Havelab a Institute of Physics, Pedagogical University, Podchorazych 2, 30 084 Krakow, Poland b Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 12116 Prague, Czech
Trang 1Review article
N.-T.H Kim-Ngana,*, L Havelab
a Institute of Physics, Pedagogical University, Podchorazych 2, 30 084 Krakow, Poland
b Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 12116 Prague, Czech Republic
a r t i c l e i n f o
Article history:
Received 15 April 2016
Received in revised form
25 April 2016
Accepted 25 April 2016
Available online 15 May 2016
Keywords:
Superconductivity
Electrical resistivity
Crystal structure
g-U phase
U-based alloys
a b s t r a c t
We succeed to retain the high-temperature (cubic)g-U phase down to low temperatures in U-T alloys with less required T alloying concentration (T¼ Mo, Pt, Pd, Nb, Zr) by means of splat-cooling technique with a cooling rate better than 106K/s All splat-cooled U-T alloys become superconducting with the critical temperature Tcin the range of 0.61 Ke2.11 K U-15 at.% Mo splat consisting of theg-U phase with
an ideal bcc A2 structure is a BCS superconductor having the highest critical temperature (2.11 K)
© 2016 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)
1 Introduction
The large interest in stabilization of U-based alloys with a cubic
g-U structure has camefirst from the viewpoint of metallurgy In
the late 1970s massive research programs were launched in USA to
develop the low enriched uranium (LEU,< 20%235U) fuels[1,2]
The research showed that the U-Mo alloys withg-U phase were
the most promising candidates for LEU fuels, e.g they have a
higher stability under irradiation and are more resistant to
swelling (thana-U alloys)[3e5] Indeed, U-10Mo (U-10 wt%Mo
(uranium alloying with 10% weight percent of molybdenum)) has
been selected for the U.S reactors, while many European reactors
have used the U-7Mo[2] This concentration (7e10 wt% Mo) in
uranium is sufficient to reach theg-U phase stability In Vietnam,
the high enriched uranium (HEU,> 90%235U) rods of the nuclear
reactor in the Central Highlands of Da Lat City have been
exchanged by LEU ones since 2011
From the fundamental research viewpoint, the 5f electronic
states in many uranium-based compounds are generally close to
the verge of localization, which brings up fascinating many-body
physics However, the fundamental physical properties of
elemental uranium have been investigated thoroughly for the orthorhombica-U phase (space group Cmcm)[6,7], since only this phase is stable at and below room temperature The supercon-ductivity of natural uranium wasfirst discovered at Tc¼ 1.3 K in
1942[8] Most recent reports gave Tc¼ 0.78 K[9,10] However, no signature of the superconductivity was found down to 0.02 K at ambient pressure in good-quality single crystals of uranium, although the charge-density-wave (CDW) states[10]were found
to be developed fully at low temperatures in those crystalline uranium specimens[11]
We remind here that pure uranium metal exhibits three allo-tropic phases The a-U with an orthorhombic structure (mentioned above) exists below 940 K down to ambient temper-ature Between 940 K and 1045 K theb-U phase with a tetragonal structure exists (space group P42/mmm), while theg-U phase with
a body-centered-cubic A2-type structure is stable only between
1049 K and 1408 K (space group Im3m)[6,7] The cubicg-U phase can be retained to the room temperature by alloying with Zr, Nb,
Mo, Pd, Pt, etc.[12] Mo has a large solubility in U (z35 at.%) and thus is considered as a good candidate to stabilizeg-U For instance the single-phaseg-U alloy has been reported for U-8 wt% Mo (y U-16.5 at.% Mo (equivalently uranium alloying with 16.5% atomic percent of molybdenum)) under normal furnace cooling condi-tions[13]
* Corresponding author Tel.: þ48 12 6627801; fax: þ48 12 6358858.
E-mail address: tarnawsk@up.krakow.pl (N.-T.H Kim-Ngan).
Peer review under responsibility of Vietnam National University, Hanoi.
Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
http://dx.doi.org/10.1016/j.jsamd.2016.04.010
2468-2179/© 2016 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license
Journal of Science: Advanced Materials and Devices 1 (2016) 121e127
Trang 2The basic thermodynamic properties ofg-U phase alloys have
been much less investigated and remained practically unknown
Except for old reports from 1960s on the superconductivity of the
g-U phase around 2 K in water-quenched U-Mo and U-Nb alloys
[14,15], there are no more detailed data on fundamental
low-temperature properties of theg-U alloys
We have been interested in stabilization of g-U alloys and
characterization of their fundamental electronic properties,
espe-cially their superconductivity It was shown earlier that the rapid
quenching (with a cooling rate of about 105K/s) of certain alloys
from the melting point could lead to a formation of new
meta-stable phases and/or amorphous solid phases [16] Indeed, the
splat-cooling technique has been used for searching novel
micro-structure or amorphous uranium [17] Recently, using ultrafast
cooling from the melt to room temperature, we were able to retain
the cubicg-U phase in U-T alloys (T¼ Mo, Pt, Pd, Nb, Zr) In our
equipment, the molten metal drops between two colliding
massive copper anvils, yielding a cooling rate better than 106K/s
We can then proceed with characterization of low-temperature
properties Starting with Mo alloying, we succeeded to suppress
thea-U phase with about 11 at.% Mo[18e20] We have extended
our investigations on other U-T alloys (T¼ Pt, Pd, Nb, Zr), focusing
in particular on their superconductivity This work is a review of
our results obtained up to date
2 Experimental
U-T alloys (T¼ Mo, Pt, Nb, Zr) with low T concentrations (up to
30 at.%) were prepared using natural U (2N8 purity or better) and T
element (3N8 or better) by arc-melting on a copper plate in argon
atmosphere The sample ingots were turned over 3 times to ensure
the homogeneity Up to 4 samples could be obtained in one
arc-melting cycle without breaking a vacuum, thanks to a special
construction of copper crucible and the chamber The splat-cooled
sample was prepared from the alloy-ingot by splat-cooling
tech-nique (using the HV splat cooler from Vakuum Praha) and had a
shape of irregular disc with a diameter of approx 20 mm and a
thickness of 100e200 mm, as shown in Fig 1 More details of
preparation of the splats have been reported earlier [18e20]
Throughout our work, the T-content is given in the atomic percent
(at.%)
The crystal structure of the splat-cooled alloys (splats) was
investigated by X-ray diffraction (XRD) using the Bruker D8
Advance diffractometer with Cu-Karadiation The resistivity and
specific heat measurements were carried out in the temperature range 0.4e300 K by means of standard techniques using e.g Closed Cycle Refrigerator system (CCR) and Quantum Design Physical Properties Measurement System (PPMS) described earlier
[18] For investigations around the superconducting transitions,
we performed those measurements in applied magneticfields up
to 7 T Additional phase purity analysis was performed by scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) analyzer The splats show in most cases a homogeneous distribution of the alloying elements with concentrations corre-sponding the nominal ones Electron backscattering diffraction (EBSD) analysis has been employed to study the microstructure and texture of several splats
3 Results and discussion 3.1 Crystal structure of U-T splats The crystal structure of U-Mo splats (with Mo concentration of
0 (pure U splat), 1, 2, 4, 6, 10, 11, 12, 13, 15 and 17 at.%) has been thoroughly investigated in order to determine precisely the min-imal Mo concentration necessary for obtaining the pure cubicg-U phase Details of our investigations of crystal structure and phase stability in U-Mo system have been reported earlier[18,19] For a necessity of a comparison with other U-T splats, we summarize briefly the main outcome obtained on U-Mo splats: 1) the (orthorhombic) a-U phase has disappeared and the (cubic)g-U phase or its tetragonally distorted variant (g0-U phase) has developed fully in the alloys with Mo larger than 11 at.% A pure cubicg-U phase without any distortion is revealed only for
U-15 at.% Mo (Fig 2a) and U-17 at.% Mo splat, and 2) the stableg-U alloys were obtained in the as-formed state without any additional sample treatment Thus, the effect of the splat cooling can be seen
in a better capability in retaining the bcc-type of structure for lower (by several at.%) Mo concentrations
No aging or phase transformation/decomposition was observed for all splat-cooled alloys when exposed to air They show even a very good resistance against any hydrogen absorption in the hydrogen atmosphere with the pressure below 2.5 bar[21,22]
