WO2016198741A1

WO2016198741A1 - A method to produce magnetic shape memory material, composites and their uses

Info

Publication number: WO2016198741A1
Application number: PCT/FI2016/050396
Authority: WO
Inventors: Ilkka Aaltio; Frans NILSÉN; Simo-Pekka Hannula
Original assignee: Aalto Korkeakoulusaatio sr
Current assignee: Aalto Korkeakoulusaatio sr
Priority date: 2015-06-07
Filing date: 2016-06-02
Publication date: 2016-12-15
Anticipated expiration: 2017-12-07

Abstract

The invention is related to a method to produce magnetic shape memory (MSM) material. In the method MSM material is provided as atomized powder particles having essentially spherical form and the particles are heat treated at temperatures from 0.66 to 0.95 times the melting point of the particles to obtain substantial grain growth. The invention is related also to MSM composite structures and their use. MSM powder obtained by the method of the invention is embedded into a strip, rod or other shaped element of another material (e.g., a polymer or elastomer) to forma composite. The powder particles in the composite structures are oriented in a selected crystal or by externally applied magnetic field to a mutual orientation or to a certain plane

Description

A method to produce magnetic shape memory material, composites and their uses

TECHNICAL FIELD

Present invention is related to a method to produce magnetic shape memory (MSM) material, composites made of the MSM material obtained by the method and use of the composites.

Introduction and state of the art

Magnetic Shape Memory Alloy (MSMA) single crystals, such as Ni-Mn-Ga, are usually manufactured by crystallization from the melt. It is commonly known that there are several different alternative methods for crystallization from the melt, such as the Czochralski method, the Bridgman method and different modified directional solidification methods from melt. Ni-Mn-Ga single crystals are presently produced by one commercial manufacturer.

MSMAs can be made into particle (or powder) form from the single crystals by cut- ting or dicing usually by EDM method or by diamond saw with subsequent mechanical milling as shown, e.g. in the publication Aaltio, I., Ge, Y., Hirvonen, S., Hannula, S-P., MSM Polymer Composite Actuator Materials, Proc. ACTUATOR 14, Messe Bremen, 23. - 25.6, 2014 p. 98-100. In addition, single crystals can be extracted from large-grain polycrystals by cutting them between or inside the grains so that the grain boundaries are eliminated. By specific processing or alloying the grain boundaries can be made more brittle, to ease the cracking at the grain boundaries. If single crystal particles are to be made, the published methods so far have been based on the previously mentioned methods. Especially the single crystal manufacturing and cutting are both slow processes with a low yield and their cost is relatively high in terms of industrial manufacturing. In order to preserve the functionality of the ma- terial, cutting is done by special methods such as EDM and the surfaces must be specially treated thereafter, for example by electropolishing. The reported state of art in manufacturing Ni-Mn-Ga single crystals is suitable mainly for small amounts, e.g., 1 crystal per day, yielding approximately max. 20 MSMA sticks per day. In ad- dition, in the method of cutting sticks from single crystal ingots, a significant amount of waste material appears. These issues have limited the feasibility for wide product use. In addition, the process such as milling of powder is a batch process and consumes resources. The resulting particle shape in milling is highly irregular and often size distribution is large, especially with martensitic alloys such as Ni-Mn- Ga.

Gas-atom ization is an industrially used process for manufacturing metallic particles. The advantages of the process are the highly spherical shape, rather uniform size distribution and rather defect-free particles. Importantly, large amounts of metallic powder can be produced quickly and economically. It is known that for many metallic materials such as steel or Ni, the grain growth can be excited by heat treatment below the melting temperature. When the existing grains grow, the growth may occur continuously (the average grain size growing evenly) or discontinuously (by so called secondary grain growth, where only some grains grow and some do not). The driving force for the grain growth is the minimizing of the internal energy by reducing the grain boundary area. Macroscopic dimensions (e.g., sheet thickness, wire diameter or particle size) are known to limit the maximum achievable grain size. Thus, during the annealing the grain growth slows down and finally may stop as the grain size approaches the smallest macroscopic dimension. However, there is a dif- ference in the way, how grain boundaries behave in sheet, wire or round particles. Only in the latter case (round particles) it is possible to almost completely get rid of the grain boundaries as the boundary movement can almost always occur to reduce their surface area. On the contrary to the round particles, e.g., in the milled powder particles, the growing grain can reach the closest edge of the irregular particle and stop growing, and consequently a multigrain structure is maintained.

