The present invention relates to magnetocaloric materials comprising manganese, iron, one or both of nickel and cobalt, phosphorus, silicon and boron, to processes for producing said magnetocaloric materials, to the use of said magnetocaloric materials in a device selected from the group consisting of cooling systems, heat exchangers, heat pumps, thermomagnetic generators and thermomagnetic switches, and to corresponding devices comprising at least one magnetocaloric material according to the present invention.

The term "magnetocaloric material" denotes a material exhibiting a magnetocaloric effect, i.e. a temperature change caused by exposing said material to a changing external magnetic field. Application of an external magnetic field to a magnetocaloric material in the vicinity of the Curie temperature of said magnetocaloric material causes an alignment of the randomly oriented magnetic moments of the magnetocaloric material and thus a magnetic phase transition, which can also be described as a field-induced increase of the Curie temperature of the material. This magnetic phase transition implies a loss in magnetic entropy, and under adiabatic conditions leads to an increase of the sum of the lattice and electronic entropies of the magnetocaloric material compensating for the loss of magnetic entropy (so that its total entropy remains constant). Thus, applying the external magnetic field under adiabatic conditions results in an increase of the lattice vibrations, and a heating of the magnetocaloric material occurs. In technical applications of the magnetocaloric effect, the generated heat is removed from the magnetocaloric material by heat transfer to a heat sink in the form of a heat transfer medium, e.g. water. Subsequent removing of the external magnetic field causes a decrease of the Curie temperature back to the normal value, and thus allows the magnetic moments to revert to a random arrangement. This causes an increase of the magnetic entropy and a reduction of the sum of the lattice and electronic entropies of the magnetocaloric material compensating for the increase of magnetic entropy. Thus, removing the external magnetic field under adiabatic conditions results in a decrease of the lattice vibrations, and cooling of the magnetocaloric material occurs. The described process cycle including magnetization and demagnetization is typically performed periodically in technical applications.

An important class of magnetocaloric materials are compounds which comprise manganese, iron, phosphorus and silicon. Such materials and processes for their preparation are generally described in WO 2004/068512. US 201 1/0167837 and US 201 1/0220838 disclose magnetocaloric materials consisting of manganese, iron, phosphorus and silicon.

WO 2015/018610, WO 2015/018705 and WO 2015/018678 disclose magnetocaloric materials consisting of manganese, iron, phosphorus, silicon and boron.

Related art is also

WO 2015/044263 A1

Guillou et al., Phys. Status Solidi C, vol. 1 1 , No. 5-6, p. 1007-1010 (2014) / DP1 10.1002/pssc.201300569

Tegus et al., Journal of Magnetism and Magnetic Materials, vol. 272-276 (2004), p. 2389- 2390

Sudhish et al. J. Phys.: Condens. Matter 19 (2007) 196217. It has been found that magnetocaloric materials consisting of manganese, iron, phosphorus, silicon and boron have improved mechanical stability and reduced thermal hysteresis, compared to magnetocaloric materials consisting of manganese, iron, phosphorus and silicon. Advantageously, these benefits are achieved without reducing the magnetocaloric effect. However, magnetocaloric materials consisting of manganese, iron, phosphorus, silicon and boron have significantly higher Curie temperatures than magnetocaloric materials consisting of manganese, iron, phosphorus and silicon. A high Curie temperature may be detrimental for certain technical applications. Thus, it would be desirable to modify the above-defined boron containing prior art magnetocaloric materials in such manner that a Curie temperature suitable for technical applications is achieved without losing the benefits resulting from the presence of boron, and without reducing the magnetocaloric effect. It is an object of the present invention to provide magnetocaloric materials having advantageous properties which facilitate technical application of the magnetocaloric effect.

According to the present invention, there is provided a magnetocaloric material comprising

- manganese, and

iron, and

one or both of nickel and cobalt, and

phosphorus, and

silicon, and

- boron.

Preferred magnetocaloric materials of the present invention consist of

manganese, and

iron, and

one or both of nickel and cobalt, and

- phosphorus, and

silicon, and

boron.

Particularly preferred magnetocaloric materials of the present invention consist of

manganese, and

- iron, and

nickel, and

phosphorus, and

silicon, and boron.

Other particularly preferred magnetocaloric materials of the present invention consist of manganese, and

iron, and

cobalt, and

phosphorus, and

silicon, and

boron.

Other particularly preferred magnetocaloric materials of the present invention consist of manganese, and

iron, and

nickel, and

cobalt, and

phosphorus, and

silicon, and

boron.

Surprisingly it has been found that magnetocaloric materials which comprise manganese, iron, one or both of nickel and cobalt, phosphorus, silicon, and boron exhibit a large magnetocaloric effect in combination with further advantages like high mechanical stability, low thermal and magnetic hysteresis and Curie temperatures in a range suitable for technical applications like cooling and refrigeration.

Typically a magnetocaloric material according to the present invention exhibits a hexagonal Fe ₂ P structure with a crystal lattice having the space group P-62m. Corresponding structures are described by M. Bacmann et al. in Journal of Magnetism and Magnetic Materials 134 (1994) 59-67 for magnetocaloric materials of the composition MnFeP-|. _y As _y .

