Thermal diffusivity and specific heat of 4H-SiC crystals as a function of temperature are measured, respectively, from room temperature to 600 °C. The thermal conductivity normal to c-axis was calculated from the measured data for both N-type and V-doped semi-insulating (SI) 4H-SiC single crystals. The thermal conductivity of N-type sample normal to c axis is proportional to T−1.26. It is approximately 280 W/mK at the room temperature. For V-doped SI sample, the thermal conductivity is proportional to T−1.256 and it is about 347 W/mK at room temperature, bigger than that of N-type sample. For semiconductor materials, total thermal conductivity is the sum of the contributions of lattice and carrier thermal conductivities. Temperature dependent Raman spectrum showed that the life time of phonons for N-type sample is shorter than that for SI sample. Accordingly thermal conductivity contributions from both lattice and carrier components are relatively small for N-type sample.
With mixed transition-metal (TM) complex, alkali-metal cations, or halogen anions as structure-directing agents, two types of two-dimensional (2D) layered inorganic-organic hybrid silver bromides were prepared and structurally characterized as K[TM(2,2-bipy)]AgBr (TM = Ni (1), Co (2), Zn (3), Fe (4)) and [TM(2,2-bipy)]AgBr (TM = Ni (5), Co (6), Zn (7), Fe (8)). Compounds 1-4 feature 2D microporous anionic [AgBr] layers composed of [AgBr] secondary building units based on AgBr tetrahedral units, and compounds 5-8 contain 2D [AgBr] layers built from the one-dimensional complex [AgBr] and [AgBr] chains. The photosensitization of TM complex dyes led to the narrow semiconducting behaviors with tunable band gaps of 1.73-2.71 eV for the title compounds, which result in excellent and stable photocatalytic degradation activities over organic pollutants under visible-light irradiation. The studies of photocatalytic mechanism based on radical-trapping experiments and electronic band structural calculation show that the TM complex cations play important roles in the photocatalytical activities and photochemical stabilities due to their excellent separating abilities for photogenerated carriers. This technique affords one new type of visible-light-driven photocatalyst and facilitates the integration of 2D layered materials and semiconducting photocatalytic properties into one hybrid d TM halogenide.
Large Ca0.28Ba0.72Nb2O6 (CBN-28) single crystals exhibit saturated ferroelectric hysteresis loops under an electric field of 115kV∕cm along the [001] direction. Their spontaneous polarization, remanent polarization and coercive field are 35.3μC∕cm2, 32.2μC∕cm2, and 38.1kV∕cm, respectively. The dielectric constant and dielectric loss tanδ of the crystals are 195 and 0.32 at 10 kHz, respectively. A diffused dielectric anomaly with relaxor characteristic was observed in the range of 325-500 °C at low frequencies, while the CBN-28 crystals experienced a first-order normal-relaxor ferroelectric phase transition around 252 °C on heating. A broad dielectric loss peak appears around 120 °C and it is interpreted in terms of the migration of oxygen vacancies. The conductance activation energy was determined to be 1.33 eV in the high temperature regime (500–560 °C).
All-optical amplify-and-forward (OAF) relaying technique can amplify and filter the attenuated optical signal in optical domain, and thus is regarded as a simple way to extend the transmission distance of free-space optical (FSO) communication systems. At the relaying node, however, the incident background radiation and the amplified spontaneous emission noise of the deployed erbium-doped fiber amplifier will deteriorate the optical signal-to-noise, therefore causing the degradation of the system's bit error ratio (BER). In this paper, we perform simulations and experiments on FSO system with on-off keying (OOK) and differential phase-shift keying (DPSK) signals, focusing on the receiving sensitivity loss (RSL) of the OAF-assisted dual-hop system. Taking the BER level of 10 −7 as the reference, we find that the DPSK system suffers more than 8 dB RSL at the data rate of 5 Gb/s in simulation, while the OOK system gets less than 1 dB RSL at the same data rate. In the following experiments, the obtained results exhibit that RSL of DPSK system is nearly 10 dB, which of OOK system is less than 1.5 dB, shown similarly property to the simulations.
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