Electrostrictive mechanism of microwave losses in a planar strontium titanate film capacitor

An analysis is made of the dielectric losses in the microwave range in a planar ferroelectric capacitor caused by electrostrictively excited sound in the ferroelectric in the presence of a bias field (induced piezoeffect). An approximate expression is obtained for the dielectric loss tangent as a function of the bias field and the frequency. Numerical estimates are made for singlecrystal strontium titanate at 78 K. This loss mechanism may be a determining factor for the fabrication of planar capacitors using high-quality ferroelectric films having properties close to those of the single crystal. Zh. Tekh. Fiz. 69, 93–99 (August 1999)     

Dielectric properties of microcomposite ferroelectrics

The dielectric response in two-phase composites of ferroelectric and dielectric materials was studied within the quasistatic approximation. Using different effective medium theories (Maxwell-Garnet theory, the effective medium approximation and the Bergman representation) the dielectric function was represented as a distribution of polar modes. One of the main results concerns the existence of modes due to geometric resonances which among other effects can lead to new low-frequency absorption peaks near the percolation threshold of the ferroelectric material. These peaks appear always higher than the transverse polar mode frequencies of the bulk components but below their corresponding longitudinal mode frequencies. The soft mode with renormalized mode strength but unchanged frequency exists only in the composite with percolated ferroelectric clusters. When the ferroelectric clusters are only finite the soft mode does not soften completely.     

Microwave dielectric tangent losses in KDP and DKDP crystals

By adding cubic and quartic phonon anharmonic interactions in the pseudospin lattice coupled mode (PLCM) model for KDP-type crystals and using double-time temperature dependent Green's function method, expressions for soft mode frequency, dielectric constant and dielectric tangent loss are obtained. Using model parameters given by Ganguliet al [9] the dielectric losses are calculated for KDP and DKDP crystals. In the microwave frequency range an increase in frequency (1–35 GHz) is followed by an increase in dielectric tangent loss (1–35) at 98 K and (1–15) × 10⁻² at 333 K for KDP and DKDP crystals respectively. The dielectric tangent loss decreases from 0.052 to 0.042 for KDP crystals with increase in temperature from 130 to 170 K and for DKDP crystals it decreases from 0.0166 to 0.0074 with an increase in temperature from 230–343 K in their paraelectric phases at 10 GHz. This shows Curie-Weiss behavior of the dielectric tangent loss     

Nonlinear properties of a medium with ellipsoidal ferroelectric nanoinclusions

A model describing the dielectric properties of a composite material with ellipsoidal ferroelectric inclusions has been developed. The dependence of the effective permittivity of the composite material on the applied external electric field is determined for different sizes of ferroelectric inclusions. The controllability of the composite material under consideration is evaluated.     

Second harmonic generation in composites of ellipsoidal particles with core–shell structure

We study the enhancement of the second-harmonic generation (SHG) coefficient in a random composite consisting of ellipsoidal particles with a core–shell structure in a linear dielectric host. The material making up the ellipsoidal core is assumed to be dielectric, but with a nonlinear susceptibility for SHG. The coating material is assumed to be metallic with a linear susceptibility. The effective SHG coefficient is derived and its expression is related to various local field factors. The numerical calculations of the effective SHG response per unit volume of nonlinear material can be greatly enhanced at certain frequencies. For coated ellipsoidal particles, the core–shell structure and the particle shape allow for tuning of the resonance through the choice of material parameters and/or the ratio of the core to shell volume fraction and the depolarization factor of the particles.     

I-V and C-V characteristics of ceramic materials based on barium strontium titanate

The properties of ferroelectric ceramics based on barium strontium titanate and doped by admixtures with low permittivity and dielectric loss tangent are studied experimentally. These ceramics are viewed as a promising material for microwave devices applied in the acceleration technology. Ceramic samples the I-V characteristics of which contain long horizontal segments are found to have good insulating properties. Various conductivity mechanisms in these ceramics are discussed, and their effect on the amount and character of dielectric hysteresis in these samples is considered.     