A material exhibiting a hexagonal Fe ₂ P structure with a crystal lattice having the space group P-62m is herein understood as a material comprising a main phase which occupies 90 % or more of the volume of the material, wherein said main phase has a hexagonal Fe ₂ P-structure with a crystal lattice exhibiting the space group P-62m. The existence of the hexagonal Fe ₂ P-structure with a crystal lattice exhibiting the space group P-62m is confirmed by X-ray diffraction patterns.

In a magnetocaloric material according to the present invention exhibiting a hexagonal Fe ₂ P structure, atoms of iron and manganese and one or both of nickel and cobalt occupy crystal sites occupied by iron atoms in Fe ₂ P, and atoms of phosphorus, silicon and boron occupy crystal sites occupied by phosphorus atoms in Fe ₂ P. Thus, nickel atoms and/or cobalt atoms as well as the boron atoms in said magnetocaloric materials according to the present invention occupy virtually exclusively crystal sites of said crystal lattice with the space group P-62m, i.e. there are virtually no nickel atoms and no cobalt atoms and no boron atoms on interstitial sites of said crystal lattice.

Herein, the term ..crystals sites" denotes positions in a given crystal structure (here Fe ₂ P) defined by the translational rules of the crystal lattice of said crystal structure which are occupied in said structure, and the term "interstitial sites" denotes positions in a given crystal structure defined by the translational rules of the crystal lattice of said crystal structure, which however are not occupied in the said structure.

Formally, certain preferred magnetocaloric materials of the present invention can be considered as being derived from a corresponding parent material which exhibits a hexagonal Fe ₂ P structure with a crystal lattice having the space group P-62m. Said parent material consists of iron, manganese, phosphorus and silicon (i.e. contains neither cobalt nor nickel nor boron). In said preferred magnetocaloric materials of the present invention, a fraction of the iron atoms and/or of the manganese atoms of the parent material is substituted by nickel and/or cobalt atoms, and a fraction of the silicon atoms and/or of the phosphorus atoms of the parent material is substituted by boron atoms. More specifically, in said preferred magnetocaloric materials of the present invention, cobalt atoms and/or nickel atoms occupy exclusively crystal sites thereby replacing manganese atoms and/or iron atoms of the corresponding parent material which consists of iron, manganese, phosphorus and silicon,

and boron atoms occupy exclusively crystal sites thereby replacing phosphorus atoms and/or silicon atoms of the corresponding parent material which consists of iron, manganese, phosphorus and silicon. Typically, the cobalt atoms and/or nickel atoms occupy crystal sites selected from the group consisting of 3g and 3f crystal sites of said crystal lattice, and/or the boron atoms occupy 1b crystal sites of said crystal lattice.

Thus, a preferred magnetocaloric material according to the present invention exhibits a hexagonal crystalline structure of the Fe ₂ P type with a crystal lattice having the space group P-62m wherein

atoms of one or both of nickel and cobalt occupy crystal sites of said crystal lattice, wherein said crystal sites are selected from the group consisting of 3g and 3f and boron atoms occupy 1b crystal sites of said crystal lattice. In preferred magnetocaloric materials of the present invention which consist of manganese, iron, nickel, phosphorus, silicon and boron,

nickel atoms occupy exclusively crystal sites thereby replacing manganese atoms and/or iron atoms of the corresponding parent material which consists of iron, manganese, phosphorus and silicon

- and boron atoms occupy exclusively crystal sites thereby replacing phosphorus atoms and/or silicon atoms of the corresponding parent material which consists of iron, manganese, phosphorus and silicon.

Typically, the nickel atoms occupy crystal sites selected from the group consisting of 3g and 3f crystal sites of said crystal lattice while the boron atoms occupy 1b crystal sites of said crystal lattice.

In preferred magnetocaloric materials of the present invention which consist of manganese, iron, cobalt, phosphorus, silicon, and boron,

cobalt atoms occupy exclusively crystal sites thereby replacing manganese atoms and/or iron atoms of the corresponding parent material which consists of iron, man- ganese, phosphorus and silicon

and boron atoms occupy exclusively crystal sites thereby replacing phosphorus atoms and/or silicon atoms of the corresponding parent material which consists of iron, manganese, phosphorus and silicon. Typically, the cobalt atoms occupy crystal sites selected from the group consisting of 3g and 3f crystal sites of said crystal lattice while the boron atoms occupy 1b crystal sites of said crystal lattice.

In preferred magnetocaloric materials of the present invention which consist of manganese, iron, nickel, cobalt, phosphorus, silicon, and boron,

nickel atoms and cobalt atoms occupy exclusively crystal sites thereby replacing manganese atoms and/or iron atoms of the corresponding parent material which consists of iron, manganese, phosphorus and silicon

and boron atoms occupy exclusively crystal sites thereby replacing phosphorus atoms and/or silicon atoms of the corresponding parent material which consists of iron, manganese, phosphorus and silicon.

