“We could potentially create new materials with unusual properties that have never existed,”
By creating atomic chains in a two-dimensional crystal, researchers at Penn State believe they have found a way to control the direction of materials properties in two and three dimensional crystals with implications in sensing, optoelectronics and next-generation electronics applications.
Whether an alloy has a random arrangement of atoms or one that is ordered can have large effects on a material’s properties. In a new paper published online in the journal Nano Letters, Nasim Alem, assistant professor of materials science and engineering, and colleagues at Penn State used a combination of simulations and scanning transmission electron microscopy imaging to determine the atomic structure of an ordered alloy of molybdenum, tungsten and sulfur. They determined that fluctuations in the amount of available sulfur were responsible for the creation of atomic chains of either molybdenum or tungsten.
“We discovered how chains form in a two-dimensional alloy as a result of fluctuations in the amount of a particular precursor, in this case sulfur,” Alem said. “Normally, when we combine atoms of different elements, we don’t know how to control where the atoms will go. But we have found a mechanism to give order to the atoms, which in turn introduces control of the properties, not only heat transport, as is the case in this work, but also electronic, chemical or magnetic properties in other alloy cases. If you know the mechanism, you can apply it to arrange the atoms in a wide range of alloys in 2D crystals across the Periodic Table.”
In the case of the molybdenum, tungsten and sulfur alloy, they showed that the electronic properties were the same in every direction, but using simulations they predict that the thermal transport properties are smaller perpendicular to the chains or stripes.
“We didn’t know why this crystal forms an ordered structure, so we worked with my colleague Dr. Vin Crespi to understand the underlying physics that causes order in this crystal. Our calculations show it was the fluctuations in the third element, sulfur, that was determining how the chains formed,” Alem said.
Vincent H. Crespi, Distinguished Professor of Physics, and professor of chemistry and materials science and engineering who developed the theoretical understanding of the phenomenon, said, “Although the interior of the flake is indifferent to whether molybdenum or tungsten occupies any site in the crystal lattice, the edge of the growing crystal does care: Depending on how much sulfur is available at a given location, the edge will prefer to be either 100% molybdenum or 100% tungsten. So as the availability of sulfur randomly varies during growth, the system alternately lays down rows of molybdenum or tungsten. We think this may be a general mechanism to create stripe-like structures in 2D materials.”
Amin Aziz, who is a Ph.D. candidate in Alem’s group and lead author on the Nano Letters paper, produced the STEM imaging and spectroscopy that showed the fine atomic structure of the alloy samples and their electronic properties.
“When we are able to directly image constitutive atoms of a substance, see how they interact with each other at the atomic level and try to understand the origins of such behaviors, we could potentially create new materials with unusual properties that have never existed,” said Azizi.
A team led by Mauricio Terrones, professor of physics, produced samples of this ordered alloy by vaporizing powders of all three elements, called precursors, under high heat.
What if it were possible to quickly and inexpensively manufacture a part simply by using a series of close-range digital images taken of the object?
Michael Immel, instructor in the Harold and Inge Marcus Department of Industrial and Manufacturing Engineering, originally started thinking about the technique, called photogrammetry, for a different purpose, but quickly realized its application in manufacturing.
In this technique, digital images of an object that have been taken at various angles are used to create a point cloud — or a large collection of points used to create 3D representation of existing structures — from which a computer-aided design (CAD) file can be generated.
The resulting CAD file and subsequent 3D model could then be used to rebuild the part, or 3D print it, to its original specifications without using traditional methods, which are both expensive and time-consuming.
“If we can take pictures of the parts and use commercial software to create the point cloud file from the images, we can come up with the dimensions within some reasonable amount of accuracy and apply it in industry,” Immel explained.
Immel received a seed grant from his department’s Entrepreneurship and Innovation Fund to explore whether photogrammetry can be a more efficient way of manufacturing low-tolerance parts — parts that have sufficient limits of variation and do not have to fit into assemblies — such as large pipes and manhole covers.
Over the summer, Immel and three engineering students — Andrew Bellows, a graduate student in mechanical engineering; Benjamin Sattler, an undergraduate mechanical engineering student and a Schreyer’s Scholar; and Xinyi Xiao, an industrial engineering graduate student — set out to test the accuracy of photogrammetry.
The group chose parts for which they already had a CAD file to compare with their photogrammetry-created point cloud files.