Theory of absorption of electromagnetic radiation by highly inhomogeneous two-component systems

Calculations are made of the permittivity of a composite formed by a mixture of a dielectric matrix and macroscopic magnetic inclusions. It is shown that the dielectric loss tangent in this material is a complex function of their concentration x, and this function is determined. The dependence of the absorption maximum on x is determined and it is shown that the maximum is only observed in the presence of foreign inclusions. Conditions for the existence of an absorption maximum are obtained for the two most interesting physical cases. A method of calculating the components of the depolarization tensor is proposed for a thin-film composite. Zh. Tekh. Fiz. 69, 89–94 (July 1999)     

Thermal behavior of Jonscher’s formalism in relaxor ferroelectrics showing diffuse phase transitions

A theoretical study of the thermal behavior of the exponent s of Jonscher’s power law in ferroelectric materials showing diffuse phase transitions (DPTs) is presented. The proposed model is applied to the experimental results of lanthanum-doped lead zirconate titanate ferroelectric ceramics. Both the proposed model and the experimental results show a critical point (a maximum) around the maximum of the dielectric loss factor, which moves to higher temperatures when the frequency increases, the typical behavior of a relaxor material with DPTs. PACS 77.22.Gm; 77.80.Bh; 77.84.Dy     

Room temperature ferroelectric effect and enhanced dielectric permittivity in Rochelle salt/PVA percolative composite films

Rochelle salt (RS) filled polyvinyl alcohol (PVA) composite films have been prepared via a simple solution casting technique. The transport, dielectric and ferroelectric properties of the samples have been studied. The dielectric permittivity decreases slowly with increasing frequency and rise gradually with increasing temperature and RS contents in the composites. As the volume fraction of the RS reaches to percolation threshold (f_c ～0.0538), an abrupt increase in the dielectric permittivity (～403 almost 80 times higher compared to pure PVA with low loss ～0.18 at 1 kHz and room temperature) occurs in the RS/PVA composite film, which is attributed to the formation of the conductive network in the matrix. Ferroelectric loops up to room temperature (300 K) and the slight increase in Curie temperature from 297 to 300 K have also been observed for percolative composite film. The developed composite material with low loss high dielectric permittivity and room temperature ferroelectric behaviors might be applied in the technological fields.     

Ku波段开腔电介质高温自动测量系统

 介绍了一种测量低损耗天线罩材料的开腔高温自动测量系统。该系统采用扫频方式程控标量网络分析仪对开腔进行扫频测量,得到介质加载前后腔体的谐振频率与Q值,并由此计算出被测材料的介电常数和损耗角正切;介质采用特制硅碳管加热,由温度控制器对其控制,最高温度可达1 000 ℃以上。用此系统对聚四氟乙烯和一种低损耗天线罩材料进行测量对比。结果表明,测得的聚四氟乙烯结果与文献结果相近,天线罩材料相对介电常数的实部随温度升高而增加。     

Discontinuous surface-treated submicron-diameter carbon filaments as an interlaminar filler in carbon fiber polymer-matrix composites for vibration reduction

Discontinuous surface-treated submicron-diameter carbon filaments are effective for use as an interlayer between continuous carbon fiber laminae in a nylon-6 matrix composite for enhancing the loss tangent (0.2 and 1.0 Hz) under flexure that involves bending of the fibers, without significant decrease of the flexural storage modulus or the tensile modulus or strength in the fiber direction. The surface treatment is oxidation using ozone. Without treatment, the filaments are not effective. The treated filaments amount to 0.64 vol.% of composite; the interlayer thickness is 77 μm. A viscoelastic interlayer is even more effective than the treated filament interlayer for enhancing the loss tangent, but the accompanying decrease in storage modulus is much more. The loss tangent for composite with viscoelastic interlayer decreases upon heating, so the loss modulus for this composite is less than that of the composite with treated filament interlayer at ?50°C.     