Typically, the nickel atoms and the cobalt atoms occupy crystal sites selected from the group consisting of 3g and 3f crystal sites of said crystal lattice while the boron atoms occupy 1b crystal sites of said crystal lattice.

Preferred magnetocaloric materials according to the present invention consist of manganese, iron, one or both of nickel and cobalt, phosphorus, silicon, and boron and have a composition according to the general formula (I)

Mn _a Fe _b T _c P _x Si _y B _z (I)

wherein

T represents one or both of nickel and cobalt

0.5 < a < 1.4, preferably 0.7 < a < 1.35,

0.65 < b < 1.598, preferably 0.75 < b < 1.34,

0.001 < c < 0.3, preferably 0.005 < c < 0.25,

1.9 < (a + b + c) < 2.1 , preferably 1.95 < (a + b + c) < 2.05

0.25 < x < 0.799, preferably 0.3 < x < 0.745,

0.25 < y < 0.65, preferably 0.3 < y < 0.6,

0.001 < z < 0.2, preferably 0.005 < z < 0.1 ,

0.95 < (x + y + z) < 1.05, preferably 0.98 < (x + y + + z) < 1.02. A magnetocaloric material having a composition according to formula (I) exhibits a hexagonal crystalline structure of the Fe ₂ P type with a crystal lattice having the space group P-62m.

In a magnetocaloric material having a composition according to formula (I), either one of nickel and cobalt or both of nickel and cobalt are present, as represented by the variable "T" in formula (I).

More specifically, preferably a magnetocaloric material according to the present invention is selected from the group consisting of magnetocaloric materials having a composition according to the general formula general formula (II)

Mn _a Fe _b Ni _d Co _e P _x Si _y B _z (II)

wherein

0.5 < a < 1.4, preferably 0.7 < a < 1.35,

0.65 < b < 1.598, preferably 0.75 < b < 1.34,

0 < d < 0.2, preferably 0.001 < d < 0.15,

0 < e < 0.3, preferably 0.001 < e < 0.25,

0 < (d + e) < 0.3, preferably 0.001 < (d + e) < 0.25,

1.9 < (a + b + d + e) < 2.1 , preferably 1.95 < (a + b + d + e) < 2.05

0.25 < x < 0.799, preferably 0.3 < x < 0.745,

0.25 < y < 0.65, preferably 0.3 < y < 0.6,

0.001 < z < 0.2, preferably 0.005 < z < 0.1 ,

0.95 < (x + y + z) < 1.05, preferably 0.98 < (x + y + + z) < 1.02.

A magnetocaloric material having a composition according to formula (II) exhibits a hexagonal crystalline structure of the Fe ₂ P type with a crystal lattice having the space group P-62m. In a magnetocaloric material having a composition according to formula (II), either one of nickel and cobalt or both of nickel and cobalt are present. If nickel is present, and cobalt is not present, then d > 0 and e = 0. If nickel is not present, and cobalt is present, then d = 0 and e > 0. If nickel is present and cobalt is present, then d > 0 and e > 0. According- ly, magnetocaloric materials having a composition according to formula (II) comprise three groups of magnetocaloric materials, namely

a first group of magnetocaloric materials having a composition according to formula (II), wherein nickel is present, and cobalt is not present

- a second group of magnetocaloric materials having a composition according to formula (II), wherein nickel is not present, and cobalt is present

a third group of magnetocaloric materials having a composition according to formula (II), wherein nickel is present, and cobalt is present.

A magnetocaloric material according to the above-defined first group is selected from the group consisting of magnetocaloric materials having a composition according to the general formula (Ma)

Mn _a Fe _b Ni _d P _x Si _y B _z (Ma)

wherein

0.5 < a < 1.35, preferably 0.7 < a < 1.25,

0.65 < b < 1.598, preferably 0.75 < b < 1.34,

0.001 < d < 0.2, preferably 0.005 < d < 0.15,

1.9 < (a + b + d) < 2.1 , preferably 1.95 < (a + b + d) < 2.05

0.25 < x < 0.799, preferably 0.3 < x < 0.745,

0.25 < y < 0.65, preferably 0.3 < y < 0.6,

0.001 < z < 0.2, preferably 0.005 < z < 0.1 ,

0.95 < (x + y + z) < 1.05, preferably 0.98 < (x + y + z) < 1.02.

A magnetocaloric material according to the above-defined second group is selected from the group consisting of magnetocaloric materials having a composition according to the general formula (Mb)

Mn _a Fe _b Co _e P _x Si _y B _z (lib)

wherein

0.5 < a < 1.35, preferably 0.7 < a < 1.25,

0.65 < b < 1.598, preferably 0.75 < b < 1.34,

0.001 < e < 0.3, preferably 0.005 < e < 0.25, 1.9 < (a + b + e) < 2.1 , preferably 1.95 < (a + b + e) < 2.05

0.25 < x < 0.799, preferably 0.3 < x < 0.745,

0.25 < y < 0.65, preferably 0.3 < y < 0.6,

0.001 < z < 0.2, preferably 0.005 < z < 0.1 ,

0.95 < (x + y + z) < 1.05, preferably 0.98 < (x + y + z) < 1.02.

