RESEARCH/RESEARCHERS - Massachusetts Institute of Technology

RESEARCH/RESEARCHERS

MRS BULLETIN • VOLUME 31 • MARCH 2006 167

From Mirror to Mist: Cracking the Secret of Fracture Instabilities

When cracks in a material propagate, bonds between atoms arebroken, generating two new surfaces. Experiments have shownthat cracks moving at low speeds create atomically flat, mirror-likesurfaces, whereas cracks moving at higher speeds create increas-ingly rough fracture surfaces. Dynamical instability leads toincreasing roughening transitioning from a mirror-like surface to aless reflective (mist) surface to a very rough irregularly faceted(hackle) surface. Using massively parallel large-scale atomisticsimulations, Markus J. Buehler from the Massachusetts Institute ofTechnology (MIT) in Cambridge, Mass., and Huajian Gao from theMax Planck Institute for Metals Research in Stuttgart, Germany,(now at Brown University) have developed a new theoreticalmodel to understand atomistic details of how cracks propagate inbrittle materials, revealing the physics of dynamical fractureinstabilities. As reported in the January 19 issue of Nature (p. 307;DOI: 10.1038/nature04408), their models indicate, in contrast tocurrent theories, that it is critical to consider the properties ofmaterials at large deformations, close to the crack tip, in order tounderstand how materials fracture. These findings have majorimplications for the understanding of fracture at different scales,ranging from the nanoscale to the larger scales of airplanes,buildings, or even earthquake dynamics, and predict that crackscan move at speeds faster than the velocity of sound, which hasso far been considered an impenetrable barrier for crack-propagation speed.

The deformation and fracture of materials has fascinated scien-tists for decades, due to both their scientific relevance as well astheir significance in engineering and to society. In the past, theclassical physics of the continuum has been the basis for mosttheoretical and computational tools in engineering analyses ofthese phenomena, and theories relying on numerous phenome-nological assumptions have been used. Scientists are now begin-ning to use atomistic simulation as a tool to study the behavior ofmaterials under extreme conditions in order to gain insightsabout the fundamental mechanisms of deformation and failure atlength and time scales unattainable by experimental measure-ments and which cannot be predicted using continuum theories.

This phenomenon of greater roughness for faster crack propa-gation is found in many different classes of brittle materials,including metals, polymers, and semiconductors, at a variety ofscales. Until now, no sound understanding of the underlyingphysics or of the particular crack speed at which the instabilityoccurs has been achieved. None of the existing theories explain

Figure 1. Atomistic simulation shows the dynamical sequence asfracture instability occurs. At a critical crack speed, straight crackmotion becomes unstable and the crack starts to wiggle, creatingincreasingly rough surfaces. Courtesy of Markus J. Buehler, MIT.

For more information, see http://www.mrs.org/bulletin_ads

www.mrs.org/bulletin


168 MRS BULLETIN • VOLUME 31 • MARCH 2006

the wide range of experimental and com-putational results, and many models con-tradict each other.

By means of large-scale atomistic simu-lations, Buehler and Gao show that hyper-elasticity, the elasticity at large strains, canplay a governing role in dynamical cracktip instabilities in the fracture of brittlematerials. They report a modified instabil-ity model that treats the dynamical frac-ture instability as a competition betweendifferent mechanisms controlled by localenergy flow and local stress field or atom-ic forces near the crack tip. Their resultssuggest that the fracture instability notonly appears in materials with defects; it isan intrinsic phenomenon of dynamicalfracture. See Figure 1 (p. 167).

“Our new theory reduces to existingmodels in limiting cases, but allows for aunified treatment of the instability prob-lem applicable to a much wider range ofmaterials,” said Buehler. “We have dis-covered that the key to understandingthe discrepancies in the literature is toconsider the material behavior close tothe breaking of bonds, rather than thematerial properties at small strains.”

