Article for magnetic heat exchange and method of fabricating an article for magnetic heat exchange

Abstract

An article for magnetic heat exchange comprising a magnetocalorically active phase with a NaZn.sub.13-type crystal structure is provided by hydrogenating a bulk precursor article. The bulk precursor article is heated from a temperature of less than 50° C. to at least 300° C. in an inert atmosphere and hydrogen gas only introduced when a temperature of at least 300° C. is reached. The bulk precursor article is maintained in a hydrogen containing atmosphere at a temperature in the range 300° C. to 700° C. for a selected duration of time, and then cooled to a temperature of less than 50° C.

Claims

1. A method of fabricating an article for magnetic heat exchange, comprising: providing a bulk precursor article comprising a magnetocalorically active phase with a NaZn.sub.13 crystal structure, performing hydrogenation of the bulk precursor article by: heating the bulk precursor article 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 bulk precursor article 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 bulk precursor article to a temperature of less than 50° C. to provide a hydrogenated article.

2. The method according to claim 1, wherein the cooling of the bulk precursor article to a temperature of less than 50° C. is in a hydrogen-containing atmosphere.

3. The method according to claim 2, wherein the selected duration of time is 1 minute to 4 hours.

4. The method according to claim 2, wherein after the hydrogenation, the article comprises at least 0.21 wt % hydrogen.

5. The method according to claim 2, wherein after the hydrogenation, the article comprises a magnetic phase transition temperature of in the range of −40° C. to +150° C.

6. The method according to claim 2, wherein the bulk precursor article is cooled at a rate of 0.1K/min to 10K/min.

7. The method according to claim 1, further comprising, before cooling the bulk precursor article to a temperature of less than 50° C., replacing the hydrogen gas by inert gas.

8. The method according to claim 7, wherein the selected duration of time is 1 minute to 4 hours.

9. The method according to claim 7, wherein after the hydrogenation, the article comprises a hydrogen content in the range of 0.02 wt % to 0.21 wt %.

10. The method according to claim 7, wherein the cooling of the bulk precursor article is cooled at a rate of 1 K/min to 100 K/min.

11. The method according to claim 1, wherein the cooling of the bulk precursor article to a temperature of less than 50° C. comprises cooling the bulk precursor article to a temperature in the range 300° C. to 150° C. in a hydrogen containing atmosphere, replacing the hydrogen by inert gas, and cooling the bulk precursor article to a temperature of less than 50° C.

12. The method according to claim 1, wherein the bulk precursor article has initial outer dimensions before hydrogenation and the article after hydrogenation has final outer dimensions, wherein a difference between the initial outer dimensions and final outer dimensions is less than 10 volume %.

13. The method according to claim 1, wherein the introducing of hydrogen gas is only when a temperature of 400° C. to 600° C. is reached.

14. The method according to claim 1, wherein the bulk precursor article has at least one outer dimension greater than 5 mm.

15. The method according to claim 1, wherein the bulk precursor article is polycrystalline.

16. The method according to claim 1, wherein the bulk precursor article is sintered or reactive sintered.

17. The method according to claim 1, wherein the magnetocalorically active phase is La.sub.1-aR.sub.a(Fe.sub.1-x-yT.sub.yM.sub.x).sub.13H.sub.z, wherein M is at least one element from the group consisting of Si and Al, T is at least one element from the group consisting of Co, Ni, Mn and Cr, R is at least one element from the group consisting of Ce, Nd and Pr, 0≦a≦0.5, 0.05≦x≦0.2, 0≦y≦0.2 and 0≦z≦3.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments will now be described with reference to the accompanying drawings.