A magnetocaloric material according to the above-defined third group is selected from the group consisting of magnetocaloric materials having a composition according to the general formula (lie)

Mn _a Fe _b Ni _d Co _e P _x Si _y B _z (lie)

0.5 < a < 1.35, preferably 0.7 < a < 1.25,

0.65 < b < 1.598, preferably 0.75 < b < 1.34,

0.001 < d < 0.2, preferably 0.005 < d < 0.15,

0.001 < e < 0.3, preferably 0.005 < e < 0.25,

1.9 < (a + b + d + e) < 2.1 , preferably 1.95 < (a + b + d + e) < 2.05

0.25 < x < 0.799, preferably 0.3 < x < 0.745,

0.25 < y < 0.65, preferably 0.3 < y < 0.6,

0.001 < z < 0.2, preferably 0.005 < z < 0.1 ,

0.95 < (x + y + z) < 1.05, preferably 0.98 < (x + y + z) < 1.02.

Specifically preferred magnetocaloric materials from the above-defined first group are selected from the group consisting of magnetocaloric materials having a composition according to the general formula (lid)

MnFe ₍ o.95-d)Ni _d Po.5iSio.45Bo.o4 (Hd)

wherein

0.001 < d < 0.2, preferably 0.005 < d < 0.15, and most preferably d is 0.06, 0.08, 0.10 or 0.12.

Particularly preferred materials having a composition according to formula (lid) are

MnFe0.89Ni0.06P0.51Si0.45B0.04

MnFe0.87Ni0.08P0.51Si0.45B0.04 M n F6Q.85N io.1 oPo.51 Sio.45Bo.04

MnF6o.83 io.12Po.5lSio.45Bo.04-

Specifically preferred magnetocaloric materials from the above-defined second group are selected from the group consisting of magnetocaloric materials having

- a composition according to the general formula (Me)

M n Feo.8 ₅ Co ₀ .i ₀ P(o.55-z)Si ₀ .45B _z (Me)

wherein

0.001 < z < 0.2, preferably 0.005 < z < 0.1 , and most preferably z is 0.02, 0.04 or 0.06

- a composition according to the general formula (llf)

MnFe ₍ o.95-e)CO _e Po.5lSio.45Bo.04 (Hf)

wherein 0.001 < e < 0.2, preferably 0.005 < e < 0.15, and most preferably e is 0.07, 0.09, 0.1 1 or 0.13

a composition according to the general formula (llg)

MnFe ₍ o.95-e)Co _e Po.44Sio.5oBo.o6 (Hg)

wherein

0.001 < e < 0.3, preferably 0.005 < e < 0.25,

and most preferably e is 0.16, 0.20 or 0.24.

Particularly preferred materials having a composition according to formula (Me) are M n Feo.ssCoo.10P0.53Si0.45B0.02

MnFe0.85Co0.10P0.51Si0.45B0.04

M n Feo.ssCoo.10P0.49Si0.45B0.06-

Particularly preferred materials having a composition according to formula (llf) are

MnFe0.88Co0.07P0.51Si0.45B0.04

M n Feo.86Coo.09Po.51 Sio.45Bo.04

M n Feo.84Coo.11 P0.51 Sio.45Bo.04

MnFe0.82Co0.13P0.51Si0.45B0.04- Particularly preferred materials having a composition according to formula (llg) are

M n Feo.7gCoo. i6Po.44Sio.50Bo.06

M n Fe0.75Co0.20P0.44Si0.50B0.06

M n Feo.7i Coo.24Po.44Sio.50Bo.06.

Preferred magnetocaloric materials according to the present invention exhibit

a Curie temperature Tc in the range of from 240 K to 350 K, preferably in the range of from 250 K to 340 K, further preferably in the range of from 260 K to 320 K, and/or

a magnetic entropy change AS _m of 3 J kg ^"1 K ^" or more, preferably of 4 J kg ^"1 K ^" or more, more preferably of 5 J kg ^"1 K ^" or more, in each case at a magnetic field change of 1 T

and/or

a thermal hysteresis AT _hys of 5 K or less, preferably of 4 K or less, more preferably of 3 K or less, in each case at zero magnetic field at a sweep rate of 2 K/min and/or

a volume change of the elementary cell during the magnetic phase transition of 0.2 % or less, preferably of 0.1 % or less, most preferably of 0.05% or less and/or

an adiabatic temperature change AT _ad of 1 K or more, preferably of 1.25 K or more, more preferably of 1.5 K or more

and/or

a magnetic hysteresis of 0.5 T or less, preferably 0.2 T or less, more preferably 0.1 T or less.

