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PubMed2002

Computational mineral physics and the physical properties of perovskite.

The inherent uncertainties in modern first-principles calculations are reviewed using geophysically relevant examples. The elastic constants of perovskite at lower-mantle temperatures and pressures are calculated using ab initio molecular dynamics. These are used in conjunction with seismic tomographic models to estimate that the lateral temperature contrasts in the Earth's lower mantle are 800 K at a depth of 1000 km, and 1500 K at a depth of 2000 km. The effect of Al(3+) on the compressibility of MgSiO(3) perovskite is calculated using three different pseudopotentials. The results confirm earlier work and show that the compressibility of perovskites with Al(3+) substituted for both Si(4+) and Mg(2+) is very similar to the compressibility of Al(3+)-free perovskite. Even when 100% of the Si(4+) and Mg(2+) ions are replaced with Al(3+), the bulk modulus is only 7% less than that for Al(3+)-free perovskite. In contrast, perovskites where Al(3+) substitutes for Si(4+) only and that are charge balanced by oxygen vacancies do show higher compressibilities. When corrected to similar concentrations of Al(3+), the calculated compressibilities of the oxygen-vacancy-rich perovskites are in agreement with experimental results.

Aluminum↗
PubMed1995

Brain injury without head injury. Some physics of automobile collisions with particular reference to brain injuries occurring without physical head trauma.

The classic kinematic formula, a=(v2-v0(2))/2s, relating deceleration to change of velocity and displacement during the change, is used to determine the acceleration experienced by the occupants of motor vehicles (and their heads) during collisions. The displacement, s, of the occupants in the accident can often be determined from the deformation of the wreckage, thus yielding this critical information about potential for brain injury. Data from 15 accidents are assembled and approximate accelerations computed. At the same time medical and neuropsychological data have been obtained for each of the accident victims. These accidents and the associated victims (except for one special case) have been selected because in no case has the victim suffered a direct head-contact injury. Nevertheless, all suffered neurologic and/or neuropsychological injuries typical of patients with closed head trauma. The paper demonstrates that with decelerations from 30 g to more than 200 g yet without skull impacts, brain damage results.

Journal Article↗