Effect of Grain Boundaries on Thermal Conductivity of Silicon Carbide Ceramic at 5 to 1300 K

The thermal conductivity of a SiC ceramic was measured as 270 W·m⁻¹·K⁻¹ at room temperature. At low temperatures ( T < 25 K), the decrease in the conductivity was proportional to T ³ on a logarithmic scale, which indicated that the conductivity was controlled by boundaries. The calculated phonon mean free path in the ceramic increased with decreased temperature, but was limited to ∼4 μm, a length almost equal to the grain size, at temperatures below 30 K. We concluded that the thermal conductivity of the ceramic below 30 K was influenced significantly by grain boundaries and grain junctions.     

EFFECT OF INTERMITTENT HEATING ON DRYING-INDUCED STRAIN-STRESS OF MOLDED CLAY

ABSTRACT

Parametric analyses of the strain-stress in the drying process are performed to study the influence of the intermittent drying on the behaviors of drying-induced strain-stress and deformation as well as the drying characteristic. The transient three-dimensional problem of strain-stress and heat and moisture transfer in a slab is solved simultaneously by the finite element method. The dimensionless parameters are introduced to generalize the problem in the conservation equations of heat and moisture transfer. The intermittent drying is modeled by applying periodically smaller and larger Biot numbers to the boundary conditions. The maximum tensile and compressive stresses fluctuate, and fall remarkably during the smaller Biot number period when a slab is heated intermittently. The peak stress of the fluctuation exceeds beyond the case in the continuous healing where the overall drying rate is almost equivalent to that in the intermittent beating, but the reduction of the stresses takes place rapidly in the low heating period.     

Grazing-incidence wide-angle X-ray diffraction study on molecular aggregation state of imprinted polyimide film before and after hard baking

Nanoimprint lithography (NIL) was carried out on precursor of polyimide (PI), poly(amic acid) film, and then hard baking to obtain imprinted PI film. The molecular aggregation states of imprinted PI films before and after hard baking were investigated by grazing-incidence wide-angle X-ray diffraction comparing with the one of flat PI film. It was found that NIL and hard baking can strongly affect the molecular aggregation states of PI film. Before hard baking, PI chain is aligned parallel to the line direction on the line. After hard baking, the alignment in ordered domain was changed to that the PI molecule of which chain axis is perpendicular to the line direction is significantly increased, while, PI molecule of which chain axis is parallel to the line direction is decreased after hard baking. Through comparing with the flat PI, crystallinity of imprinted PI film has been significantly enhanced.     

Photocatalytic effects of plasma-heated TiO2−x particles under visible light irradiation

The photocatalytic characteristics of partially reduced TiO₂ (TiO_2?x ) by plasma treatment and plasma-heated treatment were investigated in the visible-light region. For the visible-light photocatalytic activity of TiO_2?x , plasmaheated treatment shows stronger than plasma treatment significantly. The TiO_2?x by plasma-heated treatment shows broader red-shifted absorption bands than one by plasma treatment in the visible-light region. The surface color of TiO_2?x by plasma treatment and plasma-heated treatment changed from white to sky blue, and to navy, respectively. After exposure to air, the surface color of TiO_2?x changed from sky blue to white for plasma treatment and from navy to beige for plasma-heated treatment.     

Humido-sensitive conducting polymer films and applications to linear actuators

Free-standing films made of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT/PSS) with various PSS contents were newly prepared by casting water dispersion of the PEDOT/PSS colloidal particles in the presence of an extra PSS. Electrical conductivity, morphology, water vapor sorption, and electro-active polymer actuating behavior of the resulting films were investigated by means of four-point method, atomic force microscope (AFM), sorption isotherm, and electromechanical analyses. The maximum contraction of the film by application of an electric field increased with increasing both PSS content and relative humidity (RH), where the value attained 7% at 70% RH for the film with 93% of PSS. Since the isothermal sorption curve of the film was less dependent on the PSS content, the significant increase of the film contraction was explained by two mechanisms: (i) the extra PSS prevented from hydrogen bonding between adjacent PEDOT/PSS particles that suppressed dimensional changes of the film; and (ii) the higher the RH, the larger the degree of water vapor sorption, which led to the large film contraction by desorption of water vapor via Joule heating. On the basis of this phenomenon linear actuators utilizing PEDOT/PSS films were successfully developed and applied to leverage actuator and Braille cell.     