Preferred magnetocaloric materials according to the present invention are those which exhibit two or more of the above-defined preferred features in combination. Specifically preferred magnetocaloric materials according to the present invention exhibit

a Curie temperature Tc in the range of from 240 K to 350 K, preferably in the range of from 250 K to 340 K, further preferably in the range of from 260 K to 320 K, and a magnetic entropy change AS _m of 3 J kg ^"1 K ^" or more, preferably of 4 J kg ^"1 K ^" or more, more preferably of 5 J kg ^"1 K ^" or more, in each case at a magnetic field change of 1 T

and

a thermal hysteresis AT _hys of 5 K or less, preferably of 4 K or less, more preferably of 3 K or less, in each case at zero magnetic field at a sweep rate of 2 K/min and

a volume change of the elementary cell during the magnetic phase transition of 0.2 % or less, preferably of 0.1 % or less, most preferably of 0.05% or less and

an adiabatic temperature change AT _ad of 1 K or more, preferably of 1 .5 K or more, more preferably of 2 K or more

and

a magnetic hysteresis of 0.5 T or less, preferably 0.2 T or less, more preferably 0.1 T or less.

The Curie temperature Tc and the thermal hysteresis AT _hys are determined from differential scanning calorimetry (DSC) zero field measurements.

The magnetic entropy change AS _m is derived from magnetization measurements using the Maxwell relation.

The volume change of the elementary cell during the magnetic phase transition is determined from X-ray diffraction patterns as a function of temperature in a temperature range around T _c in zero field.

The adiabatic temperature change AT _ad is determined by means of an experimental setup which is designed to track the temperature of the magnetocaloric materials during magnetization and demagnetization processes while the surrounding temperature is slowly scanned over the temperature range of interest. For the direct measurements, a thermocouple is put in the middle of the sample holder, which is a small pylon-shaped plastic cup. Then, the sample holder is filled with the sample powder. The powder is compressed to increase the heat contact of the sample with the thermocouple. The sample holder is covered by a plastic cap. During the measurements, the sample holder moves in and out a magnetic field generated by two permanent magnets at a frequency of 0.1 Hz. The temperature sweep rate of a climate chamber, which regulates the surrounding temperature, is about 0.5-1.5 K/min. This is relatively low with respect to the intrinsic dT/dt related to the MCE of the sample (about 150 K/min). Hence, this setup can be considered operating under quasi-adiabatic conditions.

The magnetic hysteresis corresponds to the difference between the magnetization curves (magnetization as a function of the magnetic field strength) at increasing magnetic field strength and decreasing magnetic field strength at half of the maximum magnetization. Magnetic hysteresis is an energy loss mechanism in the magnetocaloric process cycle. It reduces the efficiency of a magnetocaloric device e.g. a heat pump. Therefore, it is desired that magnetocaloric materials exhibit a low thermal hysteresis.

Preferred magnetocaloric materials of the present invention exhibit a magnetic phase transition of first order nature (first order magnetic transition FOMT). The first order nature of the magnetic phase transition is evidenced by a more than linear variation of the magnetization upon application of an external magnetic field in the vicinity of the Curie temperature Tc.

A further aspect of the present invention relates to a process for preparing a magnetocaloric material as described above, said process comprising the steps of

(a) providing a mixture of precursors comprising atoms of the elements

manganese, and

iron, and

one or both of nickel and cobalt

phosphorus, and

silicon, and

boron

(b) reacting the mixture provided in step (a) to obtain a solid reaction product,

(c) optionally shaping of the solid reaction product obtained in step (b) to obtain a shaped solid reaction product,

(d) heat treatment of the solid reaction product obtained in step (b) or of the shaped solid reaction product obtained in step (c) to obtain a heat treated product,

(e) cooling the heat treated product obtained in step (d) to obtain a cooled product, and (f) optionally shaping of the cooled product obtained in step (e).

The mixture of precursors provided in step (a) comprises precursors comprising atoms of iron, manganese, one or both of nickel and cobalt, phosphorus, silicon and boron. In the mixture of precursors to be provided in step (a) the stoichiometric ratio of the total amounts of atoms of the elements manganese, iron, nickel, cobalt, phosphorus, silicon and boron is adjusted so that in said mixture of precursors the stoichiometric ratio of the total amounts of atoms of said elements corresponds to formula (I).

In the mixture of precursors, manganese, iron, nickel, cobalt, phosphorus, silicon and boron occur in elemental form and/or in the form of one or more compounds comprising one or more of said elements, preferably one or more compounds consisting of two or more of said elements.

Preferably, in step (a), said mixture of precursors comprises one more substances selected from the group consisting of elemental manganese, elemental iron, elemental cobalt, elemental nickel, elemental phosphorus, elemental silicon, elemental boron, phosphides of iron, phosphides of manganese, borides of iron, borides of manganese alloys of silicon and manganese (especially binary alloys of silicon and manganese, e.g. manganese silicide).

Step (a) is carried out by means of any suitable method. Preferably the precursors are powders, and/or the mixture of precursors is a powder mixture. If necessary, the mixture is ground in order to obtain a microcrystalline powder mixture. Mixing may comprise a period of ball milling which also provides suitable conditions for reacting the mixture of precursors in the solid state in subsequent step (b) (see below).

