TECH BEAT Dr. Neil Canter / Contributing Editor Vulnerability of titanium to oxygen New research reveals why oxygen causes titanium’s physical properties to decline. WITH THE CONTINUING TREND TO FIND APPROACHES TO INCREASE THE EFFICIENCY OF MACHINERY, lightweight metals are clearly being looked at more closely as a substitute for ferrous alloys. Metals such as titanium offer a superior strength-to-weight ratio and better resistance to corrosion than steel. But titanium is also more expensive. In a previous TLT article, a new approach was made to reduce the cost of titanium through a potentially lowercost manufacturing pathway using titanium hydride.1 The existing method for extracting titanium is known as the Kroll process, which is conducted through a series of steps at elevated KEY CO CONCEPTS O C S • Very low concentrations concentratio nc trat s of oxygen in titanium can cause the thee metal to harden haarden and become more brittle. • This effect occurs because one dislocation in the titanium inte interacts t acts t with th an an oxygen oxyygen atom ato tm to form multiple m ltip e dislocations. • While h l the strength tre gth of titanium tita um can increase increase c ease by a factor facto off three th ee due to multiple dislocations, this will eventually e ent ally cause ca se titanium’s titaniu ’s tita resi tance to cracking resistance acki g to decline d cline by six times. 18 • M AY 2 0 1 5 temperatures in excess of 980 C. Use of titanium hydride led to the formation of titanium in modeling studies that involved the use of much lower temperatures, thereby leading to the promise of a lower-cost, more energy efﬁcient process. One of the problems in ﬁnding a better way to produce titanium is oxygen. Andrew Minor, associate professor of materials science and engineering at the University of California in Berkeley, Calif., and faculty scientist at Lawrence Berkeley National Laboratory, says, “Very low concentrations of oxygen in titanium leads to the hardening of the metal, which eventually causes a decrease in toughness in the titanium metal that makes it less resistant to cracks.” A better understanding of how oxygen causes titanium’s physical properties to decline is needed in order to determine what can be done to ﬁgure out a more cost effective method for extracting and processing titanium. Research has now been done showing what oxygen does to titanium on the atomic level. DISLOCATION MULTIPLIER Minor and his associates at the Berkeley Laboratory, in collaboration with colleagues at Japan’s Nuclear Science and Engineering Directorate and Rolls Royce (a manufacturer of jet turbine engines), has determined how oxygen at very low concentrations will cause titanium to become harder and then eventually more brittle. The presence T R I B O LO GY & LU B R I CAT I O N T EC H N O LO GY The hope is that this work will help in the development of a lowercost process for reﬁning titanium without the metal losing its optimal characteristics. of oxygen in titanium is similar to metals containing other components that act as solutes to improve mechanical properties, but the effect is far more extreme than normal solute hardening. Minor says, “Solutes are added to every metal to make alloys. One of the main reasons for doing this is to increase the hardening of the resulting metallic alloys. This process takes place because the solutes are atoms with a different size than the plane of metallic atoms they are encountering. Solutes end up being attracted to dislocations, which represents extra spaces in the metal for foreign atoms that do not ﬁt perfectly within the matrix lattice.” An analogy to this process as explained by Minor is moving a rug along a ﬂoor by applying a ripple (the dislocation) that ﬂows through the ﬁber of the rug. There are two types of dislocations seen in metals. One is called an edge dislocation while the second one is known as screw dislocation. Both are equivalently missing W W W. ST L E .O RG Figure 1 | A moving dislocation (shown in light blue) in titanium can interact with an oxygen atom (shown in red) to form multiple dislocations (shown in dark blue) leading titanium to be much less resistant to cracking. (Figure courtesy of the University of California Berkeley.) half-planes of atoms in the crystal structure of a material. Minor explains that the solute atoms are typically either placed in a substitution manner in the same plane as the metal atoms or in an interstitial fashion in between the metal atom planes. The researchers studied the alpha form of titanium, which is readily available commercially, doped with oxygen at concentrations of 0.1%, 0.2% and 0.3% by weight using a technique known as aberration-corrected transmission electron microscopy (TEM) to determine the effect of the oxygen atoms on the titanium metal matrix over time. Minor says, “Oxygen generates a very strong negative repulsion