Yttrium barium copper oxide (YBCO) is a family of
crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
line
chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
s that display
high-temperature superconductivity; it includes the first material ever discovered to become
superconducting above the boiling point of
liquid nitrogen
Liquid nitrogen (LN2) is nitrogen in a liquid state at cryogenics, low temperature. Liquid nitrogen has a boiling point of about . It is produced industrially by fractional distillation of liquid air. It is a colorless, mobile liquid whose vis ...
[] at about .
Many YBCO compounds have the general formula (also known as Y123), although materials with other Y:Ba:Cu ratios exist, such as (Y124) or (Y247). At present, there is no singularly recognised theory for high-temperature superconductivity.
It is part of the more general group of
rare-earth barium copper oxides (ReBCO) in which, instead of yttrium, other rare earths are present.
History
In April 1986,
Georg Bednorz and
Karl Müller, working at
IBM in Zurich, discovered that certain semiconducting oxides became superconducting at relatively high temperature, in particular, a
lanthanum barium copper oxide becomes superconducting at 35 K. This oxide was an
oxygen-deficient perovskite-related material that proved promising and stimulated the search for related compounds with higher superconducting transition temperatures. In 1987, Bednorz and Müller were jointly awarded the Nobel Prize in Physics for this work.
Following Bednorz and Müller's discovery, a team led by
Paul Ching Wu Chu at the
University of Alabama in Huntsville and
University of Houston
The University of Houston (; ) is a Public university, public research university in Houston, Texas, United States. It was established in 1927 as Houston Junior College, a coeducational institution and one of multiple junior colleges formed in ...
discovered that YBCO has a superconducting transition critical temperature (''T''
c) of 93 K.
The first samples were
Y1.2 Ba0.8 Cu O4, but this was an average composition for two phases, a black and a green one. Workers at
Bell Laboratories identified the black phase as the superconductor, determined its composition YBa
2Cu
3O
7−δ and synthesized it in single phase
YBCO was the first material found to become superconducting above 77 K, the boiling point of
liquid nitrogen
Liquid nitrogen (LN2) is nitrogen in a liquid state at cryogenics, low temperature. Liquid nitrogen has a boiling point of about . It is produced industrially by fractional distillation of liquid air. It is a colorless, mobile liquid whose vis ...
, whereas the majority of other superconductors require more expensive cryogens. Nonetheless, YBCO and its many related materials have yet to displace superconductors requiring
liquid helium for cooling.
Synthesis
Relatively pure YBCO was first synthesized by heating a mixture of the metal carbonates at temperatures between 1000 and 1300 K.
:4 BaCO
3 + Y
2(CO
3)
3 + 6 CuCO
3 + (−''x'') O
2 → 2 YBa
2Cu
3O
7−''x'' + 13 CO
2
Modern syntheses of YBCO use the corresponding oxides and nitrates.
The superconducting properties of YBa
2Cu
3O
7−''x'' are sensitive to the value of ''x'', its oxygen content. Only those materials with are superconducting below ''T''
c, and when , the material superconducts at the highest temperature of ,
or in highest magnetic fields: for B perpendicular and for B parallel to the CuO
2 planes.
In addition to being sensitive to the stoichiometry of oxygen, the properties of YBCO are influenced by the crystallization methods used. Care must be taken to
sinter YBCO. YBCO is a crystalline material, and the best superconductive properties are obtained when crystal
grain boundaries
In materials science, a grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are two-dimensional crystallographic defect, defects in the crystal structure, and tend to decrease the ...
are aligned by careful control of
annealing and
quenching
In materials science, quenching is the rapid cooling of a workpiece in water, gas, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, suc ...
temperature rates.
Numerous other methods to synthesize YBCO have developed since its discovery by Wu and his co-workers, such as
chemical vapor deposition
Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high-quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.
In typical CVD, the wafer (electro ...
(CVD),
sol-gel, and
aerosol
An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be generated from natural or Human impact on the environment, human causes. The term ''aerosol'' co ...
methods. These alternative methods, however, still require careful sintering to produce a quality product.
However, new possibilities have been opened since the discovery that trifluoroacetic acid (
TFA), a source of fluorine, prevents the formation of the undesired
barium carbonate
Barium carbonate is the inorganic compound with the formula BaCO3. Like most alkaline earth metal carbonates, it is a white salt that is poorly soluble in water. It occurs as the mineral known as witherite. In a commercial sense, it is one of ...