MSMA particles may be used, e.g., for preparing hybrid materials for actuation, transducer, vibration damping, suspension, energy harvesting or sensing applica- tions (and in various 3D additive manufacturing applications). This has been partly described and demonstrated in the publication Aaltio, I., Ge, Y., Hirvonen, S., Han- nula, S-P., MSM Polymer Composite Actuator Materials, Proc. ACTUATOR 14, Messe Bremen, 23. - 25.6, 2014, p. 98-100 and in EP 2 156 445 Bl. The vibration damping can be passive, semiactive or active, depending on the specific MSM and damping properties of the material and application. The high damping property of the mar- tensite phase is based mainly on the hysteretic motion of the twin boundaries. The large magnetocrystalline anisotropy of the material allows the use of magnetic field to control its properties.

Summary of the invention

An aim of the present invention is to provide an alternative method to manufacture MSMA particles, which is significantly more economical and is suitable for large amounts. To this end the method to produce magnetic shape memory (MSM) material is characterized in that in the method MSM material is provided as atomized particles having essentially spherical form and the particles are heat treated at temperatures from 0.66 to 0.95 times the melting point of the particles to obtain substantial grain growth.

Detailed description of the invention

According to the inventive method MSM material is provided as atomized particles having essentially spherical form and the particles are heat treated at temperatures from 0.66 to 0.95 times the melting point of the particles to obtain substantial grain growth. Atomization may be carried out, e.g. by gas-atomization or vacuum- atomization which processes are known per se. Preferably the MSM material is heat treated in evacuated container and the container is manipulated during the heat treatment, to prevent sticking of the particles to each other. The powder is manipulated by rotating, vibrating or rocking the container during heat treatment.

In a preferred embodiment the heat treated MSM powder particles and a suitable spacer medium are mixed and this mixture is placed into a large container in a protective gas environment so that air inside the mixture is removed, and then the mixture is heat treated inside the container in a protective gas environment oven, and after the heat treatment, the MSM particles are separated from spacer particles. MSM particles may be separated from spacer particles, e.g. by sieving, by utilizing the magnetic nature of the MSM particles or by dissolving the spacer particles. Preferably NaCI or another medium with a suitable melting temperature and chemical stability is used as spacer material.

MSM powder obtained by the inventive method can be embedded into a strip, rod or other shaped element of another material (e.g., a polymer or elastomer) to form a composite, wherein the powder particles are rotated in a selected crystal orientation or at a certain plane by externally applied magnetic field to obtain MSM compo- site structures. Orientation by applied magnetic field has two different functions. Firstly, the rotation of the particles is based on the magnetocrystalline anisotropy of the powder and as a result the powder particles in the composite will have mainly uniform magnetic easy direction/plane. Secondly, the powder particles are oriented and moved by the applied field during the manufacturing process, when the matrix material is not too viscous to prevent the particle movement, to form chain-like structures in the matrix. The direction of the chains mainly follow the direction of the magnetic field lines (Figure 2). The orientation into chain-like structures is based on the ferromagnetic nature of the powder, rather than the magnetic anisotropy, and it can be applied to powder particles made of ferromagnetic alloys. The orienta- tion is used to obtain a functional anisotropic composite structure with, e.g., high or controlled vibration damping capacity, magnetically controlled stiffness/shape change or high magnetic field induced strain in the direction of the chains (examples are in figures 6 and 7. MSM powder obtained by the inventive method can be applied to additive manufacturing process selected from the group consisting of thermal spraying and 3D printing to obtain MSM or MSM composite structures. The powder can be mixed with the matrix before or during the printing or spraying.

In another preferable process the heat treated MSM powder and a suitable medium powder are mixed and spark-plasma-sintering (SPS) is applied to the mixture to obtain a sponge-like composite structure, and the medium is then removed from the structure and an optional heat treatment is applied to the remaining structure for obtaining a suitable microstructure. The controlled space structure can be created also by selective laser melting (SLM) or -sintering (SLS). When needed, these matrix structures can been further impregnated with a suitable material, e.g. polymer.

