High Power Characterization Oflead-free Piezoelectric Ceramics

Restricted (Penn State Only)
Author:
Gurdal, Erkan Ahmet
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 20, 2014
Committee Members:
  • Kenji Uchino, Dissertation Advisor
  • Kenji Uchino, Committee Chair
  • Clive A Randall, Committee Member
  • Shujun Zhang, Committee Member
  • Wenwu Cao, Committee Member
  • Qiming Zhang, Committee Member
Keywords:
  • Lead-Free Piezoelectric Ceramics
  • High Power Characterization
  • Piezoelectric Transformer
  • Hardening
  • Doping
Abstract:
Lead-based piezoelectric ceramics have dominated piezoelectric applications. Lead-zirconate-titanate (PZT) is especially suitable for a wide range of piezoelectric applications such as: sensors, sonars, and ultrasonic motors. This is because PZT has excellent piezoelectric properties and its compositional optimization for specific applications is well studied. However, lead-based materials are no longer welcome in electronic applications due to environmental concerns. Therefore, the research for finding alternative lead-free piezoelectric ceramics has been expanded in the past decade. One of the major piezoelectric application types is the high power application such as sonars and piezoelectric transformers. High power conditions cause degradation in material performance and heat dissipation due to increased non-linearity and losses in the material. However, the majority of the lead-free ceramic reports are focused on material properties measured at low power levels. This creates an ambiguity in the reported candidate materials’ actual high power potential. Considering the material properties measured at low power conditions, perovskite type hard-lead-free piezoelectric materials seem to have good high power potentials. Consequently, disk-shape (kp mode) hard-lead-free piezoelectric ceramics, two Bi-perovskite compositions and one alkali-niobate-based composition, were characterized at high power conditions under continuous excitation. Then, mechanical quality factors (Qm: QA at resonance and QB at anti-resonance), and temperature increase (∆T) were analyzed as a function of vibration velocity (vrms). Cu-doped (Na0.5K0.5)(Nb0.97Sb0.03)O3 (NKN-Cu) ceramics did not show any degradation in their mechanical quality factors (QA and QB) up to their maximum vibration velocity (vrms=vmax=0.45 m/s @ ∆T=20˚C), where hard-PZTs typically showed vmax=0.3 m/s with immediate degradation in QA and QB. Different QA and QB trends (NKN-Cu: QA≅QB and hard-PZT: QA<QB) showed that NKN-Cu suffered less from the piezoelectric loss (tan θ) compared to hard-PZT. Two Mn-doped Bi0.5Na0.5TiO3-based Bi-perovsikte ceramics: BNT-BT-BNMN and BNKLT-Mn also showed superior high power performances (vmax>0.5 m/s) compared to hard-PZT. QA and QB trends (i.e. QA>QB) were completely different from both NKN-Cu and hard-PZT, indicating a very low piezoelectric loss (tan θ’). These ceramics were also compared from the mechanical energy density viewpoint to eliminate the density and vibration mode effects. Even though hard-PZT was able to produce higher mechanical energy density than lead-free ceramics at identical vibration velocity, lead-free ceramics were still able to keep their stable behavior at high mechanical energy density levels. Ring-dot piezoelectric transformers (PTs) were produced from the NKN-Cu ceramics. Even though the transformer design was not completely optimized, NKN-Cu ring-dot PTs had considerably high output power density (P=25 W/cm3) compared to typical PZT ring-dot transformers, where optimized designs in the literature could reach output power density levels up around 40 W/cm3. Doping is one of the effective ways to influence domain wall motion and the material properties, especially in lead-based systems. Based on the domain stabilization studies in PZT, most low valence (acceptor) dopants can stabilize domain configuration via creation of oxygen vacancies, which is called the hardening effect. However, common acceptor dopants in lead-systems (e.g. Mn and Fe) do not seem to be as effective as Cu in hardening of the lead-free NKN system. The NKN system was doped with Yb, Ni, Mn, and Cu. Pellets were sintered around 1100˚C for 3 hours in ambient atmosphere. Polarization (P-E) and elastic strain (S-E) hysteresis loops were measured before and after poling to study the hardening in NKN via internal bias field (Ei). Doping enhanced both coercive and internal bias fields overall. NKN-Cu and NKN-Mn showed hard characteristics with lower dielectric losses (tan δ≤0.01) and higher mechanical quality factors (Qm≥300) when compared to the other doped and base NKN ceramics. Cu-doped NKN ceramics had the highest internal bias field and mechanical quality factor (Ei=0.37 kV/mm and Qm=420). Bias field and mechanical quality factor were proportional at high Ei levels. At low Ei levels (Ei<0.1 kV/mm), hardening mechanism was not clear. At higher Ei levels (Ei>0.1 kV/mm), increase in mechanical quality factor thus the hardening was proportional to the created internal bias field in the structure. This result indicated a possibility of a minimum internal bias field level required to stabilize the domain wall structure in NKN ceramics. Moreover, interesting results were obtained in S-E hysteresis loops for Cu-doped and Mn-doped NKN ceramics. Switching fields obtained from S-E hysteresis loops were different compared to the ones obtained from P-E hysteresis loops. This difference was an indication of complex domain wall dynamics in hard-NKN ceramics. These ceramics were also characterized at high power conditions. NKN-Cu and NKN-Mn showed more stable behaviors at increased excitation conditions compared to PZT4 type ceramics, which had similar hardness level. In short, the distinct high power behaviors of lead-free piezoelectric ceramics make them very promising for high power applications. Hard-lead-free piezoelectric ceramics were able to reach high excitation levels without increasing their temperatures and degrading their mechanical quality factors dramatically. Hard-lead-free piezoelectric ceramics possessed superior high power performances compared to hard-PZT piezoelectric ceramics. Additionally, hard-lead-free piezoelectric ceramics showed differences in their losses mechanisms. Piezoelectric loss was not dominant in hard-lead-free piezoelectric ceramics in general compared to hard-PZT, where the piezoelectric loss was proven to be significantly large.