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Measurement of low hysteretic losses in CVD prepared Nb3Ge
Volume 57A, number 4
PHYSICS LETTERS
28 June 1976
MEASUREMENT OF LOW HYSTERETIC LOSSES IN CVD PREPARED Nb3 Ge
*
J.D. THOMPSON, M.P. MALEY, L.R. NEWKIRK and F.A. VALENCIA Los AlaPnos Scientific Laboratory, Los Alamos, New Mexico 87545, USA Received 9 June 1976 Very low hysteretic losses have been measured in CVD prepared Nb3Ge. An apparent correlation between second phase content and low losses has been found as well as a reduction in hysteretic loss by appropriate surface treatment.
The preparation, by chemical vapor deposition (CVD), of bulk Nb3 Ge with values of Tc > 21.0 K and with critical current densities,Jc 1.8 X 2 has been reported recently(14K)> [1, 2]. We 106 A/cm have studied the hysteretic loss in this material [1] as a function of induced ac current and the dependence of loss upon surface morphology and second phase (tetragonal Nb 5 Ge3) concentration. The measurements have been made by a standard electrical method for measuring ac losses at 50 Hz and at 4.0 K. It was recently demonstrated, Bussiere et al.losses [3], could that be 2 at 60byHz) ac power low (< lOpW/cm achieved in Nb 3 Sn at rms-current-levels 500 A/cm) of practical interest. Since, at moderate current levels, the self-field in high-K materials exceeds Hc i~reduction of hysteretic losses requires high values of ‘c and the enhancement of the surface shielding field L~H [4]. The Nb3Ge material preparedby CVD has generally been characterized by a coarse microstructure with grain sizes> 1.0 pm and by the presence of large amounts (> 10%) of second-phase material. As a first step in a study of the effect of Nb5 Ge3 concentration upon pinning and uponJc, we have measured hysteretic losses on seven samples with Nb5Ge3 concentrations varying between 0 and 15%. The enhancement of surface shielding was accomplished by a combination of mechanical polishing and chemical etching. Nb3Ge was deposited onto the outside surface of 0.64 cm o.d. Nb-1% Zr tubes which were 23 cm long. The sample preparation techniques [5—7]may be briefly described as follows: Niobium pentachloride, prepared in-situ by chlorination of Nb, is mixed with appropriate amounts of H2, Ar, and GeC14 vapor introduced by bubbling Ar through GeCl4 liquid. The gases (—j
*
Work performed under the auspices of USERDA and EPRI.
are thoroughly mixed and passed rapidly into a vertical coating chamber maintained at 835—850°C.The coating theNb-l% inside Zr of tube a 2.0cm i.d. Cu tube chamber with the consists 0.64cmof o.d. mounted at the center axis starting 8—10 cm from the inlet end. Maintaining the entire coating chamber in a vertical position significantly reduces the circumferential inhomogeneities resulting from gravitational separation in the coating gas mixture. Nb3 Ge coat thicknesses were measured by optical metallography. They uniformcoiltoand within 10%inover the region underwere the pickup ranged average value from 30 pm to 50 pm for the various samples. Lattice spacings for all deposits in this study were measured on a Picker diffractometer using CuK~ radiation, and averaging the results from the (600), (610), and (611) lines. All the samples reported in this study had lattice spacings of 5.141 ±0.001 A. Second phase content was estimated by comparison of low angle diffractometer powder patterns with known mixtures of Nb3 Ge and Nb5 Ge3. These estimates were made on material taken from the coating chamber wall and can only be used to estimate the relative amounts on the sample tubes. Measurements taken on material coated on the sample tubes indicate that these numbers are an overestimate of Nb5Ge3 content. Mechanical treatment consisted of polishing for 0.4h with 6 pm-grit diamond compound followed by 0.5 h with a 1.0 pm-grit. Chemical etch was performed in a solution of 4 parts HNO3, 2 parts H2O, and 1 part HF for 30 s. The electrical loss measurement is accomplished by a technique similar to that described by Campbell [8], with some modifications. A signal proportional to the rate of change of flux through the sample surface 351