Preparation of π-conjugated polymers consisting of 1-aminopyrrole and 4-amino-1,2,4-triazole units

π-Conjugated polymers based on 1-aminopyrrole and 4-amino-1,2,4-triazole were prepared. Pd-catalyzed organometallic polycondensation gave poly(arylene-ethynylene) (PAE)-type π-conjugated polymers consisting of BOC-protected 4-amino-1,2,4-triazole (Taz(BOC)) units (BOC = t-butoxycarbonyl). The number average molecular weights (M_n) were determined to be 5200–19,200 using GPC. An alternating copolymer of Taz(BOC) and bithiophene (Th–Th) units, P(Taz(BOC)-Th-Th)_n-Pd, with an M_n of 10,800 was also prepared by Pd-catalyzed organometallic polycondensation. (Taz(BOC)-Th-Th)_n-Pd showed a UV–Vis absorption peak at λ_max = 425 nm, which is reasonable for a π-conjugated five-membered ring polymer with a coplanar structure. The deprotection of the BOC group of the polymers proceeded at 200 °C; this BOC-deprotection was investigated using a model compound. The optical and electrochemical properties of the π-conjugated polymers are reported.     

Piezoelectric properties of langasite group based on the ionic size of cation

As langasite A₃BC₃D₂O₁₄ compounds group with piezoelectric properties has no phase transition up to the melting point of 1400–1500 °C, they have been applied for the combustion pressure sensor. As they also have a larger electromechanical coupling factor compared to quartz and nearly the same temperature stability as quartz, the surface acoustic wave (SAW) filters are expected based on the digital transformation of wider bandwidth and higher-bit rates. In the case of three-element compounds such as [R₃]_A[Ga]_B[Ga₃]_C[GaSi]_DO₁₄ (R=La, Pr and Nd), the piezoelectric constant increases with the ionic radius R. In the case of four-element compounds such as [A₃]_A[B]_B[Ga₃]_C[Si₂]_DO₁₄ (A=Ca or Sr, B=Ta or Nb), |d₁₁| and k₁₂ values as a function of A_L/B_L ratio showed a linear relationship completely. There are two effects for the substitutions of A- and B-site cations: the substitution of Sr for Ca brings expansion toward [100] and enlargement in |d₁₁| and k₁₂. On the other hand, as the substitution of Ta for Nb brings not much change, the properties are similar. In this study, five-element compounds such as La_3?xSr_xTa_yGa_5?x+ySi_1+x?2yO₁₄ (0≤x≤3, 0≤y≤1) solid solutions analysed by a single crystal X-ray diffraction are compared with the three- and four-element compounds on the coordinates of oxygen ions. As the coordinates positions are located on the extended line of coordinate positions on the three-element compounds as increasing ionic radius of R-ions in A-site, the piezoelectric properties |d₁₁| and k₁₂ are expected become large.     

Fabrication of Transparent, Sintered Sc2O3 Ceramics

We report here the fabrication of transparent Sc₂O₃ ceramics via vacuum sintering. The starting Sc₂O₃ powders are pyrolyzed from a basic sulfate precursor (Sc(OH)_2.6(SO₄)_0.2·H₂O) precipitated from scandium sulfate solution with hexamethylenetetramine as the precipitant. Thermal decomposition behavior of the precursor is studied via differential thermal analysis/thermogravimetry, Fourier transform infrared spectroscopy, X-ray diffractometry, and elemental analysis. Sinterability of the Sc₂O₃ powders is studied via dilatometry. Microstructure evolution of the ceramic during sintering is investigated via field emission scanning electron microscopy. The best calcination temperature for the precursor is 1100°C, at which the resultant Sc₂O₃ powder is ultrafine (∼85 nm), well dispersed, and almost free from residual sulfur contamination. With this reactive powder, transparent Sc₂O₃ ceramics having an average grain size of ∼9 μm and showing a visible wavelength transmittance of ∼60–62% (∼76% of that of Sc₂O₃ single crystal) have been fabricated via vacuum sintering at a relatively low temperature of 1700°C for 4 h.     

Picoliter volume glass tube array fabricated by Si electrochemical etching process

Fabrication process for picoliter volume SiO₂ glass tube array partially embedded in Si wafer was developed. As a template for the glass tubes, macropore array was formed at the surface of n-Si(1 0 0) wafer by photo-assisted electrochemical etching process. The area-selective formation of the array was achieved by applying Au/Cr micropatterns formed at the back-side surface of the substrate as the shade mask, which controls the illumination condition to optimize the etching reaction conditions. Subsequently, surface of the macropores was wet-thermally oxidized to form glass layer, and the bulk Si region was removed by alkaline etching, remaining the “glass tubes”. As a result of complete removal of the bulk Si, released glass tubes were obtained. By partial removal of the bulk Si part, the glass tubes were exposed, fixed in the remaining Si substrate in the form of well-ordered array. It was confirmed that the depth, the exposed region and the wall thickness of each glass tube were controllable by adjusting the parameters such as the duration of the Si electrochemical etching, the alkaline etching and the wet-thermal oxidation, respectively. In order to demonstrate microreaction in the glass tube, aqueous rhodamine B solution was injected into the tubes and excitation light was irradiated to them. As a result, the fluorescence of rhodamine B was clearly detected, confirming the applicability of the glass tubes for various kinds of devices and systems such as microreactors.