D. L. Hester

Bio: D. L. Hester is an academic researcher from Sandia National Laboratories. The author has contributed to research in topics: Irradiation & Barium titanate. The author has an hindex of 2, co-authored 2 publications receiving 41 citations.

Effects of Radiation-Induced Damage Centers in Lead Zirconate Titanate Ceramics

Abstract: A study of the nuclear reactor irradiationinduced changes in the electrical and lattice properties of ferroelectric Pb(Zr0.65Ti0.35)O3+ 1 wt% Nb2O5 revealed that these changes in properties are caused by different damage centers. The changes observed in electrical properties were: (1) Pr decreased but P3 remained constant, (2) double (antiferroelectric-type) hysteresis loops were formed, (3) an increase by a factor of 5 in grain size did not affect the radiation damage rate in the ceramics, (4) an increase in irradiation temperature of 100°C above room temperature doubled the damage rate, (5) EC for unpoled ceramics was insensitive to irradiation until integrated fast neutron fluxes of lo18 nvt (and the associated γ-dose) were exceeded and thereafterECincreased exponentially, (6) aging of the radiation-induced damage did not occur at room tem perature, (7) the damage centers which affected the electric properties were temperature annealed, the annealing temperature depending on the particular material, (8) the introduction of lattice strain, as observed by changes in the longitudinal wave velocity (phonon mean free path length), did not appear to be related to changes in electrical properties. These changes in electrical and thermal properties are consistent with the published data for irradiated triglycine sulfate, Rochelle salt, barium titanate, and guanidinium aluminum sulfate and it is suggested that a common damage model may be constructed for all these ferroelectric materials.

Use of Ferroelectrics for Gamma-Ray Dosimetry

Abstract: A gamma-ray dosimeter employing a poled ferroelectric as the transducer element has been studied. Irradiation with gamma rays causes a release of charge by the ferroelectric element. The magnitude of the charge released has been determined experimentally to vary linearly with gamma-ray dose. The current in a shunting resistor with no external voltage applied varies linearly with gamma-ray dose rate. A constant of proportionality of 10-12 coul per rad (H2O) per cm2 of electroded ferroelectric surface has been measured for polycrystalline Pb(Zr.65 Ti.35)O3 + 1 w% Nb2O5 irradiated in the Sandia Pulsed Reactor. The contribution to the charge release from the neutron irradiation has been determined experimentally to be negligible. Irradiation in the 0.6 Mvp flash X-ray also produces a linear relationship between current and gamma-ray dose rate. A similar release of charge has been observed in poled ceramic barium titanate.

Electrical after-effects in Pb(Ti, Zr)O3 ceramics

Abstract: Pb(Ti, Zr)O3 ceramics doped with Al, Mn or Fe exhibit constricted or asymmetrically displaced hysteresis loops. This distortion is due to an internal bias field Ei which can exceed the coercive field strength. Ei depends on the Ti/Zr ratio and on the kind of dope, and it increases with increasing doping concentration.After repeated cycling of the hysteresis loop the distortion disappears. This process, called “hysteresis relaxation,” obeys a time law of the form Ei(t) ∼exp(-t/τ)and is found to behave both like a field and thermally activated process. The activation energy of all investigated specimens doped with transition metal ions (Mn, Fe) lies between 0.6 and 0.7 eV, that of Al-doped samples is about 0.8 eV.Comparison of the behaviour of the dielectric and mechanical losses with that of the internal bias shows that these effects are closely correlated. From this correlation some conclusions can be drawn concerning the nature of Ei, the physical causes of the observed after-effects, and the influence o...

Radiation tolerance of piezoelectric bulk single-crystal aluminum nitride.

Abstract: For practical use in harsh radiation environments, we pose selection criteria for piezoelectric materials for non-destructive evaluation (NDE) and material characterization. Using these criteria, piezoelectric aluminum nitride is shown to be an excellent candidate. The results of tests on an aluminum-nitride- based transducer operating in a nuclear reactor are also presented. We demonstrate the tolerance of single-crystal piezoelectric aluminum nitride after fast and thermal neutron fluences of 1.85 x 10(18) neutron/cm(2) and 5.8 x 10(18) neutron/ cm(2), respectively, and a gamma dose of 26.8 MGy. The radiation hardness of AlN is most evident from the unaltered piezoelectric coefficient d33, which measured 5.5 pC/N after a fast and thermal neutron exposure in a nuclear reactor core for over 120 MWh, in agreement with the published literature value. The results offer potential for improving reactor safety and furthering the understanding of radiation effects on materials by enabling structural health monitoring and NDE in spite of the high levels of radiation and high temperatures, which are known to destroy typical commercial ultrasonic transducers.

Irradiation Testing of Piezoelectric (Aluminum Nitride, Zinc Oxide, and Bismuth Titanate) and Magnetostrictive Sensors (Remendur and Galfenol)

Abstract: Four piezoelectric transducers with aluminum nitride (AlN), zinc oxide (ZnO), and bismuth titanate (BiTi) as the active elements and two magnetostrictive transducers were fabricated with Remendur and Galfenol as the active elements. The irradiation was for 18 months with an integrated neutron fluence of approximately $8.68 \times 10^{20}$ n/cm2 for $E >1$ MeV, temperatures in excess of 420 °C, and a gamma fluence of $7.23 \times 10^{21}$ gamma/cm2. The sensor performance is explained in the context of the pulse-echo signals. The feasibility of ultrasonic transducers in a nuclear reactor has been established. This opens the door to leave-in-place sensors for in-reactor conditions and materials.

Response of lead metaniobate acoustic emission sensors to gamma irradiation

Abstract: Piezoelectric (PE) acoustic emission (AE) sensors were exposed to hundreds of kGy from Co-60 gamma rays. Recalibration of gamma-irradiated sensors showed a sensitivity decline of 8.4% per 100 kGy. Exposure of lead metaniobate disc specimens, the sensor ferroelectric material, to /spl sim/1 MGy revealed a capacitance increase and PE voltage (g/sub 33/) coefficient decrease with gamma dose. The planar coupling factor (k/sub p/) and mechanical quality factor (Q/sub m/) exhibited small reductions. The AE sensors were also subjected to nuclear radiation from a pulse reactor.