Cement-based materials for stress sensing by dielectric measurement

The self-sensing of stress by measurement of the relative dielectric constant (κ) has been shown in cement pastes containing steel fibers of 8 μm diameter and carbon filaments of 0.1 μm diameter. The κ value increases nonlinearly and quite reversibly with compressive stress up to 6.4 MPa, although the reversibility is not complete. Inferior sensing performance was observed in cement paste with carbon fibers of 15 μm diameter, although the performance was still better than cement paste without admixture.     

热浸镀镀层厚度的动态控制方程

利用简化的动态模型过程 ,推导了热浸镀条件下的膜层厚度控制方程 ,并通过实验手段对此方程作了验证。结果表明 ,该方程作为工程快速估算是可行的     

Texturation of model clay materials using tape casting and freezing

The present work aimed to investigate the processing of textural clay based materials using tape casting together with freezing. Two model raw materials were used, namely: BIP kaolin from France and ABM montmorillonite from Mediterranean region. The mixtures of both clays were studied, whereby, the amount of montmorillonite was 0, 5, 10, 20 or 50 mass%. After tape casting, the as-obtained green bands were frozen into liquid nitrogen, lyophilized and then fired at 1050 °C or 1200 °C.The amount of montmorillonite appeared as a critical parameter that controls the cohesion of the dry products. For montmorillonite content ≥20 mass%, the products exhibited multiple cracks after lyophilisation. With lower montmorillonite content, the cohesion of the dry products was satisfactorily and a macroscopic cross-linked surface texturation was observed. After calcination at 1050 °C or 1200 °C, the texturation appeared well defined. Moreover, calcination at 1200 °C increased the densification of products and the occurrence of a glassy phase was noted.The combination of both tape casting and freezing (freeze tape casting) is a promising way to develop various clay-based and composites materials exhibiting unique microstructure organization and characteristics with potential application in the field sustainable and environmentally friendly filtration, adsorption or catalysis.     

Model for thickness effect with impact testing of viscoelastic materials

Thickness effect on impact parameters is studied and a model is developed for flat‐ended drop weight impact testing of viscoelastic materials. The model represents a relationship of specimen thickness with impact force/stress and impact energy. A polymeric material, ethylene vinyl acetate (EVA), was used for experimental verification. Experimental results for a thickness range of 1–9 mm at impact energy levels of 0.42, 0.96, and 1.54 J have been found to be in reasonable agreement with predictions based on the model with some approximated parameters. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1762–1767, 2001     

Hydrometric method for controlling the fineness of grinding ceramic materials

Glass and Ceramics -     

Aqueous tape casting processing of low dielectric constant cordierite-based glass-ceramics—selection of binder

In this paper, four different binders were investigated in the process of aqueous tape casting of cordierite-based glass-ceramics and their effects on the rheological behaviour of the suspensions and on the microstructures of the green tapes were compared. Meanwhile, a good compatibility between the dispersant and binder was found to be a predominant factor to obtain an optimised cordierite glass-ceramic tape. The microstructure of the green tape was observed by SEM and the weight loss during binder burn out process was determined by DTA/TG. The dielectric constant and dielectric loss of the sintered tapes (at 1150 °C for 2 h) was also measured.     

CAD-assisted modeling of high dielectric contrast composite materials

In this work, a CAD approach to model the effective dielectric properties of binary composite ceramics with high dielectric contrast is presented. Further, composite materials consisting of the tunable ferroelectric material barium strontium titanate (BST) with ɛ_f ≈ 2000ɛ₀ and magnesium borate (MBO) with ɛ_d ≈ 7ɛ₀ were fabricated in 10 volume percent steps. The morphology of the real materials was used to generate a micro granular sample for the CAD model. The material samples were processed to 0.5 mm thick and 8.5 mm wide discs and covered with silver electrodes, forming a parallel-plate capacitor. The dielectric properties of the composites were extracted from the capacitors by means of impedance spectroscopy at 100 MHz. The measured dielectric properties are compared to the proposed numerical model and to a widely used equivalent medium approximation model with regard to effective permittivity, quality factor and tunability.     

