Chemical composition of Biolox delta, a zirconia-toughened alumina with small additions of chromium oxide and strontium aluminate. Credit: Pawar, IJACT

Last July’s 4th International Congress on Ceramics was the setting for multiple presentations on the use of advanced ceramics in various industries. Among the application areas covered were biology and medicine. This post is a recap of a paper on the topic from the May/June issue of AcerS’ International Journal of Applied Ceramic Technology.

According to author Vivek Pawar, a materials researcher at Smith and Nephew Inc. (Memphis, Tenn.), seven presentations at the event focused on bioceramics for orthopedic, tissue engineering, and dentistry applications, as well as on innovative manufacturing techniques and novel ceramic materials for use as bearing surfaces.

Pawar writes that the bioceramics used in hard or soft tissue replacement can be classified as bioactive glasses made mainly from calcium oxide, sodium oxide, phosphorus pentoxide, and silica; apatite-based ceramics made from synthetic hydroxyapatite and calcium phosphates; and ceramics that are used as bearing surfaces for orthopedic applications.

Since development of the first bioactive glass by Larry Hench more than 40 years ago, few alterations have been made to the materials’ composition. Pawar reports current research in the area focuses on developing compositions that maintain or increase bioactivity after crystallization during sintering. The goal is to develop low-density, easily machinable materials with fracture toughness greater than 1 MPa m^1/2.

A new material aimed at meeting those criteria is a product called ‘Biosilicate’ from Vitrovita (São Carlos, Brazil), which is reported to have antimicrobial properties. In one study assessing the effectiveness of Biosilicate against a variety of microorganisms, the material displayed activity against all the bacteria except one, drastically reducing the number of viable cells in the first 10 minutes of contact.

Hydroxyapatite (HA) coatings are commonly used in orthopedic devices to promote bone in-growth on metallic implants. Pawar writes that current research focuses on increasing the material’s bioactivity by incorporating bioactive ions in the HA crystal structure. Researchers have investigated magnesium, strontium, silver, zinc, titanium, iron, sodium, and potassium cationic substitutions. Anionic substitutions considered include fluorine, chlorine, hydroxide, phosphate, and silicate ions. These ions perturb the HA crystal structure and change solubility. Current work is aimed at understanding how each of these ions affects bioactivity. Substitutions with silver, for example, have increased HA solubility and shown bactericidal effects.

Hip arthroplasty remains the predominant use for ceramic bearing surfaces in orthopedic implants, and materials used in this application have included alumina, yttria-stabilized zirconia, and zirconia-toughened alumina. Newer ceramic materials with higher strength and toughness than alumina and reduced risk of fracture include ‘Biolox delta‘ from CeramTec (Plochingen, Germany). A zirconia-toughened alumina with small additions of chromium oxide and strontium aluminate, the material is “being considered for challenging applications such as hip resurfacing femoral heads and knee femoral components,” Pawar writes.

Another potential bearing material is silicon nitride, which offers high strength and toughness, excellent wear resistance, imaging compatibility, affinity to bone, and an antibacterial surface. Produced by Amedica (Salt Lake City, Utah), Si₃N₄ is already being used in spinal devices.

In the article Pawar notes, “Although no significant clinical problems have been reported with these two materials, a long-term clinical followup will be required to evaluate the performance of these materials.”

Innovations in ceramic processing techniques are being driven by specific biomedical applications. For example, camphene freeze casting processes are being used to create a 3-D interconnected porous bioceramic scaffold with the aim of producing a bioactive glass scaffold with high strength and bioactivity.

Also proposed is a 3-D printing process for apatite-based ceramics using stereolithography. The method is said to enable production of customized solutions based on clinical needs. A limited clinical study of the technology for repair of large craniofacial bone defects is under way at 3DCeram (Limoges, France).

Finally, nanoceramics with particle sizes of 1 to 100 nm are the focus of considerable research. The unique properties of materials produced using nanoparticles—higher surface-to-volume ratios, no light scattering, and unique mechanical properties in composite form—have led to use in dental fillings and crowns. Bone grafting, bone cement, and bioactive ceramic applications also offer research opportunities. Pawar expects future research in this area will focus on development of methods to produce customized nanoceramics based on patient needs.

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Oxide CMC exhaust ground test demonstrator consists of a 1.60-m diameter nozzle and 1.14-m diameter × 2.34-m conical centerbody with titanium end cap inspection portal. Credit: Steyer; IJACT.

Ceramic matrix composite (CMC) materials can benefit aerospace in propulsion and exhaust, thermal protection, and hot primary structure applications, according to Todd E. Steyer of The Boeing Company (Huntington Beach, Calif.).

Reviewing aerospace-related presentations from last July’s 4^th International Congress on Ceramics in a recent paper in the ACerS International Journal of Applied Ceramic Technology, Steyer outlined several emerging aerospace opportunities for CMCs, including propulsion and exhaust, thermal protection, and hot primary structure applications.

In the propulsion area, gas turbines have long been dominated by the use of nickel-based superalloys and titanium alloys. According to Steyer, engine manufacturers are now taking a closer look at CMCs for use in engine hot sections. Silicon carbide-based composites can handle temperatures to 1200°C while reducing weight and cooling requirements, resulting in reduced fuel burn and improved performance.

According to an article in MIT Technology Review, new engines being developed by CFM, a partnership between GE and France’s Snecma, feature CMC components that will reduce fuel consumption by about 15 percent—enough to save nearly $1 million per year per airplane, assuming a fuel cost of $2.50 per gallon.

CFM’s LEAP engine uses SiC-reinforced CMC parts that don’t require cooling, enabling air that would normally be diverted to keep superalloy components from melting to be used to generate thrust. It also uses parts produced using a 3-D printing process, according to the MIT article.

The company already has orders for 4,500 of the new engines. In addition to saving money, the engines will help users comply with current and anticipated emissions regulations.

In engine exhaust systems, work is underway to produce an alumina-fiber reinforced aluminosilicate matrix composite centerbody and exhaust nozzle for commercial aircraft. Currently in ground testing, the ceramic nozzle will reduce weight and engine noise and increase component lifetime, Steyer wrote.

Ceramic materials have long been used in aerospace thermal protection applications—for 30 years, ceramic tiles with glass-based coatings provided thermal protection for the US’s now-retired space shuttle fleet. Initially composed of silica fibers with a nominal density of 0.14 g/cm³ and a glaze aimed at controlling emissivity and limiting catalysis for oxygen and nitrogen recombination from the plasma on reentry, the tiles provided effective insulation but required heavy maintenance between flights. Engineers improved durability over the shuttle’s service life using new tile substrates and coatings.

For new thermal protection applications, Steyer reported on CMCs developed and tested by NASA researchers for use at temperatures to 1700°C. Toughened Uni-piece Fibrous Reinforced Oxidation-Resistant Composite (TUFROC) materials build on the success of insulating fibrous tiles with high-emissivity/low-recombination-efficiency coatings using a refractory ceramic carbon-insulated layer for dimensional stability.

Supersonic and hypersonic flight vehicles present unique challenges for primary hot structural materials, and ultrahigh-temperature ceramics (UHTCs) have been emerging as a promising class of materials for leading edges for hypersonic vehicles. The refractory nature of this class of carbides, borides, and nitrides makes them good candidates for the highest heat flux areas as well as areas with high integrated heat load as a function of time, Steyer wrote.

Particulate, whisker, and chopped or continuous fiber reinforcements are resulting in improved mechanical properties, but the materials’ relatively high density and difficulty in large-scale processing are potential drawbacks. Steyer reported one recent example in which CMCs consisting of 0.5- to 1-mm long chopped Hi-Nicalon SiC fibers in a ZrB₂ matrix hot-pressed at 1700°C showed significantly improved chevron-beam fracture toughness at compositions containing up to 20 vol.% fiber.

Increased use of CMCs in aerospace will require microstructure optimization, a path to entry into service, and improved affordability. Steyer believes fundamental and applied research in damage accumulation mechanisms/models, life prediction methodologies and modeling, nondestructive inspection techniques, and robust field and depot-level repair methods will result in more CMCs in aerospace applications.

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Graphene-coated ribbons of vanadium oxide, seen in a scanning electron microscope image, show promise as electrode for lithium-ion batteries, according to researchers at Rice University. (Credit: Ajayan Group/Rice University)

Last week I told you about some fascinating work coming out of the University of Stuttgart on extremely flexible vanadium pentoxide paper like material. Researchers are interested in the material for battery electrode and supercapacitor applications.

