WO1999066541A2

WO1999066541A2 - CRITICAL DOPING IN HIGH-Tc SUPERCONDUCTORS FOR MAXIMAL FLUX PINNING AND CRITICAL CURRENTS

Info

Publication number: WO1999066541A2
Application number: PCT/NZ1999/000095
Authority: WO
Inventors: Jeffery Lewis Tallon
Original assignee: Industrial Research Ltd
Current assignee: Callaghan Innovation Research Ltd
Priority date: 1998-06-18
Filing date: 1999-06-18
Publication date: 1999-12-23
Anticipated expiration: 2000-12-18
Also published as: JP2002518287A; WO1999066541A3; EP1090398A2; EP1090398A4; AU4806499A

Abstract

A method for maximising critical current density (Jc) of high temperature superconducting cuprate materials (HTSC) which comprises controlling the doping state or hole concentration of the materials to be higher than the doping state or hole concentration of the material that provides a maximum superconducting transition temperature (Tc), and to lie at about a value where the normal-state pseudogap reduces to a minimum. Jc is maximised at hole concentration p≈0.19. HTSC compounds are also claimed.

Description

CRITICAL DOPING IN HIGH-Tc SUPERCONDUCTORS FOR MAXIMAL FLUX PINNING AND CRITICAL CURRENTS

FIELD OF THE INVENTION

The invention comprises a method for preparing a high temperature superconducting cuprate material (HTSC) to maximise the critical current density of the material, in which the doping state or hole concentration of the material is controlled so as to lie at about the point where the normal-state pseudogap reduces to a minimum.

BACKGROUND

Many High-T_c Superconducting Cuprates (HTSC) are known to have superconducting transition temperatures, T_c exceeding the temperature at which liquid nitrogen boils, 77 K. As such they have a potentially large number of applications ranging from power generation, distribution, transformation and control, to high-field magnets, motors, body scanners, telecommunication and electronics. T_c values may be of the order of 93 K for example for YBa2Cu3θ7__§,

95K for example for B-2Sr2CaCu2θg, 109 K for example for Bi2Sr2Ca2Cu3θιø_> 120K for example for TlBa2Ca2Cu₃Oi ₀ or as high as 134 K for HgBa₂Ca2Cu3θ₁₀.

For many of these applications such T_c values alone do not guarantee the utility of these HTSC at 77K or higher temperatures. Often these applications require large critical current densities, J_c, in the presence of a magnetic field. Even if the grains of the HTSC are crystallographically aligned, otherwise known as textured, and well sintered together, as is commonly achieved in thin-films, such that weak links between the grains are removed, a high critical current in the presence of a magnetic field is not guaranteed. Such high currents are only achieved if there is strong flux pinning within the individual grains. A simple measure of the flux pinning contribution to J_c is the product, Uoξ, of the condensation energy, U₀, and the superconducting coherence length, ξ. An additional measure of the intrinsic ability of HTSC materials to support high critical currents in the presence of a magnetic field is the temperature- dependent irreversibility field, H*(T). For magnetic fields exceeding H* the magnetisation is reversible and hence dissipative while it is irreversible for fields less than H* and hence substantially non-dissipative. If for a given temperature H* is large then at that temperature the critical current density for fields H<H* may be high. Many models for the irreversibility field have H* <χ λι ² where λ_L is the in-plane London penetration depth. Thus if λ is minimised (λι ² maximised) then H* may be maximised.

HTSC have a fundamentally important feature that their properties vary with the concentration of doped electronic carriers. The carriers are electron holes, referred to in short as holes. The concentration of holes may be altered by chemical substitution or by changing the oxygen concentration. In general, the hole concentration may be increased by substituting a lower valency atom for a higher valency atom or by increasing the oxygen content. The hole concentration, p, may be decreased by substituting a higher valency atom for a lower valency atom or by decreasing the oxygen content. Thus in La2C 0₄ which is an undoped insulator, the hole concentration is increased from zero by substituting Sr²⁺ for the La³⁺. In YBa Cu₃θ7 the hole concentration may be decreased by substituting La³⁺ for Ba²⁺ or by decreasing the oxygen content as in the formula YB2Cu3θ7-_δ where δ may be increased from 0 to 1. When δ = 1 this compound in an undoped insulator like La₂Cu0 . By increasing the hole concentration from the undoped insulating state the HTSC eventually becomes superconducting at low temperature. This (lower) threshold hole concentration is about p=0.05, for example. If the hole concentration is increased too high beyond an upper threshold then the HTSC material becomes a non-superconducting metal even at the lowest temperature. This upper threshold is about p=0.27. Between the lower and upper thresholds T_c rises up smoothly from zero at the lower threshold to a maximum at about p=0. 16 then falls smoothly back to zero at the upper threshold. The maximum T_c value in this variation with hole concentration is T_c>m χ and it occurs at a hole concentration frequently referred to as optimal doping. The variation in T_c with hole concentration follows a nearly parabolic variation approximated by T_c(p)=T_c,max[l-82.6(p-0.16²], so that as noted T_c maximises at p∞O.16. At optimal doping the room temperature thermoelectric power takes the value Q(290K)=+2μV/K (Tallon et al, US patent 5,619, 141 which is incorporated herein by reference). When the hole concentration is less than optimal doping the HTSC material is referred to as underdoped and when it is greater than optimal doping it is referred to as overdoped. Optimal doping then is seen as the key doping state to which other doping states are referred. Prior to the present invention the usual criterion for optimising superconductivity in HTSC was to maximise T_c.

SUMMARY OF INVENTION

In broad terms in one aspect the invention comprises a method for preparing a high temperature superconducting cuprate material (HTSC) to maximise the critical current density (J_c) thereof, comprising the step of controlling the doping state or hole concentration of the material to be higher than the doping state or hole concentration of the material that provides a maximum superconducting transition temperature (T_c) to increase the critical current density of the material.

In broad terms in another aspect the invention comprises a high temperature superconducting cuprate material (HTSC) having a doping state or hole concentration higher than the doping state or hole concentration of the material for maximum superconducting transition temperature (T_c) and at about a value where the normal-state pseudogap for the material reduces to a minimum and which maximises the critical current density (J_c) of the material.

