Xianfeng Zhang

Enhance the Yield Shear Stress of Magnetorheological Fluids

To enhance the yield shear stress of magnetorheological (MR) fluids is an important task. Since thick columns have a yield stress much higher than a single-chain structure, we enhance the yield stress of an MR fluids by changing the microstructure of MR fluids. Immediately after a magnetic field is applied, we compress the MR fluid along the field direction. SEM images show that the particle chains are pushed together to form thick columns. The shear force measured after the compression indicates that the yield stress can reach as high as 800 kPa under a moderate magnetic field, while the same MR fluid has a yield stress of 80 kPa without compression. This enhanced yield stress increases with the magnetic field and compression pressure and has an upper limit well above 800 kPa. The method is also applicable to electrorheological fluids.

ELECTRORHEOLOGICAL FLUIDS UNDER SHEAR

The behavior of an electrorheological (ER) chain under a shear force is investigated theoretically and experimentally. Contrary to the conventional assumption that the ER chain under a shear force becomes slanted and breaks at the middle, we have found that there is symmetry breaking. When the shear strain is small, the chain becomes slanted with a space gap between the first and second particles (or between the last and next last particles). As the shear strain increases, the gap becomes wider and wider. When the shear strain exceeds a critical value, the chain breaks at the gap. The experiment also confirms that an ER chain under the shear breaks at either end, not at the middle. This symmetry breaking reflects the spaces anisotropy, which is the result of the applied electric field.

The magnetic phases of the itinerant magnetic system Ce(Fe1-xCox)2

Magnetic and electrical property results for the pseudobinary compound Ce(Fe1-xCox)2 are reported. It is found that as the temperature is decreased the system goes from the para- to the ferro- to the antiferromagnetic phase. The ferro-to-antiferromagnetic phase transition is a first-order transition. An H-T phase diagram for the ferro- to antiferromagnetic transition has been determined. All the experimental results suggest that the magnetic phases in Ce(Fe1-xCox)2 may be understood in terms of itinerant electron magnetism. The authors experimental results are consistent with the magnetic phase diagram predictions of Moriya and Usamis theory of strongly interacting itinerant electron systems.

Entropy change at the ferromagnetic-antiferromagnetic transition in the intermetallic compound Ce(Fe0.8Co0.2)2

The intermetallic compounds Ce(Fe1−xCox)2 show two magnetic transitions (for x < 0.3), one occurring at Tc due to paramagnetic-ferromagnetic (PM-FM) transition, and the other arising at TN lower than Tc, supposedly a ferromagnetic-antiferromagnetic (FM-AFM) transition. We present the results of magnetization measurements around TN under various magnetic fields for Ce(Fe0.8Co0.2)2. The field dependence of the transition temperature TN has been established. The entropy change at the transition is calculated and the result shows a linear region in the entropy change versus TN. An explanation for this linear behavior has been proposed in terms of electronic band structure. The calculated coefficient difference of electronic specific heat between the FM and AFM states agrees well with the value measured by others.

Effects of Co and Y substitution on magnetic properties of CeFe2

CeFe2 orders ferromagnetically (FM) below TC=227 K; however, this FM state is inherently unstable and partial substitution of Fe by Co precipitates the instability of FM state and a ferromagnetic to antiferromagnetic (AFM) phase transition is observed at lower temperature. This second phase transition is a first‐order phase transition. The dependence of TC and TN on the concentration of Co X for Ce(Fe1−XCoX)2 is presented. For Ce(Fe0.8Co0.2)2, the FM transition is at TC=160 K, and the AFM transition is at TN=76 K. The entropy change associated with the FM to AFM phase transition has been determined from magnetization measurements and found to be primarily associated with a change in the electronic density of states at EF. If one starts with Ce(Fe0.8Co0.2)2 and partially substitutes Ce by Y, it is found that TN gradually decreases with Y concentration; and for Y concentration greater than 10%, the AFM phase disappears and FM phase is fully restored. The dependence of TC and TN on the concentration of Y Z i...

High temperature superconducting granular balls

Abstract When a strong electric field is applied to a suspension of micron-sized high Tc superconducting particles in liquid nitrogen, the particles quickly aggregate together to form balls. The millimeter-size balls hold over 106 particles each. They are sturdy, surviving constant heavy collisions with the electrodes. The ball formation is a results of superconductivity. As the c-axis coherence length is shorter than the Thomas-Fermi screening length, the electric field produced by the charged surface layer turns off the coupling between the interlayers. This loss of Josephson energy becomes a positive surface energy. Its minimization leads to the balls.