Volume 57A, number 4
PHYSICS LETTERS
is obtained from a 1.6cm long pickup coil of fine, high resistance wire which is wound directly on the sample tube. The inductive portion of the signal is bucked out by a compensating coil situated nearby. The magneticbyfield, to solenoid the axis of the sampie, isacprovided a 6.5parallel cm long wound of multifilament Nb-Ti wire embedded in a high resistance matrix. The compensated loss voltage is sent to a lock-in amplifier, where the in phase component of the fundamental is measured. The ac power loss is computed from the product of the induced sample current, derived from a shunt in series with the solenoid, and the output of the lock-in amplifier. End effects were minimized by the use of thick, 1.6 mm, wall Nb-l% Zr tubing as a substrate which extended well out into the fringing region of the solenoid. Results. Representative results of our measurements of ac power loss, as a function of induced current, are shown in fig. 1. The data for two samples, V444 and V469, are shown as measured in the as-dcposited condition and after various surface treatments. In the as-deposited condition, and at high currents, the data for both samples are described appropriately 4
..
i0
/
V444 V As deposited a Light mechanical polish A Chemically etched o f\nnealed
l0~
02 E ~
lOi
V469 Asdeposited • Mechanically polished A Chemically etched .
~ ~ ~ ~o v . £
/vn,
/
l0~
//
/
~
‘~
‘
~ ~
0
~
i
,./..
20
50
100
f0fa~/3J~, is in Hz, p where ~L is the power loss in W/m 0 = 4ir X 1 0~,u~is the peak induced current in A/rn, andis J~ is the from bulk the critical cur2. As evident figure, rent density in A/rn true bulk-loss behavior was only approached for induced current densities a> 1000 rms A/cm even in the as-deposited condition, and surface losses dominated the behavior in the lower current regime. Sampie V444 exhibited relatively high ac losses. This sampie was deposited at 900°Cand was characterized by a very coarse surface structure (2--3 surface features as determined by SEM). Sample V469, deposited at 835°C,showed a somewhat finer surface structure (0.5—0.8 pm) and significantly lower losses. Since it was possible to control the amount of second phase material in the coat at the lower deposition temperature (835°C),we initiated a study of the influence of Nb 5Ge3 content upon ac losses and uponJ~.Table 1 is a summary of the results of our measurements on six samples, of varying Nb5Ge3 concentration, all deposited under similar conditions at 83 5°C.The estimated values for bulk were derived from the Bean-London in the highofcurrent regime and represent expression only a relative figure merit for those samples which exhibited a large surface shielding field, ,
~
~
A
A A A
i..
200
500
J~(102)b)
___________
000
2000
n/cm)
Fig. 1. Power loss, ~L’ at 50 Hz and 4.0 K as a function of induced circumferential current, a. Open symbols, sample V444, are representative data for a high-loss material. Closed symbols, sample V469, are characteristic of low-loss material. The solid line corresponds to the Bean-London behavior of ‘~ a~.
352
2p
2
500 rms A/cm
A/cm
D
~ (rrns
=
Sample a)
/~
0
~L
were deposited at6X 835°C. PL(MW/cm2)c)
SA
A
a
/
/
by the Bean-London critical-state-model expression,
Table I Comparison of hysteretic loss and critical current density for samples of Nb 3Ge with various amounts of second phase Nb5Ge3 and with additional surface treatment. All samples
V
-~
28 June 1976
% Nb on coating 5Ge3 chamber d) wall
___
V475A V484A V479A V483A
2.4 3.0 4.0 5.6
49.0 45.0 27.0 10.8
V469A
6.2
9.0
________
1.0 0.5 3.0 15.0
±0.5 ±0.5 ±1 ±2
V469B 6.2 8.0 5.0 ± 1 V469C 8.0 0.4 V490A 7.0 ± 1 C — chemia) A — as 6.6 deposited; B —10.0 mechanically polished; cally etched. b) J~determined from critical state model fit at high induced current density at 4.0 K. c) Measured at 4.0 K and at 50Hz. d) Measured as described in text, indicates only relative amount of Nb 5Ge3 on samples.
