Pingchu Wang

GROWTH AND CHARACTERIZATION OF RELAXOR FERROELECTRIC PMNT SINGLE CRYSTALS

Abstract The results of relaxor ferroelectric PMNT single crystals grown by a modified Bridgman method with the size more than φ25×40 mm are reported in this paper. The crystal habit and growth mechanism of PMNT has been investigated to optimize the growth conditions. The reason that the PMNT crystals will grow along [111] direction under the condition of no seed crystals has been illustrated. It was found that the domain configuration in PMNT is much complicated and they will dominate the piezoelectric properties of PMNT single crystals. The characterization of PMNT single crystals shows that PMNT 67/33 single crystals have d33>2000pC/N. ϵ∼5500, k33-0.93.

GROWTH, CHARACTERIZATION AND PROPERTIES OF RELAXOR FERROELECTRIC PMN-PT SINGLE CRYSTALS

Abstract The present paper reviews the recent progress of studies on growth, characterization and properties of relaxor ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals in Shanghai Institute of Ceramics. Their chemical compositions were selected near the rhombohedral-tetragonal morphotropic phase boundary, more favorable in the rhombohedral phase. The dominant method used for growing the single crystals was the modified Bridgman technique, by which the single crystal boules with the size larger than 25mm diameter and 50 mm long were grown. Different orientation thin plates with thickness less than 1mm were cut from the boules and their domain structures, morphology and properties were characterized. Their phase transition were discussed as well.

Ferroelectric and piezoelectric properties of novel relaxor ferroelectric single crystals PMNT

Relaxor ferroelectric single crystals PMNT with the size of ϕ40 mm × 80 mm have been grown by a modified Bridgman method and their ferroelectric and piezoelectric properties have been characterized. The properties varied with the compositions and cut types. On the (001) cut, PMNT76/24 single crystals exhibited a dielectric constant ∈ of about 3 400, a dielectric loss of tanδ <0.7%, a piezoelectric constantd33 of 980 pC/N, an electromechanical coupling factorkt of 0.55 andTc of about 110°C. Whereas the properties of PMNT67/33 single crystals on (001) cut were better:∈ of about 5 300, tanδ <0.6%, d33 up to 3 000 pC/N,kt 0.64, k33 0.93 and Tc of about 150°C. The piezoelectric properties on other cuts such as (110) and (111) were much lower than those on the (001) cut. The rhombohedral PMNT crystals grown by this method showed more excellent piezoelectric properties than those grown by high temperature solution method and higher value of kt than the rhombohedral PZNT single crystals. It has also been found that the fluctuation in ferroelectric and piezoelectric properties was related to such factors as composition uniformity and poling degree.

Structural defects of Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals grown by a Bridgman method

The types, formation mechanism and suppressing approaches of structural defects in PMNT crystals grown by a Bridgman method were investigated in this paper. The structural defects include composition non-uniformity, scattering particles, pores, negative crystal structures, cellular structures, fissure structures and point defects. They are formed due to one or several of the following: composition deviation, incomplete melting and diffusion, high-temperature volatilization, temperature fluctuation and instability of the position and shape of growth interfaces. On understanding the formation mechanism of structural defects, one can restrain them by means of adjusting stoichiometry of starting materials, using PMNT crystal bulks, suppressing composition volatilization and modifying growth parameters.

Magnetotransport and rectifying properties in La0.67Ca0.33MnO3/yttrium-stabilized zirconia/Si heterojunction

A heterojunction has been fabricated by growing a La0.67Ca0.33MnO3 film on silicon with a buffer layer of yttrium-stabilized zirconia (YSZ). The current-voltage measurement shows that it is a diode with a good rectifying property. At low positive bias voltage, temperature dependence of the junction resistance shows a peak at a certain temperature, which shifts to low temperatures when the voltage is increased from 0.3Vto0.7V. This behavior is quite different from the previous reports on p-n junctions composed of manganites and Nb-doped SrTiO3. The heterojunction shows remarkable magnetoresistance for both positive and negative biases. The results were discussed by considering the depletion layers in both La0.67Ca0.33MnO3 and Si, and the tunneling through YSZ. This work shows the potential application of integrating manganite-based devices and semiconductor circuits.