Review of polymer materials with low dielectric constant

One of the exciting and promising developments in material science today is the design and synthesis of novel low‐dielectric‐constant polymer materials, which are found to have potential applications in the field of ultralarge‐scale integration, capacitors and other electronic circuits as insulating and/or dielectric materials. In this article the new polymer dielectric materials reported in recent years are reviewed, including aromatic (heteroaromatic) polymers, silicon‐containing polymers, fluorinated polymers, porous polymers, etc. In summarizing the review, the development, potential applications and future directions of polymer materials with low dielectric constant are discussed. Copyright © 2010 Society of Chemical Industry     

Plasma catalysis for CO2 decomposition by using different dielectric materials

Despite a large of interest in the field of plasma assisted catalytic technology (PACT), very little has been reported on the catalysis of the different dielectric barriers for a dielectric barrier discharge (DBD) reaction. In the present study, Ca_0.7Sr_0.3TiO₃ dielectric, that possesses both a high permittivity and a high dielectric strength, was prepared by a liquid phase sintering and used as a dielectric barrier to break CO₂ in order to investigate the efficiency and characteristics of this ceramic on a plasma reaction in a DBD reactor. Its results were compared with commercial alumina and silica glass that possess lower permittivities, while were widely used in the previous studies.Not only the sinterability of the Ca_0.7Sr_0.3TiO₃ dielectric was improved by the 0.5 wt.% Li₂Si₂O₅ additive, but also both the mechanical and dielectric properties of the sintered bodies were increased significantly. This ceramic was successfully used as a barrier, and the CO₂ conversion, which is proportional to permittivity, achieved 15.6%. It was much higher than with those using traditional alumina and silica glass barriers.     

Multilayer capacitor margin stresses and electrode to dielectric thickness calculations

The mismatch of thermal expansion coefficients in co-fired multilayer capacitors (MLCs) leads to development of residual stress on cooling. These stresses are of a compressive nature for the multilayer stack and a tensile character for the ceramic margin. A closed-form stress calculation of the critical ratio of electrode thickness to ceramic dielectric thickness in the stack shows a limiting ratio of approximately 1:2. Design equations are given in order to calculate the multilayer electrode to dielectric thickness ratio. The required design parameters are the linear thermal expansion coefficients, the temperature difference, the Young's moduli, the strength of the ceramic margin, and the ratio of active to inactive capacitor area.     

First-principles calculations of the dielectric properties of perovskite-type materials

We compare first-principles (FP) calculations of the ionic effective charges, phonon frequencies, and static dielectric permittivities κ_s of several perovskite-type materials. Transition metal ions have anomalously large effective charges, though in the double perovskite CaAl_1/2Nb_1/2O₃ (CAN), the effective charge of Nb is significantly lower than in the simple perovskite KNbO₃, showing different Nb–O bonding chemistry. Tolerance factors, cation chemistry, and structural phase transitions all affect the nature of the softest phonons in perovskites. For the solid solution (CaAl_1/2Nb_1/2O₃)_1−x–(CaTiO₃)_x (CAN–CT), κ_s is modeled via a cluster expansion, with the parameters determined from FP. In pure CAN, κ_s is found to increase when cation disorder increases, in agreement with experimental results on analogous systems. The dielectric constant of CAN-CT increases nonlinearly with x, in agreement with experiment.     

Evanescent microwave probe study on dielectric properties of materials

A recently developed evanescent microwave probe (EMP) technique combined with systematically quantitative analyses is demonstrated. We use a three-dimensional (3D) finite element simulation to model the electromagnetic field inside the resonant cavity and inside the sample near the tip. We also proposed an analysis model for the sample's quality factor (Q), which is usually hindered by the conductor losses of the resonator. Measurement on various dielectric samples agrees very well with the theoretical model, demonstrating the validity of analyses for the EMP resonator and providing a possibility for dielectric imaging the surface of the samples with high resolution.     