Researchers at Rice University also are looking at vanadium oxide for lithium battery electrode applications and recently reported on VO₂-graphene hybrid ribbons for cathodes in an article in Nano Letters published by the American Chemical Society (subscription required).

The work comes out of Pulickel Ajayan’s group. Ajayan is a professor in the Mechanical Engineering and Materials Science Department and in the Chemistry Department and is known for his creative thinking about batteries. Last summer, for example, we told you about his work on paintable batteries.

According to the paper’s abstract, although lithium-ion batteries have high energy density, their full potential in applications is not yet realized because “they lack suitable electrodes capable of rapid charging and discharging to enable a high power density critical for broad applications.” In a press release, Ajayan says that vanadium oxide has long interested the battery research community and that vanadium pentoxide has been used in some Li-ion batteries. However, he points out that oxides generally charge and discharge slowly because their electrical conductivities are low.

Ajayan’s group addressed the slow charge-discharge problem by “baking” high-conductivity graphene on VO₂ ribbons. The graphene forms a web like coating on the ribbons and serves as a “speedy conduit for electrons and channels for ions.”

The team reports promising results. Half-cell tests show that the cathodes fully charge and discharge in 20 seconds and retain 90 percent of their initial charge capacity even after 1,000 cycles. The team says their best cathode samples were up to 84 weight percent “lithium-slurping” VO₂ and held 204 milliamp hours of energy per gram. They also appear to be highly stable. The press release reports the “capacity for lithium storage remained stable after 200 cycles,” even at high temperatures regimes above 75°C, where the effectiveness of other cathode materials tends to attenuate. (I’m not sure how or whether the insulator-to-metal phase transition that VO₂ undergoes at 67°C is a factor. I’ll update this post when I find out.)

The ribbons are made in a simple-sounding hydrothermal process, but in the press release Subin Yang, lead author of the paper, admits, “One challenge to production was controlling the conditions for the co-synthesis of VO₂ ribbons with graphene.” They make the hybrid ribbons by heating a water suspension of graphene oxide nanosheets and vanadium pentoxide powders for hours in an autoclave. The V₂O₅ reduces completely to VO₂ and crystallizes into ribbon like structures that are 10 nanometers thick, up to 600 nanometers wide, and tens of micrometers long. Meanwhile, the graphene oxide reduces into graphene and forms a web like coating on the ribbons.

Ajayan thinks this hybrid material could be used in the paintable batteries his team is working on, too.

Full details are in the paper, “Bottom-up approach toward single-crystalline VO₂-graphene ribbons as cathodes for ultrafast lithium storage,” Shubin Yang, Yongji Gong, Zheng Liu, Liang Zhan, Daniel P. Hashim, Lulu Ma, Robert Vajtai, and Pulickel M. Ajayan, Nano Letters, DOI: 10.1021/nl400001u.

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Last fall I wrote about a small, highly focused conference in Germany I attended called, “Generation of Inorganic Functional Materials Implementation of Biomineralization Principles.” The idea behind biomineralization is to adapt natural processes to synthesize new materials and engineer them into new configurations or engineer new functionalities.

A common point of reference is nacre—also called mother of pearl. The organic-inorganic composite secreted by mollusks has a layered structure comprised of platelets of the aragonite form of calcium carbonate held together with mortar-like organic substance, such as chitin or various proteins. The composite structure of brittle platelets bound together by an elastic biopolymer make nacre an exceptionally strong natural material. The aragonite platelets have a characteristic high-aspect ratio morphology that helps make the layered structure possible.

Paper, too, is made of high-aspect ratio constituents, usually fibers. Examples exist that are based on a platelet-biopolymer type of structure (vermiculate, clay, alumina, and some research on carbon nanotube and graphene oxide papers), but few display the characteristics that make paper so useful, especially flexibility.

This appears no longer to be the case. A newly published article reports some fascinating work on “paper” made of vanadium pentoxide nanofibers and its remarkable properties. The article is “Hydrogen-bond reinforced vanadia nanofiber paper of high stiffness,” by Zaklina Burghard, et al. (I met Burghard, postdoctoral researcher of materials science at the University of Stuttgart, Germany, at the poster session of the aforementioned conference.)

