Kunquan Lu

Electrorheological fluid with an extraordinarily high yield stress

Surface modified complex strontium titanate microparticles are synthesized by means of a modified sol-gel technique. A suspension composed of these particles immersed in a silicone oil exhibits excellent electrorheological properties attractive to industry and technology applications: a yield stress as high as 27 kPa in an applied electric field of 3 kV/mm, a low leakage current, wide dynamic ranges in temperature and shear rate, and a long-term stability against sedimentation. In addition to the high dielectric constant of strontium titanate, surfactant and water-free character of the particles may be responsible for the dramatic improvement of the electrorheological properties of the suspension

Precise measurement of the densities of liquid Bi, Sn, Pb and Sb

The densities of liquid Bi, Sri, Pb and Sb have been precisely measured from the melting point up to about 1100 K using an improved Archimedean method. The densities at the melting point for liquid Bi, Sri, Pb and Sb are 10.042 x 10(3), 6.983 x 10(3), 10.635 x 10(3) and 6.454 x 10(3) kg m(-3), respectively. Comparisons between our data and those from the literature have been made and they show the present results to be more reliable. Rather than a linear fit for the temperature dependence of the density, a slight deviation from linearity in the temperature dependence of the densities has been observed.

STRUCTURE AND DEBYE TEMPERATURE OF WURTZITE GaN

High Pure Wurtzite structure GaN has been synthesized by gas reaction method. Its structure was determined by powder X-ray diffraction using the Rietveld technique. The positions in the unit cell for Ga and N were refined to be (0, 0, 0) and (0, 0, 0.3814). The Debye temperature was determined as 586 K from the refined temperature factor by using the Debye approximation.

TIO2 BASED ELECTRORHEOLOGICAL FLUID WITH HIGH YIELD STRESS

We have fabricated several TiO2 based ER fluids with doping and without designed doping, which exhibit the high yield stress up to more than 100kPa. The titanium oxide nanoparticles were synthesized by using wet chemical method. The ER effect of those materials is dominated by the special additives, such as amide or its ramification, as well as the remained molecules or ions in the sample preparation. It is found that the yield stress is also strongly dependent on the viscosity of the oil. The prepared ER fluids possess other attractive characters, for instance the current density is low and against sedimentation.

Synthesis and structure of nanocrystal-assembled bulk GaN

A new condensed form of GaN, nanocrystal-assembled bulk (NAB) GaN, was obtained directly from reactions of metal Ga and NH4Cl in liquid ammonia at 350-500 degrees C. High-resolution transmission electron microscopy observations reveal that the NAB GaN consists of well-crystallized nanocrystals with wurtzite structure. The synchronous densificated NAB GaN is transparent to visible light while the constituted nanocrystals have an average size of about 12 nm. A possible synthesis mechanism is discussed

The electrorheological fluids with high shear stress

A series of high performance ER fluids newly manufactured in our laboratory are presented. The yield stress of those ER fluids can reach several tens of kPa, 100 kPa and even 200 kPa, respectively. For understanding the high shear stress effect a model is proposed base on the electric field induced molecular bounding effect. The main effective factors in fabricating the high performance ER are discussed.

HIGH PERFORMANCE CALCIUM TITANATE NANOPARTICLE ER FLUIDS

A type of calcium titanate (CTO) nanoparticles was synthesized by means of wet chemical method [1] without coating on the particles. The CTO/silicone oil ER fluid exhibits excellent electrorheological properties: high shear stress (~50-100 kPa) under dc electric field, a low current density (less than 2μA/cm2 at 5kV/mm), and long term stability against sedimentation. Although there are not special additives in the ER fluids, it is found from the chemical analysis that a trace of alkyl group, hydroxyl group, carbonyl group and some ions is remained in the particles which may dominate the ER response.

Experimental investigation for field-induced interaction force of two spheres

An apparatus is developed to study the interaction forces between two spheres under an external ac electric field. The interaction forces of a pair of spheres as the function of interspherical spacing, electric field strength, and electric field frequency are measured precisely. The results reveal that much stronger interaction can be obtained for metal spheres and high-dielectric spheres compared with the spheres with a very low dielectric constant, such as glass and polymer. The measured forces increase very steeply as the gap of the spheres decreases, and become much larger than those of available theoretical predictions when the two single-crystalline spheres of SrTiO3 are closely spaced. The frequency dependence of the interaction force also shows an anomalous behavior. Our measurements indicate that a more accurate theoretical calculation should be performed to explain the experimental results.

The synthesis and electrorheological effect of a strontium titanyl oxalate suspension

Strontium titanyl oxalate (STO) particles were synthesized with the co-precipitation method. Suspensions made by dispersing the STO particles in silicone oil exhibit an excellent electrorheological (ER) effect with high yield stress, high shear stress at higher shear rate, and low current density. By analyzing the Fourier transform infrared spectra of STO particles heated at different temperatures and measuring the yield stresses of the corresponding ER fluids, it is confirmed that the polar molecules absorbed on the particles play a crucial role in the giant ER effect. The phenomena observed in STO ER fluids can be explained by using the model of a polar molecule dominated electrorheological (PM-ER) effect.

The electrorheological behavior of complex strontium titanate suspensions

A type of water-free electrorheological (ER) material-complex strontium titanate (STO) was synthesized by means of modified sol–gel. The ER behavior of the suspensions of STO particles in silicone oil with a 37% volume fraction was investigated systematically under both dc and ac electric fields. It is found that this ER fluid has many advantages, such as long-term stability against sedimentation, strong ER effect at low electric field, and a wide operating temperature range. The frequency dependence of ER behavior was also studied and can be partly explained on the basis of dielectric measurement.