Magnetization and time dependent effects in ErCo3Ga2

Recently, unusual magnetic properties have been reported on the ErCo3Ga2 compound. In this paper, we report on the measurements of magnetization, ac susceptibility, and the time dependence of magnetization on polycrystalline samples of ErCo3Ga2. The field cooled (FC) samples show negative magnetization with a minimum at 25 K in low magnetic fields. The FC magnetization becomes positive above 150 K. The FC and ZFC magnetizations meet at 150 K as the temperature increases. For magnetic fields above 5000 G, the FC and ZFC magnetizations are the same in the entire temperature range from 5 K to 300 K. The ac susceptibility shows a peak around 20 K. We have found that for T < 150 K, the magnetization in a constant magnetic field at constant temperature increases with time and does not attain a constant value over 64 h. The increase in magnetization is linear on the log(time) scale. The magnetic properties suggest a spin glass-type transition below 150 K and a strong temperature dependent magnetic anisotropy.

Unusual magnetic properties and time dependent magnetization in ErCo3Ga2 (abstract)

In this abstract we will present the measurements of magnetization, ac susceptibility, and the time dependent effects on the magnetization of polycrystalline samples of ErCo3Ga2 and YCo3Ga2. The field cooled (FC) and zero field cooled (ZFC) magnetization data suggest a spin glass transition in YCo3Ga2 below 160 K. The FC magnetization ErCo3Ga2 exhibits a negative minimum at 25 K and a compensation temperature at 150 K. It is suggested that ErCo3Ga2 has two magnetic phase transitions: one at 150 K (most likely a spin glass transition due to Co sublattice) and the other below 25 K (due to Er sublattice ordering). We have found an unusual time dependence of magnetization of ErCo3Ga2 at a constant temperature (below 150 K) and constant applied magnetic field. The magnetization M increases with time t and dose not attain a constant value over a period of 64 h. The time dependence of magnetization follows the relation MT,H(t)=MT,H(t0)+aT,H ln t. The slope aT,H is always positive and depends on temperature and the applied magnetic field. The field dependence of the slope shows a peak at a field Ha. The value of Ha decreases with increasing temperature. At a constant temperature the slope aT,H increases exponentially with the applied magnetic field and then decays towards zero values at high applied magnetic field. This suggests an activation energy process. Detailed experimental results on time effects will be presented.In this abstract we will present the measurements of magnetization, ac susceptibility, and the time dependent effects on the magnetization of polycrystalline samples of ErCo3Ga2 and YCo3Ga2. The field cooled (FC) and zero field cooled (ZFC) magnetization data suggest a spin glass transition in YCo3Ga2 below 160 K. The FC magnetization ErCo3Ga2 exhibits a negative minimum at 25 K and a compensation temperature at 150 K. It is suggested that ErCo3Ga2 has two magnetic phase transitions: one at 150 K (most likely a spin glass transition due to Co sublattice) and the other below 25 K (due to Er sublattice ordering). We have found an unusual time dependence of magnetization of ErCo3Ga2 at a constant temperature (below 150 K) and constant applied magnetic field. The magnetization M increases with time t and dose not attain a constant value over a period of 64 h. The time dependence of magnetization follows the relation MT,H(t)=MT,H(t0)+aT,H ln t. The slope aT,H is always positive and depends on temperature and t...

Investigation of the role of Ce atoms in the CeFe2 system

Abstract CeFe2 is a ferromagnet (FM) with cubic Laves phase structure and bears some anomalous magnetic properties such as much lower Curie temperature (Tc = 227 K) and lower magnetic moment per Fe atom (1.15 μB) as compared to the other compounds in the RFe2 series (R = Y and rare earths). The role of Ce in CeFe2 has been a controversial one. The view ranges from Ce in CeFe2 being in the tetravalent state to its having a non-integral magnetic moment with an antiferromagnetic (AFM) coupling between the iron and cerium sites. It is found that a substitution of Fe sites by Co atoms in CeFe2 destabilizes the FM order at lower temperatures with a FM to AFM transition at TN = 78 K for Ce(Fe0.8Co0.2)2. To explore the role of Ce, we substituted Ce with Y in Ce(Fe0.8Co0.2)2. It was observed that with the substitution of Y for Ce, the AFM is suppressed and FM is gradually stabilized. In Ce1−x Yx(Fe0.8Co0.2)2, complete restoration of ferromagnetism is achieved down to the lowest temperature measured for 10% of Ce substitution by Y (x=0.1).

MAGNETIC PHASE TRANSITIONS IN Ce(Fe1–xCox)2 SYSTEM

Re-entrant magnetic phase transitions in Ce(Fe1–xMx)2 systems (where M=Al, Ru, Co, etc.) are being investigated by various research groups. It has been observed that the system goes from a paramagnetic to ferromagnetic state followed by an almost complete loss of magnetization as the temperature is lowered. We have done systematic experimental measurements of magnetization, ac susceptibility, electrical resistivity, and thermal expansion on the Ce(Fe1–xCox)2 system. It is found that as the temperature is decreased, the system goes from para- to ferro- to antiferro-magnetic phase. The ferro- to antiferro-magnetic phase transition is a first order transition. Our experimental results are consistent with the magnetic phase diagram predictions by Moriya and Usamis theory of strongly interacting itinerant electron systems.