Featured Research

from universities, journals, and other organizations

Graphene is only as strong as its weakest link: Experiments determine real-world limits of two-dimensional carbon

Date:
April 29, 2014
Source:
Georgia Institute of Technology
Summary:
Scientists have tested the fracture toughness of graphene for the first time by making and measuring "pre-cracks" under stress. The results show the material to be somewhat brittle.

An electron microscope image shows a pre-crack in a suspended sheet of graphene used to measure the overall strength of the sheet in a test at Rice University. Rice and Georgia Tech scientists performed experiments and theoretical calculations and found that graphene, largely touted for its superior physical strength, is only as strong as its weakest point.
Credit: The Nanomaterials, Nanomechanics and Nanodevices Lab/Rice University

There is no disputing graphene is strong. But new research by Rice University and the Georgia Institute of Technology should prompt manufacturers to look a little deeper as they consider the miracle material for applications.

The atom-thin sheet of carbon discovered this century is touted not just for its electrical properties but also for its physical strength and flexibility. The bonds between carbon atoms are well known as the strongest in nature, so a perfect sheet of graphene should withstand just about anything. Reinforcing composite materials is among the material's potential applications.

But materials scientists know perfection is hard to achieve. Researchers Jun Lou at Rice and Ting Zhu at Georgia Tech have measured the fracture toughness of imperfect graphene for the first time and found it to be somewhat brittle. While it's still very useful, graphene is really only as strong as its weakest link, which they determined to be "substantially lower" than the intrinsic strength of graphene.

"Graphene has exceptional physical properties, but to use it in real applications, we have to understand the useful strength of large-area graphene, which is controlled by the fracture toughness," said Zhu.

The researchers reported in the journal Nature Communications the results of tests in which they physically pulled graphene apart to see how much force it would take. Specifically, they wanted to see if graphene follows the century-old Griffith theory that quantifies the useful strength of brittle materials.

It does, said Lou. "Remarkably, in this case, thermodynamic energy still rules," he said.

Imperfections in graphene drastically lessen its strength -- with an upper limit of about 100 gigapascals (GPa) for perfect graphene previously measured by nanoindentation -- according to physical testing at Rice and molecular dynamics simulations at Georgia Tech. That's important for engineers to understand as they think about using graphene for flexible electronics, composite material and other applications in which stresses on microscopic flaws could lead to failure.

The Griffith criterion developed by a British engineer during World War I describes the relationship between the size of a crack in a material and the force required to make that crack grow. Ultimately, A.A. Griffith hoped to understand why brittle materials fail.

Graphene, it turns out, is no different from the glass fibers Griffith tested.

"Everybody thinks the carbon-carbon bond is the strongest bond in nature, so the material must be very good," Lou said. "But that's not true anymore, once you have those defects. The larger the sheet, the higher the probability of defects. That's well known in the ceramic community."

A defect can be as small as an atom missing from the hexagonal lattice of graphene. But for a real-world test, the researchers had to make a defect of their own -- a pre-crack -- they could actually see. "We know there will be pinholes and other defects in graphene," he said. "The pre-crack overshadows those defects to become the weakest spot -- so I know exactly where the fracture will happen when we pull it.

"The material resistance to the crack growth -- the fracture toughness -- is what we're measuring here, and that's a very important engineering property," he said.

Just setting up the experiment required several years of work to overcome technical difficulties, Lou said. To suspend it on a tiny cantilever spring stage similar to an atomic force microscopy (AFM) probe, a graphene sheet had to be clean and dry so it would adhere (via van der Waals force) to the stage without compromising the stage movement necessary for the testing. Once mounted, the researchers used a focused ion beam to cut a pre-crack less than 10 percent of the width into the microns-wide section of suspended graphene. Then they pulled the graphene in half, measuring the force required.

While the Rice team was working on the experiment, Zhu and his team performed computer simulations to understand the entire fracture process.

"We can directly simulate the whole deformation process by tracking the motion and displacement with atomic-scale resolution in fairly large samples so our results can be directly correlated with the experiment," said Zhu. "The modeling is tightly coupled with the experiments."

