Differential thermal analysis on metallic alloys containing volatile elements
can be highly problematic. Here we show how measurements can be performed in
commercial, small-sample, equipment without modification. This is achieved by
using a sealed Ta crucible, easily fabricated from Ta tubing and sealed in a
standard arc furnace. The crucible performance is demonstrated by measurements
on a mixture of Mg and MgB$_2$, after heating up to 1470$^{\circ}{\rm C}$. We
also show data, measured on an alloy with composition Gd$_{40}$Mg$_{60}$, that
clearly shows both the liquidus and a peritectic, and is consistent with
published phase diagram data

K. W. Dennis

Loebel R.

M. Angst

Nayeb-Hashemi A. A.

Okamoto H.

P. C. Canfield

R. W. McCallum

Y. Janssen

English

arXiv

Differential thermal analysis on metallic alloys containing volatile elements can be highly problematic. Here we show how measurements can be performed in commercial, small-sample equipment without modification. This is achieved by using a sealed Ta crucible, easily fabricated from Ta tubing and sealed in a standard arc furnace. The crucible performance is demonstrated by measurements on a mixture of Mg and MgB2, after heating up to 1470°C. We also show data, measured on an alloy with composition Gd40Mg60, that clearly show both the liquidus and a peritectic and are consistent with published phase-diagram data.The following article appeared in Review of Scientific Instruments (2006): 056104 and may be found at http://dx.doi.org/10.1063/1.2202923.</p

Janssen, Y.

Angst, M.

Dennis, Kevin

Canfield, Paul

McCallum, R. William

Digital Repository @ Iowa State University (ISU)

Small sealed Ta crucible for thermal analysis of volatile metallic samplesY. Janssen, M. Angst, K. W. Dennis, P. C. Canfield, and R. W. McCallum  Citation: Review of Scientific Instruments 77, 056104 (2006); doi: 10.1063/1.2202923 View online: http://dx.doi.org/10.1063/1.2202923 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/77/5?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in On the role of Ta cap in the recrystallization process of CoFeB layers Appl. Phys. Lett. 103, 142412 (2013); 10.1063/1.4824033  Optical emission from a small scale model electric arc furnace in 250–600 nm region Rev. Sci. Instrum. 84, 043111 (2013); 10.1063/1.4802833  Effect of Mg interlayer on perpendicular magnetic anisotropy of CoFeB films in MgO/Mg/CoFeB/Ta structure Appl. Phys. Lett. 101, 122414 (2012); 10.1063/1.4754118  Influence of ZnO on the crystallization kinetics and properties of diopside-Ca-Tschermak based glasses andglass-ceramics J. Appl. 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McCallumMaterials and Engineering Physics, Ames Laboratory, Ames, Iowa 50011 and Department of MaterialsScience and Engineering, Iowa State University, Ames, Iowa 50011Received 2 November 2005; accepted 11 April 2006; published online 9 May 2006Differential thermal analysis on metallic alloys containing volatile elements can be highlyproblematic. Here we show how measurements can be performed in commercial, small-sampleequipment without modification. This is achieved by using a sealed Ta crucible, easily fabricatedfrom Ta tubing and sealed in a standard arc furnace. The crucible performance is demonstrated bymeasurements on a mixture of Mg and MgB2, after heating up to 1470 °C. We also show data,measured on an alloy with composition Gd40Mg60, that clearly show both the liquidus and aperitectic and are consistent with published phase-diagram data. © 2006 American Institute ofPhysics. DOI: 10.1063/1.2202923Differential thermal analysis DTA is an important toolfor determining both phase-diagram and thermal-processinginformation for advanced-materials development. It is par-ticularly helpful in the solution growth of single crystals.1For a typical commercial, small-sample DTA, in this case aPerkinElmer Pyris DTA 7, the standard crucible is an openAl2O3 crucible PerkinElmer. This presents two problems,one of stability and one of volatility. To close the crucibles,Al2O3 caps are available, but they do not provide sufficientsealing for samples containing highly volatile components.In addition, elements such as Li, Mg, and rare earths attackAl2O3.For metallic alloys that do not attack it, Ta is often thecrucible material of choice. Ta is easily fabricated into cru-cibles and has been found to remain inert during lengthyhigh-temperature growth experiments for a wide range ofmetallic alloys. Moreover, in growth crucibles,2 Ta caps canbe welded onto the crucibles in a standard arc furnace, re-sulting in a sealed crucible with an Ar room-temperaturepressure of 1 bar. For growth of crystals that involvesvolatile elements, such as Mg, Li, or Yb, such sealed Tacrucibles2 have been shown to yield well-formed single crys-tals. In order to replicate the growth environment,1 smallsealed Ta crucibles, usable in a commercial DTA, are re-quired. Given the high melting point of Ta, 3000 °C, andthe small dimensions of the crucible, careful considerationmust be given to the heat flow during the sealing process.aElectronic mail: yjanssen@ameslab.gov0034-6748/2006/775/056104/3/$23.00 77, 05610le is copyrighted as indicated in the article. Reuse of AIP content is subjec129.186.176.91 On: Mon, 0In the differential thermal analyzer, Al2O3 crucibles areplaced in Pt sample and reference cups with Pt-based ther-mocouples inserted through the bottom so that theycontact the crucibles. If Ta is placed in direct contact withthe Pt, there is a possibility that at elevated temperatures1000 °C, Ta may diffuse into the Pt, and thereby changethe thermocouple calibration, or it may create a diffusionbond between the Pt parts and the Ta. Therefore, the Ta cru-cibles are placed inside the standard Al2O3 crucibles, whichcan be reused multiple times in this application.The body of the crucible consists of small-diameter Tatubing annealed Ta tube, ASTM B 521 R05200. In order tosuccessfully seal volatile metals in such a crucible, it is nec-essary to keep their temperature as low as possible. However,the radiant heat produced while sealing the crucible wasfound to be sufficient to volatilize Mg. The incorporation ofa radiation shield inside the crucible adequately addressedthis problem.In Fig. 1 the components of the crucible a–h aredisplayed. The Ta crucibles are made of pieces of Ta tubing10–12 mm length, 2.15 mm inner diameter, and 3.2 mmouter diameter b. Two pieces of Ta of about 120 mg andanother one of about 75 mg are melted in a standard Ar-atmosphere arc furnace to make Ta balls, which form natu-rally upon melting because of surface tension. The larger twoballs a and e are larger than the inner diameter of the Tatube and are used to form the ends of the crucible, whereasthe smaller ball d, which just fits inside the tubing, will actas a radiation shield that protects the sample c.To form an unsealed Ta crucible, the Ta tube b is© 2006 American Institute of Physics4-1t to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:8 Sep 2014 20:49:12056104-2 Janssen et al. Rev. Sci. Instrum. 