"Richard J Kinch" 5 GPa) and temperatures (>1500°C), making it expensive (1). Two other compounds that have been synthesized recently, B6O (2) and cubic BC2N (3), rival the hardness of cubic BN. However, their syntheses also require extreme pressures, exceeding 5 GPa for B6O and 18 GPa for BC2N.
Our approach to creating ultra-incompressible superhard materials has been to optimize two design parameters: high valence-electron density and bond covalency (4, 5). High electron densities can be found among the later transition metals, whereas small first-row main-group elements, such as B, C, and N, form the strongest covalent bonds (6). Among the transition metals, Os has the highest valence-electron density (0.572 electrons/Å3) and a reported bulk odulus between 395 and 462 GPa (7?9) that rivals that of diamond (442 GPa) (10, 11). However, unlike diamond, the hardness of Os metal is only 4 GPa. This can be explained by the nondirectional metallic bonding in Os versus the short, highly covalent, directional bonds formed by sp3 hybridized C atoms in diamond. The strength and directionality of covalent bonds limit the creation and propagation of defects, which in turn causes diamond to resist plastic deformation, resulting in diamond's exceptional hardness. By optimizing covalent bonding and valence- electron density, we recently succeeded in designing, synthesizing, and characterizing an ultra-incompressible hard material, OsB2 (4). In searching for even harder materials, we looked closely at the elements surrounding Os in the periodic table. Re, which lies directly to the left of Os, has a slightly lower alence- electron density (0.4761 electrons/Å3) that produces a similar bulk modulus of
360 GPa (12). Despite being highly incompressible, the hardness of Re metal is also low, between 1.3 and 3.2 GPa, because of its delocalized nondirectional metallic bonding (13, 14). Incorporating B into the interstitial sites of Re to form ReB2 requires only a 5% expansion of the Re lattice. This results in the shortest metal-metal bonds of any known transition-metal diboride (15). In contrast, the Os lattice expands by approximately 10% upon the incorporation of B atoms to form OsB2 and undergoes a distortion to an orthorhombic phase. Applying our design criteria, ReB2 is thus considered to be the most likely candidate for improving on the mechanical properties of OsB2. ReB2 has the highest B:Re ratio among the known RexBy phases?Re3B, Re7B3, and ReB2 (16)?and therefore contains the greatest degree of covalent bonding. The structure of ReB2 consists of alternating layers of hexagonal close-packed Re and puckered hexagonal networks of B (Fig. 1). The result is a compound that is layered perpendicular to the c axis along the (00l) planes. Recent theoretical calculations have shown that there are strong covalent B-B and Os-B bonds in OsB2 (17?20). By analogy, ReB2 should contain similar covalent B-B and Re-B bonds, with the Re-B bonds in ReB2 expected to be shorter and stronger than those in OsB2 because of the minimal lattice expansion.
ReB2 was synthesized under ambient conditions via three methods, each of which may potentially be scaled up (supporting online text). First, a solid-state metathesis reaction was carried out between the metal trichloride and MgB2, because this process has been used to produce many transition-metal diborides, including OsB2 (4, 21). Without excess B, however, this process forms multiple boride phases. Second, Re and B powders were mixed together, pressed into a pellet, and then liquefied with 80 amps of current in an Ar atmosphere. The result was a solid metallic ingot of ReB2 that could be used for hardness testing. In the third method, stoichiometric quantities of Re and B powder were sealed in a quartz tube under vacuum and heated for 5 days at 1000°C. Powder x-ray diffraction, performed on a crushed portion of both the arc-melted ingot and the polycrystalline powder produced from the elements, confirmed the synthesis of phase-pure ReB2 (fig. S1). These materials were then examined by microindentation and in situ high-pressure x-ray diffraction techniques.
The covalent bonding that results in high hardness values can also contribute to the elastic incompressibility (bulk modulus) of a material. To further explore the contribution of covalent bonding as well as the possible correlation between valence-electron density and bulk elastic properties, data on the elastic volume compressibility of ReB2 were collected via in situ high-pressure x-ray diffraction studies. Samples were compressed quasi-hydrostatically up to 30 GPa in a diamond anvil cell, and in situ diffraction data were collected under pressure. From the diffraction data, the fractional volume at increasing pressures was calculated. Fitting the pressure/volume (P versus V /V0) data with a third-order Birch-Murnaghan equation of state, the bulk modulus of ReB2 was determined to be 360 GPa when Bo' (derivative of the zero-pressure bulk modulus with respect to pressure) was fixed at the canonical value of 4. This high bulk modulus is in agreement with our understanding of the correlation between valence-electron density and incompressibility (4, 6). In addition to bulk volume compressibility, the high-pressure diffraction data also revealed an anisotropy in the compressibility of the two different lattice directions of hexagonal ReB2 (Fig.
3). As seen in Fig. 3, the c axis is substantially less compressible than the a axis, and this c axis value is very similar to the analogous linear compressibility of diamond. This anisotropy results from greater electron density, and therefore greater electronic repulsions, along the c axis.