As small amount of orthorhombic a-U phase is difficult to recognize by XRD if it coexists with the cubicg-U phase, EBSD analysis has been performed on several U-Mo splats Earlier pub-lished EBSD results for pureeU and U-15 at.% Mo splats[18,20]
corroborated the XRD data For instance, the EBSD maps for
U-15 at.% Mo splat have revealed a pure cubicg-U phase with an equigranular grain structure without twinning and preferred crystallographic texture For as low as 12 at.% Mo, the EBSD maps exhibited a full crystallinity with grain size of several micrometers and no evidence fora- ora-U related phases[23]
Recently, we have extended our studies to the splat-cooled U-based alloys with other T metals (T¼ Pt, Pd, Zr, Nb) Some of the results were included in our recent publications[23,24] We pre-sent here a comparison of selected results
The XRD patterns of U-Pt splats in the as-formed state are shown inFig 2b For an easier comparison, we display normalized intensities Increasing the Pt concentration leads to merging of several reflections around 36, suppression of the low-indexa
-reflections, vanishing of the high-index a-reflections and a development ofg-reflections The situation is very similar to U-Mo alloys, showing a coexistence of botha- andg-U phase for splats with less than 10 at.% alloying level The XRD pattern of U-15 at.%
Pt revealed four characteristic reflections of theg-type structure (g(110), g(200), g(211) and g(220) respectively at 36.8, 53.0, 65.3and 78.2), indicating a stabilization of the cubicg-U phase However, unlike U-15 at.% Mo with very narrow g-reflections
Fig 1 Photograph of splat-cooled disc (right) produced by HV splat cooler from the
bulk sample ingot with a mass of approx 300 mg (top, left) prepared by the
arc-furnace.
N.-T.H Kim-Ngan, L Havela / Journal of Science: Advanced Materials and Devices 1 (2016) 121e127 122
Trang 3indicating the fundamental cubic A2 structure, there is a certain
broadening for all theg-reflections in U-15 at.% Pt, similar to those
observed in the U-13 at.% Mo splat It is interesting to compare our
findings with respective binary phase diagrams The maximum
reported solubility in g-U of Pt or Pd does not exceed 5 at.%
[12,25e27] Our results reveal that using the splat cooling we not
only retain the bcc phase to low temperatures, but also extend its
occurrence for much higher concentrations of alloying Pt/Pd
metals However, SEM analysis indicated that a small amount of
the binary phase UPt occurring at the grain boundaries, which is
accompanied by the U-Pt alloy depleted in Pt, so the splat cannot
be taken as single phase
The normalized XRD patterns of the splat-cooled U-Nb alloys in
the as-formed state are shown inFig 3a In general, the increase of
the Nb concentration leads to the suppression ofa-U reflections
and the development ofg-U reflections It causes the overlap of
low-index reflections around 36 and then the combined re
flec-tion becomes narrower for 10 at.% Nb For the U-15 at.% Nb alloy,
the splitting of theg-reflections into doublets was observed for all
four prominentg-reflections For instance, theg(110) reflection of
U-15 at.% Nb splits into doublet located around 36.3 g0(110)) and
37.0 (g0(101)) The situation is similar to that of alloying with
11e12 at.% Mo which stabilizes theg0-U phase (Theg0-U phase
has a body-centered tetragonal structure with the c/a
ratioz 0.98e0.99 It is considered as a cubic structure with a small
tetragonal distortion) In general, our results show a similarity
between the U-Nb and U-Mo systems Moreover, we expect that
using ultrafast cooling could reduce the necessary Nb
concentra-tion Indeed, it turned out that theg0-U phase is found to be
sta-bilized by 15 at.% Nb alloying, i.e lower than the minimal content
for stabilization of such a phase in water-quenched (16.8 at.% Nb)
[28]or in argon quenched ones (16.2 at.% Nb)[29] Using a
com-bined arc-melting, hot-rolling, annealing and water-quenching,
theg-U phase was stabilized in U-7 wt% Nb (i.e U-15 at.% Nb)
alloy[30]
In the case of Zr system, the situation is similar to that of
U-Nb, i.e the complete miscibility in the high-temperature bcc phase The normalized XRD patterns of the splat-cooled U-Zr alloys in the as-formed state are shown inFig 3b The results illustrate the phase transformation from thea-phase togwith increasing Zr concen-tration Unlike other T alloying, thea(110) anda(111) reflections still persist for U-11 at.% Zr and U-15 at.% Zr They become very broad for U-20 at.% Zr and then vanish for U-30 at.% Zr Existing reports indicate that the single-phaseg-alloys were obtained for Zr concentrations between 25 at.% and 80 at.%[31] In our case the single g-U phase can be considered only for U-30 at.% Zr splat Moreover, most ofg-reflections (including the main peakg(111) at 35.9) are broadened We attribute such the broadening to an additional disorder (microstrain) by randomly distributed Zr atoms especially in alloying with high Zr concentrations In all splats, UC(111) and UO2(111) impurity reflections were observed in the low-angle part of the XRD patterns attributed to surface segrega-tion Additionally for U-Zr system, ZrC presence is revealed by most intense reflections ZrC(111) and ZrC(200) at 33.4 and 38.7,
