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Hospital germs and pathogens are not always transmitted directly from person to person. They can also spread via germ-contaminated surfaces and objects.
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Empa, Swiss Federal Laboratories for Materials Science and Technology
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Newswise — An elastic material that changes color, conducts electricity, can be 3D printed and is also biodegradable? That is not just scientific wishful thinking: Empa researchers from the Cellulose & Wood Materials laboratory in Dübendorf have produced a material with these exact properties on the basis of cellulose and carbon nanotubes.
The researchers started with hydroxypropyl cellulose (HPC), which is commonly used as an excipient in pharmaceuticals, cosmetics and foodstuffs, among other things. When mixed with water HPC is known to form liquid crystals. These crystals have a remarkable property: Depending on their structure – which itself depends on the concentration of HPC, among other things – they shimmer in different colors, although they themselves have no color or pigment. This phenomenon is called structural coloring and is known to occur in nature: Peacock feathers, butterfly wings and chameleon skin get all or part of their brilliant coloration not from pigments, but from microscopic structures that “split” the (white) daylight into spectral colors and reflect only the wavelengths for specific colors.
The structural coloring of HPC changes not only with concentration but also with temperature. To better exploit this property, the researchers, led by Gustav Nyström, added 0.1 weight percent carbon nanotubes to the mixture of HPC and water. This renders the liquid electrically conductive and allows the temperature, and thus the color of the liquid crystals, to be controlled by applying a voltage. Added bonus: The carbon acts as a broadband absorber that makes the colors deeper. By incorporating a small amount of cellulose nanofibers into the mixture, Nyström’s team was also able to make it 3D printable without affecting structural coloring and electrical conductivity.
Sustainable sensors and displays
The researchers used the novel cellulose mixture to 3D print various potential applications of the new technology. These included a strain sensor that changes color in response to mechanical deformation and a simple seven-segment display. “Our lab has already developed different disposable electronic components based on cellulose, such as batteries and sensors,” says Xavier Aeby, co-author of the study. “This is the first time we were able to develop a cellulose-based display.”
In future, the cellulose-based ink could have many more applications, such as temperature and strain sensors, in food quality control or biomedical diagnostics. “Sustainable materials that can be 3D printed are of great interest, especially for applications in biodegradable electronics and the Internet of Things,” says Nyström, head of the laboratory. “There are still many open questions about how structural coloring is generated and how it changes with different additives and environmental conditions.” Nyström and his team aim to continue this line of work in the hope of discovering many more interesting phenomena and potential applications.
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BYLINE: Rainer Klose
Newswise — For many years, Thomas Geiger has been conducting research in the field of cellulose fibrils – fine fibers that can be produced from wood pulp or agricultural waste, for example. Cellulose fibrils hold great potential for sustainable production and the decarbonization of industry: they grow CO2-neutral in nature, burn without residues and are even compostable. They can be used for many purposes, for example as fiber reinforcement in technical rubber products such as pump membranes.
But can cellulose fibrils perhaps also be used to make circuit boards that reduce the ecological footprint of computers? Printed circuit boards (PCBs) in particular are anything but innocent ecologically: They usually consist of glass fibers soaked in expoxy resin. Such a composite material is not recyclable and can so far only be disposed of properly in special pyrolysis plants.
Computer mouse with an ivory look
Geiger had already produced circuit boards from cellulose fibrils and investigated their biodegradation. Mixed with water, the bio-fibrils produce a thick sludge that can be dewatered and compacted in a special press. Together with a colleague, he produced 20 experimental boards, which were subjected to various mechanical tests and finally fitted with electronic components. The test succeeded, and the cellulose board released the soldered-on components after a few weeks in natural soil.
Geiger had previously been involved in an Innosuisse project together with the OST University of Applied Sciences in Rapperswil, which produced housing parts for computer mice. The housing parts have a silky sheen and are similar in color and feel to workpieces made of ivory. But no manufacturer could be found who wanted to adopt the method. The price competition for small electronics is still too great for this – and conventional plastic injection molding processes have a clear advantage in this respect.
Circuit boards made of wood wool or cellulose fibers and fibrils
Recently, the opportunity arose to build on existing findings: Empa sustainability specialist Claudia Som was asked if she would like to collaborate on the EU research project Hypelignum. This is led by the Swedish materials research institute RISE and is looking for new ways of sustainably producing electronics. Claudia Som enlisted the help of her colleague Thomas Geiger.
