Y.B. Kim

Fabrication and comparison of PMN-PT single crystal, PZT and PZT-based 1-3 composite ultrasonic transducers for NDE applications.

This paper describes fabrication and comparison of PMN-PT single crystal, PZT, and PZT-based 1-3 composite ultrasonic transducers for NDE applications. As a front matching layer between test material (Austenite stainless steel, SUS316) and piezoelectric materials, alumina ceramics was selected. The appropriate acoustic impedance of the backing materials for each transducer was determined based on the results of KLM model simulation. Prototype ultrasonic transducers with the center frequencies of approximately 2.25 and 5MHz for contact measurement were fabricated and compared to each other. The PMN-PT single crystal ultrasonic transducer shows considerably improved performance in sensitivity over the PZT and PZT-based 1-3 composite ultrasonic transducers.

High-speed terahertz reflection three-dimensional imaging for nondestructive evaluation

We demonstrate high-speed terahertz (THz) reflection three-dimensional (3D) imaging based on electronically controlled optical sampling (ECOPS). ECOPS enables scanning of an axial range of 9 mm in free space at 1 kHz. It takes 80 s to scan a transverse range of 100 mm × 100 mm along a zigzag trajectory that consists of 200 lines using translation stages. To show applicability of the imaging system to nondestructive evaluation, a THz reflection 3D image of an artificially made sample is obtained, which is made of glass fiber reinforced polymer composite material and has defects such as delamination and inclusion, and is compared with an ultrasonic reflection 3D image of the sample.

A new non-destructive method for estimating the remanent life of a turbine rotor steel by reversible magnetic permeability

Abstract We present a new magnetic and non-destructive procedure to evaluate the remanent life of 1Cr–1Mo–0.25V steel using the value of reversible magnetic permeability. The method is based on the existence of reversible magnetic permeability in the differential magnetization around the coercive force. The measurement principle is based on the foundation harmonics voltage induced in a coil using a lock-in amplifier tuned to a frequency of the exciting one. Results obtained for reversible magnetic permeability and Vickers hardness on the aged sample show that the peak interval of reversible magnetic permeability (PIRMP) and Vickers hardness decreases as aging time increases. A softening curve is obtained from the correlation between Vickers hardness and the PIRMP. This curve can be used as a non-destructive method to evaluate the remanent life of 1Cr–1Mo–0.25V steel.

Coercivity enhancement by Zn addition in hot deformed NdFeB magnets

Anisotropic NdFeB magnets were fabricated by the single-stage hot deformation method from commercial isotropic MQPA powder. The effect of Zn addition on the magnetic properties was investigated and the considerable increase of coercivity was observed. The increment of coercivity was about 57% in the case of 0.4 wt% Zn addition compared with that for additive-free magnets.

Determination of saturation magnetic polarization of uniaxial magnetocrystalline anisotropic material from low field data

Abstract A method to determine saturation magnetic polarization of uniaxial magnetocrystalline anisotropic materials from low field data is proposed. The method is applicable for a magnetic particle system in which the magnetization easy axes of particles are aligned. In calculation, the magnetic particles were assumed as noninteracting single domains and to distribute as a Gaussian function around the alignment direction. In the magnetic system, the particle alignment can be expressed quantitatively by the ratio of remanent polarizations J r (⊥)/ J r (‖), and the saturation polarization can be calculated from the remanent polarization of the parallel curve and the particle alignment parameter.

Enhancement of permeability of Co-based amorphous alloy by two-step cooling method

Abstract The soft magnetic properties of Co-based amorphous alloy have been investigated as a function of cooling condition followed by high-temperature annealing. Furnace cooling to the Curie temperature and subsequent water quenching (F&W method) considerably increased the initial permeability by 60% and 100% higher than those of the furnace-cooled and water-quenched samples, respectively. The F&W sample also showed superior high-frequency characteristics. This phenomenon was analyzed in terms of internal thermal strees and the magnetic ordering. We confirmed the alloy annealed by F&W shows better shielding effectiveness

Hysteresis loops at different temperatures for melt-spun

Hysteresis loops of melt-spun were measured at different temperatures between 4.2 K and 250 K. The loops were analysed on the basis of the micromagnetism using the finite element technique. By fitting the calculated with the experiment, the value of the exchange constant across the inter-grain boundary was estimated to be about 90% of that across the inner-grain boundary. increases to a maximum at the spin reorientation temperature and then decreases with increasing temperature. With decrease of the exchange interaction across the grain boundary, decreases below and increases above , and the Stoner-Wohlfarth model provides the theoretical lower and upper limits of below and above , respectively. The calculations reproduce the hysteresis loops fairly well. The magnetization proceeds by non-coherent rotation, and the spin distribution becomes quite non-uniform in some grains when H decreases to near .

A graphical method to determine high-order anisotropy constants of uniaxial anisotropic material from single-crystal magnetization curves

Abstract In a uniaxial magnetic anisotropy system, the magnetization vector is proved to rotate on the plane made by easy axis and the applied field direction during the initial magnetization process. Under this restricted condition, a method to analyze high-order anisotropy constants from magnetization curves of single crystal measured in basal plane is suggested. Especially, the special curves on hard and easy directions within basal plane are proved to give the most reliable results in this method. Using the method, the anisotropy constants of Pr 2 Fe 14 B at 290 K were determined as K 1 = 5.7 × 10 7 erg/cm 3 , K 2 = − 0.26 × 10 7 erg/cm 3 , K 3 = 0.12 × 10 7 erg/cm 3 , K 4 = 0.21 × 10 7 erg/cm 3 and K 5 = − 0.12 × 10 7 erg/cm 3 . Simulated magnetization curves using these anisotropy constants coincide well with experimental data.

Magnetocrystalline anisotropy of (Sm0.5RE0.5)Fe11Ti compounds (RECe, Pr, Nd, Sm, Gd and Tb)

Abstract The intrinsic magnetic properties of (Sm 0.5 RE 0.5 )Fe 11 Ti (RECe, Pr, Nd, Sm, Gd, Tb) compounds were studied by thermomagnetic analysis, vibrating sample magnetometry and computer calculations. (Sm 0.5 RE 0.5 )Fe 11 Ti (RECe, Pr, Nd, Sm, Gd, Tb) compounds had tetragonal ThMn 12 -type structure and uniaxial magnetocrystalline anisotropy with easy magnetization c -axis. The anisotropy constant K 1 was in the range of 1.75 to 9.2 MJ/m 3 and approximately one order higher than K 2 . Among the compounds investigated, SmFe 11 Ti had the highest anisotropy0 of K 1 = 9.2 MJ/m 3 , K 2 = 0.4 MJ/m 3 and μ 0 H A = 19.8 T. Substitution of any other rare-earth element for Sm decreased the magnetocrystalline anisotropy.

Effects of Dv on magnetic properties of /spl alpha/-Fe/Nd/sub 2/Fe/sub 14/B type alloy

The effects of Dy content on magnetic and microstructural properties of melt-spun (Nd/sub 1-x/Dy/sub x/)4(Fe/sub 0.9/Co/sub 0.1/)/sub 86/B/sub 6/Nb/sub 3/Cu alloys have been investigated. The energy product of the alloys decreases with the increase of Dy content, while the coercivity increases up to 2.64 kOe at x=0.3. At a fixed annealing time condition, the optimum annealing temperature increases by Dy substitution, and grain coarsening occurs in the alloys with high Dy content. The deterioration of hard magnetic properties by Dy substitution is due to the decrease of exchange interaction between a-Fe and (Nd,Dy)/sub 2/Fe/sub 14/B phases by grain coarsening.