钛酸锶钡材料应用于超高密度动态随机存储器的研究

铁电钛酸锶钡材料具有十分优越的介电性能 :高的介电常数 ,较低的介电损耗 ,好的绝缘漏电性能 ;而且 ,通过调节材料中的Ba/Sr成分比 ,可改变材料的居里相变温度TC,以满足特定应用环境的温度需要 ,在超高密度集成的动态随机存储器 (DRAM)方面表现出广阔的应用前景 .文章概括介绍了BaxSr1-xTiO3 薄膜材料在DRAM应用中已取得的最新研究进展 ,并对这一应用所面临的问题也进行了详细讨论     

All-inkjet-printed Metal-Insulator-Metal (MIM) capacitor

BaTiO₃-resin hybrid composite is one of the most promising candidates to surmount the intrinsic process issues of pure ceramic materials such as the requirement of high temperature sintering, disability to be integrated with dissimilar materials, etc. We utilized the inkjet printing to prepare all-inkjet-printed MIM (Ag/BaTiO₃-resin hybrid film/Ag) capacitor. The microstructures of the MIM capacitor were investigated to confirm the integrity of dissimilar layers within MIM capacitor. The dielectric properties of BaTiO₃-resin hybrid film in MIM capacitor were also investigated by measuring its relative permittivity and loss tangent. The inkjet-printed BaTiO₃-resin hybrid composite film showed that its relative permittivity and loss tangent were 70 and 0.011 at 1 MHz, respectively.     

Microwave losses in ABO3 type perovskites

Using Green’s function method, microwave lossess were theoretically calculated for BaTiO₃, SrTiO₃ and KTaO₃ ferroelectric perovskites. In microwave range, an increase in frequency is followed by an increase in the dielectric loss. In the paraelectric phase, the dielectric loss decreases with increasing temperature showing the Curie-Weiss behaviour of the tangent loss.     

Dielectric, ferroelectric and piezoelectric properties of 0-3 barium titanate–Portland cement composites

In this work, barium titanate (BT) and cement composites of 0-3 connectivity were produced with BT concentrations of 30%, 50% and 70% by volume using the mixing and pressing method. The dielectric constant (ε _r ) and the dielectric loss (tan δ) at room temperature and at various frequencies (0.1–20 kHz) of the ferroelectric BT-Portland cement composites with different BT concentrations were investigated. The results show that the dielectric constant of BT-PC composites was found to increase as BT concentration increases, and that the highest value for ε _r —of 436—was obtained for a BT concentration of 70%. In addition, the dielectric loss tangent decreased with increasing BT concentration. Moreover, several mathematical models were used; the experimental values of the dielectric constants are closest to those calculated from the cube model. The 0-3 cement-based piezoelectric composites show typical ferroelectric hysteresis loops at room temperature. The instantaneous remnant polarization (P _ir ), at an applied external electrical field (E ₀) of 20 kV/cm (90 Hz) of 70% barium titanate composite, was found to have a value ≈3.42 μC/cm². Furthermore, the piezoelectric coefficient (d ₃₃) was also found to increase as BT concentration increases, as expected. The highest value for d ₃₃ was 16 pC/N for 70% BT composite.     

温度对PEEK太赫兹光谱特性影响研究

聚醚醚酮(PEEK)由于其耐热、耐腐蚀、耐辐照、抗疲劳、电绝缘性等优良性能,在许多领域可以代替金属、陶瓷等传统材料而得到广泛应用。特别是随着5G技术的发展和应用,PEEK已经成为5G热门材料。在PEEK材料实际应用中,温度的影响是一个非常重要和关键的因素。主要研究了PEEK太赫兹光谱以及温度对PEEK太赫兹光谱特性的影响。通过利用太赫兹透射光谱技术,同时结合控温装置,在温度从25～300℃均匀上升过程中,每间隔5℃测试得到PEEK片状样品的太赫兹时域光谱数据,利用光学参数提取算法可以得到PEEK的吸收系数、介电常数等光学参数,进一步得到特定频率下光学常数随温度的变化趋势,从而对材料进行表征和分析。在0.5～4 THz有效光谱范围内,实验结果表明,在常温(25℃)下,PEEK在3.5 THz具有一个明显的特征吸收峰。在25～300℃这个温度范围内,在1 THz频率下,PEEK的吸收系数、介电常数相对于室温分别有4.38%和5.0%的波动,同时PEEK在常温下在1 THz的介电损耗正切值为2.5×10^-3,相比于PMMA和PE等高分子材料,PEEK的介电损耗正切值要低...     