The project started in October 2022, and the research consortium, with participants from Austria, Slovenia, Spain, the Netherlands, Sweden and Switzerland, plans to produce and evaluate eco circuit boards made of various materials: In addition to nanofibrillated cellulose (CNF), wood wool and wood pulp are being investigated as a base; wood veneer is also being used as a base for the circuit boards.
Two Empa labs are collaborating on the project: Firstly, the sustainability specialists led by Claudia Som from the Technology and Society lab. Som will use material databases to calculate the ecological footprint of the eco circuit boards and compare the individual concepts with each other. Thomas Geiger from Empa’s Cellulose & Wood Materials laboratory will manufacture the circuit boards from renewable raw materials. Green electronics has long been a research focus of the lab, which is headed by Gustav Nyström; Nyström’s team has already developed various printed electronic components from biodegradable materials, such as batteries and displays. The requirements for industrially produced computer circuit boards, however, are not trivial: Not only must the boards have high mechanical strength, they must also not swell in humid conditions or form cracks at very low humidity.
“Cellulose fibers can be a very good alternative to glass fiber composites,” Geiger explains. “We dewater the material in a special press with 150 tons of pressure. Then the cellulose fibrils stick together on their own without any additives. We call this ‘hornification’.” The key here is at what pressure, temperature and for how long the pressing process must take place to produce optimal results.
Four demonstrators planned
The EU project Hypelignum has ambitious goals: It aims not only to study printed circuit boards made from renewable and compostable raw materials, but also to develop conductive inks for the electrical connections between individual components. These inks are often made based on silver nanoparticles. The researchers are looking for cheaper and less scarce substitute materials, as well as an ecological production method for these nanoparticles.
At the end of the project, four demonstrators ought to show what had been achieved: an ecologically exemplary printed circuit board, a large construction element made of wood that will be equipped with sensors and actuators, pieces of furniture that will be equipped with sensors in an automated production line, and finally a demonstrator that will prove the recyclability of all these components.
Infobox: Displays and batteries made from cellulose
In 2022, an Empa research group led by Gustav Nyström succeeded in building a biodegradable display based on hydroxypropyl cellulose (HPC). They used HPC as a substrate and added a small amount of carbon nanotubes, making the cellulose electrically conductive. By mixing in cellulose nanofibers (CNF), they brought the ink into a printable form. The display changes color depending on the applied electrical voltage; in addition, it can also serve as a pressure or tension sensor and has the potential to play a role as a biodegradable user interface in future eco-electronics.
https://www.empa.ch/web/s604/bioelektronik
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Newswise — Gordon Moore was right. In April 1965, the US engineer and later co-founder of Intel predicted that the number of transistors on a chip would double about every two years. To this day, this development continues with nearly the same speed – also because chip manufacturers worldwide use Moore’s Law as the basis for their strategic planning. Thus, the prophecy is self-fulfilling.
But the doubling of the number of circuits every two to three years sometimes reaches the limits of what is technically feasible. This also holds for the joining technologies, which have to keep up with the increased demands. After all, the ever smaller and more powerful electronic components still have to be integrated into larger systems, and the joints connecting the components to heat sinks or circuit boards should not fall apart during temperature changes or vibrations, or overheat during operation. A team led by Jolanta Janczak-Rusch and Bastian Rheingans from Empa’s Laboratory for Joining Technologies and Corrosion is tackling this problem.
Industry in need
“Our partners and customers, for whom we develop customized solutions, always want more, and preferably everything at the same time,” says Janczak-Rusch. A joint for a new high-performance electronic component, for example, should be made at the lowest and gentlest possible temperature – and yet survive the highest possible temperatures when the component is in operation, and efficiently dissipate waste heat from the components. This is the only way to combine miniaturization and increased performance without at the same time increasing the cost of cooling to infinity. Other advancing technologies such as photonics, sensor technology, space travel, batteries and turbine construction are also dependent on innovative joining concepts.
New materials and processes are therefore needed to meet the increasingly complex demands placed on joining. In this situation, joining with nanomaterials, so-called nanojoining, offers great potential. Industry is already using silver nanopastes, i.e. joining materials consisting of silver nanoparticles. The advantage: While the melting point for pure silver is 962 degrees Celsius, silver nanopastes can be applied to produce electrically and thermally highly conductive joints at temperatures as low as 250 degrees Celsius. And even better: Once produced, these joints can even withstand an operating temperature above their production temperature.
