-30min. After the sintering is completed, it is cooled to room temperature with the furnace. The invention pre-anneals before sintering to control the grain size of the material, and the composition distribution is more uniform, the magnetic entropy change is significantly increased, and the magnetocaloric effect is improved.

Description

A Preparation Technology of Magnetic Refrigeration Material

technical field

The invention relates to a novel preparation process of a magnetic refrigeration material.

Background technique

Magnetic refrigeration is a new type of refrigeration technology, involving huge refrigeration markets such as refrigerators and air conditioners. Magnetic material is used as the refrigerant, and the refrigeration is carried out through the magnetocaloric effect, that is, the magnetic refrigeration material releases heat to the outside when it is isothermally magnetized, and absorbs heat from the outside when it is adiabatically demagnetized to achieve the cooling effect. Because of its high efficiency, energy saving, and no greenhouse effect, it will become the most promising technology to replace traditional gas compression refrigeration in the future. Obviously, energy-saving and environment-friendly magnetic refrigeration technology will have very important significance to society, environment and economy.

In recent years, many materials with room temperature magnetocaloric effect have been developed and researched rapidly. Manganese-iron-phosphorus-germanium (MnFePGe) compounds have become the most promising new magnetic refrigeration materials for practical applications because of their giant magnetocaloric effect, abundant raw materials, low production costs, and no environmental pollution. MnFePGe series compounds undergo a first-order phase transition near the Curie temperature (Tc), and the magnetic phase transition can be induced by an external magnetic field. When a magnetic field is applied near the Curie temperature, the paramagnetic phase changes to a ferromagnetic phase, and the material releases heat; When the external magnetic field is removed, the ferromagnetic phase changes into a paramagnetic phase, and the material absorbs heat. This produces a giant magnetocaloric effect, and the material has a large magnetic entropy change. By adjusting the Mn/Fe ratio and P/Ge ratio, the Curie temperature Tc of the MnFePGe-based material can be adjusted to be close to or higher than room temperature for practical application. At present, the preparation of MnFePGe series materials at home and abroad adopts the method of ball mill ball milling + sintering. Elementary powder is used for ball milling to form Fe ₂ P phase, and then sintered to obtain MnFePGe compounds with better magnetocaloric properties. However, our research found that the powder after ball milling is nano-particles, which basically has no magnetocaloric properties. Although the crystal grains of the sintered compound have grown, there are still nano-grains that cannot produce magnetic phase transition, which affects the magnetocaloric properties. improvement.

The invention pre-anneals the ball mill powder at a certain temperature before sintering, so that the grain size of the material is controlled, and the composition distribution is more uniform, which is beneficial to the transition between the paramagnetic phase and the ferromagnetic phase, and makes the magnetic entropy change significantly The increase greatly improves the magnetocaloric effect of the material. It can be applied to magnetic refrigeration technology.

Contents of the invention

The purpose of the present invention is to provide a new type of magnetic refrigeration material that can be widely used in magnetic refrigeration technology, with a working temperature near room temperature and continuously adjustable Curie temperature; a large magnetic entropy change within the magnetic field range that the permanent magnet can provide. Preparation Process.

The general chemical formula of the magnetic refrigeration material involved in the present invention is: Mn _(2-x) Fe _(x) P _(1-y) Ge _(y) , and the range of x is: 0.8-0.9. The range of y is: 0.2～0.27.

The invention provides a preparation method of the above-mentioned magnetic refrigeration material, which adopts mechanical alloying and subsequent discharge plasma sintering technology. It includes the following steps in order:

(1) The raw materials used are commercial manganese, iron, phosphorus powder, and germanium fragments, with a purity of 99.9-99.9999wt%. The raw materials are continuously ball-milled for 0.5-4 hours by mechanical alloying to form a preliminary phase of the material, and its phase analysis Using X-ray diffractometer, Fig. 1 is the X-ray diffraction (XRD) spectrum of ball-milled powder for 0.5 hours, the Fe ₂ P crystal structure phase has basically formed, and the average grain size of the powder is about 30 nanometers.

(2) Pre-anneal the ball mill powder under vacuum or protective atmosphere at 400-600°C for 2-30min, and sinter the powder with spark plasma sintering technology. The sintering vacuum degree is higher than 6Pa, and the heating rate is 60-120°C/min. Keep warm after reaching the sintering temperature. The sintering temperature is 880-950°C, the sintering pressure is 10-40MPa, and the holding time is 2-30min. After the sintering is completed, it is cooled to room temperature with the furnace.