We have surprisingly found that the optimal doping for maximising T_c is not the optimal doping for maximising flux pinning, U₀, λv² or the critical current density J_c. We have found that J_c, flux pinning, U₀, λv² maximise at a higher doping state or hole concentration than that which maximises T_c and at about the point where the normal state pseudo-gap reduces to a minimum, particularly where the hole concentration 0. 18<p<0.20 and most particularly at about p=0. 19 or p=0. 19±0.005. We believe this is due to the presence, across the underdoped and slightly overdoped regions, of a correlated state that reduces low-energy spectral weight in the quasiparticle excitation spectrum. This reduction in spectral weight is referred to as the pseudogap and is sometimes mistakenly described as a spin gap. The pseudogap is manifested as a reduction in the normal-state entropy and susceptibility which strongly suppresses superconductivity and all measures thereof including T_c, condensation energy and superfluid density. The pseudogap energy, E_g, may be determined by fitting the normal state temperature dependence of the Knight shift, K_S(T), to an equation K_S(T) = Ko [1-tanh² (E_g/2kT) + Kc where Kc is the chemical shift. E_g may alternatively be determined by fitting the normal-state temperature dependence of the entropy, S(T) to an equation S(T)/T = g₀ [tanh²(E_g/2kT)], where g_o is a constant.

Typically E_g is found to decrease with increasing hole concentration, p. More sophisticated means of determining E_g are known (see Tallon et al J. Phys. Chem. Solids 59, 2145 (1998) which is incorporated herein by reference) but the values of E_g thus determined still reduce approximately linearly with increasing hole concentration and, most importantly, still reduce to zero at about the same value p=0.19±0.005.

The method may include overdoping the HTSC so that the grain boundary regions between individual grains in the HTSC in particular are doped to maximise the critical current density across grain boundaries. Such grain boundary regions can tend to be underdoped even if the bulk intragranular material is optimally doped or even overdoped (- see Babcock et al (Physica C 227, 183 (1994)). The pseudogap will often be locally present in the grain boundary regions, the effective superconducting order parameter thus locally reduced, and the grains weakly linked. Additionally, impurities tend to accumulate at the grain boundaries. HTSC have a d-wave order parameter which is very sensitive to the presence of impurities T_c being rapidly suppressed at a rate dT_c/dy that depends strongly on doping state. In the underdoped grain boundary regions T_c, the order parameter and the condensation energy are all much more rapidly suppressed due to impurity scattering than in the bulk intragranular material. Impurities in underdoped grain boundaries are therefore especially deleterious. Thus while the intragranular J_c may be high, the intergranular J_c may be low due to the underdoped state of the grain boundary regions.

The method of the invention may be used in producing HTSC as bulk materials, wires, tapes or other conductor elements, or thick or thin films for example. BRIEF DESCRIPTION OF THE FIGURES

The invention is further described with reference to the accompanying figures wherein:

Figure 1 is a plot for Yo sCao 2Ba2Cu3θ _-δ of the hole concentration dependence of T_c, the pseudogap energy E_g and the product U₀ξ b where ξ_ab is the ab-plane coherence length.

Figure 2 is a plot of the hole concentration dependence of T_c, E_g and U₀ for Bi₂Sr₂CaCu₂0_8+s.

Figure 3 is a plot of the hole concentration dependence of λi/² for Yo sCao ₂Ba₂Cu₃0₇.s.

Figure 4 is a plot of the p-dependence of the magnetisation (proportional to the critical current) of YBa2Cu3θ _-δ grain aligned in epoxy. Data is shown for different field strengths and temperatures as shown. The parabolic curve is T_c plotted as a function of p.

Figure 5 is a plot of the p-dependence of the irreversibility field at T=0.75T_C for Yo sCao 2Ba₂Cu3θ7-δ grain-aligned in epoxy. The parabolic curve is T_c plotted as a function of hole concentration, p.

Figure 6 is a plot of the p-dependence of the magnetisation critical current density in 0.2 Tesla at 10K and 20K as well as the irreversibility temperature at 5 Tesla for Yo sCao 2Ba2Cu3θ _-δ grain-aligned in epoxy. The parabolic curve is T_c plotted as a function of hole concentration, p.

DETAILED DESCRIPTION

In the method of the invention the doping state or hole concentration is controlled or adjusted in producing the HTSC to maximise flux pinning and critical current density, to lie at or near a critical overdoped hole concentration (herein: critical doping) at which the normal-state pseudogap is minimised or disappears, and as a consequence the superconducting condensation energy sharply maximises and the London penetration depth, X_L, sharply minimises. The critical hole concentration may be determined by use of the thermoelectric power, the temperature dependence of the resistivity, Raman measurements of the frequencies of certain phonon modes or by suitable annealing at elevated temperature in an oxygen- containing atmosphere previously determined to result in the critical doping state, or from the ratio of Tc/Tc,max where T_c,max is the maximum T_c observed in the superconducting phase curve T_c(p), estimated to occur at about p=0.16. The doping state or hole concentration of the HTSC may be controlled or adjusted by cation substitution, or by increasing the oxygen content of the material beyond that which gives maximum Tc, or a combination of both.

Cation substitution to achieve critical doping may be by altervalent substitution sucii as substitution of Ca²⁺, Li⁺, Na⁺ or K⁺ for R³⁺ in 123, 247 and 124 materials; Li⁺, Na⁺ or K⁺ for Ba²⁺ in 123, 247 and 124 materials; R³⁺ for Ca ⁺ in Bi-2212, Tl-2212, Tl- 1212 or Hg- 1212; Pb²⁺ for Bi³⁺ in Bi-2212 and Bi-2223; and Pb^*»⁺ for Tl³⁺ in Tl- 1212 and Tl-2212. The compound Tlo sPbo sSr CaCu20₇ for example has a fixed stoichiometric oxygen content so cannot be adjusted to critical doping by changing the oxygen content. As prepared, this compound is overdoped with T_C=79K and an estimated hole concentration p=0.215, and it is known that by substitution of 0.2- 0.25 Y on the Ca site or 0.2-0.25 La on the Sr site then the doping state may be reduced to optimal doping p=0. 16 where T_r is maximised to 105- 107 K. In accordance with the invention preferably 0. 12±0.04 Y is substituted for Ca or 0.12±0.04La for Sr in this compound to achieve critical doping. Other rare earth elements may be utilised in the same way in this and other HTSC compounds, noting that small rare earth elements should preferably be substituted for Ca and the larger rare earth elements preferably for Sr. Combined substitution on both the Sr and Ca sites may of course be utilised. Quite generally Li⁺ substitution in any HTSC material is suitable for increasing the hole concentration of that material.

