Composites with 3-1 connectivity for transducer applications were made by embedding extruded PZT rods in an epoxy matrix. The effects of rod diameter, volume fraction of PZT, and composite thickness on the hydrostatic properties of the cornposites were determined. Due to decoupling of the d3, and dS1 coefficients in a composite with 3-1 connectivity, thedh may be enhanced, even in composites of low volume fractions of PZT. Such composites also-have a low dielectric permittivity. The combination of high dlr values and low E,, value results in a greatly enhanced gh. Composites with 10 vol% PZT were made with values of ah and EL which are, respectively, two times ( S O x C/N) and 25 times (>70 xlO-, (V. m/N) the solid PZT values.
The reaction sequence in the formation of Pb(Zro.aTio.4)03 (PZT 60/40) was investigated as a function of PbO and ZrOt raw material variations, using powder X-ray diffraction. Particular emphasis was placed on the final stages of reaction in the formation of PZT solid solution. Based on the present work, a more detailed reaction sequence is being proposed for Pb(Zr0,6Ti0.4)03 and similar compositions. This reaction sequence is believed to account for many of the apparent discrepancies which have appeared in the literature.
The effect of aging on the temperature dependence of permittivity (K) and tan δ as a function of frequency was examined for both ferroelectric and antiferroelectric PLZT compositions. All compositions aged near their transition temperatures exhibited a marked decrease in dispersion between the aging temperature and the temperature of the high frequency (100 kHz) permittivity peak. These results are discussed in terms of the Smolenskii model for dielectric relaxation.
The effect of porosity on the hydrostatic piezoelectric sensitivity was explored for 3–1 composites with a polyurethane matrix. Hydrostatic sensitivity increases rapidly with volume of porosity, and dh increases from 12 x10−12to >200X10‐12 C/N when 40% porosity is incorporated. These composites become strongly nonlinear at >I MPa and consequently would be limited to shallow water operation.
The densification behavior of pressure sintered cubic silicon carbide containing 1 wt% boron carbide was studied as a function of temperature (1750" to 1950°C). Specimens of theoretical density were obtained at 1950" with a pressure of 3000 psi. Experimental results showed that densification proceeded by a plastic flow mechanism. Interpretation of the data in terms of Murray's equation yields an activation energy of (115+ 18) kcal/mol. At 1850°C and above, tabular grains of 6H and 2H SIC were observed in a matrix of fine grains of 3C Sic.HE good mechanical properties and corrosion resistance of T S i c make it a promising candidate for many high temperature applications. Unfortunately, it is very difficult to obtain a fully dense, fine-grained self-bonded material. It was reported that pure Sic could be densified to near theoretical density at pressures >20 kbars above 2000°C.' Densification behaviors at lower temperatures and pressures were investigated by others, who conducted an extensive study on metal additions and found that the most efficient aids were A1 (or A1203) and Fe, which gave relative densities >98% at <2oOO0C and pressures as low as 6000 psi. Densities >99% of theoretical were obtained with additions of A1203 at temperatures above 1950°C under a pressure of 4000 psi3 Effects of boron additions were investigated in Russia4 and more recently dense samples of 0.4% boron-doped S i c were obtained at 1950°C.5 In this paper, the sintering behavior of Sic doped with 1 wt% B4C is reported along with microstructural features observed on specimens hot-pressed at different temperatures.
I. Experimental Procedure
(1) Powder PreparationA blend of submicron S i c powder+ prepared by high temperature gas phase reaction under controlled atmosphere and 1 wt% submicron B4Ci was used in this study. Thep-Sic phase was present in this mixture; impurities were (in w t 8 ) B 1, Fe 0.03, Mg 0.03, Cr 0.01,Ni0.01,Ti0.02,Al0.05,Cu0.001,Ca0.001,and0,0.16. An Auger characterization of the powders revealed 0, and Si on the surface of the grains. The total O2 content being very low (0.16 wt%) and the surface area large (6.1 m*/g), the oxide layer (not necessarily SiO,) must be extremely thin.Disks 2 in. in diameter and 1/4 in. thick were sintered in a hot pressrj at from 1750" to 1950°C under a constant pressure of 3000 psi uniaxially applied to the powder by both top and bottom rams. The die walls and spacers were coated with a boron nitride film to prevent adherence and diffusion between the specimen and the graphite. All the runs were carried out under a vacuum of < torr. The heating rate selected was 150°C/min and the pressure (3000 psi) was applied at room temperature and removed at the end of the soaking time. The pressure system had previously been carefully calibrated with a load cell. Two W5Re/W26Re thermocouples were used to control and stabilize the temperature to & 2°C and were regularly checked against an optical pyrometer. Densification was monitored by two linear displacement transducers attached to rams and the...
The disappearing-phase method was used to determine the extent of the solid-solution region of the PLZT system for conventionally sintered ceramics prepared at 1100" and 1300°C in a PbO atmosphere provided by WZrO,. A decrease in the firing temperature from 1300" to 1100°C lowers the solubility limit by 5 to 8 at.% La. Beyond the limits of solubility, additional La forms LazZrzO7 and/or LazTizO,. The limit determined by the disappearing-phase method (1300°C firing) is compared to values determined by the parametric method. The Curie temperature is stabilized at 5 at.% La for modified PbZrO, (1300°C firing).
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