Pre-annealing can be carried out in a separate annealing furnace or in a spark plasma sintering equipment SPS system.

The spark plasma sintering equipment is specifically used (DR.Sinter SPS-3.2-MV).

Use FEI quanta200 scanning electron microscope to scan the sample to obtain the composition distribution map; use Netzsch204F1 differential scanning calorimeter (DSC) to test the sample, use the obtained heat flow-temperature curve to adopt the equation

$\mathrm{<span\; class="notranslate">\; \&Delta;S</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; S</span>}}_{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; S</span>}}_{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; =</span>\underset{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{\overset{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{<span\; class="notranslate">\; \&Integral;</span>}}\frac{{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}{\mathrm{<span\; class="notranslate">\; T</span>}}\mathrm{<span\; class="notranslate">\; dT</span>}$

Calculated entropy value. where Cp is the heat capacity when the magnetic field is zero.

Experiments have proved that the Mn _(2-x) Fe _(x) P _(1-y) Ge _(y) magnetic refrigeration material prepared by this method has a uniform distribution of sample components, which is conducive to the transformation of the cis-ferromagnetic phase near the Curie temperature , which increases the entropy change of the material and improves the refrigeration capacity of the material, which can be applied in magnetic refrigeration technology.

Description of drawings:

Figure 1: XRD pattern of Mn _1.2 Fe _0.8 P _0.73 Ge _0.27 ball milled powder;

Figure 2a: Scanning electron micrograph of the directly sintered sample of Mn _1.1 Fe _0.9 P _0.8 Ge _0.2 in Example 1;

Figure 2b: SEM image of Mn _1.1 Fe _0.9 P _0.8 Ge _0.2 powder in Example 1 vacuum pre-annealed and then sintered in SPS;

Figure 2c: Entropy diagrams of Mn _1.1 Fe _0.9 P _0.8 Ge _0.2 direct sintering and powder vacuum pre-annealing in SPS and then sintering samples in Example 1;

Figure 3a: Scanning electron micrograph of the directly sintered sample of Mn _1.2 Fe _0.8 P _0.73 Ge _0.27 in Example 2;

Figure 3b: The scanning electron micrograph of the Mn _1.2 Fe _0.8 P _0.73 Ge _0.27 powder in Example 2, which was pre-annealed under the protection of atmosphere (N ₂ ) and then sintered with SPS;

Fig. 3c: Entropy diagrams of samples sintered directly with Mn _1.2 Fe _0.8 P _0.73 Ge _0.27 in Example 2 and the powders were pre-annealed under the protection of atmosphere (N ₂ ) and then sintered with SPS.

Figure 4: Entropy change diagram of Mn _1.2 Fe _0.8 P _0.76 Ge _0.24 directly sintered and the powder was pre-annealed under the protection of atmosphere (Ar) in Example 3 and then sintered with SPS

Detailed ways

The present invention will be described in further detail below in conjunction with the accompanying drawings and embodiments.

Example 1. The raw materials used are manganese powder, iron powder (purity ≥ 99.99%), red phosphorus powder (purity ≥ 99.9999%) and germanium flakes (purity ≥ 99.9999%). According to the chemical ratio of the nominal composition of Mn _1.1 Fe _0.9 P _0.8 Ge _0.2 , the raw materials were ball-milled in a ball mill for 4 hours, and then the ball-milled powder was placed in the SPS system and kept at 400°C for 30 minutes under vacuum, then sintered and kept warm. The sintering vacuum degree is 5Pa, the sintering temperature is 880°C, the sintering pressure is 10MPa, and the heating rate is 60°C/min. After the temperature is raised to 880°C, it is kept for 30 minutes. After cooling to room temperature with the furnace, it is demolded to obtain a cylindrical block sample. The directly sintered samples and powder pre-annealed samples were scanned under scanning electron microscope. The SEM images and entropy curves of the samples are shown in Figure 2a, 2b, 2c, respectively.