Oxygenation to achieve critical doping may be achieved by conventional annealing in an oxygen-containing atmosphere or by titration using electrochemical means. Achieving the oxygen content required for critical doping in any HTSC may require the use of oxygen pressures in excess of 1 atmosphere or lower annealing temperatures than otherwise conventionally used to form the HTSC. Depending upon density of the HTSC such annealing may take many days to equilibrate and it may be more convenient to use higher oxygen pressures at higher temperatures where the kinetics of oxygen uptake is faster, or a combination of cation substitution with oxygenation such complete oxygenation is not required. The altervalent substitution of cations tends to alter the oxygen content. Thus the substitution of Ca²⁺ for R³⁺ results in larger oxygen deficiency (larger δ) relative to the unsubsituted compound for the same annealing conditions. This larger oxygen deficiency can be eliminated by lower temperature oxygen annealing. Thus while full oxygenation of YBa Cu3θ₇-_δ might be achieved by annealing at 380°C in oxygen, full oxygenation of Yι_-xCa_xBa2Cu3θ7- (x = 0.2 for example) requires lower temperatures 300-350°C for example. Such low temperatures require longer annealing time. The same considerations apply to R-247.

The HTSC material may be a Bi-Sr-Ca-Cu-O based material such as Bi-2212, which is of nominal composition Bi2Sr2CaCu2θ3+§ where O<δ<0.35, or Bi-2223, which is of nominal composition B-2Sr2Ca2Cu3θιø+δ where 0≤δ<0.35 (where Bi may be partially substituted by Pb, Hg, Re, Os, Ru, Tl, V, Cr, Zr, Nb, Mo, Hf, Ta, W, Co or Sm, Sr may be partially substituted by Ba or a larger lanthanide rare earth element, and Ca may be partially substituted by Y or a lanthanide rare earth element for example, and in these chemical formulae it is also recognised in the art that small variations in stoichiometry are common in such oxide materials). In Bi-2212 and Bi-2223 Bi is commonly partially replaced by Pb, such that Bi may be Biι-_zPb_z where 0≤z<0.3. Especially preferred compounds include Bi2+_eSr2CaC 2θg_+§ Bi2+_x+eSr2-_x-yCaι_+yC ₂0_8+δ) Bi2+e r2Ca2Cu3θ ₁₀+δ ^and

]3-2+x+e£>^r2-_χC_' ^a2C^"u3C' lO+δ where 0<(e and x and ±y)<0.4, and

Biι.9Pbo.2Srι,9CaιCu₂0₈+δ. The doping state of Bi-2212 and Bi-2223 may be varied by cation substitution (which increases the hole concentration p) R for Ca in Bi-2212 where R is Y or any lanthanide rare earth element, or Bi in Bi-2212 and Bi-2223 for example or by varying δ. For

for example δ may be fixed by annealing the HTSC at about (570±15)°C in an oxygen partial pressure of 0. 1 bar or at a combination of temperature and oxygen partial pressure giving the same value of δ. Only a small δ variability is possible in Bi-2223, which allows only minor changes in hole concentration. Bi-2212 may be substantially overdoped by full oxygenation to p«0.22 or 0.225 whereas B-2223 may only be doped to p«0. 17 or at most 0.18. Bi-2223 material may alternatively be prepared to incorporate intergrowths of Bi-2212 in Bi-2223. Intergrowths of Bi-2212 in Bi-2223 increase the doping state of the latter. Such hole-doping intergrowths can be introduced by substituting very small amounts (< 1% and preferably <0. 1%) of Y or a lanthanide rare-earth element on the Ca site. In preferred forms Bii sPbo 3Srι 9Ca2Cu3θιo+ may incorporate intergrowths of Bii sPbo 3Srι

Bii 7sPbo.35Srι gCa2Cu3θιo+δ may incorporate intergrowths of Bii 7sPbo 3sSrι gCaCu Os+δ" Bii ₃Pbo 3₄Sn gCai 99CU3 o₄Oιo+δ may incorporate intergrowths of Bii 73Pbo 3 Srι 9CaCu2θs+_δ;

Bii 88Pb₀23Sri 96Caι 95Cu 98θιo+δ may incorporate intergrowths of composition Bii ββPbo 23Srι 96CaCu2θs+δ; and Bii 8₄Pbo 28Srι 93Caι gsC^ gβOio+δ may incorporate intergrowths of composition Bii 8₄Pbo ₂sSrι 93CaCu θs+ .

The HTSC may be an R-Ba-Ca-CuO based HTSC such as the material R- 123, which is of nominal composition RBa2Cu3θ₇-δ where R is Y or a lanthanide rare earth element or a combination thereof and O<δ<0.5, (and R may be partially substituted by Ca, and Ba may be partially substituted by Sr, La or Nd for example, and it is also recognised in the art that small variations in stoichiometry are common in such oxide materials). Critical doping is preferably achieved by oxygenation to δ<0.05, optionally combined with cation substitution, of Ca, Li, Na or K for R, or Li, Na, or K for Ba for example. Critical doping is achieved very close to the point δ=0 for R=Y, and δ close to but >0 for a small rare earth such as Tl or Lu. Preferably where R is a larger rare earth element small amounts of Ca are substituted for R so to achieve critical doping. For larger rare earth elements it may not be possible to reach the critical doping state by merely reducing δ towards zero. This is partly because some R resides on the Ba site where R is a large rare earth element but even if suitable processing to avert such substitution is utilised the charge transfer from CuO chains to Cuθ₂ planes is insufficient to achieve critical doping. Preferred compounds are Ri. _xCaχBa₂Cu3θ₇-δ where 0<x<0.35 and 0<δ<0.5 and Rι-χLi_xBa₂Cu3θ₇-δ where 0≤x≤0.35 and 0<δ<0.5 and wherein x and δ are jointly adjusted so that the copper oxygen planes are critically doped as described above. Preferably δ is as small as possible commensurate with critical doping. Thus in this preferred form a value of x is chosen, typically x<0.1 such that critical doping is achieved for δ≤O.10. It is more preferable that x<0.05 and δ<0.03.

