Lead halide perovskite for efficient optoacoustic conversion and application toward high-resolution ultrasound imaging

Lead halide perovskites have exhibited excellent performance in solar cells, LEDs and detectors. Thermal properties of perovskites, such as heat capacity and thermal conductivity, have rarely been studied and corresponding devices have barely been explored. Considering the high absorption coefficient (104~105 cm−1), low specific heat capacity (296–326 J kg−1 K−1) and small thermal diffusion coefficient (0.145 mm2 s−1), herein we showcase the successful use of perovskite in optoacoustic transducers. The theoretically calculated phonon spectrum shows that the overlap of optical phonons and acoustic phonons leads to the up-conversion of acoustic phonons, and thus results in experimentally measured low thermal diffusion coefficient. The assembled device of PDMS/MAPbI3/PDMS simultaneously achieves broad bandwidths (−6 dB bandwidth: 40.8 MHz; central frequency: 29.2 MHz), and high conversion efficiency (2.97 × 10−2), while all these parameters are the record values for optoacoustic transducers. We also fabricate miniatured devices by assembling perovskite film onto fibers, and clearly resolve the fine structure of fisheyes, which demonstrates the strong competitiveness of perovskite based optoacoustic transducers for ultrasound imaging.

Sensitivity Analysis of the Thermal Diffusion Coefficient Effect on the Departure From Nucleate Boiling Ratio With the VIPRE Code

One of the limiting conditions during operation of a pressurized water reactor (PWR) is cladding integrity in class I (normal operations) or class II (most frequent). Cladding integrity is limited typically by the departure from the nucleate boiling (DNB), which criterion ensures an appropriate core cooling. Adequate heat transfer between the fuel cladding and reactor coolant is achieved by preventing DNB that is avoided if the local heat flux is lower than the critical heat flux (CHF). The DNB is estimated thanks to thermal-hydraulic (TH) design codes, as the VIPRE-W code that predicts the fluid behavior based on the geometry of the problem, the fuel rods and the fluid properties among others. One of the parameters that influences the DNB estimation is the thermal diffusion coefficient (TDC), which depends on the fuel design and is affected by the grid spacing. As a matter of fact, the TDC enters into the DNB calculation for thermal mixing between subchannels and in some special cases like the most primitive fuel designs, as a factor within the DNB correlation. Nevertheless, although the TDC is a variable, the TH design codes used for the DNB prediction consider the TDC as a constant. This investigation is founded on a new numerical program developed to explore the effect of the TDC on the DNB. In addition to this, variables as the effect of the turbulent momentum factor (FTM) and the correlation effect has been explored too. The most direct outcome of this research is the substantial extension of the existing studies of VIPRE-W TH code. The results show that TDC has an effect on the DNB dominated by the radial power distribution. The departure from nucleate boiling ratio (DNBR) increases up to 1.2% when TDC is a variable under normal operation radial shapes. For the design radial distribution, this effect is vanished but observable for values under 0.02 with an exponential increase of the DNBR with respect to the TDC. From this moment on, the energy exchanged between subchannels is negligible due to the flatness shape of the radial enthalpy distribution.

Measurement of Thermal Diffusion Coefficient of High Thermal Con-ductivity Pellets by Laser Scattering Method

UO2-BeO high thermal conductivity pellets have the characteristics of high thermal conductivity, high efficiency, long cycle and low recovery cost compared with traditional nuclear fuel. In this paper, the thermal diffusion coefficient of UO2-BeO pellets is characterized by laser flash method, in which the selection and optimization of test parameters are the main research part.

Growth and Thermal Properties of Mg-Doped Lithium Isotope Niobate (Mg:7LiNbO3) Crystal

An Mg-doped isotope lithium niobate (Mg:7LiNbO3) crystal was successfully grown from 7LiOH, Nb2O5, and MgO using the Crozchralski method. The weight of the as-grown crystal with good quality was about 40 g. The crystal structure was determined as an R3c space group using the X-ray powder diffraction (XRPD) method, and the crystal composition (Li%) determined using the Raman mode linewidth method was 49.29%. The average transmittance of the crystal in the range of 500–2500 nm was approximately 72%. Various thermal properties, including the specific heat (Cp), the thermal expansion coefficient (α), the thermal diffusion coefficient (λ), and the thermal conductivity (κ), were carefully determined and calculated, and the value divergences among Mg:7LiNbO3, the undoped isotope lithium niobate (7LiNbO3), and natural lithium niobate (LiNbO3) crystals were mainly related to the differences in microstructure caused by the crystal composition.

INFLUENCE OF THERMAL DIFFUSION ON KINETICS OF OSCILLATING IR DRYING

The relative thermal diffusion coefficient δt was calculated from the obtained moisture sorption isotherm for the seeds of onions. It allowed estimating the contribution of thermal diffusion with respect to the flow of mass conductivity in the considered version of the oscillating IR drying, and it turned out to be 5%. Hence, the thermal diffusion can be ignored for engineering calculations of the kinetics of the oscillating IR drying.

ANALYSIS OF THE THERMOPHYSICAL PROPERTIES AND INFLUENCING FACTORS OF VARIOUS ROCK TYPES FROM THE GUIZHOU PROVINCE

A series of analyses have been carried out on a number of rock types from the Guizhou Province to investigate their thermophysical properties. A total of 433 samples from 14 types of rock were collected, tested and analyzed. It was found that in this province, the average thermal conductivity of the samples ranged between 1.516±0.264 and 5.066±0.521 W/(m·K), the average specific heat capacity varied from 0.272±0.042 to 0.603±0.096 kJ/(kg·°C), and the average thermal diffusion coefficients were from 0.752±0.331 to 2.854±0.368 MJ/(m3·K). The older rocks always had higher thermal conductivity and thermal diffusion. Thermal conductivity and thermal diffusion of rocks are positively correlated with the mineral content of high thermal conductivity species, but the situation for the specific heat capacity is the opposite. With increasing mineral particle size, the thermal conductivity and thermal diffusion coefficient also increase, but the relationship with specific heat capacity is not obvious. The thermal conductivity and thermal diffusion coefficient of rocks increases under water saturated conditions compared to dry conditions, but the specific heat capacity decreases.

Laser-Induced Magnetization Dynamics of GdFeCo Film Probing by Time Resolved Magneto-Optic Kerr Effect

Laser-induced magnetization dynamics in Gd[Formula: see text]Fe[Formula: see text]Co[Formula: see text] (GdFeCo) film was investigated by time-resolved magneto-optical effect. After an ultrafast demagnetization and a magnetization recovery process, a second slower drop from several picoseconds (ps) to 200[Formula: see text]ps was observed. Previous report explained it as the demagnetization of Gd sublattice. However, when timescale extended up to 5 nanosecond (ns), we identified it as the part of an oscillated magnetization precession. It presents the change of magnetization orientation rather than the change of magnetization amplitude. Our results reveal that the low thermal diffusion coefficient of GdFeCo film plays an important role in the laser-induced magnetization dynamics of GdFeCo film.