(BaCO
3). Routes such as CSD (chemical solution deposition) have opened a wide range of possibilities, particularly in the preparation of long YBCO tapes. This route lowers the temperature necessary to get the correct phase to around . This, and the lack of dependence on vacuum, makes this method a very promising way to get scalable YBCO tapes.
Structure
YBCO crystallizes in a
defect perovskite structure. It can be viewed as a layered structure: the boundary of each layer is defined by planes of square planar CuO
4 units sharing 4 vertices. The planes can sometimes be slightly puckered.
Perpendicular to these CuO
4 planes are CuO
2 ribbons sharing 2 vertices. The
yttrium
Yttrium is a chemical element; it has Symbol (chemistry), symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and has often been classified as a "rare-earth element". Yttrium is almost a ...
atoms are found between the CuO
4 planes, while the
barium
Barium is a chemical element; it has symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element.
Th ...
atoms are found between the CuO
2 ribbons and the CuO
4 planes. This structural feature is illustrated in the figure to the right.
Although YBa
2Cu
3O
7 is a well-defined chemical compound with a specific structure and stoichiometry, materials with fewer than seven oxygen atoms per formula unit are
non-stoichiometric compound
Non-stoichiometric compounds are chemical compounds, almost always solid inorganic compounds, having elemental composition whose proportions cannot be represented by a ratio of small natural numbers (i.e. an empirical formula); most often, in s ...
s. The structure of these materials depends on the oxygen content. This non-stoichiometry is denoted by the x in the chemical formula YBa
2Cu
3O
7−''x''. When ''x'' = 1, the O(1) sites in the Cu(1) layer (as labelled in
the unit cell) are vacant and the structure is
tetragonal
In crystallography, the tetragonal crystal system is one of the 7 crystal systems. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the Cube (geometry), cube becomes a rectangular Pri ...
. The tetragonal form of YBCO is insulating and does not superconduct. Increasing the oxygen content slightly causes more of the O(1) sites to become occupied. For ''x'' < 0.65, Cu-O chains along the ''b'' axis of the crystal are formed. Elongation of the ''b'' axis changes the structure to
orthorhombic
In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic Lattice (group), lattices result from stretching a cubic crystal system, cubic lattice along two of its orthogonal pairs by two different factors, res ...
, with lattice parameters of ''a'' = 3.82, ''b'' = 3.89, and ''c'' = 11.68 Å.
Optimum superconducting properties occur when ''x'' ~ 0.07, i.e., almost all of the O(1) sites are occupied, with few vacancies.
In experiments where other elements are substituted on the Cu and Ba sites, evidence has shown that conduction occurs in the Cu(2)O planes while the Cu(1)O(1) chains act as charge reservoirs, which provide carriers to the CuO planes. However, this model fails to address superconductivity in the homologue Pr123 (
praseodymium instead of yttrium). This (conduction in the copper planes) confines conductivity to the ''a''-''b'' planes and a large anisotropy in transport properties is observed. Along the ''c'' axis, normal conductivity is 10 times smaller than in the ''a''-''b'' plane. For other
cuprates in the same general class, the anisotropy is even greater and inter-plane transport is highly restricted.
Furthermore, the superconducting length scales show similar anisotropy, in both penetration depth (λ
ab ≈ 150 nm, λ
c ≈ 800 nm) and coherence length, (ξ
ab ≈ 2 nm, ξ
c ≈ 0.4 nm). Although the coherence length in the ''a''-''b'' plane is 5 times greater than that along the ''c'' axis it is quite small compared to classic superconductors such as niobium (where ξ ≈ 40 nm). This modest coherence length means that the superconducting state is more susceptible to local disruptions from interfaces or defects on the order of a single unit cell, such as the boundary between twinned crystal domains. This sensitivity to small defects complicates fabricating devices with YBCO, and the material is also sensitive to degradation from humidity.
Proposed applications

Many possible applications of this and related high temperature superconducting materials have been discussed. For example, superconducting materials are finding use as
magnets
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, ...
in
magnetic resonance imaging
Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to generate pictures of the anatomy and the physiological processes inside the body. MRI scanners use strong magnetic fields, magnetic field gradients, and ...
,
magnetic levitation
Magnetic levitation (maglev) or magnetic suspension is a method by which an object is levitation (physics), suspended with no support other than magnetic fields. Lorentz force, Magnetic force is used to counteract the effects of the gravitation ...
, and
Josephson junctions. (The most used material for power cables and magnets is
BSCCO.)
YBCO has yet to be used in many applications involving superconductors for two primary reasons:
*First, although single crystals of YBCO have a very high critical current density,
polycrystal
A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. Crystallites are also referred to as grains.
