Browsing by Author "Brandt, Dan"

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NMR spin-lattice relaxation study of Cs1‑x(NH4)xH2PO4
(1997) Meschia, Steven C. L.; Zidansek, Aleksander; Brandt, Dan; Schmidt, V. Hugo
Spin-lattice relaxation time T1 for 31P was measured as a function of temperature at 28.2 MHz in a powder sample of Cs1-x (NH4) x H2PO4 (CADP) crystallized from a solution with a molar ratio x = 0.2. This crystal has the same structure as CsH2PO4 (CDP), which exhibits a low-temperature pseudo-one-dimensional ferroelectric phase transition at 159 K and a superionic transition at 504 K on heating. The measured temperature dependence is explained in terms of hydrogen bond dynamics.
Piezoelectric polymer actuator and material properties
(1996) Schmidt, V. Hugo; Brandt, Dan; Holloway, F; Vinogradov, A. M.; Rosenberg, D
This paper presents the construction and performance of a PVF/sub 2/ [poly(vinylidene fluoride), (CH/sub 2/CF/sub 2/),] thin film piezoelectric actuator. In addition, the paper discusses the methods used to characterize the viscoelastic properties of the actuator material.
Piezoelectric polymer actuators for active vibration isolation in space applications
(1999) Bohannan, G; Schmidt, V. Hugo; Brandt, Dan; Mooibroek, M
A lightweight actuator for active vibration isolation in space applications is being developed to replace the heavy electromagnetic systems now in use. The actuator has a low effective spring constant that provides for passive vibration damping down to sub-Hertz frequencies while allowing the isolated experiment to follow the near-dc bias motion of the spacecraft. The actuator is currently optimized for the vibration level of the Space Shuttle and assembled from a pair of bimorphs in a leaf-spring configuration. Changing the size and number of sheets used in construction can vary electromechanical properties. Passive damping has been demonstrated in one and two-dimensional tests. For large (greater than a few kilograms) suspended masses, the system is underdamped and relative velocity feedback must be used to remove the resonance. Real-time control of the resonance frequency is achieved by controlling the voltage applied to the actuator with feedback from a displacement sensor. A folded pendulum seismic monitoring device was adapted for use as a one-dimensional low frequency test platform and has obtained accurate measurements of the effective spring constant and damping coefficient. Single-degree-of-freedom active feedback testing is also being conducted using this device. Two-dimensional (three-degree-of-freedom) passive damping tests were conducted on NASA's KC-135 Reduced Gravity Platform in March 1998.
Temperature dependent Raman spectra of Rb1-x(ND4)xD2AsO4 mixed crystals
(1998) Tu, Chi-Shun; Gao, S.-S.; Jaw, R.-J.; Hwa, L.-G.; Schmidt, V. Hugo; Brandt, Dan; Chien, R.-M.
In the mixed ferroelectric (FE)-antiferroelectric (AFE) systemA1−x(ND4)xD2BO4 [A=Rb(orK,Cs)andB=As (or P)], there is competition between the FE and the AFE orderings, each characterized by speciﬁc conﬁgurations of the acid deuterons. [1–8] The random distribution of the Rb and ND4 ions is the main source to produce frustration which can increase local structural competition such that the long-range order of electric dipole disappears. Instead of a typical sharp FE or AFE phase transition, the phase coexistence becomes a characteristic in this type of mixed compounds. By a group theoretical analysis for the KDP-type structure (which contains two molecular units in a primitive unit cell); at zero wavevector, the vibrational modes in the tetragonal symmetry (space group I¯ 42d − D12 2d) can be decomposed into the following irreducible representations: Γvib = 4A1(R) + 5A2(Silent) + 6B1(R) + 6B2(R,IR) + 12E(R,IR). [9] The symmetry species A1, B1, B2 and E are Raman active. The situation in the mixed system D*RADA-x is more complicated than one in the parent crystals, because some Rb (or ND4) ions have been substituted by ND4 (or Rb) ions. In this case, the selection rule of the free AsO4 group is expected to be broken much easily than in the pure crystal. In the recent years, many measurements in D*RADAx system have been achieved on ferroelectric-side crystals x = 0.1, 0.10 and 0.28. [2–5] However, only a few experiments were done on antiferroelectric-side compounds (x ≥0.50).[6,7]A complete understanding for this mixed system is still lacking. This motivated us to carry out the polarized Raman scattering on D*RADA-0.55, 0.69 and 1.0. Here, we pay special attention to the stretching mode ν1 (near 755 cm−1) and the in-plane bending mode δ(O-D) (near 825 cm−1).