(2) FIG. 1 illustrates a graph of the change in entropy as a function of temperature for partially hydrogenated articles,

(3) FIG. 2 illustrates a graph of Curie temperature as a function of gas exchange temperature for the articles of FIG. 1,

(4) FIG. 3 illustrates a graph of the hydrogen content as a function of Curie temperature for the articles of FIG. 1,

(5) FIG. 4 illustrates a graph of entropy change as a function of temperature for articles dehydrogenated at 200° C. for different times,

(6) FIG. 5 illustrates a graph of Curie temperature as a function of dehydrogenation time for the articles of FIG. 4,

(7) FIG. 6 illustrates a graph of entropy change as a function of temperature for articles dehydrogenated at 250° C. for different times,

(8) FIG. 7 illustrates a graph of entropy change as a function of temperature for articles dehydrogenated at 300° C. for different times,

(9) FIG. 8 illustrates a comparison of Curie temperature as a function of dehydrogenation time for the articles of FIGS. 4, 6 and 7, and

(10) FIG. 9 illustrates a graph of entropy change as a function of temperature for three articles with differing metallic element compositions.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

(11) An article for use as the working medium in a magnetic heat exchanger may be fabricated by hydrogenating bulk precursor article comprising a magnetocalorically active phase with a NaZn.sub.13-type crystal structure.

(12) In an embodiment, the bulk precursor article comprises one or more La(Fe.sub.1-x-yT.sub.yM.sub.x).sub.13-based phases and comprises 16.87 wt % La, 3.73 wt % Si, 4.61 wt % Co and remainder iron. Each bulk precursor article has initial dimensions of around 11.5 mm×6 mm×0.6 mm and a magnetic phase transition temperature of −18.5° C., an entropy change of 9.4 J/(kg.Math.K) for a magnetic field change of 1.6 T and 5.7% alpha-Fe (α-Fe). The peak width (entropy change as a function of temperature) is 13.7° C.

(13) The bulk precursor article is polycrystalline and may be fabricated by sintering compacted powder comprising the hydrogen-free magnetocalorcially active phase or by reactive sintering precursor powders having an overall composition corresponding to the desired hydrogen-free magnetocalorically active phase to form the desired hydrogen-free magnetocalorically active phase.

(14) The α-Fe content was measured using a thermomagnetic method in which the magnetic polarization of a sample heated above its Curie Temperature is measured as the function of temperature of the sample when it is placed in an external magnetic field. The paramagnetic contribution, which follows the Curie-Weiss Law, is subtracted and the content of alpha Fe is deduced from the remaining ferromagnetic signal.

(15) The bulk precursor articles were hydrogenated by wrapping 5 bulk precursor articles in iron foil, placing them in a furnace and heating the bulk precursor articles from a temperature of less than 50° C. to a selected temperature in the range 100° C. to 700° C. in an inert atmosphere, in particular, in argon. Hydrogen gas was introduced into the furnace only when the temperature 100° C. to 700° C. was reached. Hydrogen gas at a pressure of 1.9 bar was introduced into the furnace and the article held in a hydrogen containing atmosphere at the selected temperature for a selected duration of time or dwell time. In this embodiment, the dwell time was 2 hours. Afterwards, the articles were furnace cooled in the hydrogen containing atmosphere at a mean cooling rate of about 1 K/min to a temperature of less than 50° C.

(16) The articles heat treated at a temperature of 100° C. and 200° C. were found to have disintegrated into powder and the outermost portions of the article heat treated at 300° C. were observed to have broken away. The articles heat treated at 400° C., 500° C., 600° C. and 700° C. were all found to be intact after the hydrogenation heat treatment.

(17) The magnetocaloric properties of entropy change, peak magnetic phase transition temperature and peak width as well as the measured alpha-iron content are summarized in Table 1.

