Ni-Co Complex Ionic Synergistically Modifying Octahedron Lattice of Cordierite Ceramics for the Application of 5G Microwave-Millimeter-wave Antennas.

In this work, phase-pure Mg1.8(Ni1-xCox)0.2Al4Si5O18 (0 ≤ x ≤ 1) ceramics were synthesized by a high-temperature solid-state method. On the basis of Rietveld refinement data of X-ray powder diffraction and Phillips-Vechten-Levine theory, the atomic ionicity, lattice energy, and bond energy of the compound were calculated to explore their influence on the microwave dielectric properties of ceramics. The Mg1.8Ni0.1Co0.1Al4Si5O18 (x = 0.5) ceramic exhibited the best microwave dielectric properties: εr = 4.44, Qf = 73 539 GHz@13 GHz, and τf = -23.9 ppm/°C. (Ni1-xCox)2+ complex ionic doping, compared with only Ni2+ or Co2+, is beneficial for improving the symmetry of [Si4Al2O18] hexagonal rings and reducing distortion. Subsequently, 8 wt % TiO2 was added to Mg1.8Ni0.1Co0.1Al4Si5O18, resulting in a near-zero τf and high Qf values for the composite ceramic, with εr = 5.22, Qf = 58 449 GHz@13 GHz, and τf = -2.06 ppm/°C. Finally, a 5G millimeter-wave antenna with a central operating frequency of 25.52 GHz was designed and fabricated using the Mg1.8Ni0.1Co0.1Al4Si5O18-8 wt % TiO2 ceramics. Operating in the 24.7-26.0 GHz range, it demonstrated favorable radiation characteristics with a simulated efficiency of 85.2% and a gain of 4.58 dBi. The antenna's performance confirms the high potential of the cordierite composite for application in 5G communication systems.

Synthesis of functionalized tetrahydrodibenzo[b,g][1,8]naphthyridin-1(2H)-ones through base-promoted annulation of quinoline-derived dipolarophiles and cyclic enaminones.

An eco-friendly and metal-free method for the synthesis of tetrahydrodibenzo[b,g][1,8]naphthyridin-1(2H)-ones was established. Quinoline-derived dipolarophiles and cyclic enaminones as starting materials undergo a 1,4-Michael addition/SNAr tandem annulation reaction affording the target products. This approach features transition metal-free conditions, good functional group tolerance and operational simplicity.

A quasi-one-dimensional bulk thermoelectrics with high performance near room temperature.

Pursuing efficient thermoelectricity from low-dimensional materials has been highly motivated since the seminal work of Hicks and Dresselhaus. In fact, many superior thermoelectric materials like Bi2Te3, Mg3Sb2/Mg3Bi2 and SnSe are quasi-two-dimensional (q2D), though the advantages of two-dimensionality appear to be diverse and sometimes controversial. Here, we report on a remarkably high thermoelectric performance in TlCu3Te2, which is quasi-one-dimensional (q1D) with a further reduced dimension. The thermoelectric figure of merit zT along its q1D axis amounts to 1.3 (1.5) at 300 (400) K, rivaling the best ever reported at these temperatures. The high thermoelectric performances benefit from, on one hand, large power factors derived from a center-hollowed, pancake-like Fermi pocket with q1D dispersion at the edge of a narrow band gap, and on the other hand, small lattice thermal conductivities caused by the large and anharmonic q1D lattice consisting of heavy, lone-pair-electron bearing (Tl+) and weakly-bonded (Cu+) ions. This compound represents the first bulk material with quasi-uniaxial thermoelectric transport of application level, offering a renewed opportunity to exploit reduced dimensionality for high-performance thermoelectricity.

Integrated measurement of thermoelectric properties for filamentary materials using a modified hot wire method.