Volume 57A, number 4
PHYSICS LETFERS
ML All samples deposited at 835°Cshowed approximately the same Ml of 350 Oe ±50 Oe (200 rms A/cm) in the as deposited state. The tentative conclusion reached from these results is that Nb5 Ge3 content is a very important parameter in determining ~c and hysteretic losses. The maximum value of ~c inferred from the loss measurements for6the as-deposited condition, was7% theNb 6.6 X figure for sample V490, with 106 A/cm 5Ge3 on the chamber wall. Recent measurements of critical current by conventional four-probe resistance techniques indicate the presence of a broad maximum in ~ ranging from 2% to 8% in Nb5Ge3 concentration [91. Sample V444, for which the data is presthted in fig. 1, was not included in this comparison since it was deposited at 900°Cand its X-ray pattern indicated strong orientation of the Nb5 Ge3 not seen in the material deposited at 835°C. The effect of surface treatment on the losses is also illustrated in fig. 1 and is summarized in table 1. Surface mechanical polishing produced a surface which was smooth on a scale of 1.0 pm but did not significantly reduce the losses. Previous studies [3] on Nb3Sn have shown, upon polishing, the appearance of a surface-barrier shielding field Ml, which significantly lowered the losses in the low current regime. The reason for the failure of polishing to produce shielding effects in our material could be seen in the X-ray diffrattometer trace. After mechanical treatment, for instance, the [211] line of the A-l5 structure was observed to broaden by a factor of about two and to decrease in amplitude, indicating the presence of substantial strain m the surface layer. After chemical etching removed the strained region, the lines recovered their natural widths and former amplitudes. The effect upon the ac losses after polishing and etching was most dramatic for sample V469, fig. 1. For this
28 June 1976
sample, shielding and negligible losses are apparent up to an induced current of 500 A/cm rms which corresponds to a peak magnetic field at the surface of 890 Oe. These low hysteretic losses are comparable with the lowest that have been achieved in Nb3Sn and are of considerable practical interest. Attempts to relieve the surface strain with an anneal of 1 h at 800°Cwere only minimally successful in sample V444, but a longer anneal should be attempted. Conclusions. AC power losses on CVD-prepared Nb3 Ge have been measured at 4 K which are negligible at 500 A/cm rms. Primarily responsible for the achievement of such low hysteretic losses was the discovery of a correlation between ~c and the concentration of the second phase, Nb5Ge3. Substantial surface barrier shielding was introduced by mechanical polishing followed by a chemical etch. Further efforts are being pursued to study the nature of the pinning mechanism in this material.
References [1] R.J. Bartnett, H.L. Laquer and R.D. Taylor, IEEE Trans. on Magnetics MAG-Il (1975) 405. [2] J.R. Gavaler et al., IEEE Trans. on Magnetics MAG-l 1 192. [3] (1975) J.F. Bussiere, M. Garber and M. Suenaga, IEEE Trans. on Ma~eticsMAG-li (1975) 324. [4] C.P. Bean and J.D. Livingston, Phys. Rev. Lett. 12 (1964) 14. [5] L.R. Newkirk et al, IEEE Trans. on Magnetics MAG-il [6] L.R.Newkirk, F.A. Valencia and T.C. Wallace, Proc. Fifth Intern. Conf. on Chemical Vapor Deposition (Electrochemical Society, Inc., Princeton, 1975) 704. [7] L.R. Newkirk and F.A. Valencia, to be published. [8] A.M. Campbell, J. Phys. C2 (1969) 1492. [9] R.J. Bartlett, private communication.