(18) TABLE-US-00001 TABLE 1 max, Entropy Peak Alpha change temp- Peak Fe ΔS′.sub.m.max, erature width content Sample Heat treatment [J/(kg .Math. K)] [° C.] [° C.] [%] MPS-1030 none 9.36 −18.5 13.7 VZ0821-1A1 HST = 100° C. 6.58 113.1 18.1 6.8 VZ0821-1B1 HST = 200° C. 8.97 117.1 12.1 6.6 VZ0821-1C1 HST = 300° C. 8.49 113.6 14.0 6.2 VZ0821-1D1 HST = 400° C. 7.46 112.3 16.3 7.0 VZ0821-1E1 HST = 500° C. 8.18 120.0 13.8 7.5 VZ0821-1F1 HST = 600° C. 8.67 118.5 12.9 6.6 VZ0821-1G1 HST = 700° C. 2.14 44.9 18.0 45.6

(19) Articles heated at a hydrogenation temperature between 100° C. and 600° C. have an increased magnetic phase transition temperature of between 112° C. and 120° C. compared to a value of −18.5° C. for the unhydrogenated bulk precursor article. For a hydrogenation temperature of 700° C., an increased alpha-iron proportion as well as a lower magnetic phase transition temperature of around 45° C. and increased peak width of 18° C. was observed indicating that the magnetocalorically active phase has partially decomposed.

(20) The hydrogen content was determined using chemical methods for the samples and the measured values are summarised in Table 2. The hydrogen content of all the articles lies within 0.2325 wt % and 0.2155 wt %.

(21) TABLE-US-00002 TABLE 2 Sample Hydrogen content [%] ratio La:H comparison, 0.0090 unhydrogenated state HST = 100° C. 0.2310 1:1.91 HST = 200° C. 0.2325 1:1.92 HST = 300° C. 0.2325 1:1.92 HST = 400° C. 0.2210 1:1.82 HST = 500° C. 0.2195 1:1.81 HST = 600° C. 0.2185 1:1.80 HST = 700° C. 0.2155 1:1.78

(22) The magnetic phase transition temperature of an article for use as the working medium in a magnetic heat exchanger translates into its operating temperature. Therefore, in order to be able to provide cooling and/or heating over a large temperature range, a working medium comprising a range of different magnetic phase transition temperatures is desirable.

(23) In principle, by hydrogenating bulk samples so that the hydrogen content of the article varies, i.e. by partially hydrogenating the article, different magnetic phase transition temperatures may be provided. Therefore, a plurality of articles of different magnetic phase transition temperature may be used together as the working medium in the magnetic heat exchanger so as to increase the operating range of the heat exchanger.

(24) In a first group of experiments, the hydrogenation conditions were adjusted in order to control the amount of hydrogen taken up by the article so that articles of differing hydrogen content and differing magnetic phase transition temperatures can be produced.

(25) Five bulk precursor articles having a size and composition as listed above were wrapped in iron foil and heated in inert gas to a hydrogenation temperature in the range 300° C. to 500° C. At the hydrogenation temperature, the inert gas was exchanged for 1.9 bar of hydrogen and the articles held at the hydrogenation temperature for 10 minutes. After 10 minutes, the hydrogen was exchanged for inert gas, the heating element was removed from the furnace and the working chamber of the furnace cooled with forced air as fast as possible to a temperature below 50° C.

(26) For two samples, hydrogenation was carried out at 350° C. and 450° C., respectively, and the samples cooled to 200° C. and 250° C., respectively, before the hydrogen was exchanged for argon.

(27) For hydrogenation temperatures of 350° C. and above, the articles were found to be intact. Also the two samples at which the gas exchange took place at 200° C. and 250° C., but which were initially heated in a hydrogen-containing atmosphere at a temperature above 350° C. were also found to be intact after the heat treatment.

(28) The measured magnetocaloric properties of the samples are summarised in Table 3. The entropy change of the samples was measured for a magnetic field change of 1.6 T and the results are illustrated in FIG. 1.

