Superconductors

By definition, a superconductor exhibits no resistance to electrical conductivity, and will oppose an external magnetic field, a phenomenon referred to as the Meissner effect (Figure 1) . Many pure transition metals (e.g., Ti, Zr, Hf, Mo, W, Ru, Os, Ir, Zn, Cd, Hg) and main group metals (e.g., Al, Ga, In, Sn, Pb) exhibir superconductivity, many only when esposed to high pressure conditions. These materials are referred to as Type I or soft superconductors.

Binary and ternary alloys and oxides of these elements, as well as pure V, Nb, Gd, and Tc are referred to as Type II or high-field superconductors. In contrast to Type I, these materials exhibit conductive characteristics varyng from normal metallic to superconductive, depending on the magnitude of the external magnetic field. It is noteworthy to point out that metals with the highest electrical conductivity (e.g., Cu, Au) do not naturally possess superconductivity. Although this behavior was first discover in 1911 for supercooled liquid mercury, it was not until 1957 that a theory was developed for this phenomenon.

In order to exhibit superconductive behavior, early Type I and II materials needed to be cooled below the critical temperature (Tc) ranging from 0.015 K (for W) to 23 K (for Nb3Ge). An intriguing goal of current research is to increase the Tc to room temperature ("high-temperature superconductors", HTS), which would trivialize resistence-free aplications such as power grid lines and widespread levitated trains. In 1986, Muller and Bednorz at IBN made and important discovery toward this goal -the firsr high-temperature superconductor, La2-xSrxCuO4 (LSCO), with a critical temperature of 35 K. A year later, the first material with a critical point above the boling of nitrogen (77K) was discovered, known as YBa2Cu3O7-d (YBCO), with a critical point of 92 K. In more recent years, the highest-temperature cuprate based superconductors have been synthesized with a general formula: MvNwCaxCuyOz (where M=Y, Bi, Tl, or Hg; N=Ba or Sr; v=1 or 2; w=2 or 4; x=0,1, or 2; y= 1,2, or 3; z= 3, 4, 6, 7, 9, 10, or 15). To date, the highest temperature superconductived materials are thallium (e.g., TlBa2Ca2Cu3O9, Tc= 133K), mercury (e.g., Hg0.8Tl0.2Ba2Ca2Cu3O8.33, Tc= 138K), or lead-doped (e.g., (Hg0.75Pb0.15Tl0.1)Ba2Ca2Cu3O8+, Tc= 142 K).

Reference
Fahlman, Bradley D. Materials Chemistry. Reprinted 2008. Springer. Pages 38-39.

Crystallization Pathway in the Bulk Metallic Glass Zr41.2Ti13.8Cu12.5Ni10Be22.5

A new family of multicomponent metallic alloys exhibits an excellent glass forming ability at moderate cooling rates of about 10K/s and a wide supercooled liquid region. These glasses are eutectic or nearly eutectic, thus far away from the compositions of competing crystalline phases. The nucleation of crystals from the homogeneous amorphous phase requires large thermally activated composition fluctuations for which the time scale is relatively long, even in the supercooled liquid.

In the Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 alloy therefore a different pathway to crystallization is observed. The initially homogeneous alloy separates into two amorphous phases. In the decomposed regions, crystallization probability increases and finally polymorphic crystallization occurs.


Reference
S. Schneider, P. Thiyagarajan, U. Geyer and W. L. Johnson (1996). Crystallization Pathway in the Bulk Metallic Glass Zr41.2Ti13.8Cu12.5Ni10Be22.5. MRS Proceedings, 455 , 295 doi:10.1557/PROC-455-295

New metallic glass beats steel as the toughest, strongest material yet

The development of new materials has always been a need to improve our quality of life. Hence, several researchers working on new materials for the tools and technology that will lead to greater progress.Materials scientists in California have made a special metallic glass with a strength and toughness greater than any known material, using a recipe that could yield a new method for materials fabrication. The glass, a microalloy made of palladium, has a chemical structure that counteracts the inherent brittleness of glass but maintains its strength. It’s not very dense and it is more lightweight than steel, with comparable heft to an aluminum or titanium alloy.

It's true, some tougher materials exist, but they are less strong; there are stronger materials, but they’re not as tough. To grasp this, you have to define the the difference between strength and toughness. Strength refers to how much force a material can take before it deforms. Toughness explains the energy required to fracture or break something; it describes an object’s ability to absorb energy. Most of the time, these qualities are mutually exclusive. Ideal structural materials are both strong and tough; steel is a good example. The new glass has a far better combination of strength and toughness than any steel.