353
PHYSICS LETTERS
28 June 1976
MEASUREMENT OF LOW HYSTERETIC LOSSES IN CVD PREPARED Nb3 Ge
*
J.D. THOMPSON, M.P. MALEY, L.R. NEWKIRK and F.A. VALENCIA Los AlaPnos Scientific Laboratory, Los Alamos, New Mexico 87545, USA Received 9 June 1976 Very low hysteretic losses have been measured in CVD prepared Nb3Ge. An apparent correlation between second phase content and low losses has been found as well as a reduction in hysteretic loss by appropriate surface treatment.
The preparation, by chemical vapor deposition (CVD), of bulk Nb3 Ge with values of Tc > 21.0 K and with critical current densities,Jc 1.8 X 2 has been reported recently(14K)> [1, 2]. We 106 A/cm have studied the hysteretic loss in this material [1] as a function of induced ac current and the dependence of loss upon surface morphology and second phase (tetragonal Nb 5 Ge3) concentration. The measurements have been made by a standard electrical method for measuring ac losses at 50 Hz and at 4.0 K. It was recently demonstrated, Bussiere et al.losses [3], could that be 2 at 60byHz) ac power low (< lOpW/cm achieved in Nb 3 Sn at rms-current-levels 500 A/cm) of practical interest. Since, at moderate current levels, the self-field in high-K materials exceeds Hc i~reduction of hysteretic losses requires high values of ‘c and the enhancement of the surface shielding field L~H [4]. The Nb3Ge material preparedby CVD has generally been characterized by a coarse microstructure with grain sizes> 1.0 pm and by the presence of large amounts (> 10%) of second-phase material. As a first step in a study of the effect of Nb5 Ge3 concentration upon pinning and uponJc, we have measured hysteretic losses on seven samples with Nb5Ge3 concentrations varying between 0 and 15%. The enhancement of surface shielding was accomplished by a combination of mechanical polishing and chemical etching. Nb3Ge was deposited onto the outside surface of 0.64 cm o.d. Nb-1% Zr tubes which were 23 cm long. The sample preparation techniques [5—7]may be briefly described as follows: Niobium pentachloride, prepared in-situ by chlorination of Nb, is mixed with appropriate amounts of H2, Ar, and GeC14 vapor introduced by bubbling Ar through GeCl4 liquid. The gases (—j
*
Work performed under the auspices of USERDA and EPRI.
are thoroughly mixed and passed rapidly into a vertical coating chamber maintained at 835—850°C.The coating theNb-l% inside Zr of tube a 2.0cm i.d. Cu tube chamber with the consists 0.64cmof o.d. mounted at the center axis starting 8—10 cm from the inlet end. Maintaining the entire coating chamber in a vertical position significantly reduces the circumferential inhomogeneities resulting from gravitational separation in the coating gas mixture. Nb3 Ge coat thicknesses were measured by optical metallography. They uniformcoiltoand within 10%inover the region underwere the pickup ranged average value from 30 pm to 50 pm for the various samples. Lattice spacings for all deposits in this study were measured on a Picker diffractometer using CuK~ radiation, and averaging the results from the (600), (610), and (611) lines. All the samples reported in this study had lattice spacings of 5.141 ±0.001 A. Second phase content was estimated by comparison of low angle diffractometer powder patterns with known mixtures of Nb3 Ge and Nb5 Ge3. These estimates were made on material taken from the coating chamber wall and can only be used to estimate the relative amounts on the sample tubes. Measurements taken on material coated on the sample tubes indicate that these numbers are an overestimate of Nb5Ge3 content. Mechanical treatment consisted of polishing for 0.4h with 6 pm-grit diamond compound followed by 0.5 h with a 1.0 pm-grit. Chemical etch was performed in a solution of 4 parts HNO3, 2 parts H2O, and 1 part HF for 30 s. The electrical loss measurement is accomplished by a technique similar to that described by Campbell [8], with some modifications. A signal proportional to the rate of change of flux through the sample surface 351