Fabrication and electrical properties of grain-oriented 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 ceramics

〈112〉 grain-oriented 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 (PMN–0.3PT) piezoelectric ceramics with a degree of orientation of 0.35 have been produced from melt by a directional solidification method. The dielectric, ferroelectric, and piezoelectric properties were investigated, e.g., ert∼4800, tan δ<0.6%, d33∼1500–1600 pC/N, kt∼51%, and k33∼82%, some of these values are comparable to those of PMN–PT single crystals. The strain–electric-field curve with a maximum strain of 0.23% at a field of 22 kV/cm and well-saturated hysteresis loops with a Pr∼35 μC/cm2 were recorded. The results demonstrate that the directional solidification method is a promising technique to fabricate high performance grain-oriented PMN–PT ceramics.

Large low-field magnetoresistance observed in twinned La2∕3Ca1∕3MnO3 thin films epitaxially grown on yttria-stabilized zirconia-buffered silicon on insulator substrates

La2∕3Ca1∕3MnO3 thin films have been grown on yttria-stabilized zirconia (YSZ) buffered silicon-on-insulator (SOI) substrates by the pulsed laser deposition technique. While full cube-on-cube epitaxy was achieved for the YSZ layer, the top manganite layer was multioriented in plane, with a coexistence of cube-on-cube and cube-on-diagonal epitaxial structures. Due to a combined influence from the magnetocrystalline, shape, and stain-induced magnetic anisotropy, in zero field and low temperatures the local spin orientation varies across the large-angle grain boundaries. As a result, a quite large low-field magnetoresistance (LFMR) based on spin-dependent tunneling was observed. The film shows a resistance change of ∼20% in a magnetic field <1000Oe at 50K, which is promising for real applications.

Ferroelectric phase transition in relaxor ferroelectric single crystals 0.76PMN-0.24PT

The features of the single crystals 0.76PMN-0.24PT in dietectric, ferroetectric, pyroetectric properties and domain structures indicate that they are located between typical ferroetectrics and normal ones. The unpoled crystals present a transitional domain configuration between microdomains and typical macrodomains while the crystals on (001) cuts undergo fietd-induced phase transition under poling, showing two special temperature points Td andTm during the succedent heating procedure. The dietectric constant starts to decrease drastically at Td during cooling, or the transformation from induced macrodomain to transitional domain takes place atTd during heating. Ferroetectric-paraetectric phase transition or depolarization continues within the whole temperature range ofTd-Tm, where ferroetectric phase in the form of transitional macrodomains coexists with paraetectric phase. Then the crystals macroscopically transoform into paraetectric phase containing ferroetectric microdomains at a temperature aboveTm. However, owing to the influence of crystallite orientation on fietd-induced phase transition, the temperatureTd does not appear in the same temperature-etectric fietd history in multicrystal ceramics with the same composition as the above single crystals.

The rectifying property and magnetoresistance of La0.67Ca0.33MnO3∕SiO2∕Si heterojunction

We have fabricated a heterojunction by depositing La0.67Ca0.33MnO3 film on electron doped silicon wafer with a buffer layer of natural SiO2. The current-voltage measurement shows that it is a diode with a good rectifying property in a wide temperature range. At high positive voltages, the current-voltage curve shows space charge limited (SCL) current behavior, manifesting itself as a SCL diode. At low positive and negative voltages, the resistance of the junction shows a peak at a certain temperature, which decreases with increasing positive voltage and keeps constant at negative voltages. Magnetoresistance of the junction shows a similar temperature dependence as the resistance of the junction. Calculations show that these phenomena can be attributed to the depletion layer of the La0.67Ca0.33MnO3 film. This work also demonstrates that SCL diode can be realized in La0.67Ca0.33MnO3∕SiO2∕Si with the presence of the SiO2 layer.

Field-Induced Orthorhombic Phase in Relaxor-Based Ferroelectric Single Crystals Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3

A field-induced orthorhombic phase was observed on the (110) cub cuts in 0.68Pb(Mg 1/3 Nb 2/3 )O 3 -0.32PbTiO 3 (PMNT 68/32) single crystals by using a polarizing microscope and electrical measurements. It was found that the induced monodomain orthorhombic ferroelectric state is intrinsic metastable, whose phase stability depends on the composition and poling field intensively, and it does not exhibit enhanced piezoelectricity, the piezoelectric coefficient d 33 are only about 300 pC/N. Whereas the coupling between two equivalent energy states, i.e., the rhombohedral (monoclinic) and orthorhombic phases is responsible for the high piezoelectric responses, the piezoelectric coefficient d 33 can reach up to 1600 pC/N.