Low Mechanical Loss Research Articles

A 30-MHz piezo-composite ultrasound array for medical imaging applications.

Ultrasound imaging at frequencies above 20 MHz is capable of achieving improved resolution in clinical applications requiring limited penetration depth. High frequency arrays that allow real-time imaging are desired for these applications but are not yet currently available. In this work, a method for fabricating fine-scale 2-2 composites suitable for 30-MHz linear array transducers was successfully demonstrated. High thickness coupling, low mechanical loss, and moderate electrical loss were achieved. This piezo-composite was incorporated into a 30-MHz array that included acoustic matching, an elevation focusing lens, electrical matching, and an air-filled kerf between elements. Bandwidths near 60%, 15-dB insertion loss, and crosstalk less than -30 dB were measured. Images of both a phantom and an ex vivo human eye were acquired using a synthetic aperture reconstruction method, resulting in measured lateral and axial resolutions of approximately 100 microm.

Mechanical Spectroscopy of the Carbon Snoek Relaxation in Ultra-High Purity Iron

Low and very low frequency mechanical loss spectra of the stable carbon Snoek relaxation in ultra-high purity α-Iron doped with 1000 at. ppm of carbon were carried out. The carbon Snoek peak was analyzed by various numerical methods in order to determine its relaxation parameters. The following mean values of the relaxation parameters were found: the activation enthalpy H s = 0.878 eV and the preexponential factor τ∞ = 1.71 10 -15 s. The observed height of the stable carbon Snoek peak in saturated α-Iron Q max -1 =2.8.10 -2 is to our knowledge the highest ever reported in the literature.

Low temperature internal friction spectrum of calcium doped polycrystalline YBa2Cu3O6+ x

Internal friction measurements (IF) in the kilohertz frequency range were performed on Ca doped polycrystalline YBa 2 Cu 3 O 6+ x . The samples were also characterized by DC electrical resistivity and X-ray measurements. The IF spectra obtained in the present investigation show an increase in the low temperature mechanical loss maxima at 95 K and 115 K with increasing Ca content. This could be interpreted as being due to an increase in the electrical charge density, the Y 3+-ions being replaced by the lower valent Ca 2+-ions, thus creating additional holes. Furthermore the fact that doping increases the electrical charge density is shown by a decrease in the electrical resistivity with increasing Ca content. Thus the low temperature IF peaks could be attributed to the hopping of electronic defects in the CuO 2 planes and in the CuO chains, whose density increase with Ca content.

Morphology and mechanical properties of rubber modified aromatic‐ether bismaleimide matrix resins

AbstractAn aromatic ether bismaleimide (BMI) was modified by copolymerization with various CTBN and ATBN liquid elastomers. Dynamic mechanical (DMA), flexural, and SEM fractography studies indicate that cured specimens containing various amounts of the different elastomers have widely varying morphologies and properties. The experimental parameters of interest in this study included the type of elastomer reactive end group, elastomer acrylonitrile content, elastomer concentration, and cure reaction conditions. The ATBNs are clearly more compatible than CTBNs. CTBN modified compositions show a distinct, low temperature rubber phase mechanical loss dispersion, reduced modulus and ultimate strength values, and only slight improvements in elongation. Cured compositions with small amounts of ATBN elastomers (5 phr), however, show no reduction in modulus but improved elongation and ultimate strength values. The “rubber” domains in these systems are small, typically < 5 μm, and consist of copolymerized BMI and elastomer. DMA data for these systems show no distinct low temperature elastomer peak but a broad “interphase” loss dispersion covering a wide range of temperatures. Failure in the ATBN modified BMIs involves initiation of numerous microcracks with obvious crack deflection at the rubber particles. No cavitation of rubber particles occurs, as is frequently the case with the CTBNs.

ゴム・スチレン樹脂の粘弾性

Rubber-styrene resin is a new type of polymer alloy, which can be polymerized at 60-90°C for several hours from viscous mixtures of styrene monomer, rolled rubber and divinyl benzene catalysed by benzoyl peroxide, and cured at 120-150°C for several hours after the shrinkage of the reacting system ended.This is a new type of cast resin which has a very low dielectric loss tangent, comparable to that of NBS type, and low cost and wider uses available in the field of electrical parts.Low frequency dynamic modulus and loss tangent of this resin were measured from -80°C to 100°C, relating with the composition and after cure time.The effects of composition (styrene: 70-85%; rubber 12-18%; DVB 2-8%; BPO 1%) on the dynamic mechanical properties show that DVB elevates the softening temperature, but rubber lowers it and increases mechanical loss. Measurements below room temperature reveal the remarkable mechanical absorption at -40--50°C which should be contributed from rubber molecules. The temperature at which the loss tangent has its maximum, did not seen to shift appreciably by changing composition of this resin.Secondly, for the constant composition, cure condition and after cure temperature, it has been shown that the high temperature mechanical loss decreases with increasing after cure time, but the low temperature mechanical loss increases with increasing after cure time. This fact may suggest some possibility of chain scission in rubber molecule kept at high temperature for long time.Our results have thrown some lights into the internal structure of this resin. Although the existence of some graft or network structure between rubber and polystyrene molecules cannot entirely be neglected, the results could be fairly well explained as “mixture”, that is, rubber could exist as blend in polystyrene-DVB network structure.