Volume 57A, number 4
PHYSICS LETTERS
is obtained from a 1.6cm long pickup coil of fine, high resistance wire which is wound directly on the sample tube. The inductive portion of the signal is bucked out by a compensating coil situated nearby. The magneticbyfield, to solenoid the axis of the sampie, isacprovided a 6.5parallel cm long wound of multifilament Nb-Ti wire embedded in a high resistance matrix. The compensated loss voltage is sent to a lock-in amplifier, where the in phase component of the fundamental is measured. The ac power loss is computed from the product of the induced sample current, derived from a shunt in series with the solenoid, and the output of the lock-in amplifier. End effects were minimized by the use of thick, 1.6 mm, wall Nb-l% Zr tubing as a substrate which extended well out into the fringing region of the solenoid. Results. Representative results of our measurements of ac power loss, as a function of induced current, are shown in fig. 1. The data for two samples, V444 and V469, are shown as measured in the as-dcposited condition and after various surface treatments. In the as-deposited condition, and at high currents, the data for both samples are described appropriately 4
..
i0
/
V444 V As deposited a Light mechanical polish A Chemically etched o f\nnealed
l0~
02 E ~
lOi
V469 Asdeposited • Mechanically polished A Chemically etched .
~ ~ ~ ~o v . £
/vn,
/
l0~
//
/
~
‘~
‘
~ ~
0
~
i
,./..
20
50
100
f0fa~/3J~, is in Hz, p where ~L is the power loss in W/m 0 = 4ir X 1 0~,u~is the peak induced current in A/rn, andis J~ is the from bulk the critical cur2. As evident figure, rent density in A/rn true bulk-loss behavior was only approached for induced current densities a> 1000 rms A/cm even in the as-deposited condition, and surface losses dominated the behavior in the lower current regime. Sampie V444 exhibited relatively high ac losses. This sampie was deposited at 900°Cand was characterized by a very coarse surface structure (2--3 surface features as determined by SEM). Sample V469, deposited at 835°C,showed a somewhat finer surface structure (0.5—0.8 pm) and significantly lower losses. Since it was possible to control the amount of second phase material in the coat at the lower deposition temperature (835°C),we initiated a study of the influence of Nb 5Ge3 content upon ac losses and uponJ~.Table 1 is a summary of the results of our measurements on six samples, of varying Nb5Ge3 concentration, all deposited under similar conditions at 83 5°C.The estimated values for bulk were derived from the Bean-London in the highofcurrent regime and represent expression only a relative figure merit for those samples which exhibited a large surface shielding field, ,
~
~
A
A A A
i..
200
500
J~(102)b)
___________
000
2000
n/cm)
Fig. 1. Power loss, ~L’ at 50 Hz and 4.0 K as a function of induced circumferential current, a. Open symbols, sample V444, are representative data for a high-loss material. Closed symbols, sample V469, are characteristic of low-loss material. The solid line corresponds to the Bean-London behavior of ‘~ a~.
352
2p
2
500 rms A/cm
A/cm
D
~ (rrns
=
Sample a)
/~
0
~L
were deposited at6X 835°C. PL(MW/cm2)c)
SA
A
a
/
/
by the Bean-London critical-state-model expression,
Table I Comparison of hysteretic loss and critical current density for samples of Nb 3Ge with various amounts of second phase Nb5Ge3 and with additional surface treatment. All samples
V
-~
28 June 1976
% Nb on coating 5Ge3 chamber d) wall
___
V475A V484A V479A V483A
2.4 3.0 4.0 5.6
49.0 45.0 27.0 10.8
V469A
6.2
9.0
________
1.0 0.5 3.0 15.0
±0.5 ±0.5 ±1 ±2
V469B 6.2 8.0 5.0 ± 1 V469C 8.0 0.4 V490A 7.0 ± 1 C — chemia) A — as 6.6 deposited; B —10.0 mechanically polished; cally etched. b) J~determined from critical state model fit at high induced current density at 4.0 K. c) Measured at 4.0 K and at 50Hz. d) Measured as described in text, indicates only relative amount of Nb 5Ge3 on samples.