The HTSC may be R-247, which is of nominal composition R2_-xCaχBa4Cu7θi5._s where R is Y or a lanthanide rare earth element or a combination thereof and O<δ<0.5 (and R may be partially substituted by Ca, and Ba may be partially substituted by Sr, La or Nd for example, and it is also recognised in the art that small variations in stoichiometry are common in such oxide materials). Critical doping is preferably achieved by oxygenation to δ<0.05, optionally combined with cation substitution, of Ca, Li, Na or K for R, or Li, Na, or K for Ba for example. Preferred compounds are R2-_xCaχBa₄Cu₇Oi5-δ where O≤x≤O.35 and 0≤δ<0.5 and R2-χLi_xBa4Cu₇Oi5-δ where 0<x<0.35 and 0<δ<0.5 and wherein x and δ are jointly adjusted so that the copper oxygen planes are critically doped as described above. Preferably δ is as small as possible commensurate with critical doping. Thus in this preferred form a value of x is chosen, typically x<0.1 such that critical doping is achieved for δ<0.10. It is more preferable that x<0.05 and δ<0.03. Because 247 is typically underdoped relative to the equivalent 123 materials the magnitude of Ca content given by x will be required to be greater than in the equivalent 123 materials. A small rare earth such as Tl or Lu is desirable in that a higher Ca content may be achieved than for the larger rare earth elements.

The HTSC may be R- 124, which is of nominal composition RBa Cu₄Os where R is Y or a lanthanide rare earth element and 0<δ<0.35 (and R may be partially substituted by Ca and Ba may be partially substituted by Sr, La or Nd for example, and it is also recognised in the art that small variations in stoichiometry are common in such oxide materials). Critical doping is achieved by cation substitution, of Ca, Li, Na or K for R, or Li, Na, or K for Ba for example. Preferred compounds are Rι-_xCaχBa Cu₄θ8 and Rι.xLixBa₂Cu₄08 where 0≤x<0.35. Because 124 is typically even more underdoped relative to the equivalent 247 and 123 materials a higher level of cation substitution is required than in the equivalent 247 and 123 materials. A small rare earth such as Tl or Lu is desirable to achieve the higher Ca content required.

The HTSC material may be a Tl-Sr-Ca-Cu-O based material such as Tl-1201, which is of nominal composition TlSr2CuOs+6, or Tl- 1212, which is of nominal composition TlSr2CaCu2θ7+δ, or Tl-1223 which is of nominal composition TlSr Ca₂Cu₃θ8+δ (and where Tl may be partially substituted by Pb, Sr may be partially substituted by La or Ba, and Ca may be partially substituted by R, for example, and in these chemical formulae it is also recognised in the art that small variations in stoichiometry are common in such oxide materials). Preferred compounds are

Tlo.5Pbo.5Sr₂-wBawCaCu₂θ7, Tlo.₅Pbo.5Sr2-_xLa_xCaC 2θ7, and Tlo.sPbo.sSraCai-yRyCuaOr where 0<(x and w)<2, O≤y≤l and R is Y or any of the lanthanide rare earth elements, or more generally (Tl,Pb,Bi)ι(Sr,Ba,La)2(Ca,R)ιCu₂θ7, and Tlo.5Pbo.5Sr2-χCaχCa2Cu₃Og and Tlo.5Pbo.5Sr_2- -χBa_wCa_xCa₂Cu3θ9 where O≤x, w≤l or more generally (Tl,Pb,Bi)ι(Sr,Ba,La)₂Ca₂Cu3θ9, where -0.3<δ<0.3.

The HTSC may also be a mercury-based HTSC such as Hg- 1212, which is of nominal composition HgBa2CaCu2θ6+δ, or Hg- 1223 which is of nominal composition HgBa2Ca2Cu3θs+ (and where Hg may be partially substituted by Tl, Bi, Pb or Cd, and Ba may be partially substituted by Sr for example, and in these chemical formulae it is also recognised in the art that small variations in stoichiometry are common in such oxide materials).

The critically-adjusted doping state may be quantitatively defined by the HTSC having an overdoped T_c satisfying 0.91Tc,max<Tc≤0.96T_c,max more preferably 0.92Tc,_max<Tc<0.95Tc,max and most preferably T_c = (0.93±0.005)T_c>maχ.

The critically-adjusted doping state may be quantitatively defined by the HTSC having an overdoped T_c with the room temperature thermoelectric power Q(T_RT) in units of μV/K satisfying -4 < Q(TRT) < - 1 and more preferably -3 < Q(T_RT) < -2 where 280 K < T_RT <300 K.

The critically-adjusted doping state may be quantitatively defined by the HTSC having an overdoped T with the normal-state constant-volume resistivity remaining linear in temperature from 250K down to less than 20K above T_c. In a more preferred form the normal-state constant-volume resistivity remains linear in temperature from 500K down to less than 20K above T_c. The critically-adjusted doping state may be quantitatively defined by the HTSC having an overdoped T_c with the temperature derivative of the normal-state constant- volume resistivity remaining constant within ±5% when the temperature is reduced from 250K down to less than 2 OK above T_c and, more preferably, when the temperature is reduced from 500K down to less than 2 OK above T_c

The critically-adjusted doping state may be defined by annealing the HTSC in an oxygen-containing atmosphere previously determined to result in a critically overdoped T_c with either T_c, thermoelectric power or the normal-state resistivity lying within the above noted preferred margins or with the pseudogap having been critically suppressed to zero as determined by heat capacity, NMR spectroscopy, susceptibility or other means. For example, Bi1.9Pbo.2Sr1 CaCu₂θ8+δ may be annealed at 570°C in a mixture of 10% oxygen and 90% nitrogen gases (an oxygen partial pressure of 0. 1 bar) to achieve critical over-doping with T_C=83.7K in a system with T_c,_ma_x=90.5 K. This gives a T_c reduction of 7.5%, a hole concentration of p=0.19 and a maximal condensation energy of 1.76 J/g.at as determined by heat capacity measurements. Yo,8Cao,2Ba Cu3θ7-δ may be annealed at 530°C in oxygen to achieve critical-overdoping with T_C=79K where T_c,max=85.5K, that is a 7.6% reduction in T_c, an estimated hole concentration of ρ=0.19 and a maximal condensation energy of 2.5 J/g.at. Other alternative combinations of temperature and oxygen partial pressure to achieve critical doping can be used (such as higher temperature and a consequent higher oxygen partial pressure).