Magnetic circuit parts, coils and/or permanent magnets can be embedded in or combined with the above composite structures to obtain functional composite structures for controlling the mechanical properties or shape change of the composite or for sensing a shape change, vibration and/or magnetic field strength or tempera- ture. The magnetic circuit parts here means objects with high magnetic permeability which are used for controlling or directing the magnetic flux. Functional composite structures can be used also for controllable transducer, vibration damper, suspension, energy harvesting or generation of power.

The presented method utilizes several readily developed industrially used processes. However, the essential phases are new. Furthermore, the MSMA particles can be mixed with an elastic matrix material, such as a polymer, elastomer or porous or fibrous ceramic, to form composite. The MSMA particles in the composite can be arranged, aligned and oriented by applied magnetic field during the production pro- cess of the composite, to generate functional properties, such as magneto- mechanical response or damping. The heat treatment process generates mobile twins. The mobility of the twins in the MSM particles may be further enhanced by the electropolishing of the particle surfaces. Examples of the invention

We have applied the gas-atomization for producing Ni-Mn-Ga powder to be used in the method of the present invention. The obtained gas-atomized Ni-Mn-Ga particles are spherical and the size distribution can be easily adjusted. The invention can be applied also for particles produced by other methods.

Example 1

We have observed the grain growth of gas-atomized Ni-Mn-Ga powder as a result of heat treatment in evacuated quartz ampoules at temperatures from 700 to 1000 °C. The randomly poured powder filled approximately 10 % of the ampule inside volume. The ampoule was put in a heat treatment furnace in a horizontal position.

As a result of the heat treatment, there was some attaching of the powder particles together, but more importantly, the majority of the powder amount reflected light to uniform direction, similarly to a single crystal, which suggests that the grains have a uniform orientation as in a single crystal. The powder particles could be separated from each other by mechanical means.

The x-ray diffraction experiments confirmed that the crystal structure of the heat treated powder had a nearly uniform grain orientation. It is noteworthy that the texture had appeared in the heat treatment.

The observation in Example 1 suggests that two events occur in the powder at the heat treatment, at the specific conditions: a) The grains grow at the heat treatment to a particularly large size. b) In certain conditions, there appeared sintering of particles to a certain degree, where the adjacent particles appeared to have a nearly-uniform crystal orientation.

The experiments for obtaining more detailed information of the above events is going on in our research group. For example we expect that the latter appears also in 3D-laser sintering or related annealing.

Industrial exploitation of the above invention is possible by several alternative methods, in addition to the gas-atomization which was explained above. Also, instead of using the Ni-Mn-Ga alloy, as described before, other suitable alloys can be used.

For the vibration damping use the alloy should be twinned martensite at the tem- perature of the application and the twin boundaries should be mobile enough to move hysteretically. For MSM application, the material should be ferromagnetic and it should have a high enough magnetocrystalline anisotropy energy. Examples of such alloys are Ni-Mn-Ga-Cu, Ni-Mn-Ga-Co, Ni-Mn-Ga-Co-Cu, Ni-Mn-Ga-B, Fe-Ga and Fe-Pd.

Example 2

The MSMA powder can be heat treated in several different ways which are described in the following.

A mixture of MSM powder and a suitable spacer material powder is prepared and put into a large container in a protective gas environment so that air inside the powder mixture is removed and then the powder mixture inside the container is heat treated in a protective gas environment oven. The spacer material prevents sticking or sintering of particles during the heat treatment and the protective at- mosphere stops harmful reactions such as oxidation of the MSM powder from taking place. After the heat treatment, the MSM powder can be easily separated from spacer particles for example by sieving, by utilizing the magnetic nature of the MSM particles or by simply dissolving the spacer particles to a liquid which is removed.

A fluidized bed furnace is a furnace type where typically an amount of 1 to 50 kg (in laboratory scale) of powder is placed inside the furnace to a container with gas- inlet-holes typically in the bottom of the container (Figure 1). The furnace is heated by the furnace resistors with temperature control. The mixing and dynamic fluctua- tion of the powder is adjusted by the flow of the gas to the container. In the fluidized bed, the hot particles move in relation to each other, which causes the temperature distribution in the furnace to be even. Depending on the thermal conductivity and properties of the fluidized bed, objects can be conveniently heat treated. The sticking or sintering (i.e., bonding or partial/full melting) of the particles can be pre- vented by the mixing or by adding suitable spacer particles as described before. The spacer particles can be ceramics or other non-magnetic particles, which can be easily separated from the MSMA particles. By using a suitable protective gas (e.g. Nitrogen or Argon) and a controlled gas flow the undesired oxidation is prevented. This method has been applied for heat treatment of the atomized Ni-Mn-Ga powder. The industrial scale furnaces today are large and thus the method can be used for producing large amounts of heat-treated powder.

Another way to heat treat the MSM powder is to mix the particles with the spacer material and to place it in a heat-withstanding container which is sealed. The spacer shall be of such a material, which does not chemically react with the heat treated powder and keeps its form well enough to prevent the particles from touching each other during the heat treatment, and also it shall be easily removable after the heat treatment. For Ni-Mn-Ga particles an example of such a spacer material is NaCI. Furthermore, these spacer particles prevent the evaporation of Mn from the MSMA powder at a high temperature and thus the chemical composition can be better controlled. Another way to heat treat the MSM powder is based on batch-type-process in evacuated ampoules or containers. This method resembles the already demonstrated quartz ampoule procedure. However, the ampoule size can be larger, the ampoules can be reusable and the ampoule can be manipulated (e.g., rotated or vibrated) during the heat treatment, to prevent sticking of the particles to each other.

Yet another way for the heat treatment is to use a specifically designed container in a furnace with a pre-chamber and post-chamber. The pre-chamber can be used for loading and the post-chamber for unloading, while the container inside the furnace is used for the actual heat treatment. In the case of using a round "tube shape" container, the container can be rotated while heat treated, if necessary. Alternatively, the furnace can be rotated together with the container.

Yet another way is to form a mixture of the atomized powder with a suitable medium (e.g. a salt) and to apply spark-plasma-sintering to the mixture to obtain a sponge-like composite. The medium is then removed from the structure (e.g. salt is dissolved) and a heat treatment can be applied, if necessary, for obtaining a suitable microstructure.

Example 3

This solution uses the rapid quenched MSMA ribbons as a starting material. The as- quenched ribbon is heat treated in the temperature range below the melting temperature to obtain a so-called bamboo-structure, where the grain boundaries extend across the ribbon.

The heat treated ribbon with the bamboo structure is mechanically crushed. In the crushing the ribbon breaks at the grain boundaries and single crystal particles are obtained. Grain boundary fracture of the ribbon may be facilitated by alloying. This effect is based on that the properly selected alloying element enriches at and em- brittles the grain boundary. Example 4

Due to the shape and size distribution of the atomized powder, after applying the specific process described here, it is suitable for additive manufacturing of MSM and MSM composite structures. In additive manufacturing a part and/or structure is built layer-by-layer from powder by locally sintering or melting the powder particles together. The thermal energy needed for localized sintering or melting of powder can be generated, for example, either with a guided laser or electron beam. The benefit of this method is that manufacturing of challenging shapes inside and/or outside of the part and/or structure is possible. This way it is possible to manufacture e.g. three dimensional reticulated objects with designed channels and convolutions without separate joints.

Once the layer-by-layer built part and/or structure is finished it can be easily heat treated, if needed, to achieve a suitable bamboo like grain structure. The optional heat treatment can be accomplished with any of the methods described previously in the Example 2. The bamboo like grain structure can also be achieved by careful selection of the process parameters during the additive manufacturing, in which case no heat treatment is needed to achieve suitable grain structure. Once the suitable grain structure is achieved the three dimensional part and/or structure can be used in any number of different applications e.g. as part of a magnetic flux circuit (vibration damping), sealed fully or partly inside solid and/or elastic material (valve, filter, heat exchanger, actuator or pump).