Thermoelectric materials have been rapidly developed due to the urgent need for the mutual conversion of thermal energy and electrical energy. Accurately measuring the thermoelectric properties of micro/nano thermoelectric materials is very important and highly required. Compared with traditional measurement methods, integrated measurement can avoid multiple sample preparations and reduce measurement errors. Herein, this work designed an improved integrated measurement method for the thermoelectric properties of microscale thermoelectric materials based on the hot wire method. The results demonstrated that the average ZT values of Pt and Ag2S wires are 0.75 × 10-3 and 0.44 × 10-3 with an uncertainty of ∼2.61%. It provides a novel way for the development of accurately measuring the thermoelectric properties of thermoelectric materials.

Pre-Ball-Milled Boron Nitride for the Preparation of Boron Nitride/Polyetherimide Nanocomposite Film with Enhanced Breakdown Strength and Mechanical Properties for Thermal Management.

Modern electronics not only require the thermal management ability of polymer packaging materials but also need anti-voltage and mechanical properties. Boron nitride nanosheets (BNNS), an ideal thermally conductive and high withstand voltage (800 kV/mm) filler, can meet application needs, but the complex and low-yield process limits their large-scale fabrication. Herein, in this work, we prepare sucrose-assisted ball-milled BN(SABM-BN)/polyetherimide (PEI) composite films by a casting-hot pressing method. SABM-BN, as a pre-ball-milled filler, contains BNNS and BN thick sheets. We mainly investigated the thermal conductivity (TC), breakdown strength, and mechanical properties of composites. After pre-ball milling, the in-plane TC of the composite film is reduced. It decreases from 2.69 to 2.31 W/mK for BN/PEI composite film at 30 wt% content; however, the through-plane TC of composites is improved, and the breakdown strength and tensile strength of the composite film reach the maximum of 54.6 kV/mm and 102.7 MPa at 5 wt% content, respectively. Moreover, the composite film is used as a flexible circuit substrate, and the working surface temperature is 20 ℃, which is lower than that of pure PEI film. This study provides an effective strategy for polymer composites for electronic packaging.

Organic covalent modification to improve thermoelectric properties of TaS2.

Organic semiconductors are attracting considerable attention as a new thermoelectric material because of their molecular diversity, non-toxicity and easy processing. The side chains which are introduced into two-dimensional (2D) transition metal dichalcogenides (TMDs) by covalent modification lead to a significant decrease in their thermal conductivity. Here, we describe a simple approach to preparing the side chains covalent modification TaS2 (SCCM-TaS2) organic/inorganic hybrid structures, which is a homogeneous and non-destructive technique that does not depend on defects and boundaries. Electrical conductivity of 3,401 S cm-1 and a power factor of 0.34 mW m-1 K-2 are obtained for a hybrid material of SCCM-TaS2, with an in-plane thermal conductivity of 4.0 W m-1 K-1, which is 7 times smaller than the thermal conductivity of the pristine TaS2 crystal. The power factor and low thermal conductivity contribute to a thermoelectric figure of merit (ZT) of ~0.04 at 443 K.

Thermal Rectifier and Thermal Transistor of 1T/2H MoS2 for Heat Flow Management.

Thermal rectifiers and thermal transistors are expected to be widely used for efficient thermal management and energy cascade utilization due to their excellent directional thermal management. Two-dimensional micro/nano materials have huge potential in the applications of thermal transistors, thermal logic circuits, and thermal rectifiers owing to the phase transition and thermal rectification phenomenon. Herein, a lithium intercalation method was used to transform 2H-MoS2 into the 1T phase with a purity of 76%, and a suspended microelectrode was applied to measure the thermal conductivity and thermal rectification coefficient of the same MoS2 film with 1T and 2H phases in suit. The thermal conductivity and thermal rectification effect of two-phase MoS2 couple with its phase state and structure were also obtained. The results demonstrate that the thermal conductivities of MoS2 in both 1T and 2H phases decrease with increasing temperature. It is also found that the thermal rectification coefficient has no obvious dependence on the temperature and phase change but the asymmetric structure. Furthermore, a thermal rectifier and transistor with a high thermal rectification effect are designed. The direction and magnitude of heat flow through the samples can be effectively controlled and managed by adjusting the phase, size, and structural asymmetry of the different samples. The maximum thermal rectification coefficient of the thermal rectifiers is up to 0.8.

