Structures and macrosegregation of a 2024 aluminum alloy fabricated by direct chill casting with double cooling field

Central region coarse grains and centerline segregation are common defects in aluminum ingots fabricated by direct chill(DC)casting.A double cooling field was introduced into the DC casting process to reduce these defects,whereby the external cooling was supplied by the mold and water jets,and intercooling was achieved by inserting a rod of the same alloy into the molten pool along the central axis of the ingot.Rather than forming a good metallurgical interface during solid-liquid compound casting,in the present work,the purpose of inserting the rod is to enforce internal cooling and consequently decrease the sump depth.Moreover,the insertion provides more nucleation sites with the unmoltenα-Al particles.The structure and the macrosegregation of 2024 Al alloy ingots prepared by DC casting with and without the inserts were investigated.Results show that when the inserting position is 50 mm above the upper edge of the graphite ring,significant grain refinement in the central region of the ingot and a reduced centerline segregation are achieved.

Inverse dependence of exchange bias and coercivity on cooling field caused by interfacial randomization in nanosystems with Co sparsely distributed in CoFe_(2)O_(4) matrix

We conceive a model of nanosystems where Co is uniformly,randomly distributed into a CoFe_(2)O_(4)(CFO)matrix,and study the influences of composition and CFO antiferromagnetic exchange coupling(JCFO)on zero-field-cooled/field-cooled magnetization behavior and the dependencies of exchange bias field(H_(E))and coercivity(H_(C))on Co/CFO interfacial coupling(J_(Co/CFO)),composition and cooling field(H_(FC)),based on a modified Monte Carlo method.With decreasing temperature,the zero-field-cooled and field-cooled magnetization curves are overlapped and increasing initially,and then splitting at a critical temperature(T_(P)),below which the field-cooled magnetization continues increasing while the zero-field-cooled magnetization exhibits a peak and drops,indicating a magnetically frozen state.A larger J_(CFO) induces an elevated T_(P) and a reduced magnetization,while the opposite results are obtained by a higher Co occupation fraction(P_(Co)).On the other hand,H_(E) is nonmonotonic with J_(Co/CFO )and the peak decreases and shifts to a lower J_(Co/CFO)value for a larger P_(Co).H_(C) increases monotonically with increasing J_(Co/CFO).Moreover,H_(E) is inversely proportional to P_(Co) when PCo varies from 0.11 to 0.30,indicating the exchange bias is still an interfacial effect.Finally,for a nonzero J_(CFO),H_(E) may be suppressed and accompanied by the recovery of H_(C) with increasing H_(FC).Otherwise,H_(C) for zero J_(CFO) becomes high and insensitive to H_(FC).It is ascribed to the uniform contact between Co and CFO,leading to the Co magnetization reversal and the CFO magnetization precisely controlled by H_(FC).This work opens a new avenue to control the exchange bias in nanosystems through interfacial randomization.

Automatic compensation of magnetic field for a rubidium space cold atom clock

When the cold atom clock operates in microgravity around the near-earth orbit, its performance will be affected by the fluctuation of magnetic field. A strategy is proposed to suppress the fluctuation of magnetic field by additional coils, whose current is changed accordingly to compensate the magnetic fluctuation by the linear and incremental compensation. The flight model of the cold atom clock is tested in a simulated orbital magnetic environment and the magnetic field fluctuation in the Ramsey cavity is reduced from 17 nT to 2 nT, which implied the uncertainty due to the second order Zeeman shift is reduced to be less than 2×10^（-16）. In addition, utilizing the compensation, the magnetic field in the trapping zone can be suppressed from 7.5 μT to less than 0.3 μT to meet the magnetic field requirement of polarization gradients cooling of atoms.

High piezoelectricity after field cooling AC poling in temperature stable ternary single crystals manufactured by continuous-feeding Bridgman method

After field cooling(FC)alternating current poling(ACP),we investigated the dielectric and piezoelectric properties of[001]_(pc)-oriented 0.24Pb(In_(1/2)Nb_(1/2))O_(3)(PIN)-0.46Pb(Mg_(1/3)Nb_(2/3))O_(3)(PMN)-0.30PbTiO3(PT)(PIMN-0.30PT)single crystals(SCs),which were manufactured by continuous-feeding Bridgman(CF BM)within morphotropic phase boundary(MPB)region.By ACP with 4 kVrms/cm from 100 to 70℃,the PIMN-0.30PT SC attained high dielectric permittivity of 8330,piezoelectric coefficient(d_(33))of 2750 pC/N,bar mode electromechanical coupling factorκ_(33)of 0.96 with higher phase change temperature(T_(pc))of 103℃,and high Curie temperature(7c)of 180℃.These values are the highest ever reported as PIMN-xPT SC system with Tpc>100℃.The enhancement of these properties is attributed to the induced low symmetry multi-phase supported by phase analysis.This work indicates that FC ACP is a smart and promising method to enhance piezoelectric properties of relaxor-PT ferroelectric SCs including PIMN-xPT,and provides a route to a wide range of piezoelectric device applications.