The doping state in any HTSC may be defined by Raman measurement of the frequency of a particular phonon mode, the desired frequency having been previously ascertained from a correlation with one of hole concentration, thermoelectric power, resistivity, oxygen annealing or T_c value. One such mode is the 630 cm ² Aι_g mode.

In R- 123 as well as achieving the required degree of oxygenation it is necessary to achieve ordering of the oxygens into chains with a minimum of vacancies on the chains so as to ensure sufficient charge transfer to the Cu0₂ planes. Because oxygen disordering occurs for T > 50°C it is ideally necessary to slow cool from the annealing temperature to room temperature so as to allow oxygen ordering on the chains and thus complete charge transfer and maximal doping. Moreover, such defect-free CuO chains contribute additionally to the superfluid density and thus further increase J_c.

In order to achieve optimal doping in grain boundaries the bulk material may be overdoped so that the grain boundary regions between individual grains in the HTSC are doped to maximise critical current density in the grain boundary regions of the HTSC in particular, even at some sacrifice maximal intragranular critical current density in order to maximise intergranular currents. In contrast to the bulk, cation solubility and oxygen activity will be different in grain boundaries. If for example Ca²⁺ substitution is preferred in grain boundaries relative to the bulk then there is some prospect of critically doping both the grain boundaries and the bulk simultaneously. If Ca²⁺ substitution is preferred in the bulk relative to the grain boundaries then a compromise, as discussed above, may be necessary. Because of the site disorder in grain boundaries the energy wells for oxygen occupancy will not be as deep as in the bulk and as a consequence, lower temperatures (and/or higher oxygen pressures) may be required to oxygenate the grain boundaries than is required to oxygenate the bulk. Annealing temperatures as low as 100°C to 200°C may be required to adequately oxygenate the grain boundaries to achieve critical doping therein. While at such low temperatures oxygen diffusion will be prohibitively slow within the bulk, grain boundary diffusion rates will be much higher due to the disorder therein thus making critical oxygenation of grain boundaries below 300°C or even below 300°C entirely practicable.

The method of the invention may be used in forming HTS materials into long-length flexible wires or tapes by the technique known in the art as powder-in- tube processing. This technique is especially used in the case of Bi-2212 or Bi-2223 materials. Powders of these materials or precursors to these materials are packed into a metallic tube, often made of silver metal or silver alloy, then by a process of deformation and heat treatment the tube is drawn out into a long wire and the oxide reacted to form a highly textured Bi-2223, for example, HTSC core. The wire may be rebundled once or many times to form a multifilamentary wire. Metallic alloy precursors may also be used to form such long wires and by heat treatment and deformation the metallic alloys are converted to oxides and reacted to form a highly- textured Bi-2223, for example, HTSC core. Other techniques such as coating or melt processing may also be used to form long-length wires or tapes. The materials may also be formed as thin films using any process as is known in the art or as bulk melt-processed single-domain or near-single-domain monolithic bodies.

EXAMPLES

The invention is further illustrated by the following examples:

Example 1:

Samples of Yo sCao ₂Ba Cu3θ7-δ were investigated as a function of oxygen deficiency, δ, using heat capacity, C_p, and NMR spectroscopy. This HTS material is especially useful because its doping state may be altered from heavily underdoped to heavily overdoped as δ is altered from 1 to 0. The hole concentration was determined using the thermoelectric power correlation between Q(290) and p (Tallon et al., US Patent 5,619, 141). The values were found to agree well with values determined from the parabolic relationship between T_c and p Tc(p)=T_c,max [l - 82.6(p-0. 16)²] and also from estimates of p using bond valence sums from neutron diffraction refinement of atomic coordinates. T_c,max is the maximum in the approximately parabolic hole- concentration dependence of T_c(p). The pseudogap energy, E_g, was determined from fitting the temperature dependence of the entropy and values were confirmed by fitting the temperature dependent ⁸ Y NMR Knight shifts. Both T_c and E_g are plotted in Figure 1 as functions of p. E_g is seen to fall progressively with increasing doping and reaches zero at about p=0.19 i.e. on the overdoped side and about 0.03 holes per copper higher than the optimal concentration for which T_c is maximised. Alternatively E_g reaches zero when T_c has fallen 7.4% below T_c>max on the overdoped side. The condensation energy U₀ was determined by integrating the entropy S(T) from T=0 to T=T_C between the normal state curve SNS(T) and the superconducting curve Ssc(T). U₀ is found to maximise very sharply at the point p=0. 19 where E_g reaches zero. The product Uoξab does not peak quite as sharply but the data plotted in Figure 1 shows that it stills passes through a rather sharp peak, again maximising at the point where E_g-»0. As a consequence flux pinning and critical currents will also sharply maximise at this point. Most notably the value of U₀ξ_ab at optimum doping for maximal T_c is about 39 J.nm/mole while it is nearly double that value (76 J.nm/mole) at critical doping where E_g- 0. Such a large increase for such a small decrease (6K) in T_c is most unexpected and hitherto unknown and offers great benefit for maximising critical currents with little loss in T_c value. For other HTSC materials the Tc reduction will be proportionally the same so that for Bi-2223 a reduction of T_c from T_C)ma_x= l 10 K to 103 or 102 K on the overdoped side is not a serious reduction in T_c when cooled to liquid nitrogen temperature yet potentially affords a doubling of flux pinning.