Most existing theories of fractureassume a linear elastic stress–strain lawby only considering small strain deforma-tion. However, the relation between stressand strain in real solids is strongly nonlin-ear due to large deformation near a mov-ing crack tip, a phenomenon referred to ashyperelasticity or nonlinear elasticity.

The scientists have made another sur-prising discovery. “We find that elasticstiffening materials behavior, as found inrubber-like materials, can dramaticallychange the instability dynamics ofcracks,” Buehler said. Rubber is soft atsmall deformations, and becomes harderas the stretch is increased. “In such elasti-cally stiffening materials, stable intersoniccrack motion is possible,” he said. Theseresults are in contrast to existing theories,in which the speed of elastic waves isconsidered the limiting speed of fracture,analogous to the speed of light.

“We discovered that the classical theo-ries of instability dynamics are only validin a small range of material behavior,”said Buehler. “In most real materials, thesoftening or stiffening close to bondbreaking leads to a fundamental change

in the instability dynamics, because ener-gy flow is reduced or enhanced due tochange in local wave speed.”

These results represent a breakthroughin understanding how cracks propagatein brittle materials, and the findingscould have wide impact in many scientif-ic and engineering disciplines, said theresearchers.

Scanning Ultrasound Holography Allows NanoscaleSubsurface Imaging

The characterization of deeply buriedor embedded structures and featureswith lateral resolution of under 100 nm iscritical in a number of science and engi-neering arenas. Subsurface features canbe imaged with techniques such asacoustic microscopy, but with limitedspatial resolution. More recently, scan-ning probe techniques, such as ultrasonicforce microscopy, have been used fornanomechanical mapping of elastic andviscoelastic properties of surfaces.However, these techniques still lack thesensitivity required for imaging buriedstructures. Now, Gajendra Shekhawat


Legendary Heritage In High Performance

Low Mass Materials

www.zircarceramics.com [email protected]

ZIRCAR Ceramics, Inc.100 N. Main St.

Florida, New York 10921 USATel: (1) 845 651 6600Fax: (1) 845 651 0441

Experienced people who care about your productivity bring you a world class range of ceramic fi ber based

thermal and electrical insulation solutions.

30 years of good science - an extensive selection of casthouse consumables help nonferrous metals producers make better

metal - more profi tably.

NON RCF,Al2O3

Al2O3- SiO2CaSiO3

Compositions

Rigid BoardsPrecision- Machined Shapes

BlanketsPapersCement

Temperatures 6000C to 19000C



and Vinayak Dravid from NorthwesternUniversity have reported the develop-ment of scanning near-field ultrasoundholography (SNFUH) that allows charac-terization of depth information as well asspatial resolution in the 10–100-nm range.The method can be used to image buriednanostructures in diverse materials sys-tems including microelectronic structuresand biological samples. They report thetechnique and results in the October 7,2005, issue of Science (p. 89; DOI:10.1126/science.1117694).

SNFUH is based on scanning probemicroscopy (SPM). A high-frequencyacoustic wave, on the order of a megahertzor more, is applied to the bottom of aspecimen, while a second wave islaunched on the SPM cantilever at aslightly different frequency. The interfer-ence of these two waves forms a surfaceacoustic standing wave. Buried featuresperturb the specimen acoustic wave, andthese perturbations to the phase andamplitude of the surface acoustic stand-ing wave are monitored by the SPM can-tilever. A pictorial representation of theacoustic wave perturbation is recorded

that includes quantitative informationabout the internal features. The re-searchers used a model polymer–goldnanoparticle composite to demonstrate

the spatial resolution and depth sensitivi-ty of the technique. The nanoparticleswith an average diameter of 15 nm wereburied beneath a 500-nm-thick polymer

a b

Figure 1. (a) Typical atomic force microscope topography image shows a low-dielectricpolymer coating covering trenches in a model microelectronics trench structure. (b) Thecorresponding scanning near-field ultrasound holography phase image reveals embeddedvoiding in the polymer coating over the nitride. Reprinted with permission from Science(Oct. 7, 2005) p. 89. ©2005 AAAS.