Volume 57A, number 4
PHYSICS LETFERS
ML All samples deposited at 835°Cshowed approximately the same Ml of 350 Oe ±50 Oe (200 rms A/cm) in the as deposited state. The tentative conclusion reached from these results is that Nb5 Ge3 content is a very important parameter in determining ~c and hysteretic losses. The maximum value of ~c inferred from the loss measurements for6the as-deposited condition, was7% theNb 6.6 X figure for sample V490, with 106 A/cm 5Ge3 on the chamber wall. Recent measurements of critical current by conventional four-probe resistance techniques indicate the presence of a broad maximum in ~ ranging from 2% to 8% in Nb5Ge3 concentration [91. Sample V444, for which the data is presthted in fig. 1, was not included in this comparison since it was deposited at 900°Cand its X-ray pattern indicated strong orientation of the Nb5 Ge3 not seen in the material deposited at 835°C. The effect of surface treatment on the losses is also illustrated in fig. 1 and is summarized in table 1. Surface mechanical polishing produced a surface which was smooth on a scale of 1.0 pm but did not significantly reduce the losses. Previous studies [3] on Nb3Sn have shown, upon polishing, the appearance of a surface-barrier shielding field Ml, which significantly lowered the losses in the low current regime. The reason for the failure of polishing to produce shielding effects in our material could be seen in the X-ray diffrattometer trace. After mechanical treatment, for instance, the [211] line of the A-l5 structure was observed to broaden by a factor of about two and to decrease in amplitude, indicating the presence of substantial strain m the surface layer. After chemical etching removed the strained region, the lines recovered their natural widths and former amplitudes. The effect upon the ac losses after polishing and etching was most dramatic for sample V469, fig. 1. For this
28 June 1976
sample, shielding and negligible losses are apparent up to an induced current of 500 A/cm rms which corresponds to a peak magnetic field at the surface of 890 Oe. These low hysteretic losses are comparable with the lowest that have been achieved in Nb3Sn and are of considerable practical interest. Attempts to relieve the surface strain with an anneal of 1 h at 800°Cwere only minimally successful in sample V444, but a longer anneal should be attempted. Conclusions. AC power losses on CVD-prepared Nb3 Ge have been measured at 4 K which are negligible at 500 A/cm rms. Primarily responsible for the achievement of such low hysteretic losses was the discovery of a correlation between ~c and the concentration of the second phase, Nb5Ge3. Substantial surface barrier shielding was introduced by mechanical polishing followed by a chemical etch. Further efforts are being pursued to study the nature of the pinning mechanism in this material.
References [1] R.J. Bartnett, H.L. Laquer and R.D. Taylor, IEEE Trans. on Magnetics MAG-Il (1975) 405. [2] J.R. Gavaler et al., IEEE Trans. on Magnetics MAG-l 1 192. [3] (1975) J.F. Bussiere, M. Garber and M. Suenaga, IEEE Trans. on Ma~eticsMAG-li (1975) 324. [4] C.P. Bean and J.D. Livingston, Phys. Rev. Lett. 12 (1964) 14. [5] L.R. Newkirk et al, IEEE Trans. on Magnetics MAG-il [6] L.R.Newkirk, F.A. Valencia and T.C. Wallace, Proc. Fifth Intern. Conf. on Chemical Vapor Deposition (Electrochemical Society, Inc., Princeton, 1975) 704. [7] L.R. Newkirk and F.A. Valencia, to be published. [8] A.M. Campbell, J. Phys. C2 (1969) 1492. [9] R.J. Bartlett, private communication.
353