Example 2:

Samples of overdoped Bi2Sr2CaCu θs+δ with different δ and underdoped Bi₂Sr2Cao 7Y03Cu2θs+_δ with different δ were substituted with a range of Co on the Cu site. From the rate of impurity depression of T_c, dT_c/dy, due to the fraction y of Co one may determine the value of the value of γ=C_p/T at T=T_C. Using scaling relations (Tallon, Phys. Rev. B - 58, R5956 (1998)) one may thence determine E_g and U_D which are plotted in Fig. 2 by the filled symbols. The open triangles are the values of E_g determined for Yo sCao 2Ba2Cu3θ₇-δ from NMR measurements and these show that the two methods give comparable results for E_g. Again one may see that for Bi2Sr₂CaCu208+6, as in Y₀ sCao 2Ba2Cu3θ₇-δ, U₀ passes through an unexpectedly sharp maximum in the lightly overdoped region where E_g becomes zero. Again U₀ approximately doubles on progressing from optimal doping to critical doping. This data for Bi-2212 is confirmed by direct heat capacity measurements. It is believed that the maximising of U₀ and U₀ξ at critical doping where E_g becomes zero is a universal behaviour amongst the HTSC materials as has been confirmed in La2-_xSr_xCu0₄ and in Tlo 5Pbo 5Sr2Caι-_xY_xCu2θ₇. It is thus reasonable that this behaviour is also common to the thallium-based and mercury-based HTSC.

Example 3:

Samples of Yo sCao ₂Ba2Cu3θ7- were investigated as a function of oxygen deficiency, δ, using muon spin relaxation. The muon spin relaxation rate at T=0, σ₀, is proportional to λι ². Figure 3 shows a plot of v² as a function of hole concentration, p. λi ². may be seen to pass through a sharp maximum at the critical point p-sO.19 where E_g becomes zero. λ ² is a key parameter in determining the magnitude of the irreversibility field. As a consequence critical doping at p-sO.19 not only sharply maximises flux pinning but will result in high irreversibility fields.

Example 4:

Seven pellets of Y₀8Cao.2Ba₂Cu3θ7-δ were synthesized and annealed under various temperatures and oxygen partial pressures then quenched into liquid nitrogen to freeze in the equilibrium oxygen content. Thus 7 samples with different values of oxygen deficiency δ were obtained which accordingly have 7 different hole concentrations spanning the range 0.09<p<0.23. These pellets were each ground to a very fine powder with a mortar and pestle using isopropyl alcohol as a lubricant and milling for a period of 1 hour. The powder was then suspended in isopropyl alcohol and settled for a period of 30 minutes and the fluid with the remaining suspension was drawn off and dried by evaporation. In this way the large particles were settled out and the remaining particles were of size < 5μm. These small particles were well mixed with epoxy. The mixture was then placed in rubber moulds and mounted in an electromagnet to align the particles with c-axes parallel to the field while the epoxy set. The aligning field was approximately 1 tesla. After setting the alignment was confirmed by x-ray diffraction which showed primarily c-axis reflections on the face normal to the aligning field. Small samples were cut from each of the 7 aligned samples and their magnetisation was determined as a function of field strength and temperature using a vibrating- sample magnetometer. Figure 4 shows a plot of the magnetisation in units of emu as a function of p for measurements at 0.2T, 0.5T and 2T as well as at temperatures of 20K, 40K and 60K. The data can be seen to pass through a sharp maximum at the critical doping concentration of p=0.19. For reference the parabolic curve of T_c as a function of p is also plotted to show that the sharp maximum occurs in the lightly overdoped state. This parabolic curve is calculated using the formula T_c = T_c,max [1 - 82.6(p-0. 16)²] which well describes T_c(p) for many of the HTS cuprates, and probably all. The criticality of doping is very evident in this plot, especially at low temperature. The ureversibility line H*(T) was also determined for each sample at T=0.75T_C and is plotted in Fig. 5. Here H* is also seen to pass through a sharp maximum but the maximum occurs beyond critical doping because H* is governed by the product λ_ab ².λ_c ² where λ_ab is the in-plane penetration depth and λ_c is the c-axis penetration depth. λ_ab ² is proportional to the superfluid density which QC U₀ and hence passes through a sharp maximum at critical doping. On the other hand λ_c oc p_c, the c-axis resistivity which monotonically decreases with doping, p. Thus the maximum in irreversibility field occurs beyond critical doping just as shown in Fig. 5. However it is to be noted that because in Fig. 5 T=0.75T_C this data is not for fixed temperature. For conditions of fixed temperature (and fixed field) critical current maximises at critical doping. It is further to be noted that in the samples of this example all grains are separated from each other. The currents are all intragranular currents and do not reflect on grain boundary currents.

Example 5:

The samples of Example 4 were examined in a scanning electron microscope to determine the distribution of particle sizes. The total mass in each sample was then determined by pyrolising off the epoxy. The magnetisation data of Example 4 was then converted to critical current density, J_c, using the well-known method of the Bean formula. The thus obtained J_c values for an applied field of 0.2 Tesla and at temperatures of 10K and 20K are plotted as a function of hole concentration, p, in Figure 6. The irreversibility temperature, T_ιrr, at 5 Tesla is also plotted in the figure together with the p-dependence of transition temperature,T_c. Both J_c and T_ιrr are seen to maximise close to the critical doping state ρ=0. 19 and well beyond the point p=0.16 where T_c maximises.

Example 6:

Samples of Tl₀.5Pbo.5Sr2Caι-_xY_xCu2θ₇ and

0.05, 0.1, 0.2, 0.3 and 0.4, and y=0, 0.05, 0.1, 0.2, 0.25, 0.3 and 0.4 were synthesized by solid-state reaction of pellets in oxygen at 1060°C of a stoichiometric mixture of the oxides of Tl, Pb, Y, La and Cu and the carbonates of Ca and Sr. The precursor materials were first reacted without the TI2O3 which was then added for a further reaction at 1060°C. The reacted material was ground and resintered under the same conditions. A small excess (5%) of TI2O3 was used because of the volatility of this material and the pellets were sealed in a gold tube under oxygen by crimping the ends. Samples were then investigated by measuring the resistivity, ac susceptibility and heat capacity. When x or y=0 the materials are overdoped and increasing x or y results in decreasing hole concentration. T_c values were found to follow an approximately parabolic dependence upon the amount of substituted Y or rare-earth element passing through a maximum of T_c,max= 107 K when x = 0.25. The jump in heat capacity however passed through a sharp maximum when x=0. 15. This corresponds to the critical-doping point. The jump in heat capacity at the superconducting transition is directly related to the condensation energy and hence to the flux pinning. Similar results are obtained for Tlo.5Pbo.5Sr₂-_yLa_yCaCu θ₇. Thus, again, properties relating to critical currents are found in these materials to maximise in the lightly overdoped region at critical doping and not at optimal doping where T_c passes through a maximum.