Example 2. The raw materials used are manganese powder, iron powder (purity ≥ 99.99%), red phosphorus powder (purity ≥ 99.9999%) and germanium flakes (purity ≥ 99.9999%). According to the chemical ratio of nominal composition Mn _1.2 Fe _0.8 P _0.73 Ge _0.27 , the raw materials were put into a ball mill and ball milled for 0.5 hour, then the ball milled powder was put into a N ₂ atmosphere-protected furnace and kept at 600°C for 2 minutes, and then molded and used SPS is sintered and then kept warm. The sintering vacuum degree is 6Pa, the sintering temperature is 950°C, the sintering pressure is 40MPa, the heating rate is 120°C/min, the temperature is raised to 950°C and then kept for 2 minutes, and then demolded after cooling to room temperature with the furnace to obtain a cylindrical block sample. The direct sintered samples and the powder pre-annealed samples were scanned separately. The scanning patterns and entropy changes of the samples are shown in Fig. 3a, 3b, 3c, respectively.

Example 3. The raw materials used are manganese powder, iron powder (purity ≥ 99.99%), red phosphorus powder (purity ≥ 99.9999%) and germanium flakes (purity ≥ 99.9999%). According to the chemical ratio of nominal composition Mn _1.2 Fe _0.8 P _0.76 Ge _0.24 , the raw materials were put into a ball mill for ball milling for 2 hours, and then the ball milled powder was put into an Ar atmosphere-protected furnace and kept at 500°C for 15 minutes, and then molded with SPS Carry out sintering, and then keep warm. The sintering vacuum degree is 6Pa, the sintering temperature is 920°C, the sintering pressure is 25MPa, and the heating rate is 90°C/min. After the temperature is raised to 920°C, it is kept for 20 minutes. After cooling to room temperature with the furnace, it is demolded to obtain a cylindrical block sample. The entropy change of the samples is shown in Fig. 4.

Taking the Mn _1.1 Fe _0.9 P _0.8 Ge _0.2 sample prepared in Example 1 as an example, the scanning electron microscope images of the sample show that the powder pre-annealed sample has a larger grain size than the directly sintered sample, and the grain size is relatively uniform and the composition of the material is distributed More uniform, as can be seen from the entropy change-temperature curves, the entropy changes of the directly sintered sample and the powder pre-annealed sample were 22.2 J/Kg K and 26.5 J/Kg K, respectively, and the entropy change of the powder pre-annealed sample increased by 19.4 %. Taking the Mn _1.2 Fe _0.8 P _0.73 Ge _0.27 sample prepared in Example 2 as an example, by scanning the sample, it can be seen that, as in Example 1, the powder pre-annealed sample has a larger grain size than the direct sintered sample, the grain size is uniform, and the composition of the material The distribution is more uniform. From the entropy change-temperature curve, it can be seen that the entropy changes of the directly sintered sample and the powder pre-annealed sample are 22.4J/Kg K and 29.4J/Kg K, respectively, and the entropy change of the powder pre-annealed sample increases 31.2%. Taking the Mn _1.2 Fe _0.8 P _0.76 Ge _0.24 sample prepared in Example 3 as an example, the same as the previous two examples, the entropy change of the sample obtained by pre-annealing and then sintering increased significantly, and the entropy change of the directly sintered sample and the powder pre-annealed sample 20.2J/Kg·K and 23.6J/Kg·K respectively, the entropy change of the powder pre-annealed sample increased by 16.8%.

Claims (1)

1. a kind of preparation technology of magnetic refrigerating material, the chemical general formula of described magnetic refrigerating material is: Mn _(2-x)fe _(x)p _(1-y)ge _(y), the scope of x is: the scope of 0.8 ~ 0.9, y is: 0.2 ~ 0.27; It is characterized in that comprising the following steps:

(1) raw materials was manganese, iron, phosphor powder, germanium fragment, and purity is 99.9 ~ 99.9999wt%, by starting material continuous ball milling 0.5 ~ 4 hour;

(2) by ball-milled powder preannealing 2 ~ 30min under 400 ~ 600 DEG C of vacuum or protective atmosphere; discharge plasma sintering technique is adopted to sinter powder; sintering vacuum tightness is higher than 6Pa; heat-up rate is 60 ~ 120 DEG C/min; be incubated after being warming up to sintering temperature, sintering temperature is 880 ~ 950 DEG C, and sintering pressure is 10 ~ 40MPa; soaking time is 2 ~ 30min, cools to room temperature with the furnace after having sintered.