Shaping the future of electronics: nanowires!

Nanowires (NWs) can be defined as cylindrical-shaped nanostructures with diameter in the order of nanometres and that very recently, have been attracting the attention of scientific community due to its unique set of properties which were found to be promising to compose the next generation of nano-electronic devices.

One particular challenge is the synthesis and design of new materials that can be used to manufacture new transistors at the nano-scale, thus revisiting and giving extra-life to the hyper-saturated Moore’s law for electronics and computers processor.  In this context, one-dimensional nanowires have been recently considered the most promising candidates.

However, semiconductor materials are generally produced upon the incorporation of dopants by using ion implantation: a technique that can introduce several different types of defects within the material.

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The IIB phenomenon: (a) a Si NW before irradiation and (b) the same NW during irradiation. Credits Dr. I. Hanif (Sweden).

Our research group led by my french friend O. Camara, has been focusing its attention to the phenomenon of Ion-Induced Bending (IIB) of nanowires and in a recent publication at the Advanced Materials and Interfaces journal, we have demonstrated that  the IIB phenomenon can be mitigated (AND EVEN REVERSED!) by means of Solid-Phase Epitaxial Growth. In order to support the discussion, we have developed a MATLAB-based implementation of the Stopping and Range of Ions on Matter (SRIM) that now is known as IDRAGON: Ion Damage and RAnge in Geometry Of Nanowires. For the whole nanowires community interested in to get a copy of our code, fell free to contact me (m.a.tunes[at]physics.org).

IDRAGON

Atomic displacement profile of nanowire with a diameter of 50 nm obtained with the IDRAGON code.

 

Featured image credits on the top of the post: CVD equipment coorporation.

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Protective coatings on Zr alloys: a pathway towards Accident Tolerant Fuels in Light-Water Reactors technology

The nuclear accident at Fukushima-Daiichi nuclear power complex in 2011 has started a discussion within the nuclear materials community regarding the safety and the operational limits of Zirconium alloys nuclear fuel rods under extreme conditions.

Zirconium is the holy grail of Light-Water Reactors (LWRs) and it is directly responsible for the success of nuclear reactors technology worldwide. Deeply researched by Admiral Hyman G. Rickover and his team in early 1960s [1], this success is mainly attributed to the properties that Zr and its alloys have within the context of a nuclear reactor operation: desirable mechanical properties, good corrosion resistance and low thermal neutron absorption cross section [2-3]. The last topic is of paramount importance as if the nuclear fuel rod material is relatively transparent to thermal neutrons, the efficiency of a nuclear reactor is not penalised (that is the case for stainless steels).

In the Fukushima-Daiichi nuclear power complex, the nuclear reactor lost its coolant material (i.e. water) due to the occurrence of external events: an earthquake and a tsunami [4]. In this operational condition, the temperature inside the nuclear reactor core rose abnormally and the known oxidation reaction between steam and the Zr rods have generated huge quantities of hydrogen gas (H2). The accumulation and leak of H2 was the cause behind several explosions in the nuclear power complex.

Replacing Zirconium in the already consolidated nuclear technology would require the design, test and licensing of a new metallic alloy: a task that would require millions of dollars in investments and in evaluation and tests as well as countless efforts to find a material with similar properties to Zr. One possible solution has been to coating Zirconium alloys with protective materials.

Conventional Titanium Nitride (TiN) and Titanium Aluminium Nitride (TiAlN) thin films are currently some candidate systems [5-6], but recently, the demonstrated feasibility for the synthesis of high-quality nanocrystalline high-entropy alloy thin films has added another perspective regarding protective coatings on Zirconium alloys [7]. Highly-concentrated alloys have been subjected to intense research in the past three years and due to their unique properties and superior resistance to energetic particle irradiation, these alloys are currently looking for some space to be applied in real nuclear systems.

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Cross-sectional ion beam image of a high-entropy alloy thin film (B – HEATF) deposited on Zircaloy-4 (A) with a protective Pt layer on top (A). MA Tunes own work.

