Complete elastic constants of α-BaB2O4: Schaefer-Bergmann acousto-optic diffraction and resonant ultrasound spectroscopy.

Both Schaefer-Bergmann diffraction and resonant ultrasound spectroscopy were used to measure the six independent elastic-stiffness coefficients of the trigonal, non-piezoelectric crystal α-BaB2O4. The two measurement sets resulted in a root-mean-square variance of 1.2%. This paper provides a detailed analysis of the two different measurement techniques and discusses the similarities and differences.

Observation of higher stiffness in nanopolycrystal diamond than monocrystal diamond.

Diamond is the stiffest known material. Here we report that nanopolycrystal diamond synthesized by direct-conversion method from graphite is stiffer than natural and synthesized monocrystal diamonds. This observation departs from the usual thinking that nanocrystalline materials are softer than their monocrystals because of a large volume fraction of soft grain-boundary region. The direct conversion causes the nondiffusional phase transformation to cubic diamond, producing many twins inside diamond grains. We give an ab initio-calculation twinned model that confirms the stiffening. We find that shorter interplane bonds along [111] are significantly strengthened near the twinned region, from which the superstiff structure originates. Our discovery provides a novel step forward in the search for superstiff materials.

Alpha-plutonium's Grüneisen parameter.

Reported Grüneisen parameters γ of alpha-plutonium range from 3.0 to 9.6, which is remarkable because typical Grüneisen parameter uncertainty seldom exceeds ± 0.5. Our six new estimates obtained by different methods range from 3.2 to 9.6. The new estimates arise from Grüneisen's rule, from Einstein model and Debye model fits to low-temperature ΔV/V, from the bulk modulus temperature dependence, from the zero-point-energy contribution to the bulk modulus, and from another Grüneisen relationship whereby γ is estimated from only the bulk modulus and volume changes with temperature (or pressure). We disregard several high estimates because of the itinerant-localized 5f-electron changes during temperature changes and pressure changes. Considering all these estimates, for alpha-plutonium, we recommend γ = 3.7 ± 0.4, slightly high compared with values for all elemental metals.

Elastic-Stiffness Coefficients of Titanium Diboride.

Using resonance ultrasound spectroscopy, we measured the monocrystal elastic-stiffness coefficients, the Voigt C ij, of TiB2. With hexagonal symmetry, TiB2 exhibits five independent C ij: C 11, C 33, C 44, C 12, C 13. Using Voigt-Reuss-Hill averaging, we converted these monocrystal values to quasiisotropic (polycrystal) elastic stiffnesses. Briefly, we comment on effects of voids. From the C ij, we calculated the Debye characteristic temperature, the Grüneisen parameter, and various sound velocities. Our study resolves the enormous differences between two previous reports of TiB2's C ij.

Elastic constants of layers in isotropic laminates.

The individual laminae elastic constants in multilayer laminates composed of dissimilar isotropic layers were determined using ultrasonic-resonance spectroscopy and the linear theory of elasticity. Ultrasonic resonance allows one to measure the free-vibration response spectrum of a traction-free solid under periodic vibration. These frequencies depend on pointwise density, laminate dimensions, layer thickness, and layer elastic constants. Given a material with known mass but unknown constitution, this method allows one to extract the elastic constants and density of the constituent layers. This is accomplished by measuring the frequencies and then minimizing the differences between these and those calculated using the theory of elasticity for layered media to select the constants that best replicate the frequency-response spectrum. This approach is applied to a three-layer, unsymmetric laminate of WpCu, and very good agreement is found with the elastic constants of the two constituent materials.

Elastic constants of natural quartz.

The elastic constants of a natural-quartz sphere using resonance-ultrasound spectroscopy (RUS) are measured. The measurements of the near-traction-free vibrational frequencies of the sphere are matched with the predicted frequencies from the dynamic theory of elasticity, with optimized estimates for the elastic constants driving the differences between these sets of frequencies to a minimal value. The present computational model, although based on earlier approaches, is the first application of RUS to trigonal-symmetry spheres. Quartz shows six independent elastic constants, and our estimates of these constants are close to those computed by other means. Except for C14, after a 1% mass-density correction, natural quartz and cultured quartz show the same elastic constants. Natural quartz shows higher internal frictions.