with the screw dislocations in titanium at a very small concentration. Normally, for most alloys, a solute concentration between 0.1%-0.3% does not make much difference in the resulting properties. But with titanium, the difference between three and one oxygen W W W. ST L E .O RG atoms within 1,000 titanium atoms will cause a very different motion of the dislocations.” The TEM data shows that dislocation in titanium interacts with an oxygen atom (shown in red in Figure 1) to generate additional dislocations. Minor says, “This oxygen atom acts as a multiplier to produce additional dislocations leading to more tangling of the dislocations and therefore results in the titanium becoming more brittle.” The researchers used in situ TEM nanocompression testing to assess the impact of these dislocations in real time on the mechanical properties of titanium. Minor says, “We found that the strength of titanium increases by a factor of three due to the multiplier dislocation effect of oxygen atoms— this leads to the effect of six times less resistance to cracking.” Future work will concern ﬁnding techniques for preventing oxygen atoms from causing titanium to become brittle. Minor says, “We will try to use compuT R I B O LO GY & LU B R I CAT I O N T EC H N O LO GY tational means to determine how to lock up oxygen atoms to mitigate this effect, such as ﬁnding elements that can bind with the oxygen preventing the dislocation multiplier effect.” The hope of the researchers is that this work will help in the development of a lower-cost process for reﬁning titanium without the metal losing its optimal characteristics. Additional information can be found in a recent article2 or by contacting Minor at [email protected] berkeley.edu. REFERENCES 1. Canter, N. (2014), “Cost-effective titanium extraction,” TLT, 70 (7), pp. 12-13. 2. Yu, Q., Qi, L., Tsuru, T., Traylor, R., Rugg, D., Morris, J., Asta, M., Chrzan, D. and Minor, A. (2015), “Origin of dramatic oxygen solute strengthening effect in titanium,” Science, 347 (6222), pp. 635-639. M AY 2 0 1 5 • 19 TECH BEAT Removing metals from aqueous waste streams with hydroxyfullerene Included are a variety of main group and transition metals. FULLERENE, WHICH IS ALSO KNOWN AS A BUCKYBALL, exhibits a hollow sphere cage structure similar to a soccer ball that contains 60-carbon atoms. This compound is prepared by vaporization of graphite and is remarkably stable. Since fullerene was discovered in 1985, this compound has been evaluated for use in such applications as solar cells and hydrogen gas storage. One area where other carbon-based materi- KEY CONCEPTS • Hydroxyfullerenes, Hydroxyf ll nes, a derivative deri t e of the 60-carbon atom attom buckyball structure known as full fullerene, e, exhibit promise p i e in in removing metals from aqueous waste streams. • A wide range of metals can be extracted, but the ones preferred are a e +2 charge cations ar withh small s all a atomic atom t icc radii d i and +3 charge cations with large atomic radii. • The reason for this preference f ce is that hydroxyfullerenes are mi t s of different mixtures d fferent molecules mol l with thh hydroxyl groups placed in multiple orientations. 20 Initial work with hydroxyfullerenes is very promising because it shows great afﬁnity for zinc, which is a difﬁcult metal to waste treat. als such as graphene oxide have been evaluated is in removal of heavy metals from water through adsorption. This application is very important because water is a precious resource and removal of metals from efﬂuent streams generated in manufacturing plants is mandatory. In a previous TLT article, a new compound known as copper hydroxide ethanedisulfonate was found to have promise in removing anions from water though ion exchange.1 Among the anions removed were dicarboxylates and metal oxo anions such as permanganate. Andrew R. Barron, the Charles W. Duncan Jr.-Welch Chair of Chemistry and professor of materials science and nanoengineering at Rice University in Houston, indicates that past research has focused on looking for compounds that extract speciﬁc precious metals. He says, “There have been lots of studies on nanomaterials to evaluate their ability to extract specific precious metals. One material that has shown promise is graphene oxide, which has been found in the literature to remove transition metals, lanthanides and actinides from water.” None of these nanomaterials have the ability to extract all of the metals that may be present in a speciﬁc efﬂuent stream. Barron says, “Based on the work conducted with carbon nanomaterials, we decided to look at hydroxyfullerenes. Our initial work showed that hydroxyfullerenes chelate ferric ions (Fe3+) to form a water insoluble complex.” The other attractive reason for working with hydroxyfullerenes is that they function in a similar manner to the phenolic derivative, catechol in forming cross-linked