Bacillite is a type of crystallite. It is rodlike with parallel longulites.
S ...
s have a very low critical
current density: only a small current can be passed while maintaining superconductivity. This problem is due to crystal
grain boundaries
In materials science, a grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are two-dimensional crystallographic defect, defects in the crystal structure, and tend to decrease the ...
in the material. When the grain boundary angle is greater than about 5°, the supercurrent cannot cross the boundary. The grain boundary problem can be controlled to some extent by preparing thin films via
CVD or by texturing the material to align the grain boundaries.
*A second problem limiting the use of this material in technological applications is associated with processing of the material. Oxide materials such as this are brittle, and forming them into
superconducting wire
Superconducting wires are electrical wires made of superconductive material. When cooled below their transition temperatures, they have zero electrical resistance. Most commonly, conventional superconductors such as niobium–titanium are u ...
s by any conventional process does not produce a useful superconductor. (Unlike
BSCCO, the
powder-in-tube process does not give good results with YBCO.)
The most promising method developed to utilize this material involves deposition of YBCO on flexible metal tapes coated with buffering metal oxides. This is known as . Texture (crystal plane alignment) can be introduced into the metal tape (the RABiTS process) or a textured ceramic buffer layer can be deposited, with the aid of an ion beam, on an untextured alloy substrate (the
IBAD process). Subsequent oxide layers prevent diffusion of the metal from the tape into the superconductor while transferring the template for texturing the superconducting layer. Novel variants on CVD, PVD, and solution deposition techniques are used to produce long lengths of the final YBCO layer at high rates. Companies pursuing these processes include
American Superconductor, Superpower (a division of
Furukawa Electric),
Sumitomo,
Fujikura,
Nexans Superconductors,
Commonwealth Fusion Systems, and European Advanced Superconductors. A much larger number of research institutes have also produced YBCO tape by these methods.
The superconducting tape is used for
SPARC, a
tokamak
A tokamak (; ) is a device which uses a powerful magnetic field generated by external magnets to confine plasma (physics), plasma in the shape of an axially symmetrical torus. The tokamak is one of several types of magnetic confinement fusi ...
fusion reactor design that can achieve
breakeven energy production.
Surface modification
Surface modification of materials has often led to new and improved properties. Corrosion inhibition, polymer adhesion and nucleation, preparation of organic superconductor/insulator/high-''T''
c superconductor trilayer structures, and the fabrication of metal/insulator/superconductor tunnel junctions have been developed using surface-modified YBCO.
These molecular layered materials are synthesized using
cyclic voltammetry. Thus far, YBCO layered with alkylamines, arylamines, and
thiol
In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl grou ...
s have been produced with varying stability of the molecular layer. It has been proposed that amines act as
Lewis bases and bind to
Lewis acidic Cu surface sites in YBa
2Cu
3O
7 to form stable
coordination bonds.
Mass production

In 1987, shortly after it was discovered, physicist and science author
Paul Grant published in the U.K. Journal ''
New Scientist
''New Scientist'' is a popular science magazine covering all aspects of science and technology. Based in London, it publishes weekly English-language editions in the United Kingdom, the United States and Australia. An editorially separate organ ...
'' a straightforward guide for synthesizing YBCO superconductors using widely-available equipment. Thanks in part to this article and similar publications at the time, YBCO has become a popular high-temperature superconductor for use by hobbyists and in education, as the magnetic levitation effect can be easily demonstrated using liquid nitrogen as coolant.
In 2021, SuperOx, a Russian and Japanese company, developed a new manufacturing process for making YBCO wire for fusion reactors. This new wire was shown to conduct between 700 and 2000 Amps per square millimeter. The company was able to produce 186 miles of wire in 9 months, between 2019 and 2021, dramatically improving the production capacity. The company used a plasma-laser deposition process, on a electropolished substrate to make 12-mm width tape and then slit it into 3-mm tape.
References
External links
Diagram of YBCO structure*
Synthesis of YBCO using somewhat common lab equipment-
NileRedNew World Record For Superconducting Magnet 26.8T April 2007Superconductivity in everyday life : Interactive exhibition – little if any specific to YBCO.
{{DEFAULTSORT:Yttrium Barium Copper Oxide
Yttrium compounds
Barium compounds
Copper compounds
High-temperature superconductors
Mixed valence compounds
Transition metal oxides
Non-stoichiometric compounds
Articles containing video clips
Ceramic materials