Grady S. White

Intelligent Ceramic Materials: Issues of Brittle Fracture

In this paper we review the brittle fracture behavior of piezoelectric and electrostrictive ceramics of interest for intelligent material systems. Properties of importance, such as critical fracture toughness, strength, and susceptibility to moisture enhanced crack growth, are addressed, especially as they are affected by the materials microstructure and chemistry. Local strains and ferroelectric domains produced by phase transformations in these materials are shown to have a direct influence on both fracture toughness and strength. The cyclic stressing of a piezoelectric ceramic is discussed in terms of the structural damage produced by this mode of loading. Finally, tradeoffs in improved reliability, which may be achievable through suitable variations in composition or microstructure, are compared against possible losses in the piezoelectric properties of interest.

Thermal transport through thin films: Mirage technique measurements on aluminum/titanium multilayers

Thermal transport properties of multilayer thin films both normal and parallel to the layers were measured. Al/Ti multilayer films 3 μm thick, with individual layers systematically varied from 2.5 to 40 nm, were studied on Si substrates. Layers of Al and Ti were nominally equal in thickness, with actual composition determined for each specimen using energy dispersive spectroscopy. The thermal diffusivity both in the plane and normal to the plane of the films (thermal conductivity divided by specific heat per volume) was found to decrease significantly with decreasing bilayer thickness. Pure Ti and Al films as well as Cu films from 0.1 to 5 μm thick were also studied. In-plane electrical conductances of the Al/Ti multilayers were also measured.

Effects of crystal bonding on brittle fracture

A series of unistructural materials was used to investigate dependence of fracture energy upon crystal bond ionicity. Fracture energy values obtained from indentation measurements were found to agree with those obtained from a simplistic sine function force law calculation. In addition, an indentation procedure to determine whether environments enhance fracture is discussed.

Scattering from a V-shaped groove in the resonance domain

Radiation scattered from a V-shaped groove in the resonance domain was measured and compared to predictions from a rigorous theory. The calculated scattering accurately predicted the number of peaks and general shape of the observed scattering. Small variations in groove shape were found to alter peak heights noticeably and peak positions slightly. Finite beam size caused unavoidable discrepancies between calculated and measured scattering.

Atomic force microscope observations of domains in fine-grained bulk lead zirconate titanate ceramics

This study describes sample preparation techniques that allow atomic force microscope observation of domains on the scale of tens of nanometers in fine-grained bulk lead zirconate titanate (PZT) ceramics. It is shown that carefully conducted light polishing and etching results in no significant disturbance to domain morphologies in hard PZT ceramics.

Detection of subsurface damage: studies in sapphire

A variety of nondestructive characterization techniques has been used to detect and measure subsurface damage in single- crystal sapphire to develop methods suitable to inspect high performance optics for sub-surface damage. These techniques include polarized light microscopy, x-ray diffraction topography, transmission electron microscopy (TEM) and Raman spectroscopy. TEM examination shows that for ground surfaces damage can extend up to 6 - 7 micrometers into the bulk and includes cracks, twins and dislocations, while under polished surfaces only dislocations are seen. X-ray diffraction topography can image defects such as long-range strain, dislocations, residual surface scratches (not visible optically) and low-angle grain boundaries (lineage). Polarized light is also sensitive to strain and provides a relatively easy method for detecting defects such as cracks and lineage. Of all of the techniques Raman spectroscopy offers the best potential for quantifying strain in terms of stress.

Calculated potential for water enhanced crack growth in silicon

Abstract Ab initio molecular orbital calculations have been performed on the model molecules Si 8 H 17 OH (taking into consideration of the crack wall species -OH and -H) for the crack-tip strain simulation study of silicon and its reaction with water. The calculated equilibrium structure of silicon using a 6-31G ∗∗ basis set agreed well with experimental data. It was found that, regardless of the manner strain was applied, the values of net charge on the two crack-tip Si atoms are similar and that both become slightly more positive as strain increases. In addition to this absence of polarization of the crack-tip SiSi bond under strain, steric hindrance also inhibited the approach of the water molecule to the SiSi crack tip region. These calculations explain why moisture does not enhance crack-growth via chemical reaction in Si, which is different from that observed in silica.

Processing flaws in PZT transducer rings

In industrial fabrication, many PZT transducer rings fail to meet the specifications of physical properties, and some of them even fracture during poling, the final stage of fabrication. The microscopic examination of fractured surfaces, using SEM, shows the presence of pores and microcracks that intersect with pores of size significantly larger than the average grain size. Evaluation of the semi-finished products at various stages of fabrication indicates that the pores formed during pressing and the microcracks were initiated during poling. The strong correlation of microcracks with sharp wedges, each formed by two adjacent grains at the edge of a pore, suggests that these microcracks originated from the sharp wedges where severe stress concentrations usually occur. Significant inhomogeneities in composition of the as-sintered samples were dectected using EDS, and these inhomogeneities are attributed to the inhomogeneous evaporation of Pb and the presence of a liquid phase during sintering. The x-ray diffraction patterns indicate a reduction of the Ti/Zr molar ratio during sintering that, together with observation of second-phase particles at some triple grain junctions, suggests the presence of a liquid phase during sintering.

Nonlinear resonant acoustic detection of cracks in multilayer ceramic capacitors

Measurements of resonant ultrasonic frequencies of multilayer ceramic capacitors (MLCCs) were performed as a function of excitation amplitude to assess the potential of nonlinear acoustic methods for sensing the presence of cracks. The ultimate objective of this work is to establish an approach for nondestructively screeningMLCCs for susceptibility to failure that arises from the evolution of structural flaws into electrical pathways during service. Direct ferroelectric tone-burst transduction and time-domain signal analysis were used to measure the frequency of an extensional mode near 1 MHz in BaTiO3-based MLCCs with interleaved Ag electrodes. The capacitors were subjected to thermal stress by quenching from 189 °C into ice water, which led to generation of visible surface-breaking cracks in a fraction of the MLCCs. The amplitude dependence of the resonant frequencies was found to be strongly correlated with the presence of visible cracks.

Biophotonic Tools in Cell and Tissue Diagnostics

In order to maintain the rapid advance of biophotonics in the U.S. and enhance our competitiveness worldwide, key measurement tools must be in place. As part of a wide-reaching effort to improve the U.S. technology base, the National Institute of Standards and Technology sponsored a workshop titled “Biophotonic tools for cell and tissue diagnostics.” The workshop focused on diagnostic techniques involving the interaction between biological systems and photons. Through invited presentations by industry representatives and panel discussion, near- and far-term measurement needs were evaluated. As a result of this workshop, this document has been prepared on the measurement tools needed for biophotonic cell and tissue diagnostics. This will become a part of the larger measurement road-mapping effort to be presented to the Nation as an assessment of the U.S. Measurement System. The information will be used to highlight measurement needs to the community and to facilitate solutions.