The glass has been obtained and described features, but still very expensive in their manufacture, but the methods to create new materials is a reality that will lead to significant improvements of tools and so further development in the world of science and technology.

Reference
POPSCI. New metal glass beats steel as the thoughest, strongest material yet. Online (http://www.popsci.com/technology/article/2011-01/new-metallic-glass-toughest-strongest-material-yet)

Future and actual applications of carbon nanotubes

Carbon nanotubes, similar to small graphite coiled sheets with nanometric diameters and lengths about microns, constitute a unique material with exceptional mechanical, electrical, optical, thermal and chemical properties which make them suitable to improve numerous already existing products and to even generate other new ones. Many are the applications that can take advantage of the properties of carbon nanotubes. Composite materials reinforced with nanotubes, flat screens that use the nanotubes as field emitters, biological and chemical sensors used to detect polluting substances, drug administration or fuel cells are only some of them. In general, sectors like electronics, materials, sensors, biotechnology, chemistry, energy, mechanics, scientific instrumentation and photonics could get many advantages from the introduction of carbon nanotubes in many of their products.

The yearly ascending tendency in the number of publications that treat of carbon nanotube applications shows the great interest existing about them. EE.UU. is the world-wide leader in number of publications, but Asia is also an important region, partly due to the presence in it of very important electronic companies which can obtain great benefits when incorporating carbon nanotubes to their products. Although Europe appears in third place in number of publications, its contribution to the research and development of carbon nanotube applications is also very important, clearly betting on these new technologies.

The market of carbon nanotube applications is still very incipient. Only composite materials reinforced with nanotubes appear in sport accessories like tennis rackets or bicycles. Electronic applications are very promising since carbon nanotubes will allow them to continue with the progressive miniaturization typical of this area which is at present threatened by the physical limits of operation of silicon, near to be reached. Nevertheless there are only electronic prototypes that incorporate carbon nanotubes, there are no commercial products due to the lack of suitable industrial processes for their elaboration. The other technologies that incorporate carbon nanotubes show different degrees of maturity in their access to the market, but they are not commercialized yet. Carbon nanotubes appear like an interesting alternative for the manufacturers of multiple products who are interested in innovating, since they promise to produce incredible benefits and to revolutionize the market when they burst into it.

Reference:

Rivas Martínez, María Jesús; Román Ganzer, José; y Cosme Huertas, María Luisa. Aplicaciones actuales y futuras de los nanotubos de carbono. Informe de vigilancia tecnológica. Vt miod 11.

Design, synthesis and characterization of self-assembled As2L3 y Sb2L3 cryptands

Fabrication of complex structures from simple components has been a research topic of great interest in recent years. Supramolecular self-assembly processes and dynamic covalent chemistry offer a powerful set of tools for the bottom-up synthesis of complex structures with new properties and emergent functionality. A defining feature of these synthetic strategies is that information contained within relatively simple components determines the formation of much more complex structures.

Chemists have compiled an enormous library of self-assembled supramolecular complexes utilizing the directing properties of the transition metals. In comparison, there are relatively few examples of assemblies that were designed to use the main group elements as directing components, showing that main group supramolecular chemistry is still developing the tools for the predictable formation of well-defined structures.

The main group elements generally prefer unusual “coordination” geometries compared to the transition metals, making them attractive targets when seeking novel properties, new topologies and alternative functionalities in self-assembled molecules and materials. However, there exists perhaps a misconception that bonds to main group elements are not labile enough for self-assembly. Contradictory to this belief, thiolate bonds shown to the Group 15 elements phosphorus, arsenic, antimony and bismuth are sufficiently reversible and can be used to drive the formation of supramolecular assemblies.

Full synthetic details and characterization of newAs2L3 and Sb2L3 cryptands are avaliable. There are many squemes that represents the ligands used in synthesis of cryptands, such as representations of the X-Ray crystal structures of As and Sb cryptands and chemical properties of them. Moreover, it could be find studies of X-ray diffraction, where we can obtain distances between atoms of the criptands, symmetries and angles.

Reference:

Fontenot et al. Design, synthesis and characterization of self-assambled As₂L₃ and Sb₂L₃ cryptands. Dalton Trans., 2011, 40, 12125