respectively Traces of carbon are ubiquitous in uranium metal However, it seems that it couples preferentially only with Zr (among all investigated T alloying) and has a high surface segre-gation tendency
The lattice parameters estimated for theg-U phase alloys are given inTable 1 The atomic radii of Nb (1.47Å), Pd (1.37 Å) and Pt (1.39Å) are equal or close to that of Mo (1.40 Å), all which are lower than the nominal atomic radius of U (1.56Å), while the Zr atomic radius (1.60 Å) is larger[32] The lattice parameters of the alloys can
be compared with that ofg-U at 1050 K (3.52 Å) and the value extrapolated to room temperature considering the thermal expansion (3.48 Å) It is evident that the largest lattice parameters for the Zr alloying are related to the Zr atomic diameter A remarkable fact is the large tetragonal distortion for the Nb alloy-ing, which apparently exhibits c> a, i.e opposite than for theg0-U phase at U-Mo alloys
Fig 2 X-ray diffraction (XRD) patterns of the as-formed splat-cooled U-Mo alloys (a) and U-Pt alloys (b) Each curve was normalized to the maximal intensity of the most intense peak at 2q¼ 36 o
e37 and then shifted upwards with respect to that of pure U splat to provide a better visual comparison The color vertical ticks indicate the main XRD lines of orthorhombic (blue) and cubic (red) structures and of the surface impurities (black) The four maing-U reflections are also indicated.
N.-T.H Kim-Ngan, L Havela / Journal of Science: Advanced Materials and Devices 1 (2016) 121e127
Trang 43.2 The electrical resistivity of the cubicg-U phase
For a brief summary of the change of the temperature coefficient
in splat-cooled U-T alloys with increasing T content in the normal
state in the temperature range 3e300 K, we show inFig 4a the
temperature dependence of the (normalized) electrical resistivity
of U-Mo splats (We show the data of all investigated U-Mo splats in
one Figure here, while they were already reported separately
earlier [18,33,34]) We concentrate on the two limit cases which
reveal a striking difference, i.e the pure-U splat (consisting ofa-U
phase) and the U-15 at.% Mo splat (consisting of theg-U phase) The
pure-U splat exhibits a quadratic temperature dependence below
50 K and then an almost linear dependence up to 300 K, i.e with a
positive temperature coefficient (dr/dT> 0) Unlike such a common
metallic behavior, for U-15 at.% Mo, the resistivity weakly decreases
with increasing temperature in the normal state in the whole
temperature range, i.e with a negative temperature coefficient (dr/
dT< 0) The temperature dependence of the resistivity of other
U-Mo splats lies between such the two limits The U-U-Mo alloys
con-sisted of botha- andg-U phase (with<10% Mo alloying) have still
positive dr/dT, all U-Mo alloys with (cubic)g-U phase (with11%
Mo alloying) have the negative dr/dT As such a change of the
temperature dependence appears in conjunction with increase of the absolute resistivity value, we can deduce that a large disorder effect plays an important role in the splat-cooled alloys, similar to a strong disorder observed e.g in some (superconducting) amor-phous systems or disordered alloys and compound[35e37] The reason for the negative slope can be seen in the weak localization, i.e a quantum interference effect (e.g the anomalous dispersion of the conduction electrons) occurring in strongly disordered systems
[38] In our case, there is certainly still some extra contribution to the disorder produced by ultrafast cooling, affecting the grain size
It is interesting to review the resistivity behavior of all splat-cooled U-T alloys (T¼ Mo, Pt, Pd, Nb, Zr) formed in the (cubic)g
-U structure The temperature dependence of the resistivity of these alloys in zero-field and in the temperature range of 3e300 K is shown inFig 4b The resistivity values at 300 K and 4 K are given in
Table 1 Ther(T) curves of U-15 at.% Mo and U-15 at.% Nb splat are quite similar Besides, the residual resistivityr0(r4K) and the re-sistivity at room temperature (r300K) are also similar For the
U-15 at.% Pt splat, although the resistivity values are twice higher, the relative change of the resistivity in U-15 at.% Pt (ther/r300K (T) curve) is very similar to that of U-15 at.% Mo (as well as U-15 at.% Nb) Namely, from room temperature down to temperature just