Utilizing nanoeffects
There’s a lot of materials science know-how behind this innovative solution. “Here we are replacing a classic soldering process with a sintering process,” explains Rheingans. This means that the particles in the joining zone are not melted, but grow together into larger particles and grains by diffusion, thereby reducing their surface energy. Diffusion, i.e. the movement of individual atoms, is very rapid at surfaces and interfaces. Since nanoparticles have an extremely large surface area in relation to their volume, sintering is particularly pronounced on the nanoscale and can be exploited even at comparatively low temperatures. In the case of very small nanoparticles or thin nanolayers, the amount of easily moving, “liquid” surface atoms becomes so large that the melting point can drop several hundred degrees below the melting point of the solid material. The researchers call this effect MPD (Melting Point Depression) – and use it to develop innovative and efficient joining processes.
The race continues
“We are working on nanopastes with multiple components to optimize the properties of the joining compound and to open up new areas of application,” Rheingans says. “For example, we are investigating combinations of copper and nickel nanopastes.” These metals are less expensive than silver and exhibit very interesting electrical and thermal properties – but because they are less noble metals, they oxidize much more easily. That has to be prevented in the joining process. “So we put the nanoparticles in a paste of organic adjuvants that evaporate during the joining process and reduce the oxide on the particle surface. Or we coat the particles with a protective coating,” explains the Empa researcher. Using special analytical methods such as X-ray diffraction (XRD) or X-ray photoelectron spectroscopy (XPS), the researchers can verify whether the postulated method of protecting the nanoparticles works as intended.
But innovation is also possible with the well-known silver nanopaste: “In an Empa research project for the development of oxide membranes for microelectronics, we were able to effectively support our colleagues with our know-how: using the nanopaste, we could transfer the ultra-thin membranes onto a carrier substrate without introducing any damage,” says Rheingans. This method could also be applied to other 2D materials.
An oven on the nanoscale
For particularly temperature-sensitive components, the researchers have another nanojoining method that they are continuously developing further: so-called reactive joining. In this process, reactive foils replace the soldering oven as a local source of heat. The foils consist of a large number of individual nanolayers, for example of nickel or aluminum. When these nano-multilayers are ignited, the nickel and aluminum react and form a new chemical compound – and release a great deal of heat that drives the process and makes it travel at speeds of up to 50 meters per second over the entire foil. Only layer thicknesses in the nano-range enable such a fast and self-perpetuating reaction. Locally, temperatures of up to 1000 degrees Celsius can be reached, but because of the low thickness of the reactive foil, the total amount of heat remains small and limited to the adjacent solder layers. In this way, sensitive electronic elements can be gently and firmly attached to copper heat sinks.
Nanolayer systems to combat heat buildup
An important focus in recent years has been the development of nanomultilayer systems starting from classic brazing filler metal/alloys such as copper, silver, silver-copper or aluminum-silicon: “Due to the lowering of the melting point and the rapid diffusion on the nanoscale, these bonding materials offer the possibility of carrying out joining processes much faster and at significantly lower temperatures than with conventional brazing techniques” explains Janczak-Rusch.
Nanomultilayers can also be used elsewhere in the joining process: With the recently approved SNF-NCN Lead Agency project “Development of submicro- and nanostructured Cu-Mo composites with tailored properties for thermal management,” the Advanced Joining Technologies team is addressing the burning issue of heat dissipation in miniaturized electronic components.
“The interesting properties of copper-molybdenum composites have already been used in the design of an ion source for the JUICE mission of the European Space Agency ESA,” says Empa researcher Hans Rudolf Elsener, who specializes in space missions. Together with Polish researchers, the potential of nanostructured Cu-Mo multilayer systems as heat sinks will now be specifically investigated and suitable joining processes for their integration will be developed.
Glossary Joining techniques
Soldering/Brazing: The base materials are joined together by melting an additional material, the solder/brazing filler alloy. The workpieces themselves are not melted or fused during the process. Up to 450 degrees Celsius, this is referred to as soldering, and above 450 degrees as brazing.
Welding: In contrast to brazing, the workpieces are partially melted and are immediately joined after cooling. Filler materials are often introduced into the weld seam to increase the amount of molten metal.
Nanojoining is a new scientific discipline. It includes joining techniques for joining nano-objects, but also novel, high-performance joining processes that utilize nano-effects. Empa is one of the main players in this new discipline, as well as a founding member and headquarters of the international Nano- & Microjoining Association.
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