Claims

CLAIMS:

1. A method for preparing a high temperature superconducting cuprate material (HTSC) to maximise the critical current density (J_c) thereof, comprising the step of controlling the doping state or hole concentration of the material to be higher than the doping state or hole concentration of the material that provides a maximum superconducting transition temperature (T_c) to increase the critical current density of the material.

2. A method according to claim 1 comprising the step of controlling the doping state or hole concentration to lie at about a value where the normal-state pseudogap reduces to a minimum.

3. A method according to either of claims 1 or 2 comprising the step of controlling the hole concentration p to be in the range 0. 18<p<0.20.

4. A method according to claim 3 comprising the step of controlling the hole concentration p to be about 0.19.

5. A method according to claim 3 comprising the step of controlling the hole concentration p to p=0.19±0.005.

6. A method according to any one of claims 1 to 5 comprising the step of controlling the hole concentration so that T_c for the HTSC is in the range 0.91T_c,_ma_x < Tc < 0.96T_c,max where T_c,_maχ is the maximum value taken by T_c in its variation with hole concentration.

7. A method according to claim 6 comprising the step of controlling the hole concentration so that T_c is in the range 0.92T_c,_maχ ≤ T_c < 0.95T_c,_maχ.

8. A method according to claim 6 comprising the step of controlling the hole concentration so that T_c = (0.93±0.005)T_c,_maχ.

9. A method according to any one of claims 1 to 8 comprising the step of controlling the hole concentration p so that the room-temperature thermoelectric power Q(TRT) for the HTSC is in the range -4<Q(TRT)<- 1 where the units of Q(T_RT) are μV/K, where room temperature, T_RT , is in the range 280K<TRT<300K.

10. A method according to claim 9 comprising the step of controlling the hole concentration p so that the thermoelectric power Q(TRT) for the HTSC is in the range - 3<Q(T_RT)<-2.

11. A method according to any one of the preceding claims comprising the step of controlling the hole concentration so that the temperature derivative of the normal-state resistivity of the HTSC remains constant within ±5% when the temperature is reduced from 250K down to less than 2 OK above T_c.

12. A method according to any one of the preceding claims comprising the step of controlling the hole concentration so that the temperature derivative of the normal-state constant-volume resistivity remains constant within ±5% when the temperature is reduced from 500K down to less than 2 OK above T_c.

13. A method according to any one of claims 1 to 12 comprising the step of overdoping the HTSC so that the grain boundary regions between individual grains in the HTSC are doped to maximise critical current density.

14. A method according to any one of claims 1 to 13 wherein said HTSC is a Bi-Sr-Ca-Cu-O based HTSC.

15. A method according to claim 14 comprising the step of controlling the hole concentration by partial substitution of Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu or any combination thereof for Ca, or Pb for Bi.

16. A method according to claim 14 or claim 15 comprising the step of controlling the hole concentration by adjustment of the oxygen content of the HTSC.

17. A method according to claim 14 wherein said HTSC is of composition (Biι-_zPb_z)2+x+eSr -χ-yCaι_+yCu2θ8+_δ wherein 0< (e and x and ±y)<0.4, 0<z≤0.3, and 0≤δ<0.35.

18. A method according to claim 17 wherein said HTSC is of composition Biι.9Pbo.2Srι.gCaιCu θ8+δ and comprising the step of fixing δ by annealing the HTSC at a temperature of about (570±15)°C in an oxygen partial pressure of about 0.1 bar or a combination of temperature and oxygen pressure or partial pressure giving an equivalent value of δ.

19. A method according to claim 14 wherein said HTSC is of composition (Biι-_zPb_z)2_+x+eSr₂-χCa2Cu₃Oιo₊₅ wherein 0<(e and x)<0.4, 0<z<0.3 and 0<δ<0.35.

20. A method according to claim 19 and comprising the step of preparing the material to incorporate a fraction of intergrowths of (Biι-_zPb_z)2+x+eSr2- -yCaι+yCu2θ8+_δ where 0<(e and x)<0.4, 0<z<0.3, and 0<δ<0.35.

21. A method according to claim 20 comprising the step of causing said intergrowths to occur by partially substituting R for Ca in the HTSC where R is Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu or any combination thereof.

22. A method according to any one of claims 1 to 13 wherein said HTSC is an R-Ba-Cu-O based HTSC wherein R is Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu or any combination thereof.

23. A method according to claim 22 comprising the step of controlling the hole concentration by partial substitution of Ca, Li, Na, or K or a combination thereof for R, or of Li, Na, K for Ba, or a combination thereof.

24. A method according to claim 22 or 23 comprising the step of controlling the hole concentration by adjustment of the oxygen content of the HTSC.

25. A method according to claim 22 wherein said HTSC is of composition

Rι_-xCa_xBa₂Cu3θ7-_δ wherein O≤x≤O.35 and 0<δ<0.5.

26. A method according to claim 23 wherein said HTSC is of composition Yo.8Cao.2Ba2Cu3θ7-δ and comprising the step of fixing δ by annealing the HTSC at a temperature of about (530±15)°C in oxygen or a combination of temperature and oxygen pressure or partial pressure giving an equivalent value of δ.

27. A method according to claim 22 wherein said HTSC is of composition Rι_-xLiχBa2Cu3θ -δ wherein 0<x<0.35 and 0<δ<0.5.

28. A method according to claim 22 wherein said HTSC is of composition

R₂-χCaχBa₄Cu7θi5-_δ and 0≤x<0.35 and 0<δ<0.5.

29. A method according to claim 22 wherein said HTSC is of composition Rι_-xCaχBa2Cu₄θ8 wherein O. lO≤x≤O.35.

30. A method according to any one of claims 1 to 13 wherein said HTSC is a Tl-Sr-Ca-CuO based HTSC.

31. A method according to claim 30 comprising the step of controlling the hole concentration by partial substitution of Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,

Yb or Lu or any combination thereof for Ca or Sr, or Pb for Tl.

32. A method according to claim 30 or claim 31 comprising the step of controlling the hole concentration by adjustment of the oxygen content of the HTSC.

