It was founded in 1665 as the Academia Holsatorum Chiloniensis by Christian Albert, Duke of Holstein-Gottorp and has approximately 24,000 students today.
The University of Kiel is the largest, oldest, and most prestigious in the state of Schleswig-Holstein. Until 1864/66 it was not only the northernmost university in Germany but at the same time the 2nd largest university of Denmark.
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How metals can be used depends particularly on the characteristics of their surfaces. A research team at Kiel University has discovered how they can change the surface properties without affecting the mechanical stability of the metals or changing the metal characteristics themselves.
This fundamentally new method is based on using an electro-chemical etching process, in which the uppermost layer of a metal is roughened on a micrometer scale in a tightly-controlled manner. Through this “nanoscale-sculpturing” process, metals such as aluminium, titanium, or zinc can permanently be joined with nearly all other materials, become water-repellent, or improve their biocompatibility. The potential spectrum of applications of these “super connections” is extremely broad, ranging from metalwork in industry right through to safer implants in medical technology. Their results have now been published in the prestigious journal “Nanoscale Horizons” of the Royal Society of Chemistry.
“We have now applied a technology to metals that was previously only known from semiconductors. To use this process in such a way is completely new,” said Dr. Jürgen Carstensen, co-author of the publication. In the process, the surface of a metal is converted into a semiconductor, which can be chemically etched and thereby specifically modified as desired. “As such, we have developed a process which – unlike other etching processes – does not damage the metals, and does not affect their stability,” emphasised Professor Rainer Adelung, head of the “Functional Nanomaterials” team at the Institute for Materials Science. Adelung stressed the importance of the discovery: “In this way, we can permanently connect metals which could previously not be directly joined, such as copper and aluminium.”
How does the “nanoscale-sculpturing” process work exactly?
The surfaces of metals consist of many different crystals and grains, some of which are less chemically stable than others. These unstable particles can be specifically removed from the surface of a metal by a targeted etching. The top surface layer is roughened by the etching process, creating a three-dimensional surface structure. This changes the properties of the surface, but not of the metal as a whole. This is because the etching is only 10 to 20 micrometers deep – a layer as thin as a quarter of the diameter of human hair. The research team has therefore named the process “nanoscale-sculpturing”.
The change due to etching is visible to the naked eye: the treated surface becomes matt. “If, for example, we treat a metal with sandpaper, we also achieve a noticeable change in appearance, but this is only two-dimensional, and does not change the characteristics of the surface,” explained Dr. Mark-Daniel Gerngroß of the research team on materials sciences from Kiel.
Through the etching process, a 3D-structure with tiny hooks is created. If a bonding polymer is then applied between two treated metals, the surfaces inter-lock with each other in all directions like a three-dimensional puzzle. “These 3-D puzzle connections are practically unbreakable. In our experiments, it was usually the metal or polymer that broke, but not the connection itself,” said Melike Baytekin-Gerngroß, lead author of the publication.
Surfaces with multifunctional properties
Even a thin layer of fat – such as that left by a fingerprint on a surface – does not affect the connection. “In our tests, we even smeared gearbox oil on metal surfaces. The connection still held,” explained Baytekin-Gerngroß. Laborious cleaning of surfaces, such as the pre-treatment of ships’ hulls before they can be painted, could thus be rendered unnecessary.
In addition, the research team exposed the puzzle connections to extreme heat and moisture, to simulate weather conditions. This also did not affect their stability. Carstensen emphasised: “Our connections are extremely robust and weather-resistant.” A beneficial side-effect of the process is that the etching makes the surfaces of metal water-repellent. The resulting hook structure functions like a closely-interlocked 3D labyrinth, without holes which can be penetrated by water. The metals therefore possess a kind of built-in corrosion protection. “We actually don’t know this kind of behaviour from metals like aluminium. A lotus effect with pure metals – i.e. without applying a water-repellent coating – that is new,” said Adelung.
Potentially limitless applications
“The range of potential applications is extremely broad, from metalworking industries such as ship-building or aviation, to printing technology and fire protection, right through to medical applications,” said Gerngroß. Because the “nanoscale-sculpturing” process not only creates a 3D surface structure, which can be purely physically bonded without chemicals – the targeted etching can also remove harmful particles from the surface, which is of particularly great interest in medical technology.