New Progress on Fiber-Based Thermoelectric Materials: Performance, Device Structures and Applications.

With the rapid development of wearable electronics, looking for flexible and wearable generators as their self-power systems has proved an extensive task. Fiber-based thermoelectric generators (FTEGs) are promising candidates for these self-powered systems that collect energy from the surrounding environment or human body to sustain wearable electronics. In this work, we overview performances and device structures of state-of-the-art fiber-based thermoelectric materials, including inorganic fibers (e.g., carbon fibers, oxide fibers, and semiconductor fibers), organic fibers, and hybrid fibers. Moreover, potential applications for related thermoelectric devices are discussed, and future developments in fiber-based thermoelectric materials are also briefly expected.

High-contrast photochromic Eu-doped K0.5Na0.5NbO3 ceramics with prominent pellucidity.

The Eu-doped K0.5Na0.5NbO3 pellucid ceramics were first prepared via a conventional solid-state reaction, and they exhibited light illumination-induced high-contrast photochromism of both optical transmittance and photoluminescence behaviors. Through thermal treatment, the optical performances could return to their initial states and displayed excellent reversibility. Eu3+ ions were selected as the luminescent activator for detecting the local environment of the K0.5Na0.5NbO3 host. Meanwhile, the effects of the amount of Eu3+ present on phase structures, microstructures, optical transmittance and photoluminescence intensities were systematically investigated. The results suggest that Eu-doped K0.5Na0.5NbO3 transparent ceramics possess multifunctionality including photochromism, photoluminescence and optical switching properties, and that they exhibit promising potential for non-destructive optical data storage application.

Experimental Study on Thermal Conductivity and Rectification in Suspended Monolayer MoS2.

Thermal rectification is an attractive phenomenon for thermal management, which refers to a specific behavior in a heat transfer system where heat flow in one direction is stronger than that in the opposite direction under the same conditions. Two-dimensional monolayer molybdenum disulfide (MoS2) synthesized by chemical vapor deposition (CVD) has exhibited exceptional thermal, optical, and electrical properties due to its special structure; however, the thermal rectification in monolayer MoS2 is still not achieved by experimental measurement. Here, we successfully transferred monolayer MoS2 samples with three geometrical morphologies to the suspended microelectrodes by the PMMA approach. Through further heating the suspended microelectrodes with AC power in the opposite directions of these three monolayer MoS2 samples, we experimentally measured the thermal conductivity and first obtained the thermal rectification of monolayer MoS2. The rectification coefficients of monolayer MoS2 with three different geometrical morphologies are 10-13, 11-4, and 69-70%. Moreover, a theoretical model was also applied to discuss the dependence of thermal rectification on the geometrical asymmetry (angle and spacing). The results demonstrate that the monolayer MoS2 has an obvious thermal rectification phenomenon owing to the asymmetric structure, and it would have great potentials in the application of thermal energy control and management.

A novel super thermal insulation material: bamboo-like polymer nanotubes.

One-dimensional nanomaterials have widely applied in the fields of energy conversion and storage due to their exceptional performance because of the nano-size effect. Herein, we synthesized bamboo-like polymer nanotubes based on cationic polymerization using immiscible initiator nanodroplets that results in a hollow structure. The suspended microelectrode is fabricated to measure the axial thermal conductivity of a single polymer nanotube, which demonstrates that this hollow structure can reduce its effective thermal conductivity. The experimental results show that its effective thermal conductivity is close to 0.03 W m-1 K-1, and decreases to 0.02 W m-1 K-1 with increasing temperature of the heating microelectrode, which may be due to the increasing lattice vibration and inelastic scattering between phonons. Its effective thermal conductivity is smaller than that of air, indicating that the synthetic method is an effective way to fabricate thermal insulating polymer nanotubes by significantly lowering the effective thermal conductivity. Hence, the method offers a new strategy in the fields of thermal insulation and protection.