complexes with metals. A catechol derivative known as 3,4-dihydroxyphenylalanine is used by marine mussel proteins to chelate metals. Barron says, “Hydroxyfullerenes are a larger version of the phenolic ligands seen in biological systems.” Hydroxylation of fullerene takes place through reaction with sodium hydroxide in the presence of tetrabutylammonium hydroxide. Barron says, “Depending upon the reaction conditions, over 20 hydroxyl groups can be placed on a single fullerene molecule. Some of the groups are alkoxides that are neutralized with sodium cations.” A recently completed study has now shown that hydroxyfullerenes can extract The Apollo 11 spacecraft consisted of the command module, Columbia, and the lunar module, Eagle. ing of hydroxyfullerenes with nickel and iron salts. The choice of the anion appears to have no effect NANOAGGREGATES on the ability of the hydroxyfullerene to bind to Barron and his associthe metal. Barron says, ates evaluated the abil“We tried a series of anity of hydroxyfullerenes ions that included aceto cross-link a series of tates, carbonates, citrates, metal salts. The process nitrates and sulfates. None used is to mix solutions of these anions impacted of the metal salts and the cross-linking process.” the hydroxyfullerenes at Future work will inroom temperature, obvolve trying to boost the serve the formation of metal: hydroxyfullerene a precipitate, isolate the ration used and to work solid and then analyze with carbon nanotubes. the metal ligand through Barron says, “Our objecthe use of ultraviolettive is to determine if carvisible spectroscopy. bon nanotubes bound to a Experiments were membrane ﬁlter can act as done using individual tentacles to bind metals in salts and then comparing the efﬂuent stream.” a speciﬁc metal salt with Figure 2 | Nanoaggregates formed from the cross-linking of Initial work with hyferric nitrate in a series of hydroxyfullerenes with metals (the example shown is with iron droxyfullerenes is very competitive cross-linking and nickel) explain how this fullerene derivative is effective in extracting metals from aqueous streams. (Figure courtesy of promising because it reactions. The researchers Rice University.) shows great affinity for evaluated salts of alumizinc, which is a difﬁcult num, cadmium, calcium, metal to waste treat. Addicobalt, copper, mangational information can be nese, nickel, silver and found in a recent article2 zinc. sistent with the empirical results.” Barron says, “We found that hyor by contacting Barron at [email protected] The reason for the discrepancy is droxyfullerenes prefer to bind with +2 edu. that hydroxyl groups attached to the charge cations that have small atomic fullerene cage are not only in the 1,2 radii and with +3 charge cations that orientation but also in the 1,3 and 1,3,5 have large atomic radii. For our exREFERENCES orientations. Barron says, “Fullerene periments, hydroxyfullerenes have the 1. Canter, N. (2011), “Water is functioning as a mixture of different most afﬁnity for zinc of the +2 charge treatment of anions,” TLT, 67 molecules and this explains the binding cations followed by cobalt, manganese, (12), pp. 12-13. behavior for +2 and +3 charged cations.” nickel, cadmium and copper. The +3 2. Heimann, J., Morrow, L., Transmission electron microscope charge cation that has most afﬁnity to Anderson, R. and Barron, A. images of the cross-linked hydroxyhydroxyfullerenes is lanthanide fol(2015), “Understanding the fullerene metal samples show nanoaglowed by iron and aluminum.” relative binding ability of hydroxyfullerene to divalent and gregates that contain a series of metals The reason for this effect was at ﬁrst trivalent metals,” Dalton Transacand fullerenes. Barron says, “In dilute puzzling for Barron and his colleagues tions, 44, pp. 4380-4388. solutions, we found that these nanoagbecause they did a computational study gregates are dozens of nanometers long assuming that the hydroxyfullerenes because chains initially made from one doing the binding mainly had diols adhydroxyfullerene molecule tend to agjacent to each other in the 1,2 position. Neil Canter heads his own consulting company, Chemical gregate to other chains. The aggregates He says, “We initially did ab initio calSolutions, in Willow Grove, Pa. can contain as many as 20-30 hydroxyculations on catechols and found that Ideas for Tech Beat can be fullerene molecules.” binding is based on size as expected and submitted to him at Figure 2 shows an image of a nanonot on charge. For hydroxyfullerenes, [email protected] aggregate formed from the cross-linkthe computational studies were not cona variety of main group and transition metals from an aqueous environment. 22 Connect with STLE: Like us on Facebook (www.facebook.com), Follow us on Twitter (@STLE_Tribology), join our LinkedIn group (www.linkedin.com).
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