172 MRS BULLETIN • VOLUME 31 • MARCH 2006

cover layer on a silicon substrate. Whilenormal atomic force microscopy (AFM)showed a smooth, featureless surface ofthe top polymeric layer, the SNFUHphase image clearly showed the dis-persed, buried gold nanoparticles.

SNFUH is particularly well suited fordetecting defects and voids in microelec-tronics structures. For example, the scale offabrication continues to shrink and inter-connect metal lines are approaching 60-nmwidths. Model shallow trench structureswere fabricated in a dielectric materialwith a 50-nm-thick silicon nitride layerdeposited as a capping layer and etchedinto the trenches. A 500-nm-thick low-dielectric polymer layer was deposited byspin-coating and annealing. A convention-al topography scan showed a uniform andcontinuous polymer coating at the bottomsof the trenches as well as on the tops of thelines, as seen in Figure 1 (p. 169). However,the corresponding SNFUH phase imagerevealed embedded voids within the poly-mer and at the SiN–polymer interfaces.

The SNFUH technique can also beused to image embedded or buried sub-

structures in biological samples. A near-contact mode was used for imaging softstructures, wherein the probe tip is liftedby 2–5 nm after it touches the surface.This was used to obtain high-contrast,high-resolution images of malaria para-sites inside infected red blood cells. Thedirect real-space in vitro imaging was per-formed without labeling or sectioning thecells under physiologically viable condi-tions. SNFUH was thus demonstrated tobe a versatile technique for nondestruc-tive, high-resolution, real-space imagingof diverse materials systems. In particu-lar, it fills the spatial resolution gap at the10–100-nm scale for nondestructive sub-surface imaging.

GOPAL RAO

Silica Nanoparticles Doped withMultiple Fluorescent DyesDemonstrated as Potential NewBarcoding Tags

Often in biological imaging experiments,it is desirable to view several componentsof a structure simultaneously in real time.This can be done with small-molecule fluo-

rescent tags, but this technique is limitedby the number of available dyes that areexcited at the same wavelength but havedistinguishable emission spectra. Quantumdots, polymer microspheres, and othermaterials have been used to overcome thislimitation and create “bio-barcoding” sys-tems, but many of these have problemswith performance. In the January 11 issueof Nano Letters (p. 84; DOI: 10.1021/nl052105b), researchers L. Wang andW. Tan from the University of Florida atGainesville have presented a novel solu-tion to this problem. The researchersincorporated combinations of three com-mon fluorophores into silica nanoparticles(NPs) to create NPs that respond tomonochromatic illumination with a rangeof distinguishable emission signatures.They also demonstrated the potential forusing these materials as tags for biologicalmolecules by functionalizing the NPswith biotin and binding them to avidin-functionalized microspheres.

The researchers incorporated the organicdyes FITC, R6G, and ROX into silica NPsin a variety of ratios. Alone, these dyes

For more information, see http://www.mrs.org/bulletin_ads For more information, see http://www.mrs.org/bulletin_ads

www.mrs.org



have different excitation maxima, butwhen incorporated together they form atandem system for fluorescent resonantenergy transfer (FRET) so that when excit-ed at the optimal wavelength for FITC, thedye-impregnated silica NPs emit uniquecolors based on their dye ratios. Theresearchers synthesized the NPs by a mod-ified Stöber synthesis route, in which thedyes were incorporated into the structureas the NPs grew. The NPs thus preparedwere found to be uniform in size, colloidal-ly stable, and strongly fluorescent.

To demonstrate the utility of the mate-rial, the researchers derivatized amine-modified NPs with biotin and attachedthem to avidin-derivatized microspheres.Under confocal fluorescent microscopy,these microsphere–NP complexes emittedlight at the characteristic wavelength of theparticular NP they contained. The re-searchers said that this approach can begeneralized to label the NPs with otherbiopolymers for use in barcoding assaysand multiplex bioanalysis. According to theresearchers, the NPs can also be used asoptical materials for display technologies.