In step (b) the mixture provided in step (a) is reacted in the solid and/or liquid phase. Accordingly, step (b) comprises

(b-1 ) reacting the mixture provided in step (a) in the solid phase obtaining a solid reaction product

and/or

(b-2) transferring the mixture provided in step (a) or the solid reaction product obtained in step (b-1 ) into the liquid phase and reacting it in the liquid phase obtaining a liq- uid reaction product, and transferring the obtained liquid reaction product into the solid phase obtaining a solid reaction product. In certain processes according to the invention, reacting is carried out in the solid phase (b-1 ) over the whole duration of step (b) so that a solid reaction product is obtained. In other processes according to the invention, reacting is carried out exclusively in the liquid phase (b-2) so that a liquid reaction product is obtained which is transferred into the solid phase obtaining a solid reaction product. Alternatively, reacting according to step (b) comprises one or more periods wherein reacting is carried out in the solid phase and one or more periods wherein reacting is carried out in the liquid phase. In preferred cases the reacting in step (b) consists of a first period wherein reacting is carried out in the solid phase (b-1 ) followed by a second period wherein reacting is carried out in the liquid phase (b-2) obtaining a liquid reaction product which is transferred into the solid phase obtaining a solid reaction product. Preferably, step (b) is carried out under a protective gas atmosphere.

In a preferred process according to the present invention, in step (b-1 ) reacting of the mixture in the solid phase comprises ball-milling so that a solid reaction product in the form of a powder is obtained.

In another preferred process according to the present invention, in step (b-2) reacting of the mixture comprises reacting of the mixture in the liquid phase by melting together the mixture of precursors, e.g. in an induction oven, preferably under a protecting gas (e.g. argon) atmosphere and/or in a closed vessel. Step (b-2) also comprises transferring said liquid reaction product into the solid phase obtaining a solid reaction product. Transferring said liquid reaction product into the solid phase is carried out by means of any suitable method, e.g. by quenching, melt-spinning or atomization.

Quenching means cooling of the liquid reaction product obtained in step (b-2) in such manner that the temperature of said liquid reaction product decreases faster than it would decrease in contact with resting air.

The technique of melt-spinning is known in the art. In melt-spinning the liquid reaction product obtained in step (b-2) is sprayed onto a cold rotating metal roll or drum. Typically the drum or roll is made of copper. Spraying is achieved by means of elevated pressure upstream of the spray nozzle or reduced pressure downstream of the spray nozzle. Typically the rotating drum or roll is cooled. The drum or roll preferably rotates at a surface speed of 10 to 40 m/s, especially from 20 to 30 m/s. On the drum or roll, the liquid composition is cooled at a rate of preferably from 10 ² to 10 ⁷ K/s, more preferably at a rate of at least 10 ⁴ K/s, especially with a rate of from 0.5 to 2 ^* 10 ⁶ K/s. Preferably, melt spinning is carried out under a protecting gas (e.g. argon) atmosphere. Melt spinning enables a more homogeneous element distribution in the obtained reaction product which leads to an improved magnetocaloric effect.

Atomization corresponds to mechanical disintegration of the liquid reaction product obtained in step (b-2) into small droplets, e.g. by means of a water jet, an oil jet, a gas jet, centrifugal force or ultrasonic energy. The droplets solidify and are collected on a substrate.

In a preferred process according to the present invention, in step (b-2) transferring the obtained liquid reaction product into the solid phase is carried out by quenching, melt- spinning or atomization. Step (c) is carried out by means of any suitable method. For instance, when the reaction product obtained in step (b) is a powder, in step (c) said powder obtained in step (b) is shaped by pressing, molding, rolling, extrusion (especially hot extrusion) or metal injection molding.

Step (d) is carried out by means of any suitable method. In step (d) the maximum temperature to which the solid reaction product obtained in step (b) or the shaped solid reaction product obtained in step (c) is exposed is below its melting temperature. Step (d) is performed in order to cure structural defects and to thermodynamically stabilize the reaction product obtained in step (b) and/or to strengthen and compact the shaped solid reaction product obtained in step (c) by fusing together the material grains. Preferably, in step (d) the heat treatment comprises sintering the solid reaction product obtained in step (b) or the shaped solid reaction product obtained in step (c), preferably under a protective gas atmosphere.

Particularly preferably, in step (d) the heat treatment includes a heat treatment at temperatures in the range of from 850 °C to 1250 °C, preferably of from 950 °C to 1 150 °C and most preferably of from 1025 °C to 1 125 °C, preferably for a duration of from 1 hour to 30 hours, preferably from 5 hours to 25 hours, most preferably of from 10 hours to 20 hours.