Fig 3 (Normalized) X-ray diffraction (XRD) patterns of the as-formed splat-cooled and U-Nb alloys (a) and U-Zr alloys (b) The same notation of the color vertical ticks are used as those in Fig 2
Table 1
Summary of low-temperature properties of U-T splat alloys havinggeU structure: resistivity values at 300 K and at 4 K (r300K ,r4K ), superconducting transition temperatures (T c ) determined from ther(T) jump and/or from the specific heat C(T), the width of the superconducting transition in the resistivity (DT r ), the Sommerfeld coefficient of electronic specific heat (ge ) and Debye temperature (QD ) The structure types (the orthorhombica-U, the cubicg-U and the tetragonalg0 -U (or the cubic with a small tetragonal distortion)) and lattice parameters (a,c) are given as well.
T Content
(at.%)
(Å)
r300K
(mUcm)
r4 K
(mUcm)
T c (K) (r(T))
DT r (K)
T c (K) (C(T))
ge
(mJ/K 2 mol)
QD
(K)
3.565 (c)
N.-T.H Kim-Ngan, L Havela / Journal of Science: Advanced Materials and Devices 1 (2016) 121e127 124
Trang 5above the superconducting transition, the electrical resistivity
ex-hibits a negative temperature coefficient (dr/dT< 0) For U-30 at.%
Zr containing theg-U phase, the negative slope does not develop
yet Instead, we found a very small (close to zero) but still positive
slope of the temperature dependence in this splat It should be
mentioned that a negative temperature coefficient (dr/dT< 0) was
indeed reported for U-Zr system, but for sample with 70 at.% Zr
[38] We assume that the negative slope can be also observed for
higher Zr concentrations than 30 at.%
3.3 Superconductivity in U-T splats withg-U phase
All investigated U-Mo splats become superconducting at low
temperatures below 2.2 K The superconducting transitions
revealed by abrupt resistivity drops in zero magnetic field are
shown inFig 5 We focusfirst on the two cases: the pure U splat
and the U-15 at.% Mo splat (Fig 5a) The transition is manifested by
a single drop at Tc¼ 1.24 K and 2.11 K, respectively[19,33] We
remind here a very small width of the transitionDTr ¼ 0.02 K
observed for U-15 at.% Mo, while a wider transitionDTr¼ 0.2 K was
found for pure U splat However, unlike al-type anomaly for
U-15 at.% Mo, the superconducting transition in the pure U splat was
revealed only as a small feature around 0.65 K in the specific heat
[19]which is a clear evidence against the bulk nature of
super-conductivity We assume that only a small fraction of the sample
becomes superconducting As the impurity phase has to form a 3D network to reach a zero-resistance state, it must be related to the grain boundaries For otherg-U alloys, such as 11 at.% Mo and
U-12 at.% Mo, the superconducting transition also appears as a single resistivity drop, although broader than that in U-15 at.% Mo We pay particularly attention to the superconducting transition in the
U-6 at.% Mo splat [23], i.e the intermediate range of Mo alloying consisted of botha- andg-U phases The phase coexistence is re-flected by a flat but still a metallic-type overall temperature dependence (dr/dT> 0) In the low-T range, the resistivity starts to decrease rapidly below 1.6 K This decrease ends in an abrupt drop into the zero resistance state at Tc¼ 0.78 K The obtained results suggest that there are two different superconducting phases in the U-6 at.% Mo splat (we have to assume the coexistingaand g-U phase), each of them exhibiting its own superconductivity The lower Tc may be associated to theg-U phase, as it revealed by a sizeable anomaly in the specific heat[23]
The low-temperaturer(T) dependence of U-15at.% T (T¼ Nd, Pt) splats measured in zerofield is shown inFig 5b We add in the samefigure the data for U-30 at.% Zr splat consisting ofg-U phase
In all cases, a very sharp resistivity drop was observed at Tc The estimated values for TcandDTrare given inTable 1 U-15 at.% Nb becomes superconducting at similar critical temperature (Tc¼ 1.90 K withDTr¼ 0.15 K) as for other splat alloys consisting of
g0-U structure (with 11e12 at.% Mo) U-30 at.% Zr exhibits a superconducting transition revealed by a single drop at Tc¼ 0.81 K (withDTr¼ 0.08 K)[24] The superconductivity in U-15 at.% Pt is characterized by a sharp drop at Tc¼ 0.61 K (withDTr¼ 0.04 K)
Fig 4 Temperature dependence of electrical resistivity in zero-field in the normal
state of all investigated U-Mo splats (a) and of splat-cooled U-T splats having theg-U
phase (b) For an easier comparison the curves were normalized to respective
re-sistivity values at T ¼ 300 K All U-Mo splats with theg-U phase (11 at.% Mo alloying)
have a negative temperature coefficient (dr/dT < 0) Other alloys with 15 at.% T alloying
(T ¼ Nb, Pt) have a negative dr/dT, while U-30 at.% Zr having a positive one but close to
zero.
Fig 5 Temperature dependence of electrical resistivity in zero-field of U-Mo splats (a) and of splat-cooled U-T alloys having theg-U phase (b) around the superconducting transition temperature For an easier comparison the curves were normalized to respective resistivity values at T ¼ 4 K The pure singleg-U phase U-15% Mo splat has a highest critical temperature (T c ¼ 2.11 K) and sharpest resistivity drop (DT r ¼ 0.02 K) N.-T.H Kim-Ngan, L Havela / Journal of Science: Advanced Materials and Devices 1 (2016) 121e127
Trang 6Despite of a similarity in the crystal structure (g-U) and lattice
parameter between U-15 at.% Mo and U-15 at.% Pt (resulted from
alloying with elements with a similar atomic radii), U-15 at.% Pt
becomes superconducting at much lower temperature In addition,
a second small drop was observed at Tc(h) ¼ 0.95 K (with
DTr¼ 0.08 K) As a complicated phase situation was detected for the
U-15 at.% Pt splat at the grain boundaries (a small amount of
ferromagnetic UPt phase plus U-Pt matrix depleted in Pt), we
cannot be conclusive about intrinsic behavior of U-Pt alloys More
detailed investigations of superconducting phase transition in
U-15 at.% Pt are in progress in order to understand the two transitions
below Tcand Tc(h) We note here that, even if for the U-5 at.% Pt
splat consisted of a mixeda-U andg-U phase, the superconducting
phase transition is revealed by only a single drop in the resistivity at
0.7 K[23] One can also see a certain parallel to recently observed
two transitions in the skutterudite-related La3Rh4Sn13and La3
R-u4Sn13[39]
Applying external magneticfields, the superconducting
transi-tions shift towards lower temperatures, as expected The estimated
values for critical magneticfields at zero temperature (m0Hc) and
the critical slopes at Tcof the Hc2vs T curves (m0(dHc2/dT)Tc) for
selected U-Mo splats were reported earlier[18,19] InTable 1we
listed only the values for pure U and U-15 at.% Mo splat, for a
comparison with other T-alloying splats The estimated values for
(m0Hc) and for (m0(dHc2/dT)Tc) are respectively in the range of
2e7 T and 2e4 T/K These values are close to that found for the
strongly interacting Fermi liquid superconductor U6Fe (m0(dHc2/
dT)Tc ¼ 3.42 T/K) [40]and Chevrel-phase superconductors (2 T/
K (m0(dHc2/dT)Tc) 8 T/K)[41] One difference is that for those
splat-cooledg-U alloys, the Tcvalues are lower than 2.2 K, while
Chevrel-phase superconductors have much higher Tc(>10 K)
The temperature dependence of specific heat, Cp(T), has been
studied for selected splats over the whole temperature range,
including both the low-T and high-T parts for characterizing the
superconducting behavior as well as the electronic and phonon
contribution The estimated values for Sommerfeld coefficient of
electronic specific heat (ge) and the Debye temperature (QD) are
given inTable 1 A clear evidence of an increase of density of states
at the Fermi level forg-U is observed only for U-15 at.% Mo, as
shown by an enhancement of the ge value by Mo alloying
(ge¼ 16 mJ/K2mol (¼18.8 mJ/K2mol U for U-15 at.%Mo, in a
com-parison with that for pure Uge¼ 11 mJ/K2mol U)) It is ascribed to
the increasing atomic volume and higher UeU spacing The
enhancement of thegevalue is found to be larger for Pt alloying,
while it was smaller for Nb and Zr alloying (seeTable 1)
The temperature dependence of the specific heat and its field
variations have been performed down to 0.3 K for selected
splat-cooled UeT alloys The jump in the specific heat at Tcwithin the
BCS theory in the weak coupling approximation is:
DC¼ 1:43geTc
We estimated the height of the experimentally observed