KRISTA L. NIECE

Highly Ordered TiO2 Nanotube ArrayImproves Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) arepotentially a cheap and environmentallyfriendly alternative to silicon-based photo-voltaics. Based on an effect analogous tophotosynthesis, DSCs have achieved light-to-electricity conversion efficiencies ofmore than 11%. In the February 8 issue ofNano Letters (p. 215; DOI: 10.1021/nl052099j), G.K. Mor, C.A. Grimes, andcolleagues at the Pennsylvania StateUniversity have reported the fabrication ofa DSC based on an ordered array of TiO2nanotubes. Although the photocurrentefficiency of their device is only about 3%,their results suggest that nanotube-basedDSCs may achieve conversion efficiencies10 times as high, significantly exceedingsilicon-based solar cells.

Standard DSCs are typically based on a10-μm-thick film of randomly arrangedTiO2 nanoparticles. The nanoparticles arecoated with a molecular layer of an organicdye (often extracted from blackberries,raspberries, or pomegranates) that releaseselectrons when illuminated with sunlight.The large surface area of the dye-coatednanoparticles enhances the efficiency of thelight collection, and the released electronsbecome the device’s photocurrent. How-ever, the mobility of the released electronsis limited by scattering at the disorderedboundaries between nanoparticles, whichreduces the overall efficiency. To addressthis problem, the Penn State team grew

ordered arrays of TiO2 nanotubes 360 nmin length on glass substrates by anodizinga 500-nm-thick titanium film, and thenimmersed the arrays in a ruthenium-baseddye. When exposed to full sunlight, theresulting devices generated photocurrentwith a relatively low efficiency of 2.9%.However, the lifetime of liberated electronswas significantly longer than in nanoparticledevices, implying much less electron scat-tering by the ordered nanotube array.

The team concluded that the conver-sion efficiency was limited by the rela-tively short length of the nanotubes, andthat by improving the fabrication proce-dure to grow micrometer-scale nano-tubes, the efficiency could be boosted toclose to the ideal limit of 31%. If thisproves to be the case, high-efficiency,dye-sensitized solar cells could becomean important competitor to standard sili-con solar cells, both in terms of lower



MRS BULLETIN • VOLUME 31 • MARCH 2006

production costs and reduced environ-mental pollution during fabrication.

COLIN MCCORMICK

Researchers Examine Growth and Optical Properties of GoldNanoparticles in Stained Glass

Stained glass is best known from itsextensive use in Gothic churches, although

its production dates back to ancient civi-lizations of Egypt and Rome. A colored“stain” is achieved when colorless metal-doped glass is heated at high tempera-tures. Annealing of gold-doped glassresults in the formation of colloidal gold,which gives rise to a characteristic ruby-red color through surface plasmon reso-nance (SPR). However, tuning the SPR

Octahedral Nanocontainer Molecules Formed SpontaneouslyApplications for nanocontainer molecules include stabilizing short-lived chemi-

cal species, accelerating chemical reactions, and directing regioselectivity andstereochemistry of reaction products. Supramolecular approaches to the self-assembly of nanocontainers, which involve hydrogen bonding or metal coordina-tion chemistry, are efficient and quantitative. In contrast, nanocontainers con-structed from cavitands (i.e., molecules whose constrained structure accommo-dates a cavity) typically require multiple steps. Recently, however, researchersfrom the Department of Chemistry and Chemical Biology at Rutgers Universityhave used dynamic covalent chemistry to synthesize nanocontainers from 18 com-ponents in a single step and in high yield, greatly improving the simplicity andefficiency of nanocontainer synthesis.