77, 056104 2006 This articclamped in the water-cooled copper hearth of an Ar-atmosphere standard arc furnace, and one of the larger Taballs a is placed on top of it. The ball is carefully meltedwith a low operating current, so that it forms a closed lid ofthe crucible without making it wider than the original tube.This crucible is the standard Ta crucible that we have usedbefore.1,3,4 A similar crucible can be made by mounting thetubing in a lathe and spinning one end closed, but this re-quires a skilled machinist and can thin the walls and leave asmall hole at the tip.Next, the sample c is placed inside, and a pair of cut-ters is used to pinch indentations in the crucible f. Thepurpose of the indentations is to position the radiation shieldd, the smaller Ta ball, above the sample so that the shield isin contact with the crucible but with neither the sample northe lid e. The assembly is then sealed by carefully meltingthe lid, the second larger Ta ball, on top of the crucible, asabove, to form the sealed crucible h. Finally, the standardAl2O3 crucible g will hold the crucible in the DTA.We have made no attempt to accurately determine themaximum sample temperature as the crucible is sealed withthe radiation shield in place. However, we have used thistechnique to encapsulate several pieces of Mg and thenopened the crucible to inspect the contents. The results indi-cated that partial melting of some of the Mg pieces occurred,but the pieces were still distinct. This suggests that, althoughthere was sufficient radiative heat to bring the irregularpieces of Mg to near their melting temperature, upon theoccurrence of surface melting, the increase in thermal con-tact with the crucible prevented complete melting of thesample. Since the vapor pressure of Mg at its melting tem-perature 650 °C is only about 3 mbar Ref. 5, the in-crease in pressure within the crucible is not sufficient to dis-place the molten ball of Ta Fig. 1e, so that the crucible issuccessfully sealed. Assuming that the average temperatureinside the crucible was about 600 °C, the room-temperatureAr pressure inside the sealed crucible was approximately1/3 bar.In order to qualify the crucibles, both a standard BaCO3sample NIST reference material 8760 and a Ag 99.999%sample were measured, both in a Ta crucible and in a stan-dard Al2O3 crucible at heating rates of 5, 10, and 20 °C/min.The instrument was calibrated using Al2O3 crucibles at aheating rate of 10 °C/min. At this rate, the onset temperaturefor the crystallographic phase transformation in BaCO3809 °C was found equal to 809.5 °C in the Al2O3 crucibleFIG. 1. The different steps in manufacturing the sealed volatile-elementDTA crucible, as described in the text.and 806.8 °C in the Ta crucible. The variation with heatingle is copyrighted as indicated in the article. Reuse of AIP content is subjec129.186.176.91 On: Mon, 0rate of onset temperatures was smaller for the Ta cruciblesthan for the Al2O3 crucibles: about 3.3 °C for Ta and 5.4 °Cfor Al2O3. The onset temperature of melting for Ag 962 °Cwas, measured at 10 °C/min, found to be equal to 959.3 and959.4 °C in the Al2O3 and the Ta crucibles, respectively.Here, the variation with heating rate of onset temperatureswas also smaller for the Ta crucibles than for the Al2O3crucibles: 2.5 °C for Ta and 4 °C for Al2O3. The integratedarea under the DTA peak for the Ag samples was found to besmaller, by about 45%, for the Ta crucibles, probably due tothe extra thermal barrier that the Ta–Al2O3 interface formssee Fig. 1. The variation with heating rate of the integratedarea under the DTA peak for Ag was less than 2% for bothAl2O3 and Ta crucibles.The effectiveness of the Ta crucibles in holding volatilesamples was tested by a series of DTA measurements madein an attempt to determine solution-growth parameters forgrowing MgB2, from a mixture of Mg and MgB2. 