In particularly preferred processes according to the present invention, wherein step (b) involves melt-spinning, a duration of the heat treatment of 5 hours or less is sufficient, because melt spinning provides for a rather homogeneous element distribution in the obtained reaction product. In particularly preferred processes according to the present invention, in step (d) the heat treatment includes

sintering the solid reaction product obtained in step (b) or the shaped solid reaction product obtained in step (c) at a temperature in the range of from 1000 °C to 1200 °C

optionally annealing of the sintered product at a temperature in the range of from 800 °C to 950 °C

cooling down of the sintered and optionally annealed product to room temperature with cooling rates up to 100 K/s

- optionally re-heating the cooled product and re-sintering at a temperature in the range of from 1000 °C to 1200 °C.

Further preferably in step (d) the heat treatment includes

sintering the solid reaction product obtained in step (b) or the shaped solid reaction product obtained in step (c) at a temperature in the range of from 1000 °C to 1200 °C

annealing of the sintered product at a temperature in the range of from 800 °C to 950 °C

cooling down of the sintered and annealed product to room temperature with cooling rates up to 100 K/s

- re-heating the cooled product and re-sintering at a temperature in the range of from 1000 °C to 1200 °C.

In this preferred mode of carrying out step (d), during the stage of sintering the material grains are fused together so that the cohesion between the material grains of the shaped solid reaction product is increased and the porosity is reduced, and during the stage of annealing, the crystal structure is homogenized and crystal defects are cured.

Within step (d), cooling down of the sintered and optionally annealed product may be carried out by turning off the oven (known to the specialist as "oven cooling").

Step (e) is carried out by means of any suitable method. In a preferred process according to the present invention, step (e) includes contacting the heat treated product obtained in step (d) with a liquid or gaseous medium, preferably at a quenching rate of 200 K/s or less, preferably < 100 K/s or less, most preferably < 25 K/s. Particularly preferably, in step (e) quenching is carried out by means of contacting the heat treated product obtained in step (d) with oil or water or aqueous liquids, for example cooled water or ice/water mixtures. For example, in step (e), the heat treated product obtained in step (d) is allowed to fall into ice-cooled water, or the heat treated product obtained in step (d) is quenched with sub-cooled gases such as liquid nitrogen or liquid argon.

Step (f) is carried out by means of any suitable method. For instance, when the cooled product obtained in step (e) is in a shape not suitable for the desired technical application (e.g. a powder), in step (f) said cooled product obtained in step (e) is transferred into a shaped body by means of pressing, molding, rolling, extrusion (especially hot extrusion) or metal injection molding. Alternatively, the cooled product obtained in step (e) which is in the form of a powder or has been transferred into the form of a powder is mixed with a binding agent, and said mixture is transferred into a shaped body in step (f). Suitable binding agents are oligomeric and polymeric binding agents, but it is also possible to use low molecular weight organic compounds, for example sugars. The shaping of the mixture is achieved preferably by casting, injection molding or by extrusion. The binding agent either remains in the shaped body or is removed catalytically or thermally so that a porous body with a monolith structure or a mesh structure is formed.

Preferred processes according to the present invention are those which exhibit two or more of the above-defined preferred features in combination.

In a further aspect, the present invention relates to the use of a magnetocaloric material according to the present invention in a device selected from the group consisting of cooling systems, heat exchangers, heat pumps, thermomagnetic generators and ther- momagnetic switches. Preferably, said magnetocaloric material is one of the preferred magnetocaloric materials described above, preferably a magnetocaloric material having a composition according to any of formulae (lla)-(llg) described above.

In a further aspect, the present invention relates to a device selected from the group consisting of cooling systems, heat exchangers, heat pumps, thermomagnetic generators and thermomagnetic switches, wherein said device comprises at least one magnetocaloric material according to the present invention. Preferably, said magnetocaloric material is one of the preferred magnetocaloric materials described above, preferably a magnetocaloric material having a composition according to any of formulae (lla)-(llg) described above. The present invention is now further illustrated by the following examples.

Examples

Preparation of magnetocaloric materials Step (a) For the preparation of magnetocaloric materials having a composition according to formulae (lid), (Me), (llf) and (llg) as defined above, in each case 15 g of a precursor mixture consisting of the precursors elemental manganese, elemental iron, one of elemental nickel and elemental cobalt, elemental red phosphorus, elemental silicon and elemental boron (each in the form of a powder) was provided. For a comparison material not com- prising boron (z = 0) 15 g of a precursor mixture consisting of the precursors elemental manganese, elemental iron, one of elemental nickel and elemental cobalt, elemental red phosphorus, and elemental silicon (each in the form of a powder) was provided.

Step (b)

Magnetocaloric materials according to the present invention and comparison materials were prepared by reacting the mixtures provided in step (a) in the solid phase using a planetary ball mill (Fritsch Pulverisette) with four grinding bowl fasteners. Each grinding bowl (80 ml volume) contains seven balls (10 mm diameter) made of tungsten carbide and 15 grams of a mixture of precursors prepared in step (a). The mixtures were ball milled for 10 hours with a constant rotation speed of 380 rpm in an argon atmosphere. (The total time in the ball mill is 16.5 hours, the machine stops milling for 10 minutes after every 15 minutes of milling).