As reported in Angewandte Chemie International Edition (DOI: 10.1002/anie.200504049), Rutgers University researcher R. Warmuth and co-researchers dis-covered a spontaneous formation of an octahedral nanocontainer (B), composed ofsix cavitands (A) linked together with 12 diamino bridges via 24 newly formed iminebonds (see Figure 1). Yields of up to 82% were achieved. The researchers used 1H and13C nuclear magnetic resonance (NMR) spectroscopy and electrospray ionizationmass spectroscopy to validate the structure of B. Although the researchers wereunable to isolate and purify B, they were able to reduce all 24 imine bonds and purifythe trifluoroacetate salt of the resulting polyamino nanocontainer (C) with reverse-phase high-pressure liquid chromatography and isolate it in an overall 63% yield.Although crystals suitable for x-ray structure determination could not be obtained,the researchers used molecular mechanics (MM) calculations to estimate the cavityvolume at 1700 Å3, which is large enough to encapsulate multiple guest molecules.Pulse-field gradient spin-echo NMR measurements and application of theStokes–Einstein equation yielded a solvodynamic diameter of 3.2 nm, which is con-sistent with the MM model. The researchers also used MM to show that the forma-tion of B requires each ethylenediamine to be in an anti conformation.

The researchers said, “We see potential uses for [C] and analogues in drug- orpesticide-delivery systems, wastewater detoxification, separation technology, andas molecular reactors for controlled oligomerizations of organic and inorganic mol-ecules.” In addition, the researchers said that “other covalent nanoassemblies withspherical or tubular shapes and different properties could be accessible throughsuch multicomponent synthesis.”

STEVEN TROHALAKI

Figure 1. An octahedral nanocontainer (B) composed of six cavitands (A) linkedtogether with 12 diamino bridges via 24 newly formed imine bonds; B is reduced topolyamino nanocontainer (C).




peak in stained glass requires very longannealing times that also result in a deteri-orated shape of the gold clusters. In theDecember 9, 2005, issue of AngewandteChemie International Edition (p. 7905; DOI:10.1002/anie.200502174), M. Eichelbaumat Humboldt-Universität zu Berlin,R. Müller of the German Federal Institutefor Materials Research and Testing, andtheir colleagues have shown that activat-ing the gold-doped glass with hard x-rayradiation reduces the annealing timeneeded for tuning the SPR peak, therebyminimizing shape degradation of thegold clusters.

Gold-doped glass was obtained bymelting 0.02 mol% AuCl3 with soda-limesilicate glass. Annealing at 590°C for 1 hproduced ruby-colored glass with theSPR peak at 574 nm. Another 4 h ofannealing was required to observe a shift

of 6 nm (to 580 nm) in the SPR peak, andhigh-resolution transmission electronmicroscopy images confirmed that thiscaused distinct deviation of particle shapefrom spherical symmetry. In contrast,when the gold-doped glass was activatedby x-ray radiation (32 keV) for 5 min,only 30 min of annealing produced a red-colored glass with a SPR peak at 540 nm.Also, substantial changes in SPR peakpositions were observed for short anneal-ing periods: the SPR peak shifted to 547 nmafter 10 min of annealing at 590°C, and to554 nm after 15 min of annealing at 630°C.Although the shifts in SPR peaks wereexplained by the deviation of the gold clus-ters from their spherical shape, samplesactivated by x-rays still exhibited a sharpsize distribution of gold clusters because ofthe shorter annealing period. Short anneal-ing periods are due to the presence of a

very high density of gold atoms that areproduced when cationic gold is reducedby x-ray radiation. X-ray near-edge spec-troscopy confirmed that the x-ray-activatedsamples have very small quantities of Au3+

ions and mostly solid gold. The researcherssaid that x-ray–activated stained glassesare sturdy components for realizing opti-cally tunable devices in integratednanophotonic applications.

TUSHAR PRASAD

Willard S. Boyle and George E. Smith,both formerly of Bell Laboratories, sharethe Charles Stark Draper Prize—a$500,000 annual award that honors engi-neers whose accomplishments have sig-nificantly benefited society—“for theinvention of the charge-coupled device(CCD), a light-sensitive component at theheart of digital cameras and other widelyused imaging technologies.”

