摘要:

The effects of the Pr additions (10, 20, 25, and 30 at.&percent;) on the magnetothermal properties of polycrystalline metallic Er have been studied using low temperature, high magnetic field heat capacity(4∼120 K, 0∼75 kOe),dc magnetization and ac magnetic susceptibility. These properties include the volumetric heat capacity, the magnetocaloric effect (MCE), the magnetic transition temperatures, the paramagnetic Curie temperatures and the effective magnetic moments. It has been found that Pr additions increase the Curie temperature and decrease the Ne´el temperature of Er, as well as wipe-out the 26 K and 52 K transitions apparently by merging with the former two transitions. This gives rise to only one MCE peak in each of the Er-Pr alloys, from the broad table-like shape for theEr90Pr10alloy to the typical narrow caret-like shape for theEr70Pr30alloy. Pr additions also increase the heat capacity of Er significantly below 18 K. The implications of this higher heat capacity as potential cryocooler regenerator materials are discussed. ©2002 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1472519

出版商:AIP    

年代:1902

数据来源: AIP

摘要:

The crystal structure and magnetothermal properties of the potential magnetic refrigerant materials,Tb5(SixGe4−x)alloys wherex⩾2,have been studied. The crystal structure of theTb5(SixGe4−x)alloys varies from the orthorhombicGd5Si4-type to the monoclinicGd5(Si2Ge2)-type as silicon concentration decreases fromx=4tox=2.The dc magnetic susceptibility and magnetization data indicate that a paramagnetic to ferromagnetic phase transition occurs in these alloys. The heat capacity data measured in magnetic fields varying from 0 to 100 kOe show that a second order(x=4,3)or a first order phase transition(x=2)occurs in the alloys. The magnetic entropy change and the adiabatic temperature change calculated from the heat capacity data indicate that theTb5(SixGe4−x)alloys are promising materials for magnetic refrigeration from about 100 to 250 K when their chemical composition varies fromx=2tox=4.©2002 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1472520

出版商:AIP    

年代:1902

数据来源: AIP

摘要:

The magnetocaloric properties (the adiabatic temperature change) of the high purity single crystalline dysprosium have been measured directly over the temperature range from 78 to 220 K in magnetic fields from 0 to 14 kOe applied along the easy magnetization direction (a-axis). These results are in good to excellent agreement, except for two regions (105 to 127 K, and 179 to 182 K), with the previous magnetocaloric effect data reported on lower purity dysprosium samples. The magnetic phase diagram of Dy has been refined based on the results of these measurements and two new high magnetic field phases have been identified. ©2002 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1472521

出版商:AIP    

年代:1902

数据来源: AIP

摘要:

The character of a magnetic entropy change under the influence of the magnetic field for different isobaric thermodynamic processes is discussed. The general equations describing the relationship of adiabatic, isothermal and field constant ways of changing of magnetic entropy are presented. It is shown that a value of total magnetic entropy adiabatic change of can be close to isothermal one (for, example, at elevated temperature and/or a small value of magnetocaloric effect) as well as to constant field one (in the case of large magnetocaloric effect and/or a small value of magnetization and low temperature). ©2002 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1472522

出版商:AIP    

年代:1902

数据来源: AIP

摘要:

A ceramic magnetic regenerator material GAP(GdAlO3)has been developed for cryocoolers below 4 K region. A new fabrication method has achieved Vickers hardness of ∼900, relative density of 99.5&percent; to that of the single crystal, and smooth surface on the spherical particles in diameters between 133 &mgr;m and 500 &mgr;m. The cooling tests with a GM cycle cryocooler have been done for4Heand3Heas working fluids individually. The 2nd regenerator constitution was adjusted by partially replacing theHoCu2with the GAP. The cooling capacity increases up to 30&percent; around 3.4 K and the minimum temperature reached 2.2 K for the4Hetest.3Heoperation with the GM cycle was investigated to provide temperatures below 2 K without the superfluid transition appearing with4He.The test results showed a remarkable improvement on the cooling characteristics; the cooling capacity increased 1.8 times at 2.0 K and 2.4 times at 1.8 K, compared to those without the GAP. The minimum temperature without heat load was lowered from 1.67 K to 1.57 K at the cycle speed of 60 rpm. ©2002 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1472523

