多壁碳纳米管/聚偏氟乙烯高介电常数复合材料的制备与性能

为制备具有高介电常数的复合材料,采用注射成型法制备了原始多壁碳纳米管(P-MWCNTs)/聚偏氟乙烯(PVDF)复合材料和石墨化多壁碳纳米管(G-MWCNTs)/PVDF复合材料。然后,对P-MWCNTs和G-MWCNTs进行了Raman光谱表征,对MWCNTs/PVDF复合材料进行了断面形貌、力学性能和电学性能测试。结果表明:G-MWCNTs比P-MWCNTs具有更高的纯度和结晶度,两种不同的MWCNTs都能均匀分散在PVDF基体中,添加MWCNTs会显著影响PVDF的力学行为。MWCNTs/PVDF复合材料的介电性能随MWCNTs含量的增加而提高,与P-MWCNTs相比,G-MWCNTs有效降低了复合材料的渗流阈值。当频率为100 Hz时,纯PVDF的介电常数为7.0;当P-MWCNTs的含量为5wt%时,复合材料的介电常数为23.8;当G-MWCNTs的含量为5wt%时,复合材料的介电常数高达105.0。注射成型法制备的MWCNTs/PVDF复合材料仍保持相对较低的电导率,进而导致复合材料的能量损耗较低,对电荷存储应用具有重要意义。     

Effects of substrate temperature on dielectric and structural properties of Ti and Er co-doped HfO2 thin films

We report dielectric and structural properties of Ti and Er co-doped HfO₂ (HfTiErO_x) thin films at different substrate temperatures. The film at 400 °C substrate temperatures has the highest k value of 33, improved flat band voltage of −0.3 V, small hysteresis voltage and the significant interface-state density, which shows better dielectric properties for new high-k microstructure. XPS and XRD results reveal that Hf-Ti-Er-O bond may exist in addition with Hf-O, Hf-Er-O and Hf-Ti-O bonds, while the change in chemical structure and degradation of crystallization quality of HfO₂ thin films are directly related to Ti and Er co-doping.     

CF4 plasma effect combined with rapid thermal annealing for high-k Er2O3 dielectrics

In this study, the influence of the duration of CF₄ plasma treatment of rapid thermal annealing on high-k Er₂O₃ dielectrics deposited on polycrystalline silicon was investigated using electrical and material analyses. Results demonstrate that Er₂O₃ dielectric films annealed at 800 °C and plasma treated with CF₄ for a period of 1 min exhibited excellent dielectric performance, including a higher breakdown electric field, lower charge trapping rate, and a larger charge-to-breakdown than the as-deposited sample. Performance improvements were caused by the incorporation of fluorine atoms and the reduction of dangling bonds and defect traps.     

Hydrothermal synthesis and characterization of (Bi,K)TiO3 ferroelectrics

A lead-free ferroelectric (Bi,K)TiO₃ (BKT) was synthesized by a hydrothermal process and characterized systematically at various temperatures. Well-crystallized BKT in the tetragonal phase was identified at a hydrothermal temperature over 220 °C. Small cubic particles were observed, regardless of hydrothermal temperature. The BKT sintered at 1050 °C was observed to be a typical relaxor behavior and very stable against frequency and temperatures, respectively. The sintered-BKT ceramics exhibited a high temperature of maximum dielectric permittivity (T_max = 356 °C at 10⁶ Hz) with piezoelectric constant (d₃₃ = 65 pC/N) and electromechanical coupling factors (k_p = 0.22, k_t = 0.43). Thus, the sintered-BKT showed excellent temperature stability with a high-T_max and piezoelectric properties.     

Investigation of thermal conductivity and dielectric properties of LDPE-matrix composites filled with hybrid filler of hollow glass microspheres and nitride particles

The hybrid filler of hollow glass microspheres (HGM) and nitride particles was filled into low-density polyethylene (LDPE) matrix via powder mixing and then hot pressing technology to obtain the composites with higher thermal conductivity as well as lower dielectric constant (D_k) and loss (D_f). The effects of surface modification of nitride particles and HGMs as well as volume ratio between them on the thermal conductivity and dielectric properties at 1 MHz of the composites were first investigated. The results indicate that the surface modification of the filler has a beneficial effect on thermal conductivity and dielectric properties of the composites due to the good interfacial adhesion between the filler and matrix. An optimal volume ratio of nitride particles to HGMs of 1:1 is determined on the basis of overall performance of the composites. The thermal conductivity as well as dielectric properties at 1 MHz and microwave frequency of the composites made from surface-modified fillers with the optimal nitride to HGM volume ratio were investigated as a function of the total volume fraction of hybrid filler. It is found that the thermal conductivity increases with filler volume fraction, and it is mainly related to the type of nitride particle other than HGM. The D_k values at 1 MHz and microwave frequency show an increasing trend with filler volume fraction and depend largely on the types of both nitride particles and HGMs. The D_f values at 1 MHz or quality factor (Q × f) at microwave frequency show an increasing or decreasing trend with filler volume fraction and also depend on the types of both nitride particle and HGM. Finally, optimal type of HGM and nitride particles as well as corresponding thermal conductivity and dielectric properties is obtained. SEM observations show that the hybrid filler particles are agglomerated around the LDPE matrix particles, and within the agglomerates the smaller-sized nitride particles in the hybrid filler can easily form thermally conductive networks to make the composites with high thermal conductivity. At the same time, the increase of the value D_k of the composites is restricted due to the presence of HGMs.     