Clearly coatings on Zirconium alloys will have to exhibit compatibility with the HCP matrix and prove suitable tribological properties. They will also have to cope with the limitations of materials for nuclear structures: reduced activation, good corrosion resistance, improved radiation resistance and desirable mechanical properties. Therefore, these new efforts towards new accident tolerant fuel concepts are just expressing how important the materials research is for our civilisation and how consolidated technologies can always be improved, modified and enhanced upon advances in technology.

References
[1] RICKOVER, Hyman George; GEIGER, Lawton D.; LUSTMAN, Benjamin. History of the development of zirconium alloys for use in nuclear reactors. Energy Research and Development Administration, 1975.
[2] ZAIMOVSKII, A. S. Zirconium alloys in nuclear power. Soviet Atomic Energy, v. 45, n. 6, p. 1165-1168, 1978.
[3] GRIFFITHS, M. A review of microstructure evolution in zirconium alloys during irradiation. Journal of Nuclear Materials, v. 159, p. 190-218, 1988.
[4] HOLT, Mark; CAMPBELL, Richard J.; NIKITIN, Mary Beth. Fukushima nuclear disaster. Congressional Research Service, 2012.
[5] ALAT, Ece et al. Ceramic coating for corrosion (c3) resistance of nuclear fuel cladding. Surface and Coatings Technology, v. 281, p. 133-143, 2015.
[6] ALAT, Ece et al. Multilayer (TiN, TiAlN) ceramic coatings for nuclear fuel cladding. Journal of Nuclear Materials, v. 478, p. 236-244, 2016.
[7] TUNES, Matheus A.; VISHNYAKOV, Vladimir M.; DONNELLY, Stephen E. Synthesis and characterisation of high-entropy alloy thin films as candidates for coating nuclear fuel cladding alloys. Thin Solid Films, v. 649, p. 115-120, 2018.

Do dislocations exist as a real physical entity?

When I was attending in the introductory course in materials science and metallurgy during my master of sciences, one particular question had intrigued myself for long time: are dislocations real physical entities?

As a curious student at the time, during the class, I have respectfully raised my hand and asked the professor: — Do dislocations really exist? And of course, the entire audience started to laugh on me. Students very often have such curiosities inside themselves, but are always reluctant to ask questions thinking they are only stupid doubts.

If you look in a materials science textbook, in most of the cases, the concept of dislocation will be properly described, but the figures often induce the reader/student to think that a dislocation is a real physical entity in the sense that they are something external to the crystal structure and which is added to it somehow. The textbooks sometimes give the impression that the dislocation is something real, something like an artefact.

The physical understanding behind the concept of “dislocation” is from 1930s when the mechanisms of plastic deformation of crystals were under deep investigation in Europe. The Royal Society Yarrow Professor Geoffrey Ingram Taylor was one of the pioneers in this field of research. In his paper entitled: “The Mechanism of Plastic Deformation of Crystals – Part 1 – Theoretical” [1], Taylor defined for the first time what is a dislocation.

Taylor’s early concepts on dislocations were motivated by very interesting experimental observations made by  Joffe et al. [2] in analysing deformed rock salt crystals in nicol prisms (a kind of portable homemade polarized optical microscope). The latter authors noted that in deformed rock salt crystals , a very bright line from side to side of a crystal attracted the attention of that experimentalists whose concluded that such line was a representation of a “crystal breakdown (…) indicating distorted material.”

The theoretical interpretation given by Taylor was made by means of picturing a “crystal block” that under stress, the propagation of a line of slipping atoms within a slip plane would result in a perfectly well ordered crystal structure, but deformed by the unit slip, or the dislocation.

For Taylor a dislocation can be viewed as a kinetic phenomenon of the “passage” of a strain field from side to side of a crystal within a slip plane! This strain field, is of course, caused by the external stresses acting on the crystal. Dislocations do not exist! Dislocation is rather a physical concept that defines a defective region of a crystal structure, not a physical entity.

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The movement of dislocations in recrystallised Au recorded in situ at 1000 K using Bright-Field Transmission Electron Microscopy.