Having observed that the different planes of ReB2 are able to support varying amounts of stress in the conventional and radial geometry high-pressure experiments, we anticipated observing this effect in our hardness measurements as well. Indeed, Fig. 2A shows that there is a substantial difference between the highest and lowest measured hardness across grains of ReB2 under the same load. This observed spread in hardness values at constant load can be attributed to the anisotropic structure of ReB2, combined with our inability to control the crystallographic orientation of the tested grains. For example, under a load of
4.9 N, the highest measured hardness is 32.5 GPa and the lowest obtained value is
26.0 GPa. We can begin to understand this variation in hardness by using electron backscattering diffraction to measure the orientations of the grains studied (Fig. 2, C and D). The results indicate that indentations parallel to the (00l) planes yielded the lowest average hardness, a value of 27 GPa. In contrast, indentations along directions that contained a larger component parallel to the c axis [that is, perpendicular to (00l)] resulted in measurements with an average hardness of 31 GPa, an increase of 15%. The dependence of hardness data on crystallographic orientation can be explained by the presence of the same slip planes described above. Furthermore, because similar anisotropic behavior is observed in the high-pressure data, we conclude that the radial diffraction study elucidates the plastic behavior of the material, giving an indication of the yield strength rather than merely measuring elastic behavior.
In our microindentation experiments to date, we have found no grains with pure (00l) orientation. As a result, our data demonstrate a minimum average hardness. It is likely that these planes parallel to (00l), which we were unable to directly measure, will have an even higher hardness and are responsible for scratching diamond (Fig. 2B).
References and Notes
donald j haarmann
---------------------------- The turbo-encabulator in industry.
For more then 50 years the Arthur D. Little Industrial Bulletin has endeavored to interpret scientific information in terms that he lay person could understand. "The turbo-encabulator in industry" is the contribution of J.H. Quick, graduate member of the Institution of Electrical Engineers in London, England, and was, first published in the Institution's Students' Quarterly Journal in December 1944, It is here reprinted without the kind permission of that publication and of the author in a further salute to Quick.
For a number of years now, work has been proceeding to bring perfection to the crudely conceived idea of a machine that would not only supply inverse reactive current for use in unilateral phase detractors, but would also be capable of automatically synchronizing cardinal grammeters.
Such a machine is the "turbo-encabulator." Basically, the only new principle involved is that instead of power being generated by the relative motion of conductors and fluxes, it is produced by the medial interaction of magneto-reluctance and capacitive directance.
The original machine had a base plate of prefabulated amulite, surmounted by a malleable logarithmic casing in such a way that the two spurving bearings were in direct line with the pentametric fan. The latter consisted simply of six hydrocoptic marzelvanes, so fitted to the ambifacient lunar waneshaft that side fumbline was effectively prevented. The main winding was of the normal lotus-0-delta type placed in panendermic semiboiloid slots in the stator, every seventh conductor being connected by a nonreversible tremie pipe to the differential gridlespring on the "up" end of the grammeters.
Forty-one manestically spaced grouting brushes were arranged to feed into the rotor slipstream a mixture of high S-value phenylhydrobenzamine and 5% remanative tetryliodohexamine. Both of these liquids have specific pericosities given by P=2.5C.n(exponent)6.7 where n is the diathetical evolute of retrograde temperature phase disposition and C is Chlomondeley's annular grillage coefficient. Initially, n was measured with the aid of metaploar refractive pilfrometer (for a description of this ingenious instrument, see Reference 1), but up to the present, nothing has been found to equal the transcendental hopper dadoscope (2).
Electrical engineers will appreciate the difficulty of nubing together a regurgitative purwell and a supramitive wennelsprock. Indeed, this proved to be a stumbling block to further development until, in 1942, it was found that the use of anhydrous nangling pins enabled a kryptonastic boiling shim to the tankered.
The early attempts to construct a sufficiently robust spiral decommutator failed largely because of a lack of appreciation of the large quasipiestic stresses in the gremlin studs; the latter were specifically designed to hold the roffit bars to the spamshaft. When, however, it was discovered that spending could be prevented by a simple addition to the living sockets, almost perfect running was secured.
The operating point is maintained as near as possible to the h.f. rem peak by constantly fromaging the bitumogenous spandrels. This is a distinct advance on the standard nivel-sheave in that no dramcock oil is required after the phase detractors have remissed.
Undoubtedly, the turbo-encabulator has not reached a very high level of technical development, It has been successfully used for operating nofer trunnions. In addition, whenever a barescent skor motion is required, it may be employed in conjunction with a drawn reciprocating dingle arm to reduce sinusoidal depleneration.
References
(1) Rumpelvestein, L.E., Z. Elektro-technistatisch-Donnerblitz vii. (2) Oriceddubg if the Peruvian Academy of Skatological Sciences, June 1914.