The combination of modeling and experiment provides a level of detail that allowed the researchers to better understand the fracture process -- and the tradeoff between toughness and strength in the graphene. What the scientists have learned in the research points out the importance of fabricating high quality graphene sheets without defects -- which could set the stage for fracture.

"Understanding the tradeoff between strength and toughness provides important insights for the future utilization of graphene in structural and functional applications," Zhu added. "This research provides a foundational framework for further study of the mechanical properties of graphene."

Lou said the techniques they used should work for any two-dimensional material. "It's important to understand how defects will affect the handling, processing and manufacture of these materials," he said. "Our work should open up new directions for testing the mechanical properties of 2-D materials."

Lou is an associate professor of Materials Science and Nanoengineering and of Chemistry at Rice. Zhu is an associate professor in the George W. Woodruff School of Mechanical Engineering at Georgia Tech.

The Welch Foundation, the National Science Foundation, the U.S. Office of Naval Research and the Korean Institute of Machinery and Materials supported the research.


Story Source:

The above story is based on materials provided by Georgia Institute of Technology. Note: Materials may be edited for content and length.


Journal Reference:

  1. Peng Zhang, Lulu Ma, Feifei Fan, Zhi Zeng, Cheng Peng, Phillip E. Loya, Zheng Liu, Yongji Gong, Jiangnan Zhang, Xingxiang Zhang, Pulickel M. Ajayan, Ting Zhu, Jun Lou. Fracture toughness of graphene. Nature Communications, 2014; 5 DOI: 10.1038/ncomms4782

Cite This Page:

Georgia Institute of Technology. "Graphene is only as strong as its weakest link: Experiments determine real-world limits of two-dimensional carbon." ScienceDaily. ScienceDaily, 29 April 2014. <www.sciencedaily.com/releases/2014/04/140429085118.htm>.
Georgia Institute of Technology. (2014, April 29). Graphene is only as strong as its weakest link: Experiments determine real-world limits of two-dimensional carbon. ScienceDaily. Retrieved July 28, 2014 from www.sciencedaily.com/releases/2014/04/140429085118.htm
Georgia Institute of Technology. "Graphene is only as strong as its weakest link: Experiments determine real-world limits of two-dimensional carbon." ScienceDaily. www.sciencedaily.com/releases/2014/04/140429085118.htm (accessed July 28, 2014).

Share This




More Matter & Energy News

Monday, July 28, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Europe's Highest Train Turns 80 in French Pyrenees

Europe's Highest Train Turns 80 in French Pyrenees

AFP (July 25, 2014) Europe's highest train, the little train of Artouste in the French Pyrenees, celebrates its 80th birthday. Duration: 01:05 Video provided by AFP
Powered by NewsLook.com
TSA Administrator on Politics and Flight Bans

TSA Administrator on Politics and Flight Bans

AP (July 24, 2014) TSA administrator, John Pistole's took part in the Aspen Security Forum 2014, where he answered questions on lifting of the ban on flights into Israel's Tel Aviv airport and whether politics played a role in lifting the ban. (July 24) Video provided by AP
Powered by NewsLook.com
Creative Makeovers for Ugly Cellphone Towers

Creative Makeovers for Ugly Cellphone Towers

AP (July 24, 2014) Mobile phone companies and communities across the country are going to new lengths to disguise those unsightly cellphone towers. From a church bell tower to a flagpole, even a pencil, some towers are trying to make a point. (July 24) Video provided by AP
Powered by NewsLook.com
Algonquin Power Goes Activist on Its Target Gas Natural

Algonquin Power Goes Activist on Its Target Gas Natural

TheStreet (July 23, 2014) When The Deal's Amanda Levin exclusively reported that Gas Natural had been talking to potential suitors, the Ohio company responded with a flat denial, claiming its board had not talked to anyone about a possible sale. Lo and behold, Canadian utility Algonquin Power and Utilities not only had approached the company, but it did it three times. Its last offer was for $13 per share as Gas Natural's was trading at a 60-day moving average of about $12.50 per share. Now Algonquin, which has a 4.9% stake in Gas Natural, has taken its case to shareholders, calling on them to back its proposals or, possibly, a change in the target's board. Video provided by TheStreet
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:

Breaking News:
from the past week

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile: iPhone Android Web
Follow: Facebook Twitter Google+
Subscribe: RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins