Fabrication of c-axis oriented zinc oxide by electrophoretic deposition in a rotating magnetic field

The fabrication of c-axis oriented zinc oxide was attempted by electrophoretic deposition (EPD) in a rotating magnetic field. The EPD was conducted in a small container which was placed on a turntable arranged in a superconducting magnet. The suspension was rotated at 0–90 rpm in a 12 T magnetic field during the deposition. The deposits were dried and then sintered at 1400 °C for 2 h. The degree of the c-axis orientation was evaluated by the Lotgering factor calculated from the X-ray diffraction data.     

Grain oriented titania ceramics made in high magnetic field

Microstructure with grain orientation has a high potential for greatly improving characteristics of ceramics. This paper shows a very promising approach for developing the structure of grain orientation through para- and dia-magnetic interaction of ceramics particles in an extremely strong magnetic field. We used titania as a raw material. Grain oriented titania ceramics of high density was successfully developed through a processing in a high magnetic field. X-ray diffraction shows that the c-axes of particles of fine powder particles were oriented parallel to the applied magnetic field. The orientation factor is 0.05 as determined using the Lotgering method. High density was achieved by the subsequent sintering. Relative density is 98% of theoretical density.     

Beta-sialon phosphor deposits fabricated by electrophoretic deposition (EPD) process in a magnetic field

Phosphor deposits of β-sialon:Eu²⁺ were prepared by electrophoretic deposition (EPD) process within a magnetic field. Under the action of the magnetic force, which was parallel to the direction of the electric field of the EPD, the β-sialon:Eu²⁺ crystals were aligned along the c-axis of the hexagonal cell unit to form an oriented deposit via the EPD fabrication. Higher orientation degree was obtained at longer depositing time (300 s) and stronger applied magnetic field (12 T). The oriented deposit aligned along the c-axis obtained higher relative deposit density than the randomly fabricated deposit. Due to the improved relative density, the oriented deposit prepared within the magnetic field possessed an enhanced external quantum efficiency (η_ex). Also, because of different relative densities of the deposits prepared within and without the magnetic field, they presented different chromaticity coordinates.     

Effect of seeding layer on orientation control of potassium niobate thin film by CSD

This paper focuses on the orientation control of the KN thin film on Si wafer by chemical solution deposition (CSD). We selected the PbO layer and PZT layers as the seeding layer in order to control the crystal orientation of the resulting KN thin film. Crystalline phase in KN thin film was identified by XRD, and the degree of c-axis orientation was calculated from XRD analysis. The resultant KN thin film was orthorhombic perovskite single phase. As a result, highly c-axis oriented thin film (about 90%) was deposited by using PbO seeding layer. The dielectric constant of the resultant KN thin film was measured by impedance analyzer. The dielectric constant of highly c-axis oriented KN thin film was compared with that of the c-axis of KN single crystal.     

Anisotropy in activation energy of textured LiCoO2 for the initial stage of sintering

The anisotropic initial sintering of oriented LiCoO₂ green compacts prepared by slip casting in a strong magnetic field was studied. The activation energy and the mechanism of the initial stage of sintering were analyzed in order to understand the effect of the orientation. The analysis was carried out using constant rate heating (CRH). The activation energy of the initial stages of sintering for random, a, b-axes and c-axis direction was evaluated from the Arrhenius plots of the dilatometric measurements. It was confirmed that the activation energy of the c-axis direction was larger than that of the a, b-axes direction at the initial stage of sintering. The anisotropic shrinkage was attributed to the differences in the surface energies.     

Effect of wet-jet milling on the c-axis orientation of hydroxyapatite ceramics fabricated using a strong magnetic field

Controlling the crystal orientation of hydroxyapatite, an inorganic material that is a major component of human hard tissues is important to fabricate better artificial bones and artificial tooth roots. To obtain highly oriented hydroxyapatite ceramics under a strong magnetic field, a good dispersion of the raw materials in the slurry must be obtained. This study investigates the effect of wet-jet milling of a slurry on the orientation of hydroxyapatite ceramics fabricated using a strong magnetic field. Although the prolonged ball milling with ZrO₂ balls of the raw powder fractures the primary particles of hydroxyapatite, wet-jet milling is used to successfully pulverize agglomerated hydroxyapatite raw powder without changing the morphology of the primary particles. Evidently, ceramics with a highly oriented c-axis of hydroxyapatite are obtained by molding the wet-jet milled slurry in a strong rotating magnetic field. They exhibit anisotropy in fracture toughness, and the fracture toughness calculated from the crack length perpendicular to the rotating axis is higher than that calculated from the crack length parallel to it. Furthermore, these values are higher than those of randomly oriented hydroxyapatite ceramics, which may be owing to the crystal orientation dependence of the fracture toughness of the hydroxyapatite grains and grain boundaries.     

Fabrication of c-axis oriented hydroxyapatite ceramics in a rotating high magnetic field using photopolymerization

C-axis oriented hydroxyapatite (HAp) was prepared by colloidal processing using a photopolymerization reaction in a rotating magnetic field with a UV-curable binder as the solvent. This technique achieved a short processing time of 150 s in a magnetic field as a result of rapid solidification induced by the photopolymerization. The slight difference in refractive index between the HAp material and UV curable resin allowed the fabrication of 1.6 mm thick green compacts. The oriented structure in the compact was maintained from the surface down to a depth of 700 μm, but was randomized at depths of 700 μm and beyond. The orientation degree was retained after sintering at 1250 °C, and the relative density of the compact was approximately 97 %. This technique can be effectively utilized for the manufacture of high-performance biomaterials.     

Highly c-axis oriented W-type hexaferrites SrNi2ScxFe16-xO27 (x = 0.70, 0.75, 0.80 and 0.85) for the design of self-biased circulator at Ku band

Highly oriented W-type hexaferrites SrNi₂Sc_xFe_16-xO₂₇ (SrW, x = 0.70, 0.75, 0.80 and 0.85) are fabricated via ceramic process and magnetic orientation to meet design requirements of self-biased circulators. The orientation degree reaches 0.81 when Sc³⁺ substitution content x is 0.75. The XRD refinement results in consideration of orientation illustrate the strong c-axis anisotropy texture of SrW. SEM images show significant uniformity in grain growth with c-axis stays vertical to sample plane. Consequently, the corresponding remanence ratio M_r/M_s reaches as high as 0.92, which is indispensable to reduce low field loss (LFL) as much as possible in the design of self-biased circulators. According to simulation results, the self-biased microstrip circulator based on this SrW material (x = 0.75) realizes ideal functions in circular transmission operating at Ku band.     

Enhanced properties of tailored MoAlB ceramics by hot forging

In this work, hot forging texture of pre-sintered MoAlB ceramics (relative density, 97.9%) were achieved by spark plasma sintering (SPS). The radial compression and lateral expansion rates reached ? 54.8% and 25.8%, respectively. The X-ray diffraction results exhibited a preferred orientation of tailored samples. The b-axis of grains were prone to be parallel to the pressure and the grains grew along the a- and c-axis directions. Therefore, the Lotgering orientation factor of f_(0k0) on the textured top surface (TTS) get 0.66. The properties of tailored MoAlB exhibited obvious anisotropic characteristics. Remarkably, the electrical conductivity perpendicular to the b-axis reached 4.34 × 10⁶ S/m (25 °C). Compared with untextured MoAlB (5.45 MPa·m^1/2), the fracture toughness parallel to the b-axis reached 8.04 MPa·m^1/2. Additionally, the tailored surface perpendicular to the b-axis presented better oxidation resistance with thinner oxide layer when exposed at 1400 °C for 20 h in air.     

A study on the crystallization behavior of poly(β-hydroxybutyrate) thin films on Si wafers

The exact molecular chain orientation of poly(β-hydroxybutyrate) (PHB) in ultrathin films was successfully probed using surface-sensitive, grazing incidence X-ray diffraction techniques. The crystal orientation of spin-coated PHB films was very sensitive to free surface and thermal annealing. In pristine films, the free surface easily exerted its influence on PHB crystallization and caused lamellar orientation with the b-axis perpendicular to the film surface. The effect of the buried interface increased with temperature. With the increase in thermal annealing temperature, the lamellar orientation changed from the b-axis being perpendicular to the film surface to the c-axis becoming perpendicular to the film surface. As film thickness increased, the temperature, at which the lamellae with the b-axes oriented normal to the film surface disappeared, increased. The thickness and temperature dependence of the crystallization behavior of PHB in an ultrathin film could be attributed to the competition between the effects of the free surface and the buried interface.     

A novel strategy for c-axis textured silicon nitride ceramics by hot extrusion

Highly c-axis textured β-silicon nitride (β-Si₃N₄) ceramic with fine grains was prepared by a new method of hot extrusion for the first time. The (002) pole figure on the section plane vertical to extruding direction showed a characteristic of center rotational symmetry. The average cline angle between elongated β-Si₃N₄ grains and hot extruding direction was about 14.4°. The degree of c-axis texturing by hot extrusion was comparable to that achieved by rotating magnetic field. The hardness and toughness anisotropy in different direction was apparent and relatively higher hardness was achieved in the present work mainly due to the finer grain size. Therefore, many different compositions of c-axis aligned Si₃N₄-based ceramics with tailored mechanical properties could be achieved by the strategy of hot extrusion.     

Unique orientation textures formed in miscible blends of poly(vinylidene fluoride) and poly[(R)-3-hydroxybutyrate]

The oriented crystallization of poly[(R)-3-hydroxybutyrate] (PHB) in the miscible blends with poly(vinylidene fluoride) (PVDF) was investigated with various compositions. The PVDF/PHB blend films were prepared by solution casting and subsequent melt-quenching in ice water. Oriented films of the blends were prepared by uniaxially stretching the melt-quenched film at 0 °C in ice water using a hand-operated stretching apparatus. The oriented blend films were heat-treated at a fixed length in order to crystallize PHB in the oriented state. The crystal orientation and the lamellar textures of the obtained samples were studied with wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS), respectively. The SAXS measurements showed that a considerable amount of molecular chains of PHB are excluded from the lamellar stacks of PVDF and exist in the interfibrillar regions in the oriented films of the blends. The cold crystallization of PHB in the interfibrillar region results in the orientation of PHB crystals, and the type of crystal orientation depends upon the composition of the blends. For the PVDF/PHB=4/6-7/3 blends, the crystal a-axis of PHB is highly oriented parallel to the drawing direction and the crystal c-axis (molecular chain axis) in PHB crystals is perpendicular to the drawing direction, i.e. orthogonal to the chain axis of the crystals of PVDF. It is considered that the a-axis orientation is induced by the confinement of crystal growth in the interfibrillar nano-domains. For the PVDF/PHB=2/8-3/7 blends, however, the crystal c-axis of PHB is primarily oriented in the drawing direction, suggesting that the stressed molecular chains of PHB are crystallized with the molecular orientation retained.     

Plastic deformation of oriented lamellae: 1. Cold rolling of β-phase isotactic polypropylene

Isotactic polypropylene was crystallized by the oriented growth method and the oriented β-phase obtained. This has unidirectional lamellar orientation with the lamellar long axis parallel to the growth direction, the lamellae being twisted along this direction. The sample plates were cold-rolled in three orthogonal directions, and the deformation behaviour of each case was investigated chiefly by wide-angle and small-angle X-ray diffraction methods. It was revealed that deformation takes place by a different mechanism in each case, including rotation of lamellae, interlamellar slip, chain-directional and transversal chain slip. These results are discussed in connection with the anisotropic structure of these samples due to the lamellar orientation.When the β-phase samples are rolled, α-phase crystals appear with c-axis orientation and the proportion increases with draw ratio. For crystallographic reasons it is concluded in this case that by stretching the c-axis orientation is brought about not through block formation of the original β-phase lamellae and incorporation of these blocks into microfibrils, but by melting or unfolding of the original β-phase lamellae and recrystallization to the c-axis-oriented new α-phase.     

Textured M-type barium hexaferrite Ba(ZnSn)xFe12−2xO19 with c-axis anisotropy and high squareness ratio

A series of oriented M-type barium hexaferrites (BaM) of composition Ba(ZnSn)_xFe_12?2×O₁₉ (where x?=?0, 0.2, 0.4, and 0.6) was fabricated. Pure BaM phase formation for all samples was confirmed by the X-ray diffraction patterns. The scanning electron microscopy images revealed that the grains in the oriented BaM had realigned along the hexagonal crystallographic c-axis, which was in good agreement with the results of magnetic hysteresis loops. Moreover, by treating under appropriate sintering conditions, oriented BaM with a high squareness ratio (M_r/M_s =?0.81) was obtained. Owing to the Zn–Sn substitution, the magnetocrystalline anisotropy field (H_k) decreased dramatically while maintaining the c-axis anisotropy, except for x?=?0.6. A narrow ferromagnetic resonance linewidth ΔH of 667?Oe was measured when x?=?0.4. The oriented BaM are potential candidates for the design of self-biased microwave devices such as circulators and isolators operated at low frequencies.     

Effect of Al2O3 addition on texturing in a rotating strong magnetic field and densification of B4C

The properties of ceramics can be improved by controlling the microstructure through texturing ceramics in a strong magnetic field. Fabricating dense boron carbide (B₄C) requires high temperature sintering, therefore sintering additives are often used in order to densify B₄C ceramics at lower temperatures. However, combined effect of texturing and sintering additives on densification of B₄C has not been made clear yet. Here we report the effect of alumina (Al₂O₃) sintering additive on texturing in a strong magnetic field and densification of B₄C. Texturing was performed by rotating superconducting magnet at 12 T during slip casting process. Electron backscatter diffraction (EBSD) was used to observed the texturing projection. {0001} plane is clearly oriented in the plane parallel to rotating magnetic field. In addition, Lotgering factor was also calculated as quantitatively evaluation of texturing degree. Results on densification showed that addition of Al₂O₃ successfully increased density of B₄C sintered by spark plasma sintering (SPS) at 1800^oC to 97.8%. Formation of aluminum borate (Al₅BO₉) as secondary phase was detected by X-Ray diffraction (XRD). It is considered that the generation of Al₅BO₉ assisted finer densification of B₄C ceramic. Textured B₄C sintered at 1700^oC by SPS without alumina addition exhibited the highest orientation of c-axis. Addition of alumina caused decrease in degree of orientation of c-axis.     

Orientation of lamellar crystals and its correlation with switching behavior in ferroelectric P(VDF-TrFE) ultra-thin films

The ferroelectric properties of poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) thin films mainly depend on the crystal orientation and crystallinity. We demonstrate here the thermal history and thickness dependent crystal orientation in P(VDF-TrFE) ultrathin films, and its correlation with local polarization reversal. Upon annealing in the paraelectric phase after spin-cast, the lamellar crystals grow preferentially along the crystallographic a axis, with the c axis (chain axis) parallel to the substrate. In contrast, when crystallized in the paraelectric phase from melt, the lamellar crystals take a flat-on orientation with the crystallographic c-axis normal to the substrate. In addition, local measurement by piezoresponse force microscopy indicates that the flat-on crystals do not display any polarization switching, whereas the edge-on crystals exhibit proper switching upon application of a vertical electric field. Importantly, the coercive field measured from the piezoresponse hysteresis loops does not change with the film thickness in the edge-on oriented lamellar crystals.     

Highly textured and strongly anisotropic TiB2 ceramics prepared using magnetic field alignment (9T)

Highly textured TiB₂ ceramics were prepared by slip casting an aqueous suspension in a magnetic field of 9 T, followed by sintering using Field Assisted Sintering Technology (FAST). Particle size refinement by ball milling improved both the degree of texturing and densification of the material (RD > 98 %). The sintered material exhibited a Lotgering orientation factor of 0.90, with the c-axis of TiB₂ oriented parallel to the magnetic field and FAST pressing direction. The texturing effect induced by the uniaxial pressing was negligible. The textured TiB₂ material exhibited a significant anisotropy in mechanical properties; the values of hardness and indentation elastic modulus measured along directions transverse to the c-axis of TiB₂ were 37 % and 13 % higher than the ones measured along the c-axis direction. Moreover, the specific wear rate of a surface of textured TiB₂ parallel to the field was one order of magnitude lower than a surface perpendicular to the field.     

Control of molecular orientation

I. Amorphous polymers . The mechanical performance of a glassy amorphous polymer is strongly dependent upon molecular orientation. The pattern of molecular orientation is governed by the kinematics (and temperature) of mechanical forming operations. Three types of controllable orientation are: (a) uniaxial, (b) biaxial, and (c) “crossed.” The optimum pattern of orientation in a part is one which is appropriate for the mechanical stresses encountered in service. For a fiber subjected to tensile and bending loads, uniaxial orientation is appropriate. A shell structure, subjected to multiaxial stresses, requires either biaxial or crossed orientation for maximum performance. As a rule, the maximum achievable multidirectional strength in such a structure is less than the maximum strength of a uniaxially oriented fiber. II. Crystalline polymers . Oriented crystalline polymer structures can be created in two distinct ways. An isotropic polycrystalline polymer can be deformed below the melting point, with extensive reorganization of the crystal morphology, or an oriented amorphous melt can undergo crystallization to yield oriented crystalline polymer. Performance of an oriented semicrystalline polymer depends upon orientation of the amorphous portion as well as orientation of the crystallites. As with amorphous polymers, orientation can be uniaxial, biaxial, or crossed. “Orientation” usually denotes c-axis orientation only, but drawing followed by rolling can result in double orientation—orientation of a-axis, b-axis, and c-axis.     

Analysis of abnormal grain growth of oriented LiCoO2 prepared by slip casting in a strong magnetic field

The relationship between the development of the crystallographic orientation and the grain growth behavior were studied. The degree of orientations of the green compacts and sintered samples were evaluated by the Lotgering factor. The f(0 0 l) of all the samples were drastically increased with the increasing applied magnetic field strength. The f(0 0 l) of the samples sintered at 1223 K were improved in comparison to those of the green compacts. However, the f(0 0 l) value of the samples sintered at 1273 K were not increased at 4 T or lower. To characterize the grain growth process, these samples were analyzed using electron backscatter diffraction (EBSD). The sintered samples prepared in the magnetic field at 4 T or lower showed abnormal grain growth. The samples with an applied magnetic field of 8 T or higher had no abnormal grain growth. It was revealed that the orientation angle of the particles has an effect on the grain growth.     

Preparation of c-axis-oriented Bi4Ti3O12 thick films by templated grain growth

Highly c-axis-oriented Bi₄Ti₃O₁₂ thick films were successfully fabricated by templated grain growth. The effects of template particles and sintering conditions on grain orientation in thick films were investigated. SEM micrographs and X-ray diffraction (XRD) patterns exhibited that thick films were c-axis-oriented. The degree of grain orientation (Lotgering factor, f) increases with increasing sintering temperature and soaking time. Highly c-axis-oriented thick film (orientation degree of ∼ 0.98) is obtained with the use of only 5 wt.% template particles by sintering at 1000 °C for 2 h. This film exhibits a better temperature-independent dielectric constant and a lower dielectric loss.