According to the paper, vanadium pentoxide (V₂O₅) differs from other transition metal oxides in that it can be synthesized into crystalline nanofibers with extremely high aspect ratios. The group reports making fibers with diameters 1-10 nanometers and lengths ranging from 100 nanometers to tens of micrometers.

The ribbon like fibers are made by a polycondensation process in an aqueous solution and are composed of two V₂O₅ layers with a layer of water in between. They have a rectangular cross section with oxygen groups and water bonded to the surface, both of which contribute to the bonding between fibers, similar to the role of biopolymer in nacre. The paper is made by slow drying vanadia nanofiber sols, which are then floated off the substrate.

The resulting paper is a dark orange color and can be made with a high degree of fiber alignment. The paper has extraordinary flexibility and can be bent or rolled as shown in the image. Burghard reports rolling cylinders with diameters as tight as 1 millimeter.

Because of the hydrogen bonds between the fibers, the team investigated the sensitivity of the papers to water content. They found that drying the paper at 40˚C was an important first step, which was followed by an annealing heat treatment at 100˚C or 150˚C. Without the drying step, the papers cracked. The annealing removes the weakly adsorbed water between the nanofibers, and by removing it slowly, the fibers are mobile enough to pack tightly, “most likely through the structure-directly capability of the hydrogen bonded between the V₂O₅ fibers.”

The importance of optimized thermal processing is evident in the tensile strength of the paper. The tensile strength of as-prepared paper is about 76 MPa. After drying, it is about 132 MPa and, after annealing, it pushes toward 200 MPa. The team suggests that the slow drying interlocks the basal planes of the fibers and increases the hydrogen-bond density between -OH groups on the surface. The article explains, “The excellent mechanical performance… can be attributed to the alternating layer structure of the vanadia paper, comprising a stiff inorganic oxide component combined with “flexible” layers of water in between, strongly resembling the brick-and-mortar architecture of structure biomaterials like nacre.”

The ordered structure is also reflected in the electrical conductivity. Conductivity increases after drying, and decreases slightly after annealing with the removal of ionic contributions to the overall conductivity. The results also correlated in expected ways with fiber alignment and in-plane-and out-of-plane directions.

Because of the dramatic flexibility of the paper and its promising properties, especially mechanical, the vanadia paper could be used in a range of applications relating to energy and electronics. The authors suggest the a wide range of potential applications including stretchable electronics, energy storage, flexible electrodes in chemical sensors, actuators, electrochromic devices, batteries, and supercapacitors. They note that the toughness of the paper could reduce the crack formation from swelling that tends to occur during ion intercalation of electrodes, for example.

The article is “Hydrogen-bond reinforced vanadia nanofiber paper of high stiffness,” by Zaklina Burghard, et al., Advanced Materials, doi: 10.1002/adma.201300135.

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Last week a National Science Foundation-sponsored workshop addressed multidisciplinary approaches to the Materials Genome Initiative. From left: Gregory Rohrer, Abby Kavner, Young-Shin Jun, and Amy Walker. Credit: ACerS.

Genome (noun; origin 1930s: blend of gene and chromosome): the complete set of genes or genetic material present in a cell or organism.

The Materials Genome Initiative has gotten lots of attention since it was announced by the White House Office of Science and Technology Policy (OSTP) in June 2011. Its stated goal (pdf) is simple: “… to discover, develop, manufacture, and deploy advanced materials at least twice as fast as possible today, at a fraction of the cost.”

In my opinion, “genome” is an unfortunate choice of word. Why? A genome is a constrained set. Perhaps a very large set, but the genome is limited to the genes that define the organism. In the human organism, that is about 23,000 genes. The Human Genome Project (HGP) sequenced and mapped the 23,000 genes in the human genome, which ended up involving about 3.3 billion base pairs.

The HGP narrowed the description of what we are made of by painting in the fine-strokes details. It was never about expanding the diversity of our basis or of life—no new organisms were “invented,” no additions to the genome were desired (for example, adding feathers to the anatomy), nor were any such innovations intended.

The idea of the MGI, in contrast, is to start with basic building blocks of matter—the fine structure—and discover new combinations of elements, i.e., materials, with end properties and functionalities in mind right from the start. The MGI is all about expanding the possibilities beyond that which is already mapped. And, with 116 elements comprising the periodic table of the elements, the possible combinations make 3.3 billion look paltry. (According to my “go to” mathematician, the number of ways you can combine up to just six elements or less of the 116 known elements exceeds three billion.)