出版商:AIP    

年代:1902

数据来源: AIP

摘要:

Testing the critical current of superconducting cables under compression is a means to assess the performance of the final magnet. However, these cable tests are complicated and expensive. A fixture to assess the superconducting performance of aNb3Snstrand within a reacted and impregnated cable under compression was designed and built. It is currently being commissioned. This device was designed to operate in liquid helium at 4.2 K and in high magnetic fields. A cable sample is compressed between two plates. A hydraulic cylinder mounted on the top flange allows applying a pressure up to 200 MPa to the cable sample. The copper current leads to the sample were designed to carry 2000 A. This paper illustrates the fixture assembly, describes the analyses performed, details the cable sample preparation, and shows preliminary test results. ©2002 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1472524

出版商:AIP    

年代:1902

数据来源: AIP

摘要:

Tensile tests to measure the ultimate tensile strength of full-section high strength composite wire have been improved by using tabs that are bonded directly to the metal. Reduced section specimens are not always desirable due to the non-uniform properties of the composite wire. The use of friction grips on tensile tests of full-section wire without tabs creates combined stresses in the material in the grip region usually resulting in premature failure. For a tabbed specimen, the stress concentration is decreased making failure within the gage section possible. Analysis is included to show that, by using tabs, direct contact of the wire to serrated friction grips is eliminated, reducing the high stress-concentration factor. The net results are higher ultimate strength values that better approximate the wire’s actual strength. ©2002 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1472525

出版商:AIP    

年代:1902

数据来源: AIP

摘要:

Since 1986, a series of international interlaboratory comparisons on the evaluation of mechanical properties of cryogenic structural materials have been performed among the participants of VAMAS (the Versailles Project on Advanced Materials and Standards) TWA (Technical Working Area) 17 in order to establish unified test methods. Through these international collaborations and Round-Robin Tests (RRTs), we have accumulated knowledge about mechanical tests at 4 K, and have prepared a draft of an international standard for tensile testing in liquid helium. The draft was submitted to ISO TC164/SCI as a new work item, and testing conditions, strain measurements, and other technical points have been discussed. The outline, development, and discussion of the document so far, with the results of RRTs, are presented. Objectives of other international standardization programs on mechanical testing in high magnetic field, J-Evaluation on Tensile Test, and for composite material are summarized also in this paper. ©2002 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1472526

出版商:AIP    

年代:1902

数据来源: AIP

摘要:

The mechanical properties and hydrogen embrittlement behaviors (HE) of Zr-4 alloy in the temperature range of 40-4.2 K have been investigated. The hydrogen free Zr-4 alloy sample exhibits a very good tensile strength and superior ductility as high as a 21&percent; increase at liquid hydrogen temperature (20 K) over that at room temperature. It also shows the typical features that appeared in austenitic stainless steel, such as unstable plastic flow, strengthening and multi-necking behaviors. The difference is that the temperature of these behaviors occurred much lower than those in austenitic stainless steel. The experiment on hydrogen charged sample shows a heavy hydrogen embrittlement behavior at low temperatures. The hydride cracking mechanism of HE has been discussed in the paper. ©2002 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1472527

出版商:AIP    

年代:1902

数据来源: AIP

摘要:

Titanium alloys are employed in the design of superconducting magnet support systems for their high mechanical strength associated with their low thermal conductivity. But their use requires a careful attention to their crack tolerance at cryogenic temperature. Measurements have been performed on two extra low interstitial materials (Ti-5Al-2.5Sn ELI and Ti-6Al-4V ELI) with different thickness and manufacturing process. The investigation includes the tensile properties at room and liquid helium temperatures using smooth and notched samples. Moreover, the fracture toughness has been determined at 4.2 K using Compact Tension specimens. The microstructure of the different alloys and the various fracture surfaces have also been studied. After a detailed description of the experimental procedures, practical engineering characteristics are given and a comparison of the different titanium alloys is proposed for cryogenic applications. ©2002 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1472528

出版商:AIP    

年代:1902

数据来源: AIP