(29) TABLE-US-00003 TABLE 3 Entropy Peak Peak change temperature width ΔS′.sub.m.max, T.sub.C ΔT.sub.WHH Sample Heat treatment [J/(kg .Math. K)] [° C.] [° C.] MPS-1030 None starting material 9.4 −18.5° C.  13.7 VZ0821-1L1 TH = 500° C., 10 min, gas exchange 6.6 −3.2° C. 20.3 VZ0821-1M1 TH = 450° C., 10 min, gas exchange 6.6 −2.9° C. 19.8 VZ0821-1N1 TH = 400° C., 10 min, gas exchange 6.9 14.2° C. 19.4 VZ0821-1O1 TH = 350° C., 10 min, gas exchange 7.4 17.8° C. 17.5 VZ0821-1P1 TH = 300° C., 10 min, gas exchange 6.8 37.6° C. 18.9 VZ0821-1R1 TH = 450° C., OK auf 250° C. 7.3 88.7° C. 16.8 gas exchange VZ0821-1S1 TH = 350° C., OK auf 200° C. 7.7 97.0° C. 15.9 gas exchange

(30) The relationship between the magnetic phase transition temperature and the gas exchange temperature is also illustrated in FIG. 2. FIG. 2 shows a general trend that with increasing gas exchange temperature, the magnetic phase transition temperature decreases. In the temperature region from 250° C. to 300° C. a strong dependence of the magnetic phase transition temperature with the gas exchange temperature is observed.

(31) The hydrogen content of the samples was determined using chemical techniques and the results are summarised in Table 4 and FIG. 3. FIG. 3 illustrates a generally linear relationship between the magnetic phase transition temperature and the measured hydrogen content of the samples.

(32) TABLE-US-00004 TABLE 4 sample Hydrogen content [%] ratio La:H comparison 0.0090 GAT = 500° C. 0.0324 1:0.27 GAT = 450° C. 0.0337 1:0.28 GAT = 400° C. 0.0576 1:0.48 GAT = 350° C. 0.0621 1:0.51 GAT = 300° C. 0.0818 1:0.68 GAT = 250° C. 0.1615 1:1.33 GAT = 200° C. 0.1750 1:1.44

(33) Curie temperatures in the range of −3.2° C. and 97° C. and hydrogen contents in the range of 0.0324 wt % and 0.1750 wt % were obtained.

(34) This method therefore, enables polycrystalline sintered or reactive sintered articles for use as the working medium in the heat exchanger to be fabricated with differing magnetic phase transition temperatures and differing hydrogen content.

(35) A set of articles having differing Curie temperatures may be used together as the working medium of a magnetic heat exchanger in order to extend the operating range of the magnetic heat exchanger. The magnetic heat exchanger is able to heat and/or cool over a temperature range generally corresponding to the range of the magnetic phase transition temperatures of the working medium.

(36) In a second set of embodiments, articles with differing magnetic phase transition temperatures were fabricated by dehydrogenating fully hydrogenated or near fully hydrogenated bulk precursor articles comprising the magnetocalorically active phase described above.

(37) The hydrogenated bulk precursor articles were fabricated by heating the samples in an inert gas to 450° C. and, at 450° C., exchanging the inert gas for 1.9 bar of hydrogen. After a dwell time of two hours at 450° C. in the hydrogen atmosphere, the samples were furnace cooled in a hydrogen atmosphere to a temperature of less than 50° C.

(38) To partially dehydrogenate the now fully hydrogenated or near fully hydrogenated articles, the articles were heated at one of three different temperatures 200° C., 250° C. and 300° C. for different times in air. In particular, 10 samples were placed in a preheated oven and then the samples removed individually after a different dwell time in a range of 10 minutes to 1290 minutes. The magnetocaloric properties of the samples were measured.

(39) The results for samples heated at a temperature of 200° C. are summarised in Table 5. The entropy change at 1.6 T measured for these articles is illustrated in FIG. 4 and the dependence of the magnetic phase transition temperature as a function of dwell time at 200° C. is illustrated in FIG. 5.