Boosting Upconversion Photoluminescence and Multielectrical Properties via Er-Doping-Modulated Vacancy Control in Ba0.85Ca0.15Ti0.9Zr0.1O3.

The lead-free 0.5(Ba0.7Ca0.3)TiO3-0.5Ba(Ti0.8Zr0.2)O3 (BCTZ) ceramics with Er doping have shown good upconversion photoluminescence (PL) and desirable optical temperature sensing properties. To bridge a relationship between the structure/intrinsic defects and properties of rare-earth-doped ferroelectrics, we designed and fabricated a series of BCTZ ceramics doped with 1 mol % Er3+ by combining the first-principles calculations and experimental measurements. Theoretically, we discovered that Er can occupy both A sites (i.e., replacing Ba or Ca) and B sites (i.e., replacing Ti or Zr) in the BCTZ lattice and highlighted that the Er-doping-induced vacancy concentration decreases for both the oxygen vacancies (V o) and cation vacancies (V c). Experimentally, the enhanced PL performance and the dielectric, ferroelectric, and piezoelectric properties of the Er-doped BCTZ ceramics have been observed. Finally, the physical origin of Er-induced property enhancement in BCTZ has been elaborated according to the charge density and chemical bonding analysis. These results open up a path to investigate the effects of site substitution and vacancies on optoelectronic properties of multifunctional rare-earth-doped ferroelectrics.

Stethoscope-type 3ω independent detector for fast measurement of material thermal conductivity.

As one of the most basic properties of materials, thermal conductivity is a key parameter to investigate the analysis and design of the particular thermal process, such as the analysis of the thermal/cool energy storage and release process, the thermal/cool energy storage system design, and so on. Also, it is the foundation of energy technology development. In order to overcome the problem that the measurement frequency of a polyimide substrate independent detector is lower and the measurement time is longer, based on the relationship among 3ω detection frequency, thermal wave penetration depth, and thermal properties of the substrate material, this paper developed a stethoscope-type 3ω independent detector based on a sapphire substrate for fast measurement of material thermal conductivity. Nickel, with a high resistance temperature coefficient, was used as the metal detector. The influence of the width of the nickel metal detector and the thickness of the sapphire substrate on the measurement accuracy was analyzed by introducing an effective heat flow ratio. The sapphire substrate independent detector was calibrated by using copper, aluminum, 304 stainless steel, quartz glass, ethylene glycol, and plexiglass with known thermal conductivities. The experiment shows that the measurement frequency of the stethoscope detector can be ten times higher than that of a Kapton film independent detector. Its measurement time is only one tenth of that of the Kapton film independent detector.

Remarkably enhanced thermal transport based on a flexible horizontally-aligned carbon nanotube array film.

It has been more than a decade since the thermal conductivity of vertically aligned carbon nanotube (VACNT) arrays was reported possible to exceed that of the best thermal greases or phase change materials by an order of magnitude. Despite tremendous prospects as a thermal interface material (TIM), results were discouraging for practical applications. The primary reason is the large thermal contact resistance between the CNT tips and the heat sink. Here we report a simultaneous sevenfold increase in in-plane thermal conductivity and a fourfold reduction in the thermal contact resistance at the flexible CNT-SiO2 coated heat sink interface by coupling the CNTs with orderly physical overlapping along the horizontal direction through an engineering approach (shear pressing). The removal of empty space rapidly increases the density of transport channels, and the replacement of the fine CNT tips with their cylindrical surface insures intimate contact at CNT-SiO2 interface. Our results suggest horizontally aligned CNT arrays exhibit remarkably enhanced in-plane thermal conductivity and reduced out-of-plane thermal conductivity and thermal contact resistance. This novel structure makes CNT film promising for applications in chip-level heat dissipation. Besides TIM, it also provides for a solution to anisotropic heat spreader which is significant for eliminating hot spots.