When dislocations are viewed in a transmission electron microscope, they often appear in a form of lines or loops whose are of diffraction contrast, but the contrast is generated because in the analysed region of a dislocation, the crystal structure is defective (it has an extra plane of atoms). Therefore, the contrast does not exhibit something that is external to the crystal lattice itself, but it only shows that there is a defect in the local atomic arrangement which was supposed to be periodic and ordered.

Coming back to my class in the introductory course of materials science and metallurgy, I have asked this question to my professor at the time, he said to me that dislocations were real indeed, but I should be concerned in to study for the tests rather than asking stupid questions.

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Dark-Field Transmission Electron Microscope (DFTEM) micrograph colourised in a computer showing dislocation lines in recrystallised Au at ~ 1000 K

References:

[1] Taylor, Geoffrey Ingram. “The mechanism of plastic deformation of crystals. Part I. Theoretical.” Proceedings of the Royal Society of London. Series A 145.855 (1934): 362-387.

[2] Joffe, Abram Fedorovich, and Leonard Benedict Loeb. “Physics of crystals.” (1928).

 

The multidisciplinary interplay in materials sciences: can we understand and mimic the nature?

Nature is smart and brilliant. In nature, we can find the best examples on how to biosynthesise a material and use it for several purposes with efficiency. An astonishing case study is the spider silk which is a form protein fibre structurally similar to cellulose and human hair. Looking at the nanoscale, Transmission Electron Microscopy (TEM) analysis performed by Simmons et al. [1] has revealed an intriguing microstructure without any resemblance with typical man’s made materials: the spider silk is composed of small crystallites with sizes around of 30 nm separated by amorphous regions cross-linking those nanocrystals.

This semi-crystalline microstructure is responsible to deliver a unique set of properties which express the complexity and the beauty of such spider fibres. The tensile strength, 1.30 GPa, is in the order of the stainless steels, 1.65 GPa, with its density corresponding to a sixth of the steels [2]. With extreme ductility, these fibres can be stretched almost up to six times of their unloaded length. In terms of structural integrity, the mechanical properties of the spider silks are hold within the temperature range of -40 to 220°C, unlikely relevant for the natural environments. Darwin’s bark spider silk can reach up to 520 MJ/m³ of toughness: at around 10 times higher than Kevlar toughness [3].

Are the metallurgists and material scientists able to invent a material inspired by the semi-crystalline spider silk microstructure and make use of its properties? An interesting multidisciplinary field that, nowadays, has attracted the attention of the scientific community is known as biomimetics.  Consists in how scientists can interpret nature aiming at the design and synthesis of materials and structures by means of mimicking biological process.

In material sciences, the benefits of adopting an interdisciplinary biomimetic approach could bring enormous consequences to the next generation of researchers and engineers. The simple fact that a biological spider silk with mixed amorphous-crystalline microstructure can give birth to a protein-based material with comparable mechanical strength of the human top-steels is an astonishing achievement.

As pointed out in this blog before, superspecialisation has created an entire generation of professionals without any commitment with nature, environment and wellbeing. We need to go back to our roots. To observe nature as scientists and learn with it. To extract the best that nature can give us, and as a result, produce bio-inspired materials with enhanced properties. The triad metals-ceramics-polymers is old fashion and may condemn the creativity of our students for their entire carrers.

[1] SIMMONS, Alexandra H.; MICHAL, Carl A.; JELINSKI, Lynn W. Molecular orientation and two-component nature of the crystalline fraction of spider dragline silk. SCIENCE, p. 84-87, 1996.

[2] SHAO, Zhengzhong; VOLLRATH, Fritz. Suprising strength of silkworm silk. NATURE, 418, 741, 2002.

[3] BLACKLEDGE, Todd; KUNTNER, Matjaz; AGNARSSON, Ingi. Bioprospecting finds the toughest biological material: extraordinary silk from a giant riverine orb spider. Plos one, v. 5, n. 9, p. 1, 2010.

Featured Image: Darwin’s back spider. Credits: BBC London.