specific-heat jump (DC) and then compared to the estimated BCS
values by using thegeand Tcvalues determined from our
experi-ments InFig 6, we shown the C-T curves in zerofield for selected
investigated U-T splats Only a very small feature related to the
superconducting transition was revealed at 0.65 K in the specific
heat for the pure-U splat (Fig 6a) The results suggest that only a
small fraction of the sample is really superconducting For U-15 at.%
Mo splat (consisting of singleg-U phase with ideal bcc A2
struc-ture), a pronounced l-type specific-heat anomaly was observed
The height of the experimentally observed specific-heat jump (DC)
is in a good agreement with that estimated from BCS theory For
other U-Mo splats with lower Mo contents (<15 at.%), a broader
peak with a smaller (but non-negligible) specific-heat jump was observed close to the superconducting transition temperature Tc
defined from the resistivity measurements The experimentally estimated jump for instance for U-6 at.% Mo splat amounts to only about 55% of the BCS value[23] The specific heat of other U-T splats containing theg-U phase measured down to 0.4 K in zero magnetic field is shown inFig 6b Only a weak and broad bump with a small height was observed in the C(T) curve of U-15 at.% Nb[24] The crystal structure, the resistivity jump and the Tcvalue of this splat are similar to that of U-12 at.% Mo splat, but a much larger peak was observed for U-12 at.% Mo in the C(T) curve The specific heat peak related to the superconducting transition in U-30 at.% Zr splat is visible at Tcdetermined from the resistivity jump, proving that the superconductivity in this splat is a real bulk effect
4 Conclusions
We have stabilized theg-U phase in the U-T alloys by a combi-nation of ultrafast cooling and alloying with 15 at.% T content (T¼ Mo, Pt, Nb) and 30 at.% Zr content An ideal bcc A2 structure was found only in the U-15 at.% Mo splat It is crucial that using ultrafast cooling we are able to reduce the necessary concentration
of the T elements (T¼ Mo, Nb, Zr), i.e theg-U phase can be sta-bilized by a lower concentration of alloying elements Moreover, ultrafast cooling could also extend the solubility of Pt metal (up to
at least 15 at.%) and thus we are able to stabilize alsog-U phase in U-15 at.% Pt splat We emphasize again that all splat-cooled alloys
Fig 6 Specific-heat anomalies related the superconducting phase transition for U-Mo splats (a) and of selected U-T splats (b) A pronouncedl-type specific-heat anomaly was observed only for U-15 at.% Mo splat consisting of singleg-U phase with ideal bcc A2 structure The bars show the estimated jumps from BCS theory.
N.-T.H Kim-Ngan, L Havela / Journal of Science: Advanced Materials and Devices 1 (2016) 121e127 126
Trang 7were obtained without any additional treatment and that they are
very stable when exposing to ambient conditions
All the U-T splats become superconducting with the lowest and
highest Tcof 0.61 K and 2.11 K respectively for 15 at.% Pt and
U-15 at.% Mo The prediction of BCS superconductivity for the specific
heat jump at Tcwas found to be entirely fulfilled in the U-15 at.% Mo
among all investigated splats
Our investigations have provided new data to the data-base for
low-temperature properties of the U-T system with low-T content
(< 30 at.%, T ¼ Mo, Pt, Pd, Zr, Nb)
Acknowledgments
We express our thanks to all colleagues, in particular our four
Ph.D students from Prague and Krakow (Ilya Tkach, Mykhaylo
Paukov, Magdalena Krupska, Sylwia Sowa), who have performed
the experiments in the scope of our‘splat-cooling’ project
This review paper is a tribute to Peter Brommer
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