RELAXOR-PT SINGLE CRYSTALS FOR BROAD BANDWIDTH, HIGH POWER SONAR PROJECTORS

Open Access
Author:
Sherlock, Nevin Paul
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 01, 2010
Committee Members:
  • Richard Joseph Meyer Jr., Dissertation Advisor
  • Richard Joseph Meyer Jr., Committee Chair
  • Thomas R Shrout, Committee Chair
  • Shujun Zhang, Committee Member
  • Thomas B Gabrielson, Committee Member
Keywords:
  • ferroelectric
  • piezoelectric
  • single crystal
  • sonar
  • transducer
Abstract:
The high piezoelectric response of the ferroelectric relaxor (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMNT) in single crystal form has generated significant interest in producing broad bandwidth SONAR systems. Both the piezoelectric coefficient (d33 > 2000 pC/N) and coupling coefficient (k33 > 0.90) are superior to those of conventional piezoelectric ceramics. Within the context of a high power acoustic projector, its high losses and low temperature stability have limited its development. Second generation single crystals with compositions modified from the base PMNT have been recently developed to decrease the electromechanical losses and mitigate the thermal property dependence. In this work, the electromechanical properties were measured using single crystals which have been modified in various ways. The modified crystals exhibit electromechanically "hard" behavior with lower losses (tand = 0.1-0.2% and QM = 230-950) than unmodified PMNT (tand = 0.26% and QM = 190). The measured d33 values of modified single crystals (d33 = 760-1490pm/V) are also lower than unmodified PMNT (d33 = 1540pm/V), but the lower piezoelectric response is compensated by the greater stability of the modified single crystals. These modified single crystal properties were also compared to conventional high power piezoelectric ceramics (d33 = 240pm/V and QM = 1050) to show similar losses but significantly greater response in the modified PMNT single crystals. Although most piezoelectric materials are measured under small signal conditions (small signal defined by a completely linear relationship between the input and output signals), the high power nature of SONAR projectors demands that these modified single crystals also be evaluated under high power conditions. A test procedure was developed to measure the electromechanical properties of each material as a function of applied electric field over a frequency range which includes the resonance frequency. Modified single crystals showed twice the dynamic strain of unmodified PMNT as a function of electric field, and in many cases also showed greater maximum strain at failure (0.3% compared to 0.15% for unmodified PMNT). When QM was measured as a function of drive level, it was shown to sharply decrease under high dynamic strain. Modified single crystals with greater small signal QM values than unmodified PMNT maintain higher QM values under high drive, with QM = 50-150 immediately prior to sample failure (QM = 20 for base PMNT immediately prior to failure). The temperature dependence of modified PMNT single crystal electromechanical properties was also determined, and it was shown that modified crystals possess greater property stability than unmodified PMNT. While the base composition shows a limiting rhombohedral-tetragonal transition at 95C, modified single crystals using ternary PIN and PZT components show increased transition temperatures of 125C and 144C, respectively. The greater phase stability of the PIN ternary crystal was also examined through the coercive field, which was shown to be much greater than that of unmodified PMNT over the temperature range of interest (Ec = 5 kV/cm and 2 kV/cm, respectively, at room temperature). From the combined set of property measurements, the heat generation of each material was predicted for an arbitrary projector device. As a consequence of the lower losses, modified single crystals showed as little as 25% of the heat generation value for unmodified PMNT single crystals. Using this prediction as a performance metric, the crystals with the lowest heat generation were selected for device testing. Transducers with base PMNT and modified single crystals were designed using a finite element modeling approach. This model predicted approximately two octaves of bandwidth for the transducer geometry under investigation. A 5 dB decrease in acoustic output was observed when moving from base PMNT to highly modified crystals, but that result does not account for nonlinear material behavior. Transducers fabricated using modified PMNT were compared to both base PMNT and conventional PZT4 ceramic. In-water analysis at the resonance frequency showed that heavily modified single crystals showed a maximum source level 5 dB greater than base PMNT and comparable to the conventional PZT4. Off resonance, the modified crystal showed a 3 dB improvement over base PMNT and a 6 dB improvement over PZT4. Broadband frequency sweeps confirmed the superior bandwidth of single crystal transducers relative to the conventional piezoelectric ceramic. The previous heat generation predictions were confirmed, with modified crystals showing 3-4x reduction in heat generation relative to base PMNT when measured in water under isothermal conditions of 50C. Combined transducer measurements demonstrate that modified PMNT single crystals may combine broad bandwidth and high power stability in an underwater acoustic projector.