Douglas B. Chrisey, formerly of theU.S. Naval Research Laboratory, hasbeen named the Deputy Director forResearch and Development at the Centerfor Nanoscale Science & Engineering atNorth Dakota State University.

Joseph A. Gardella Jr., State Universityof New York at Buffalo, received the 2005Presidential Award for Excellence in Science,Mathematics, and Engineering Mentoring.

This presidential award recognizes indi-viduals who have demonstrated a com-mitment to mentoring students andincreasing the participation of minorities,women, and disabled students in science,mathematics, and engineering.

Ashok Puri, University of New Orleans,received the 2005 Presidential Award forExcellence in Science, Mathematics, andEngineering Mentoring. This presidentialaward recognizes individuals who havedemonstrated a commitment to mentor-ing students and increasing the participa-tion of minorities, women, and disabledstudents in science, mathematics andengineering.

David H. Sliney, program manager forthe U.S. Army Center for Health Pro-motion and Preventive Medicine Laser/Optical Radiation Program, received the

24th Arthur L. Schawlow Award fromthe Laser Institute of America (LIA) dur-ing ICALEO 2005 (the International Con-gress on Applications of Lasers & Electro-Optics), which was held last October.The award is in recognition and apprecia-tion of Sliney’s extensive contributions asa pioneering leader in the field of lasersafety for more than 30 years and as oneof LIA’s most valuable supporters.

Daniel Zajfman has been nominated asthe next president of the WeizmannInstitute of Science in Rehovot, Israel. Inaccordance with the rules of the Institute,the election of the president will beapproved by the full board. Zajfman willserve as the 10th president of theWeizmann Institute, replacing the Insti-tute’s current president, Ilan Chet, whoseterm of office ends in December 2006.

News of MRS Members/Materials Researchers

Scientific American named its top con-tributors to science and technology in2005, including:

Edward H. Sargent (University ofToronto), Michael Grätzel (SwissFederal Institute of Technology), andTsutomu Miyasaka and Takurou N.Murakami (Toin University ofYokohama) for their research in energyand to Hydrogen Solar for its business inenergy;

Inez Y. Fung (University of California,Berkeley) for research in protection fromthe Earth’s climate;

Paul W.M. Blom and Ronald C.G.Naber (University of Groningen and

Philips Research Eindhoven), JohnRogers (University of Illinois at Urbana-Champaign), and Samuel I. Stupp(Northwestern University) for researchon plastic; and

Bradley J. Nelson (Swiss FederalInstitute of Technology) and the U.K.Royal Society and the Royal Academyof Engineering for research in nano-tubes, and James E. Jaskie (MotorolaPhysical Sciences Research Laboratory,DuPont Central Research and Develop-ment, and Hewlett-Packard Labora-tories for their business contributions inthe area of nanotubes.

Corrections

In the December 2005 issue of MRSBulletin 30 (12) p. 965, the reference imme-diately following 105 was incorrectlynumbered as 109; it should be 106. In thesame issue, pp. 964–966, in References 17,23–25, 30, 44, 52, 71, 73, 82, 104, 106 (cor-rected number), 108, 109, 111, and 112, thename T.P. Russell should have appearedinstead of P.F. Green.

The Center for Nanoscale Materials atArgonne National Laboratory will begininitial operations in April 2006 and fulloperations in September 2007. Incorrectdates were published in the UP CLOSEarticle in the January 2006 issue of MRSBulletin 31 (1) p. 44.

For more research news on MaterialsScience, access the Materials Research Society

Web site:

www.mrs.org/connect ions

For more research news on Materials Science, access the

Materials Research Society Web site: www.mrs.org/connect ions

www.mrs.org/bulletin

RESEARCH/RESEARCHERS - Massachusetts Institute of Technology

Documents

Transcript of RESEARCH/RESEARCHERS - Massachusetts Institute of Technology