(40) TABLE-US-00005 TABLE 5 Entropy Peak Alpha Dwell change Peak width Fe time at ΔS′.sub.m.max, temperature ΔT.sub.WHH content sample 200° C. [J/(kg .Math. K)] T.sub.C [° C.] [° C.] [%] VZ0826-1A1 none 8.30 113.6 14.3 6.7 VZ0826-1B1  10 min 7.91 111.3 15.0 7.6 VZ0826-1C1  30 min 7.62 101.5 15.7 8.6 VZ0826-1D1  60 min 7.37 93.1 16.1 8.2 VZ0826-1E1 120 min 7.00 95.6 17.3 8.4 VZ0826-1F1 240 min 6.87 82.3 18.5 8.3 VZ0826-1G1 390 min 6.45 64.0 19.2 8.9 VZ0826-1H1 810 min 6.30 55.9 20.0 8.6 VZ0826-1I1 1290 min  6.32 46.9 19.9 8.6

(41) The entropy change measured at 1.6 T for samples heated for different times at 250° C. and 300° C. are illustrated in FIGS. 6 and 7 and summarized in Tables 6 and 7.

(42) TABLE-US-00006 TABLE 6 Entropy Peak Alpha- Dwell change Peak width Fe time at ΔS′.sub.m.max, temperature ΔT.sub.WHH content Sample 250° C. [J/(kg .Math. K)] T.sub.C [° C.] [° C.] [%] VZ0826-1A1 none 8.30 113.6 14.3 6.7 VZ0827-1B1  10 min 6.24 98.2 21.0 8.3 VZ0827-1C1  30 min 4.42 85.4 33.0 8.8 VZ0827-1D1  60 min 6.01 55.9 20.4 10.1 VZ0827-1E1 120 min 5.68 46.5 22.5 9.5 VZ0827-1F1 240 min 5.37 29.6 23.7 10.4 VZ0827-1G1 480 min 4.95 17.4 26.2 11.1 VZ0827-1H1 960 min 4.20 15.5 33.3 11.9

(43) TABLE-US-00007 TABLE 7 Entropy Peak Alpha- Dwell change Peak temp- width Fe time at ΔS′.sub.m.max, erature T.sub.C ΔT.sub.WHH content Sample 300° C. [J/(kg .Math. K)] [° C.] [° C.] [%] VZ0828-1A1 none 8.16 117.0 14.5 6.9 VZ0828-1B1 10 min 5.38 81.8 23.4 10.1 VZ0828-1C1 30 min 4.76 56.2 29.0 11.2 VZ0828-1D1 60 min 4.99 45.0 26.1 10.4 VZ0828-1E1 120 min  4.63 26.8 29.5 10.5 VZ0828-1F1 240 min  4.44 4.0 30.3 12.5

(44) The Curie temperature as a function of dwell time for articles heated at the three different temperatures are illustrated in the comparison of FIG. 8.

(45) Generally, the magnetic phase transition temperature is reduced for increasing dwell time. Furthermore, for increased temperature, the reduction in the magnetic phase transition temperature occurs more quickly. The relationship between magnetic phase transition temperature and dwell time is approximately logarithmic for all three temperatures.

(46) For a temperature of 250° C. and 300° C., the change in entropy is slightly reduced and the peak width is increased for the partially dehydrogenated samples in comparison to the fully hydrogenated precursor sample. This indicates that the dehydrogenation may be more inhomogeneous than that achieved at 200° C. although the dehydrogenation occurs more quickly. Additionally, the alpha iron content was found to increase at 250° C. and 300° C. which may indicate that some of the magnetocalorically active phase has decomposed due to oxidation.

(47) FIG. 9 illustrates a graph of entropy change as a function of temperature for three articles with differing metallic element compositions. The magnetocaloric properties are summarized in Table 8.