Chances are you have seen the OSTP’s schematic of the “materials development continuum.” The beginning point is discovery of new materials, and this is where science researchers must take the lead in finding approaches to discovering the few hundred or thousand materials that are worth developing out of the many billion possibilities.

Last week, to address this issue, a group of researchers who work on ceramic materials gathered for a National Science Foundation-sponsored workshop, “The Materials Genome Initiative in Ceramics, Geosciences, and Solid-State Chemistry.” The workshop grew out of two previous “future directions” workshops: a 2011 workshop on “Materials by Design” at the University of California, Santa Barbara and the 2012 workshop on “Emerging Research Areas in Ceramic Science” in Arlington, Va.

About 20 researchers, mostly from academia, participated. They were an interesting mix of well-established scientists and new young-bloods, all working with ceramic materials, whether as materials scientists or as researchers from the geoscience, earth science, and solid-state chemistry and physics communities.

Alexandra Navrotsky, the brilliant and witty professor from the University of California, Davis, set the stage with her opening presentation by observing that there are striking commonalities between earth science and materials science. “The Materials Genome Initiative,” she said, “means different things to different people. It is up to the communities to define.” In her assessment, MGI means starting from first principles and CalPhaD-type calculations, mining data from databases, and designing new, smarter experiments. “Putting these together in a holistic fashion is the MGI,” she says.

Much common ground exists between the geosciences and materials science—both need structural, thermodynamic, and physical property data. Phase diagrams are very important in both fields. Both fields need kinetic and mechanistic information for modeling of impossible-to-observe phenomena, like the geoscience of Jupiter, for example.

Narrowing the idea somewhat, Krishna Rajan from Iowa State University, said, “MGI is about doing new science to solve pressing issues,” implying that new science should be driving solutions to new and urgent issues, not to incrementally improving existing technology. Incremental improvements may not be compelling to the business world, anyhow. For example, why would a company that specializes in refurbishing thermal barrier coatings be interested in a coating that lasts twice as long? To them, that looks like half as much work (read revenue). Similarly, a materials improvement that would require expensive retooling of a fabrication line may not be worth the trade-off.

The materials science and geoscience community share some common frustrations, for example, with the difficulty of modeling across multiple scales, especially length scales. This is true whether modeling the transition from the nanoscale to the mesoscale, or from meters to kilometers. Time scales matter, too. Modeling of processes that occur in picoseconds is challenging, but so is modeling of processes that occur in light years.

Given the challenges, what can modeling offer? Ram Seshadri from the University of California, Santa Barbara (and coorganizer of the workshop with Carnegie Mellon’s Gregory Rohrer) says, “looking at large data sets tells you where you will be wasting your time.” And the role of computation, according to Michelle Johannes of the Naval Research Laboratory, is to give experimentalists a “rough directional map, an explanation of trends, and suggestions for optimization of properties.” In return, the materials mathematicians need property data and characterization information (such as crystal structures) from the experimentalists.

All in attendance seemed to agree that access to data (that they liked) was a challenge with no easy (or cheap) solutions. There also seemed to be consensus that multidisciplinary dialog is extremely valuable, and this is an area where technical societies such as ACerS or the American Geophysical Union can help, perhaps by organizing multidisciplinary symposia, special issues of journals, or workshop-like meetings. (Last October, ACerS, took a step in this direction by setting up a structure for “Technical Interest Groups.” The idea of the TIGS is to provide a way for Society members and non-members to come together on a topic of mutual interest that does not really have a “home” in any particular society.)

Much of the discussion on data focused on the need for single crystal property data. The single crystal data gets you as close as possible to values for intrinsic properties. However, with a few exceptions, most engineered materials are not single crystals. Does that mean “junky” data is without value? Hardly! As one participant noted, much of the functionality of engineered materials we use today arises from their “junkiness,” whether from impure compositions or process effects or elsewhere.

There was much more, of course. The group will publish a report on the workshop in the ACerS Bulletin, which will capture in detail the issues, challenges, opportunities—and payoffs—of multidisciplinary approaches to materials discovery.

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