9.7 mgMg and 0.71 mg MgB2 which amounts to Mg0.93B0.07 weresealed in a crucible as described above and subjected to sev-eral DTA runs. The highest temperature reached was1470 °C, which lies outside of the calibration range of theinstrument. The curve obtained from one of the runs after thesample had been at 1470 °C is displayed in Fig. 2. Thesample was heated and cooled between 450 and 1200 °C ata rate of 10 °C/min. Upon heating, only one endothermicevent, with an onset temperature of about 640 °C, was ob-served, and upon cooling only one exothermic event wasobserved, also with an onset temperature of about 640 °C.These results are consistent with the published Mg–B phasediagram,6 which shows no exposed liquidus for MgB2 atambient pressures and a eutectic essentially just the meltingof pure Mg at 650 °C. Furthermore, the results indicatedthat solution-growth parameters cannot be determined in thisway.This experiment demonstrates quite clearly the good per-formance of the crucible. The sealed crucible allowed re-peated heating cycles up to 1470 °C, and its appearance didnot change visibly due to the temperature cycles. Thestrength of the DTA signal due to the melting and solidifica-FIG. 2. Relevant part of the DTA curves of a sample of Mg and MgB2measured upon heating and cooling with a 10 °C/min rate. The meltingand solidification event near 640 °C can clearly be distinguished.tion did not visibly change in any of the several runs that wet to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:8 Sep 2014 20:49:12056104-3 Notes Rev. Sci. Instrum. 77, 056104 2006 This articperformed, consistent with no Mg loss during the experi-ments. Note that at 1470 °C, the partial pressure of Mg isabout 12 bar and the Ar partial pressure is estimated at about2 bar.A second example demonstrates the usefulness of thesealed Ta DTA crucible for the determination of phase-diagram data. A total of 40.9 mg of Gd and Mg, correspond-ing to the composition Gd40Mg60, was sealed in a crucible, asdescribed above. After alloying the elements by heating thecrucible to above 1200 °C, the DTA signals were recorded.The DTA results measured upon heating to 1100 °C andcooling to below 400 °C at 10 °C/min are shown in Fig. 3.In both curves, two events can clearly be observed. Uponheating, a sharp peak occurs at 750 °C, followed by anevent typical of a liquidus event,1 ending near 850 °C.Upon cooling, an event that is also typical of a liquidus oc-curs with an onset temperature of 840 °C, followed by asharp peak with an onset temperature of 750 °C. No fur-FIG. 3. Relevant part of the DTA curves of a sample of Gd40Mg60 measuredupon heating and cooling with a 10 °C/min rate. The melting and solidi-fication events near 750 and 850 °C can clearly be distinguished.ther events were observed at lower temperature. These re-le is copyrighted as indicated in the article. Reuse of AIP content is subjec129.186.176.91 On: Mon, 0sults are consistent with published phase-diagram data,7where the liquidus temperature for the compositionGd40Mg60 is reported near 850 °C, below which the com-pound GdMg solidifies, followed by the peritectic forma-tion of GdMg2, at 755 °C.Both sealed and unsealed Ta crucibles fabricated as de-scribed above have been used routinely for DTA measure-ments in our laboratory for over one year. With experience,the failure rate in sealing the crucibles is minimal. Rare fail-ures in sealing are easy to identify and discard and no suc-cessfully sealed crucible has failed during measurement, sothat the instrumentation has not been exposed to reactivevapors. It should be noted that the use of Ta crucibles re-quires an extremely clean environment within the DTA,since Ta will oxidize rapidly and become brittle. The overallcost of the Ta crucibles is approximately the same as that ofthe commercially available ceramic crucibles.The authors are grateful to M. Huang, J. Frederick, S. A.Law, and S. Jia for useful discussions and sample prepara-tion. Ames Laboratory is operated for the U.S. Departmentof Energy by Iowa State University under Contract No.W-7405-Eng-82. This work was supported by the Directorfor Energy Research, Office of Basic Energy Sciences.1 Y. Janssen, M. Angst, K. W. Dennis, R. W. McCallum, and P. C. Canfield,J. Cryst. Growth 285, 670 2005.2 P. C. Canfield and I. R. Fisher, J. Cryst. Growth 225, 155 2001.3 Y. Janssen, S. Chang, B. K. Cho, A. Llobet, K. W. Dennis, R. W. McCal-lum, R. J. McQueeney, and P. C. Canfield, J. Alloys Compd. 389, 102005.4 M. Huang, D. Wu, K. W. Dennis, J. W. Anderegg, R. W. McCallum, andT. A. Lograsso, J. Phase Equilib. Diff. 26, 209 2005.5 R. Loebel, in CRC Handbook of Chemistry and Physics, edited by R. C.Weast CRC, Cleveland, OH, 1969.6 A. A. Nayeb-Hashemi and J. B. Clark, in Desk Handbook Phase Dia-grams for Binary Alloys, edited by H. Okamoto ASM International, Ma-terials Park, OH, 2000.7 H. Okamoto, in Desk Handbook Phase Diagrams for Binary Alloys, editedby H. Okamoto ASM International, Materials Park, OH, 2000.t to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:8 Sep 2014 20:49:12

Small sealed Ta crucible for thermal analysis of volatile metallic samples

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A small sealed Ta crucible for thermal analysis of volatile metallic
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A small sealed Ta crucible for thermal analysis of volatile metallic samples

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