Perspectives paper: First principles modeling of high-k gate dielectrics

Aggressive scaling has led to silicon dioxide (SiO₂) gate dielectrics as thin as 15 Å in state-of-the-art CMOS technologies. As a consequence, static leakage power due to direct tunneling through the gate oxide has been increasing at an exponential rate. As technology roadmaps call for sub-10 Å gate oxides within the next five years, a variety of alternative high-k materials are being investigated as possible replacements for SiO₂. The higher dielectric constants in these materials allow the use of physically thicker films, potentially reducing the tunneling current while maintaining the gate capacitance needed for scaled device operation. Recognizing that the current Si/SiO₂ system benefits from nearly 30 years of research, developing a replacement material for SiO₂ presents an immense challenge. This has prompted recent interest in novel computational approaches, such as first principles density functional theory (DFT) simulations, to computationally screen candidate dielectrics by predicting their properties based on the microscopic interactions within the system. This paper provides perspectives on the application of DFT simulations to address challenging problems of high-k gate dielectric research. We provide background and motivation for the development of high-k materials and highlight opportunities for theoretical study of such materials. We also describe specific examples of recent first principles work related to two particularly promising materials systems: silicates and aluminates.     

Development of hafnium based high-k materials—A review

The move to implement metal oxide based gate dielectrics in a metal-oxide-semiconductor field effect transistor is considered one of the most dramatic advances in materials science since the invention of silicon based transistors. Metal oxides are superior to SiO₂ in terms of their higher dielectric constants that enable the required continuous down-scaling of the electrical thickness of the dielectric layer while providing a physically thicker layer to suppress the quantum mechanical tunneling through the dielectric layer. Over the last decade, hafnium based materials have emerged as the designated dielectrics for future generation of nano-electronics with a gate length less than 45 nm, though there exists no consensus on the exact composition of these materials, as evolving device architectures dictate different considerations when optimizing a gate dielectric material. In addition, the implementation of a non-silicon based gate dielectric means a paradigm shift from diffusion based thermal processes to atomic layer deposition processes. In this report, we review how HfO₂ emerges from all likely candidates to become the new gold standard in the microelectronics industry, its different phases, reported electrical properties, and materials processing techniques. Then we use specific examples to discuss the evolution in designing hafnium based materials, from binary to complex oxides and to non-oxide forms as gate dielectric, metal gates and diffusion barriers. To address the impact of these hafnium based materials, their interfaces with silicon as well as a variety of semiconductors are discussed. Finally, the integration issues are highlighted, including carrier scattering, interface state passivation, phonon engineering, and nano-scale patterning, which are essential to realize future generations of devices using hafnium-based high-k materials.     

Tailoring magnetic and dielectric properties of rubber ferrite composites containing mixed ferrites

Rubber ferrite composites containing various mixed ferrites were prepared for different compositions and various loadings. The magnetic and dielectric properties of the fillers as well as the ferrite filled matrixes were evaluated separately. The results are correlated. Simple equations are proposed to predetermine the magnetic and dielectric properties. The validity of these equations is verified and they are found to be in good agreement. These equations are useful in tailoring the magnetic and dielectric properties of these composites with predetermined properties.     

Giant multiferroic effects in topological GeTe-Sb2Te3 superlattices

Multiferroics, materials in which both magnetic and electric fields can induce each other, resulting in a magnetoelectric response, have been attracting increasing attention, although the induced magnetic susceptibility and dielectric constant are usually small and have typically been reported for low temperatures. The magnetoelectric response usually depends on d-electrons of transition metals. Here we report that in [(GeTe)₂(Sb₂Te₃)_l]_m superlattice films (where l and m are integers) with topological phase transition, strong magnetoelectric response may be induced at temperatures above room temperature when the external fields are applied normal to the film surface. By ab initio computer simulations, it is revealed that the multiferroic properties are induced due to the breaking of spatial inversion symmetry when the p-electrons of Ge atoms change their bonding geometry from octahedral to tetrahedral. Finally, we demonstrate the existence in such structures of spin memory, which paves the way for a future hybrid device combining nonvolatile phase-change memory and magnetic spin memory.     

石墨烯多壁碳纳米管/超高分子量聚乙烯导电复合材料的制备及性能

为了比较超高分子量聚乙烯(UHMWPE)在单一填充和混合填充时, 复合材料导电性的差别。在超声和肼的作用下, 通过对氧化石墨烯(GO)、 多壁碳纳米管(MWCNTs)和超高分子量聚乙烯水/乙醇分散液减压蒸馏及热压制备了隔离型MWCNTs/UHMWPE、 石墨烯(GNS)/UHMWPE和MWCNTs-GNS/UHMWPE导电复合材料。经SEM、 TEM测试发现, 导电填料分散于UHMWPE颗粒表面, 热压后形成隔离结构。隔离型的MWCNTs/UHMWPE和GNS/UHMWPE复合材料均表现出较低的导电逾渗(0.148%和0.059%, 体积分数,下同), 但MWCNTs/UHMWPE复合材料的电导率(2.0×10^-2 S/m, 1.0%, 质量分数, 下同)明显高于相同填料含量下的GNS/UHMWPE复合材料。 MWCNTs-GNS/UHMWPE复合材料表现出了更低的逾渗(0.039%) 和较高导电性能(1.0×10^-2 S/m, 1.0%), 其拉伸强度和断裂伸长率随填充剂含量的增加呈现出先上升后下降的趋势。