Method of fabricating a working component for magnetic heat exchange

1. A method of fabricating a working component for magnetic heat exchange, comprising:

selecting for each of two or more portions of the working component amounts of La, Fe, Si, and one or more elements T and R suitable to produce a La 1-a R a (Fe 1-x-y T y Si x ) 13 H z phase, wherein T is one or more of the elements from the group consisting of Mn, Co, Ni and Cr and R is one or more of the elements from the group consisting of Ce, Nd, Y and Pr, wherein the amount of the one or more elements T and R and the amount of Si that are selected for each of the two or more portions provides each of the two or more portions with a different Curie temperature, but with a similar sinter activity, wherein the two or more sintered portions of the working component comprise differing silicon contents after being sintered,

mixing the amount of the selected elements T, R and Si with La, Fe or precursors thereof in amounts suitable for producing the La 1-a R a (Fe 1-x-y T y Si x ) 13 H z phase to produce two or more precursor powder mixtures, wherein a and y are such that 0≦a≦0.5, 0.003≦y≦0.2, or 0.05≦a≦0.5, 0≦y≦0.2, or 0.05≦a≦0.5, 0.003≦y≦0.2, x is such that 0.05≦x≦0.2, and z is the hydrogen content, which ranges between 0 and z sat , wherein z sat is the saturation hydrogen concentration,

producing a single monolithic green body from the two or more precursor powder mixtures, wherein the single monolithic green body is heat treated at a temperature T sinter to produce a single monolithic working component comprising the two or more sintered portions

each having a differing T c and a density, d, within a range of ±5% of an average density, d av , of a total number of the sintered portions.

2. The method according to claim 1 , further comprising hydrogenating the sintered portions comprises heat treating under a H 2 partial pressure of 0.5 to 2 bar.

3. The method according to claim 2 , wherein the hydrogenating comprises:

heating the working component from a temperature of less than 50° C. to at least 300° C. in an inert atmosphere,

introducing hydrogen gas only when a temperature of at least 300° C. is reached,

maintaining the working component in a hydrogen containing atmosphere at a temperature in the range 300° C. to 700° C. for a selected duration of time, and

cooling the working component to a temperature of less than 50° C. to provide a second working component.

4. The method of claim 3 , wherein the working component is cooled to a temperature of less than 50° C. in a hydrogen-containing atmosphere.

5. The method according to claim 3 , wherein hydrogen gas is introduced only when a temperature of 400° C. to 600° C. is reached.

6. The method according to claim 3 , wherein after hydrogenating, the second working component comprises at least 0.18 wt % hydrogen.

7. The method according to claim 2 , wherein the hydrogenating comprises heat treating at a temperature in the range 0° C. to 100° C.

8. The method according to claim 7 , wherein the hydrogenating comprises heat treating at a temperature in the range 15° C. to 35° C.

9. The method according to claim 2 , wherein the hydrogenating comprises a dwell at a temperature T hyd , wherein 300° C.≦T hyd ≦700° C.

10. The method according to claim 9 , wherein the hydrogenating comprises a dwell at a temperature T hyd , wherein 300° C.≦T hyd ≦700° C. followed by cooling to a temperature of less than 100° C.

11. The method according to claim 2 , wherein the H 2 partial pressure is increased during hydrogenating.

12. The method according to claim 1 , wherein T comprises Mn and can optionally further comprise Co, and wherein the amount of silicon is selected according to Si m =3.85−0.0573×Co m −0.045×Mn m 2 +0.2965×Mn m , wherein Si m is the metallic weight fraction of silicon, Mn m is the metallic weight fraction of manganese, Co m is the metallic weight fraction of any cobalt that is present.

13. The method according to claim 12 , wherein the sintered portions comprise a silicon content Si, Si act , that lies within ±5% of Si m .

14. The method according to claim 13 , wherein Si act , that lies within ±2% of Si m .

15. The method according to claim 1 , wherein a multistep heat treating process is used to heat treat the green body.

16. The method according to claim 15 , wherein the multi-step heat treatment comprises a first dwell at T sinter for a time t 1 in vacuum and a time t 2 in argon, followed by cooling to a temperature T 1 , wherein T 1 <T sinter , followed by a second dwell at T 1 for a time t 3 followed by rapid cooling.

17. The method according to claim 16 , wherein 1000° C.≦T 1 ≦1080° C. and/or 0.5 h≦t 1 ≦10 h and/or 0.5 h≦t 2 ≦10 h and/or 1 h≦t 3 ≦20 h and/or the rapid cooling takes place at a rate of 5 to 200° C./min.

18. The method according to claim 1 , wherein the two or more sintered portions of the working component comprise differing values of a and y.

19. The method according to claim 1 , wherein the amount of one or more of the elements R and T is selected within the ranges 0≦a≦0.5 and 0.003≦y≦0.2.

20. The method according to claim 1 , wherein the amount of one or more of the elements R and T is selected within the ranges 0.05≦a≦0.5 and 0≦y≦0.2.

21. The method according to claim 1 , wherein the amount of one or more of the elements R and T is selected within the ranges 0.05≦a≦0.5 and 0.003≦y≦0.2.

22. The method according to claim 1 , wherein 1050° C. ≦T sinter ≦1200° C.

23. The method according to claim 1 , wherein the mixing is carried out using steel balls and optionally isopropanol.

24. The method according to claim 1 , wherein producing the green body comprises pressing the precursor powder mixtures to form the green body.

25. The method according to claim 1 , wherein density, d, is within a range of ±2% of the average density, d av , of the total number of the sintered portions.