33. A method according to any one of claims 1 to 13 wherein said HTSC is an Hg based HTSC.

34. A method according to claim 33 including controlling the hole concentration by partial substitution of Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu or any combination thereof for Ca, Sr, or Ba, or Pb for Hg.

35. A method according to claim 32 or claim 33 including controlling the hole concentration by adjustment of the oxygen content of the HTSC.

36. A high temperature superconducting cuprate material prepared by the method of any one of claims 1 to 35.

37. A high temperature superconducting cuprate material (HTSC) having a doping state or hole concentration higher than the doping state or hole concentration of the material for maximum superconducting transition temperature (T_c) and at about a value where the normal-state pseudogap for the material reduces to a minimum and which maximises the critical current density (J_c) of the material.

38. An HTSC according to claim 37 in which the hole concentration p of the material is in the range 0.18<p<0.20.

39. An HTSC according to claim 38 wherein the hole concentration p is about 0.19.

40. An HTSC according to claim 38 wherein the hole concentration p=0.19±0.005.

41. An HTSC according to any one of claims 37 to 40 having a Tc in the range

0.91T_c,max≤Tc≤0.96Tc,_maχ where T_c,max is the maximum value taken by T_c in its variation with hole concentration.

42. An HTSC according to claim 41 wherein T_c is in the range 0.92T_c,max--Tc--0.95Tc,maχ.

43. An HTSC according to claim 41 wherein T_c = (0.93±0.005)T_c,maχ.

44. An HTSC according to any one of claims 37 to 43 wherein the room- temperature thermoelectric power Q(TRT) for the HTSC is in the range -4<Q(T_RT)<- 1 where the units of Q(T_RT) are μV/K, where room temperature, T_RT , lies in the range 280K<T_RT<300K.

45. An HTSC according to claim 44 wherein the thermoelectric power Q(T_RT) for the HTSC is in the range -3<Q(T_RT)<-2.

46. An HTSC according to any of claims 37 to 45 wherein the temperature derivative of the normal-state resistivity of the HTSC remains constant within ±5% when the temperature is reduced from 250K down to less than 2 OK above T_c.

47. An HTSC according to any one of claims 37 to 45 wherein the temperature derivative of the normal-state constant-volume resistivity of the HTSC remains constant within ±5% when the temperature is reduced from 500K down to less than 2 OK above T_c.

48. An HTSC according to any one of claims 37 to 47 which is overdoped so that the doping state or hole concentration of the material in grain boundary regions between individual grains in the HTSC is at about said value which maximises critical current density of the material.

49. An HTSC according to any one of claims 37 to 48 wherein said HTSC is a Bi-Sr-Ca-Cu-O based HTSC.

50. An HTSC according to claim 49 and of composition (Biι-_zPb_z)_2+x+eSr2_-x-yCaι+_yCu2θ8+δ wherein 0<(e and x and ±y)<0.4, 0<z<0.3, and 0≤δ<0.35.

51. An HTSC according to claim 50 of composition Bii gPbo ₂Sri 9Caι Cu₂Oβ+δ-

52. An HTSC according to claim 49 of composition (Biι-_zPb_z)₂+_x+_eSr₂__xCa2Cu3θιo+δ wherein 0<(e and x)<0.4 and 0<z<0.3.

53. An HTSC according to claim 52 and incorporating a fraction of intergrowths of (Biι_-zPb_z)₂+_x+eSr₂-_x-yCaι+yCu₂θ8+δ where 0<(e and x and ±y)<0.4 and O≤x<0.3.

54. An HTSC according to claim 53 of composition Bii sPbo 3Srι 9Ca₂Cu3θιo+_δ and said intergrowths are of composition Bii sPbo 3Srι 9CaCu Os+δ.

55. An HTSC according to claim 53 of composition Bii sPbo 3sSri gCa₂Cu3θιo_+δ and said intergrowths are of composition Bii sPbo 3sSrι 9CaCu₂Os+δ.

56. An HTSC according to claim 53 of composition

Bii 73Pbo 3₄Srι gCai 99CU3 cwOio+δ and said intergrowths are of composition

57. An HTSC according to claim 53 of composition Bii sβPbo 23Srι g₆Caι 95CU2 gβOio+δ and said intergrowths are of composition Bii ββPbo ₂3Srι g₆CaCu₂θ8+δ-

58. An HTSC according to claim 53 of composition Bii 8₄Pbo ₂8Srι 3Cai 98Cu₂96θιo+δ and said intergrowths are of composition Bii 8₄Pbo 2βSrι 93CaCu2θs+δ-

59. An HTSC according to any one of claims 37 to 48 wherein said HTSC is an R-Ba-Cu-O based HTSC wherein R is Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu or any combination thereof.

60. An HTSC according to claim 59 of composition Rι-χCa_xBa₂Cu30₇-δ wherein 0≤x≤0.35 and 0<δ<0.5.

61. An HTSC according to claim 60 of composition Yo sCao ₂Ba₂Cu3θ_{7 δ} wherein 0. 16<δ<0.24.

62. An HTSC according to claim 60 wherein δ = x ± 0.04.

63. An HTSC according to claim 59 of composition Rι_-xLi_xBa₂Cu3θ7-δ wherein 0<x<0.3 and 0<δ<0.5.

64. An HTSC according to claim 63 wherein δ = (x/2) ± 0.04.

65. An HTSC according to claim 59 of composition R2-χCaχBa₄Cu₇Oi5-_δ wherein 0<x<0.35 and 0<δ<0.5.

66. An HTSC according to claim 59 of composition Rι-χCa_xBa2Cu Os wherein 0.1<x<0.35.

67. An HTSC according to any one of claims 37 to 48 wherein said HTSC is a Tl-Sr-Ca-Cu-O based HTSC.

68. An HTSC according to claim 67 of composition Tlo.5Pbo.5Sr₂Caι__xR_xCu₂θ7 and

0.06<x<0.18.

69. An HTSC according to claim 68 wherein x = 0.12±0.04.

70. An HTSC according to claim 67 of composition Tlo.5Pbo.5Sr₂._xR_xCaCu₂0₇ wherein R is Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu or any combination thereof, and 0.06<x<0.18.

71. An HTSC according to claim 70 wherein x = 0. 12±0.04.

72. Wires, tapes, thin films, coated conductors, bulk materials or any other conductor comprising or containing the materials of claims 37 to 71.