To get started, Bellows created a studio setup to take consistent and replicable photographs of the part. The environment included even lighting, to eliminate shadows, and a contrasting background to ensure the photo obtained enough data from the part. Additionally, Bellows took overlapping photos around the part at a specific angle and from different distances to be sure he had enough images to create the point clouds.
Bellows, Sattler and Xiao then each used a version of software used for photgrammetry — Photomodeler Scanner, AutoDesk ReMake and Mathworks MatLab — to create point cloud files, which were then compared to each other and to the original CAD and point cloud files.
“We looked at the variance between the original point cloud files and photogrammetry point clouds to see if there are discrepancies between them and to determine how accurate this technique would be if it were to be used in manufacturing,” said Immel. “Photogrammetry has proven to be an accurate approach for applications where tight tolerances are not necessary.”
In a traditional manufacturing process, large quantities of parts are made in quick succession and then go from the manufacturing line through an inspection process. A quality control engineer or specialist then measures the parts with handheld tools and check for any abnormalities, making sure all of the dimensions of the part are within tolerance so they operate as the part was originally designed.
“The ideal application of photogrammetry in the industry setting would be to have a vision system in a manufacturing plant that included cameras fixed on the machines making the parts, taking continuous photos,” explained Immel. “Live data could be sent back to an engineer or a quality control employee and they could compare the point cloud that has been derived from the digital images to the point cloud of the original file and determine if the part is within tolerance or not.”
Immel and his team have concluded that photogrammetry has the potential to make the quality control process quicker, less expensive and more efficient for manufacturers.
Saurabh Basu, an assistant professor of industrial engineering, recently joined Immel’s research group and is interested in looking at photogrammetry from an empirical research standpoint.
“Now that we have a process in place that works, we need to hone it. Dr. Basu will help us by providing us with the empirical data before we take the process to industry to test it out,” said Immel.
A team of chemical engineers at Penn State has developed a beneficial biofilm with the ability to prevent the biofouling of reverse osmosis (RO) membranes.
The biofilm allows membranes to limit their own thickness via a quorum-sensing circuit, and ultimately to reduce the occurrence of biofouling in membrane-based water treatment systems by releasing chemicals that repel undesirable bacteria.
“We realized that the accumulation of microbial films in water treatment membranes is unavoidable,” said Manish Kumar, assistant professor of chemical engineering and the principal investigator on the project. “But just like good bacteria exists in your gut to keep you healthy, we predicted that helpful bacteria in RO may be able to prevent the unchecked reproduction of harmful biofilms. Essentially, our method is a ‘probiotic-approach’ to combat biofouling.”
With the demand for access to safe and clean water escalating globally, membrane filtration technologies are quickly becoming popular ways to utilize low quality and readily abundant water sources such as seawater, brackish water and recycled wastewater.
Complications with these systems arise frequently, however, most often in the form of biofouling—a buildup of microbes and bacteria on membrane surfaces that causes clogging and leads to decreased membrane permeability and an overall increase in energy consumption.
To find a solution to the problem, Kumar teamed with collaborators and co-investigators Thomas Wood, Endowed Biotechnology Chair Professor of Chemical Engineering, and Tammy Wood, research associate in the Department of Chemical Engineering, to study membrane-biofilm interactions and their biochemical attributes.
A new type of 3-D printing will make it possible for the first time to rapidly prototype and test polymer membranes that are patterned for improved performance, according to Penn State researchers.
Ion exchange membranes are used in many types of energy applications, such as fuel cells and certain batteries, as well as in water purification, desalination, removal of heavy metals and food processing. Most of these membranes are thin, flat sheets similar to the plastic wrap in your kitchen drawer. However, recent work has shown that by creating 3-D patterns on top of the 2-D membrane surface, interesting hydrodynamic properties emerge that can improve ion transport or mitigate fouling, a serious problem in many membrane applications.
Currently, making these patterned membranes, also called profiled membranes, involves a laborious process of etching a silicon mold with the desired pattern, pouring in the polymer and waiting until it hardens. The process is both time-consuming and expensive, and results in a single pattern type.
“We thought if we could use 3-D printing to fabricate our custom-synthesized ion exchange membranes, we could make any sort of pattern and we could make it quickly,” says Michael Hickner, associate professor of materials science and engineering, Penn State.
The ROAR project is aimed at showing how machines can communicate with each other and how, in the future, they will be able to carry out tasks now undertaken by humans
The system uses drones to locate refuse bins and robots to collect and empty them. The aim is to show how machines can communicate with each other and how, in the future, they will be able “to facilitate everyday life in a large number of areas.”