A New Process for Sintering Ceramics at Low Temperatures Could Save Money and Cut CO2 Emissions

A new technology developed by Penn State researchers, called Cold Sintering Process (CSP), has opened a window on the ability to combine incompatible materials, such as ceramics and plastics, into new, useful compound materials, and to lower the energy cost of many types of manufacturing.

 “In this day and age, when we have to be incredibly conscious of the CO2 budget, the energy budget, rethinking many of our manufacturing processes, including ceramics, becomes absolutely vital,” said Clive Randall, professor of materials science and engineering at Penn State who developed the process with his team. “Not only is CSP a low temperature process (room temperature up to 200 ° Celsius), but we are also densifying some materials to over 95 percent of their theoretical density in 15 minutes. We can now make a ceramic faster than you can bake a pizza, and at lower temperatures.”

The making of ceramics using heat is the oldest of manmade materials processes, dating back tens of thousands of years. Ceramic products are found in every corner of the Earth and even in orbit, from table settings to the tiles used on the space shuttle to protect it from the heat of re-entry. What once was the stuff of grain storage casks and clay figurines is now the highly engineered material used in semiconductor electronics, biomedical implants, and jet engines.

The common denominator of all these manmade ceramic objects is heat, very high heat. Penn State professor of engineering science and mechanics Michael Lanagan points out, “There is a long trend in reducing the process temperature in ceramics. The dream is to get ceramics to consolidate at much lower temperatures. Then all sorts of things open up.”

Within the space of a year, the Randall group has shown that the long-sought process of consolidating, also called sintering, ceramics at low temperature has been achieved. In around a dozen peer-reviewed articles published in 2016 and early 2017, the researchers described the process of sintering ceramics at room temperature up to around 200 ° C, far below the normal temperatures of 1000 ° C and above. As of early 2017, Randall’s team has successfully sintered over 50 composite systems using CSP.

Just add water

According to the researchers, the process involves wetting ceramic powder with a few drops of water or acid solution. The solid surfaces of the particles decompose and partially dissolve in the water to produce a liquid phase at particle-particle interfaces. Adding temperature and pressure causes the water to flow and the solid particles to rearrange in an initial densification process. Then in a second process, clusters of atoms or ions move away from where the particles are in contact, which aids diffusion, which then minimizes surface free energy, allowing the particles to pack tightly together. The key is knowing the exact combination of moisture, pressure, heat and time required to capture the reaction rates so the material fully crystallizes and gets to very high density.

“I see cold sintering process as a continuum of different challenges,” Randall says. “In some systems, it’s so easy you don’t need pressure. In others you do. In some you need to use nanoparticles. In others, you can get away with a mixture of nanoparticles and larger particles. It really all depends on the systems and chemistries you are talking about.”

The Penn State team has begun building a library of the precise techniques required to use CSP on various materials systems, with 50 processes verified to-date. These include ceramic-ceramic composites, ceramic-nanoparticle composites, ceramic-metals, as well as ceramic-polymers.

Other areas that are now open to exploration by CSP include architectural materials, such as ceramic bricks, thermal insulation, biomedical implants and many types of electronic components.

Why is sintering at low temperatures so important?

There have been hints of this low-temperature sintering capability before, Lanagan says. Penn State is well known for its work in chemically bonded ceramics and has a long history in ceramics processing. “Like anything in science and technology, you build on things you’ve learned from others. In my mind, that is the root of this discovery.”

Having so much prior knowledge of ceramics, the team is able to follow a set of chemical guidelines to understand the possibilities for CSP. “We know all these materials really well,” Lanagan says. “We know where their applications are, and now we are starting to make them through the cold sintering process.”  

Ceramics are produced in high-temperature ovens that gobble up electricity, much of it produced by coal, gas, and oil combustion, adding substantially to the burden of greenhouse gases in the atmosphere. Cement, which is produced at high temperatures, is alone responsible for around 5 percent of global CO2 emissions. It is entirely possible that cold sintering process could be used in the manufacturing of cement, and a group in Zurich, Switzerland, is already experimenting with CSP for this purpose.

Reducing CO2 is of global importance, but reducing the cost of manufacturing ceramics and composites is what makes business people pay attention. Energy is expensive, and reducing the time and temperature required to produce a densified product in an energy intensive manufacturing process could strongly impact a company’s bottom line. 

Richard Clark, senior technical specialist for Morgan Advanced Materials, an international company that is building a Center of Excellence at Penn State, speculated that the savings in energy costs could be worth billions to industry, if the process could be ramped up to factory floor scale.

“I still have some skepticism,” he said. “Will there be problems with scaling up? Is there a reason it wasn’t discovered 50 or 100 years ago? Will we replace a floor of high temperature furnaces with a similar number of hot press machines? But if this is the real thing, it could mean billions, or more likely tens of billions, in value.”

CO2 reduction and cost savings are both highly commendable outcomes of low temperature sintering, but from a curiosity driven perspective, CSP opens up a dizzying array of possibilities for new material composites – composites that could never be sintered in a single process because their temperature requirements were incompatible.

In November 2016, The Department of Energy’s ARPA-E program announced a $1M award to Penn State researchers to develop safe and reliable polymer-ceramic composites for next generation battery and fuel cells using the cold sintering process. Associate professor of chemical engineering Enrique Gomez is the lead PI on the project. This project leverages a new approach to processing ceramics that has been recently developed by the Randall group at Penn State,” Gomez said in a press release.

In a 2016 article in the journal Advanced Functional Materials, Randall and his coauthors described the co-sintering of ceramic and thermoplastic polymer composites using CSP. Three types of polymer were selected to complement the properties of three types of ceramics, a microwave dielectric, an electrolyte and a semiconductor, in order to highlight the diversity of applicable materials. These composite materials demonstrate new possibilities for dielectric property design, and both ionic and electronic electrical conductivity design. These composites can be sintered to high density at 120 ° C in a timeframe of 15 to 60 minutes.

Other areas that are now open to exploration by CSP include architectural materials, such as ceramic bricks, thermal insulation, biomedical implants and many types of electronic components.

“My hope is that a lot of the manufacturing processes that already exist will be able to use this process, and we can learn from polymer manufacturing practices,” Randall concluded.

Along with Randall and Lanagan, postdoctoral scholars Jing Guo and Hanzheng Guo, and research and development engineer Amanda Baker are part of the team developing the patent-pending technology.        

Their work is supported by the National Science Foundation as part of the Center for Dielectrics and Piezoelectrics and the NSF-ERC ASSIST program, the 3M Science and Technology Fellowship, and the Department of Energy GATE Fellowship. 

Contact Prof. Clive Randall at car4@psu.edu.