Titanium is often used for medical implants. To mechanically fix the titanium in place, small quantities of aluminium are added. However, the aluminium can trigger undesirable side-effects in the body. “With our process, we can remove aluminium particles from the surface layer, and thereby obtain a significantly purer surface, which is much more tolerable for the human body. Because we only etch the uppermost layer on a micrometer scale, the stability of the whole implant remains unaffected,” explained Carstensen.
The researchers have so far applied for four patents for the process. Businesses have already shown substantial interest in the potential applications. “And our specialist colleagues in materials sciences have also reacted enthusiastically to our discoveries,” said a delighted Adelung.
UV-sensors from the oven
Placed in fire detectors and water treatment units UV-sensors can save lives; also in many areas of industry and environment the demand for these devices is rising steadily. Scientists of Kiel University have been able to ”bake” nanostructures within seconds, in order to fabricate very fast UV-sensors. This new technique totally diminishes the need to use sophisticated equipments and toxic chemicals. It will therefore be highly interesting for companies. The scientists have published their results today (November 19) in Advanced Materials, a very renowned scientific magazine.
When building a sensor device from nanostructures, one of the biggest challenges is how to interconnect them into electrical contacts in chips because of their extremely small dimensions in nanoscale range, says Dawit Gedamu, the first author of the paper. Most of the existing synthesis methods, such as Chemical Vapour Deposition or Vapor-Liquid-Solid (VLS) growth allow synthesis of different nanostructures only under specific conditions. For instance, the presence of catalytic particles, particular substrates, complex temperature, atmospheric conditions and many more factors must be met. Furthermore, to integrate the synthesized nanostructures with these techniques in the chips requires another very sophisticated step. There are silicon or gallium nitride based UV detectors already available in the market but they lack a certain level of selectivity and also they cannot function in harsh environments. High production costs, multistep processes and the requirement of specific operating conditions limit the field of application for these sensors.
“Extremely promising” for various applications are the sensors that are based on zinc oxide, says Dr. Yogendra Kumar Mishra, scientific assistant with the work group “Functional Nanomaterials” at Kiel University and main author of the study. “Nanostructures made from zinc oxide are highly interesting for multifunctional applications, due to their sensibility to UV light and their electrical and mechanical properties”, says Mishra. Also, the material is relatively inexpensive and easy to synthesize. Since up to a certain level zinc is necessary for human organisms, these zinc oxide nano-microstructures could be of potential interest for biomedical engineering.
The scientists have fabricated a network of interconnected zinc oxide nano-tetrapods as a bridge between electrodes on a chip by a new single step flame transport synthesis process: In a simple oven or airbrush gun-type burner it only needs high temperature to convert zinc microparticles into nano-micro tetrapods. This process takes place in normal air environment and the necessary amount of oxygen is regulated by the flame itself. “This burner-flame transport synthesis method allows us to grow the zinc oxide nano-microstructures directly on the chip – and that only takes a few seconds, it is just a matter of driving the chip through the flame while the nano tetrapods assemble themselves onto it!” Mishra is excited to report. The high temperature of the flame ensures contacts of good quality between chip and the nanostructures, which is highly desirable for a better performance of the device.
The result: the sensor produced by the Kiel University scientists reacts to UV light within milliseconds of its exposure. Additionally, it also works in rather rough environments. These simple and inexpensive manufacturing conditions as well as the usage of pure zinc microparticles make this production method at the laboratories at Kiel University highly attractive for manufacturing companies. “We already had regional companies inquiring about our work. It shows that our basic research can be transferred into concrete applications”, Professor Rainer Adelung, head of the research team, explains. The next logical step for the material scientists is therefore to find the ways to produce these nano-tetrapods on a larger scale.
One curious fact: Zinc oxide nanostructures started their career as waste from conventional VLS growth experiments for zinc oxide. One day, Yogendra Mishra examined the crystals that looked like artificial snow under the microscope: “Their particular intertwining structure and their ability to detect light implied an enormous potential”, says the scientist, who was holding a fellowship from the Alexander von Humboldt Foundation while developing the new method in the years following this discovery.