(48) TABLE-US-00008 TABLE 8 Entropy change Peak Peak Alpha-Fe ΔS.sub.m.max, temperature width content [J/(kg .Math. K)] T.sub.PEAK [° C.] ΔT.sub.WHH [° C.] [%] Nr. 1 11.10 29.81 9.76 3.53 Nr 2 20.24 70.64 6.24 4.35 Nr. 3 8.97 117.06 12.09 6.58

(49) Sample Nr. 1 has a composition of 17.88 wt % La, 4.34 wt % Si, 0.03 wt % Co and 1.97 wt % Mn, rest Fe. The Co and Mn is substituted for Fe. Sample 1 was sintered at 1120° C. and then annealed at 1050° C. Sample Nr 1 was subsequently hydrogenated by heating it from room temperature to 500° C. in an argon atmosphere and exchanging the gas for 1.9 bar of hydrogen at 500° C. After a dwell time of 15 min in the hydrogen atmosphere at 500° C., the sample was furnace cooled at an average cooling rate of 1K/minute in the hydrogen atmosphere to a temperature of less than 50° C.

(50) Sample Nr. 2 has a composition of 17.79 wt % La, 3.74 wt % Si, 0.06 wt % Co and 0 wt % Mn, rest Fe. The Co is substituted for Fe. Sample 2 was sintered at 1100° C. and then annealed at 1040° C. Sample Nr 2 was subsequently hydrogenated by heating it up from room temperature to 500° C. in an argon atmosphere and exchanging the gas for 1.9 bar of hydrogen at 500° C. After a dwell time of 15 min in the hydrogen atmosphere at 500° C., the sample was furnace cooled at an average cooling rate of 1K/minute in the hydrogen atmosphere to a temperature of less than 50° C.

(51) Sample Nr. 3 has a composition of 18.35 wt % La, 3.65 wt % Si, 4.51 wt % Co and 0 wt % Mn, rest Fe. The Co is substituted for Fe. Sample 1 was sintered at 1080° C. and then annealed at 1030° C. Sample Nr 3 was subsequently hydrogenated by heating it from room temperature to 500° C. in an argon atmosphere and exchanging the gas for 1.9 bar of hydrogen at 500° C. After a dwell time of 15 min in the hydrogen atmosphere at 500° C., the sample was furnace cooled at an average cooling rate of 1K/minute in the hydrogen atmosphere to a temperature of less than 50° C.

(52) Table 8 illustrates that as the Co content is increased, the magnetic transition temperature increases. Sample 1 which includes Mn substitutions has a lower magnetic transition temperature.

(53) A working medium for a magnetic heat exchanger is provided which comprises at least one article which comprises a NaZn.sub.13-type crystal structure and hydrogen. The article may have at least one outer dimension which is at least 5 mm. For a working medium which includes two or more of these articles, the articles may have differing hydrogen contents and differing Curie or magnetic phase transition temperatures. The articles may be fully- or near fully hydrogenated as well as partially hydrogenated.

(54) The partially hydrogenated articles may be produced by adjusting the temperature at which hydrogenation is carried out as well as by exchanging the hydrogen atmosphere for an inert atmosphere at the hydrogenation temperature or at temperatures above about 150° C. during the cooling of the article from the hydrogenation temperature.

(55) For both fully-hydrogenated as well as partially hydrogenated articles, hydrogen is introduced into the furnace containing the articles only once the furnace has been heated up to a temperature above 300° C. This prevents the physical disintegration of the bulk precursor article so that a solid bulk article comprising hydrogen can be provided. Furthermore, the entropy change is largely unaffected by the hydrogenation treatment so that the hydrogenated article can provided an efficient working medium for a magnetic heat exchanger.

(56) In a further method, fully or near fully hydrogenated articles are dehydrogenated to remove some or all of the hydrogen. Since the magnetic transition temperature depends on the hydrogen content, articles of different magnetic phase transition temperature may be provided by controlling the degree of dehydrogenation. Increased dwell times at temperatures in the range 150° C. and 400° C. lead to decreasing hydrogen content and decreasing magnetic transition temperature.

(57) The invention having been described herein with respect to certain of its specific embodiments and examples, it will be understood that these do not limit the scope of the appended claims.