Step (c)

After ball-milling the obtained reaction product which is in the form of a powder was compacted to small tablets (diameter 12 mm, height 5-10 mm) in a hydraulic pressing system with a pressure of 1.47 kPa (150 kgf cm ^"2 ). Step (d)

After pressing, the tablets were sealed inside quartz ampoules in an argon atmosphere of 20 kPa (200 mbar). Then, the samples were sintered at 1 100 °C for 2 h and annealed at 850 °C for 20 h. The annealed samples were cooled down slowly to room temperature by turning off the oven and thereafter re-sintered at 1 100 °C for 20 h to achieve a homogeneous composition.

Step (e)

The thermal treatment of step (e) was finished by contacting the ampoules with water.

The composition of the magnetocaloric materials prepared in the above-described manner and the composition of the corresponding precursor mixtures (weight of each precursor in g) is given in tables 1-4 below:

Table 1 : Materials according to formula MnFe(o.95-d ₎ Ni _d Po.5iSio.45B ₀ .o4 (Nd):

Table 2: Materials according to formula MnFeo.85Coo.ioP(0.55-z)Sio.45B _z (Me) and comparison material with z = 0 z Mn / [g] Fe / [g] Co / [g] P / [g] Si / [g] B / [g] z = 0.00 5.9724 5.1606 0.6407 1.8520 1.3736 0.0000 z = 0.02 5.9903 5.1751 0.6424 1.7897 1.3780 0.0236 z = 0.04 6.0080 5.1917 0.6448 1.7276 1.3823 0.0474 z = 0.06 6.0252 5.2067 0.6467 1.6646 1.3863 0.071 1 Table 3: Materials according to formula MnFe ₍ o.95-e ₎ Co _e Po.5iSio.45B ₀ .o4 (Nf):

Table 4: Materials according to formula MnFe( ₀ .95-e)Co _e Po.44Sio.5oBo.o6 (Ng):

Determination of magnetocaloric properties

Before the measurements, the samples were precooled in liquid nitrogen to remove the virgin effect. Then the samples were manually crushed by means of a mortar to prepare powders for the measurements.

The parameters Curie temperature Tc, thermal hysteresis AT _hys , adiabatic temperature change AT _ad and isothermal magnetic entropy change AS _m of the materials according to tables 1-3 are listed in tables 5-7 below. Regarding the methods to determine these parameters, reference is made to the disclosure provided above.

The isothermal magnetic entropy change AS _m and the adiabatic temperature change AT _ad are two characteristic parameters to evaluate the MCE of a magnetic material. AS _m is a measure of how much heat can be transported (at a given temperature) by magnetic means, while AT _ad is a measure of how big the temperature difference is that can be achieved in the transfer of the heat to and from the heat transfer fluid. In other words, AS _m determines the cooling capacity, and AT _ad is directly associated with the temperature span in magnetic refrigerators. Table 5: Materials according to formula MnFe(o.95-d ₎ Ni _d Po.5iSio.45B ₀ .o4 (Nd):

Table 7: Materials according to formula MnFe(o.95-e ₎ Co _e Po.5iSio.45Bo.o4 (Nf): e T _c (K) upon |AS _m |(Jkg- ¹ K- ¹ ) ΔΤ^ (K) Δ^,(Κ) cooling

0.5 T 1.0 T 1.5T 2.0 T

0.07 316 2.7 5.3 6.8 8.1 1.3 1.82

0.09 304 5.0 9.1 10.7 11.9 1.7 1.90

0.11 295 3.7 7.7 10.0 11.4 2.5 1.94

0.13 272 5.7 9.2 10.6 11.5 1.9 2.04 Table 6 shows that with increasing boron content the thermal hysteresis is reduced, while the Curie temperature increases. Tables 5 and 7 show that with increasing content of nickel resp. cobalt the Curie temperature is reduced, while the thermal hysteresis remains low and the magnetic entropy change as well as the adiabatic temperature change re- main in a range suitable for technical applications. It is also important to note that for most materials the thermal hysteresis is smaller than their adiabatic temperature change, thus rendering these materials suitable for magnetocaloric devices with cyclic operation.

Figure 1 shows the temperature dependence of the specific magnetization (magnetization per mass) recorded on cooling and heating (sweeping rate 2 k/min) in a magnetic field of 1 T for the materials according to table 1.

Figure 2 shows the temperature dependence of the specific magnetization (magnetization per mass) recorded on cooling and heating (sweeping rate 2 k/min) in a magnetic field of 1 T for the materials according to table 2.

Figure 3 shows the temperature dependence of the specific magnetization (magnetization per mass) recorded on cooling and heating (sweeping rate 2 k/min) in a magnetic field of 1 T for the materials according to table 3.

Figure 4 shows the temperature dependence of the specific magnetization (magnetization per mass) recorded on cooling and heating (sweeping rate 2 k/min) in a magnetic field of 1 T for the materials according to table 4. Significant reduction of thermal hysteresis due to the presence of boron is evident from figure 2. Figures 1 , 3 and 4 show that the Curie temperature decreases with increasing content of nickel resp. cobalt, while the thermal hysteresis remains low.