Example 5

The MSM powder is embedded into a composite strip, rod or other shaped element, in which the powder particles are oriented in a selected crystal or by externally applied magnetic field orientation or at a certain plane. An example of the polymer composite structure with oriented MSM particles is shown in Figure 2, where the particles have been oriented into chains along the direction of the magnetic field. A macroscopic object can be formed by gluing, melting or otherwise attaching two or more elements together in an appropriate way so that the uniform orientation is preserved. This method can be utilized by a suitable 3D printer. Magnetic coils, permanent magnets or ferromagnetic parts can be embedded into the structure as intermediate layer or layers or at the surface. The required field can be generated using them. Also, the coils and ferromagnetic parts can be used to obtain electrical signal when the shape of the part changes. The structure can thus act as a deformation, temperature- or magnetic field sensor. When connected to suitable electric circuit an energy harvester can be made.

Example 6

The MSM powder composite, as explained above, can be combined with layer(s) or part(s) of another material with piezoelectric properties, thus forming a hybrid composite with enhanced high-frequency properties.

Example 7

The MSM powder is placed into a graphite mold of the Spark Plasma Sintering (PECS) system, in which the powder is compressed and heated to form an object. The MSM powder composite, can be produced by mixing the MSM powder with a different matrix material before the PECS processing. The oriented crystal structure can be created by pre-orienting the powder in the mold by an applied field, which can be enhanced by a suitable mechanical vibration during the orientation.

Demonstration of the invention In an experiment the gas-atomized Ni-Mn-Ga powder test material had an average grain size typically 1-2 pm and the average particle size of 28 μιτι. This powder was mixed with NaCI particles by mechanical stirring and the mixture (approximately 0.5 I volume in total) was poured into AI203 cup. Ti metal chips were added and a layer of NaCI was spread on the top of the mixture to better protect the powder from compositional changes and oxidation. The cup was then placed into a fluidized bed furnace filled with Si02 sand and heat treated using Ar gas flow.

A cross-sectional scheme of the test setup is shown in Fig. 1. The gas-atomized powder is denoted by ATO. The lid on the top of the U-shaped cup is not shown in the figure. Resistors and insulation are near the side wall of the outer shell.

The chemical composition was measured by a scanning electron microscope and EDS analysis. The phase transformation temperatures are measured by the low-field ac magnetic susceptibility method. The grain size and diameter are determined from the microscope images.

Table 1 and Fig. 4 shows that the applied heat treatment process increases significantly the grain size but the chemical composition remains almost unchanged, which is supported by the similarity of the measured phase transformation temperatures in Table 2. The heat treatment has produced a more homogenous crystal structure, which is shown in the XRD results of Fig. 3.

Table 1. Measured average grain sizes and chemical composition of the ATO pow- der. as-atomized P2 (770 °C / 22 h) P3 (770 °C / 26 h) grain size (μιτι) 2 20.15 ±7.3 17.94 ±8.53

Ni (at-%) 49.28 49 49

Mn (at-%) 29.63 29.6 29.7

Ga (at-%) 21.08 21.4 21.3 aver, particle size (prn) 51

Table 2. The martensite (MT) and austenite (AT) transformation, Curie transition (T_c) and the width of the phase transformation temperatures before and after processing the powder by the heat treatment process P2 (780 °C / 22 h) and P3 (780 °C / 26 h).

Brief description of figures

Fig. 1 shows a cross-sectional scheme of the test setup. The gas-atomized powder is denoted by ATO. The lid on the top of the U-shaped cup is not shown in the figure. Resistors and insulation are near the side wall of the outer shell.

Figure 2 shows an optical microscope image of a magnetic-field-aligned composite. The MSM particles are arranged or oriented during the composite manufacturing to chains along the applied field (dark lines from upper-left to lower right corner. The inset shows a magnified detail of a chain. The round particles are connected to each other in the epoxy polymer matrix.

Figure 3 shows an X-ray diffraction spectra for the ATO powder measured at room temperature. The powder heat treated for 26 h is slightly more homogenous than the one treated at the same temperature (770 °C) for 22 h. Cu k-a radiation was used. The spectra show 10M martensite peaks. Lattice parameters are approximately: a = 5.95, b = 5.91, c = 5.58 nm. Figure 4 shows an average grain diameter of ATOl powder as a function of heat treatment time at 3 different annealing temperatures. The results show the grain growth due to the heat treatment.