Fine grains ceramics of PIN-PT, PIN-PMN-PT and PMN-PT systems: drift of the dielectric constant under high electric field

Lead-based ferroelectric ceramics with (1-x)Pb(B1 B2)O3-xPbTiO3 formula have emerged as a group of promising materials for various applications like ultrasonic sonars or medical imaging transducers. (1-x)PMN-xPT, (1-x)PIN-xPT and ternary solutions xPIN-yPMN-zPT ceramics are synthesised using the solid state reaction method. Our objective is to achieve higher structural transition temperatures than those of PMN-PT ceramics with as good dielectric, piezoelectric and electromechanical properties. Ceramics capacitance and loss tangent are measured when the ac field of measurement increases up to E=500 V/mm. Behaviours of these materials under ac field are related to their coercive field and Curie temperature.     

Application of Bruggeman and Maxwell Garnett homogenization formalisms to random composite materials containing dimers

The homogenization of a composite material comprising three isotropic dielectric materials was investigated. The component materials were randomly distributed as spherical particles, with the particles of two of the component materials being coupled to form dimers. The Bruggeman and Maxwell Garnett formalisms were developed to estimate the permittivity dyadic of the homogenized composite material (HCM), under the quasi-electrostatic approximation. Both randomly oriented and identically oriented dimers were accommodated; in the former case the HCM is isotropic, whereas in the latter case the HCM is uniaxial. Representative numerical results for composite materials containing dielectric–dielectric dimers demonstrate close agreement between the estimates delivered by the Bruggeman and Maxwell Garnett formalisms. For composite materials containing metal–dielectric dimers and metal–metal dimers with moderate degrees of dissipation, the estimates of the two formalisms are in broad agreement, provided that the dimer volume fractions are relatively low. In general, the effects of intradimer coupling on the estimates of the HCM’s permittivity are relatively modest but not insignificant, these effects being pronounced by anisotropy when all dimers are identically oriented.     

Effect of polymer relaxation on dielectric spectroscopic study of polymer–ferroelectric liquid crystal composites

Polymethylmethacrylate (PMMA) and ferroelectric liquid crystal (P–FLC) composite films (~ 4 μm) with varying proportions of ferroelectric liquid crystal have been prepared and the dielectric response of the composite films as a function of wide frequency (100 Hz–10 MHz) and temperature (127–40 °C) have been analyzed. The dielectric spectra of such composite systems are considerably modified compared to those of the corresponding pure LC or PMMA materials. The observed paraelectric to ferroelectric transition temperature of the composites is found to be lower (~ 85 °C) compared to that of the pure LC system (~ 98 °C), which makes these composites technologically more significant. The P–FLC composites also exhibit higher relaxation frequency and smaller dielectric strength for the Goldstone mode compared to those of the corresponding pure FLC. Depending on the percentage of the constitute materials, the molecular dynamics of the composite materials changes. Dielectric contributions of both liquid crystal and PMMA matrix forming the composites have been analyzed on the basis of Havriliak and Negami function.     

Laser induced air breakdown using 0.355, 0.532, and 1.06 µm radiation

Studies of breakdown threshold intensity for air at various pressures in the range of 24–760 torr using 0.355, 0.532 and 1.06 μm radiation are reported. We observep ^−0.8 scaling ofI _th at 1.06 μm and a weak scaling ofp ^−0.4 at 0.532 and 0.355 μm radiation. Strong dependence of breakdown spot size on laser power but weak dependence on air pressure is observed.