Figure 5 shows a SEM micrograph of the ATO particles after P2 processing. The particles were molded into epoxy resin (dark area) and mechanically grinded and polished before examination.

The heat treated powder was mixed with epoxy and oriented by magnetic field in a mold. The magnetomechanical and dynamic properties of the resulting composite were studied in the laboratory. The magnetically controlled properties are described shortly in Figures 6 and 7.

Figure 6 shows measured compressive displacement during and after loading vs. time of the magnetic-field-oriented ATO-epoxy composite without field and in perpendicularly applied field. The unloading time varies slightly due to manual unloading operation. The results show that under applied field the composite compression displacement is smaller and the shape recovery after compression is faster than without field. The recovery at unloading is a result of the elasticity of the epoxy res- in.

Figure 7 shows uniaxial cycling fatigue test loop of the ATO-epoxy composite (filling ratio 35 vol-%) measured in a perpendicular applied field of 0.44 T. The loop is a computed average of 50 cycles which were recorded after 20 thousand cycles at 1 MPa sinusoidal tensile and compressive stress. The magnetic field biases the strain of the loop because of the elongation by the MSM effect in the composite. This can be seen as a positive average strain.

Claims

1. A method to produce magnetic shape memory (MSM) material, characterized in that in the method MSM material is provided as atomized powder particles having essentially spherical form and the particles are heat treated at temperatures from 0.66 to 0.95 times the melting point of the particles to obtain substantial grain growth.

2. The method of claim 1, wherein the MSM material is heat treated in evacuated container.

3. The method of claim 2, wherein the powder is manipulated in the container during the heat treatment, to prevent sticking of the particles to each other.

4. The method of claim 3, wherein the container is rotated, vibrated or rocked.

5. The method of claim 1, wherein the MSM powder particles and a suitable spacer medium are mixed and this mixture is placed into a container in a protective gas environment so that air inside the mixture is removed, and then the mixture is heat treated inside the container in a protective gas environment oven the spacer medium preventing sticking or sintering of the MSM particles to each other during heat treatment, and after the heat treatment, the MSM particles are separated from spacer medium.

6. The method of claim 5, wherein the MSM particles are separated from spacer particles by sieving, by utilizing the magnetic nature of the MSM particles or by dissolving the spacer particles.

7. The method of claim 5 or 6, wherein NaCI or another medium with a suitable melting temperature and chemical stability is used as spacer medium.

8. The method of claim 1, wherein the MSM powder and a suitable medium powder are mixed and spark-plasma-sintering is applied to the mixture to obtain a space structure, and the medium is then removed from the structure and an optional heat treatment is applied to the remaining structure for obtaining a suitable microstruc- ture.

9. The method of claim 1, wherein the MSM powder is used in selective laser melting or selective laser sintering to create a space structure and an optional heat treatment is applied to the remaining structure for obtaining a suitable microstruc- ture.

10. MSM composite structure, wherein MSM powder obtained by a method of any of claims 1 to 7 is embedded into a strip, rod or other shaped element of another material (e.g., a polymer or elastomer) to form a composite, the powder particles being oriented in a selected crystal orientation or at a certain plane by externally applied magnetic field

11. MSM composite structure, wherein MSM powder obtained by a method of any of claims 1 to 7 is embedded into a strip, rod or other shaped element of another material (e.g., a polymer or elastomer) to form a composite, the powder particles being oriented to form chain-like arrangements by externally applied magnetic field.

12. MSM or MSM composite structures, wherein MSM material obtained by a method of any of claims 1 to 7 is added to base object by additive manufacturing process such as gluing, 3d printing, cold- or thermal spraying.

13. MSM composite structure wherein a space structure obtained by the method of claim 8 or 9 has been impregnated with polymer or other matrix forming material.

14. The MSM composite structure according to any of claims 8 to 13, wherein mag- netic circuit parts, coils and/or permanent magnets are embedded in or combined with the composite.

15. Use of the MSM composite structure according to any of claims 8 to 14 for controlling the mechanical properties or shape change of the composite or for sensing a shape change, vibration and/or magnetic field strength or temperature.

16. Use of the MSM composite structure according to any of claims 8 to 14 for controllable transducer, vibration damper, suspension, energy harvesting or generation of power.