Property control of microwave dielectric materials using self-flux compositions

In order to improve the sinterability and controllability of dielectric properties, we focused on self-flux composition. In the case of Ba(Mg_1/3Nb_2/3)O₃, we selected Ba_(1 − β)Nb_βO_δ as self-flux system and investigated correlations between Q-factor and the β value. Interestingly, high Q-value was obtained only at the β = 0.45. Moreover, the dielectric constant (ɛ_r) and temperature coefficient of resonant frequency (τ_f) change linearly with the quantity of Ba_0.55Nb _0.45O_δ by keeping up high Q-value. As a result, it was indicated that the dielectric properties could be controlled by the assumption of stoichiometric composition and the liquid phase consisted of “self-flux”. In a similar way of thinking, it seems that this concept of property designing will be applied to the complex-perovskite-type materials.     

Simultaneous thermal analysis (STA) of dielectric materials

Polymeric Dielectrics are usually chemically complex, and it is attractive to establish techniques capable of providing a general assessment of material composition and thermal behaviour. Simultaneous thermal analysis (STA) is capable of providing such an assessment and complementing flammability studies. The use of STA in both of these roles is demonstrated for a number of polymer materials used in high-voltage cables. The materials investigated include various polyethylenes, ethylene-copolymers and filled polyvinylchloride formulations. Fingerprint identification of materials and the influence of chemical structure and additives on thermal behaviour is discussed. In particular the influence of antimony trioxide-chlorinated hydrocarbon flame retardants is examined. The application of STA information in guiding flammability studies is shown by example, and some potential routes for further development are also discussed.     

Viscoelastic analysis applied to the determination of long‐term creep behavior for magnetic tape materials

Creep‐compliance experiments were performed for three representative magnetic tapes. Two of these tapes used a magnetic particle (MP) coating, and one used a metal‐evaporated (ME) coating. The MP tapes used the following polyester substrates: semitensilized poly(ethylene naphthalate) (PEN) and supertensilized poly(ethylene terephthalate). The ME tape used an aromatic poly(amide) or aramid substrate. Time–temperature superposition was used to make creep‐compliance predictions at 30 and 50°C reference temperatures. Comparisons were made with dimensional stability requirements based on position error signal (PES) specifications for magnetic tape drives along with in‐cartridge creep specifications based on PES measurements. Circumferential and lateral creep strains were determined that account for storage of the tapes in a reel, and creep strains were predicted for future tapes with thinner, lower compliance coatings. A rule of mixtures method was also used to extract compliance information for individual layers of MP‐PEN tapes, and stress profiles through the thickness of the tapes were determined. Additional measurements and analyses were performed to determine the creep recovery and shrinkage characteristics for the magnetic tapes. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1106–1128, 2006     

Reduced temperature-dependent thermal conductivity of magnetite thin films by controlling film thickness

We report on the out-of-plane thermal conductivities of epitaxial Fe₃O₄ thin films with thicknesses of 100, 300, and 400 nm, prepared using pulsed laser deposition (PLD) on SiO₂/Si substrates. The four-point probe three-omega (3-ω) method was used for thermal conductivity measurements of the Fe₃O₄ thin films in the temperature range of 20 to 300 K. By measuring the temperature-dependent thermal characteristics of the Fe₃O₄ thin films, we realized that their thermal conductivities significantly decreased with decreasing grain size and thickness of the films. The out-of-plane thermal conductivities of the Fe₃O₄ films were found to be in the range of 0.52 to 3.51 W/m · K at 300 K. For 100-nm film, we found that the thermal conductivity was as low as approximately 0.52 W/m · K, which was 1.7 to 11.5 order of magnitude lower than the thermal conductivity of bulk material at 300 K. Furthermore, we calculated the temperature dependence of the thermal conductivity of these Fe₃O₄ films using a simple theoretical Callaway model for comparison with the experimental data. We found that the Callaway model predictions agree reasonably with the experimental data. We then noticed that the thin film-based oxide materials could be efficient thermoelectric materials to achieve high performance in thermoelectric devices.     

Inkjet printing of polyimide insulators for the 3D printing of dielectric materials for microelectronic applications

In this article, we report the first continuous fabrication of inkjet‐printed polyimide films, which were used as insulating layers for the production of capacitors. The polyimide ink was prepared from its precursor poly(amic) acid, and directly printed on to a hot substrate (at around 160 °C) to initialize a rapid thermal imidization. By carefully adjusting the substrate temperature, droplet spacing, droplet velocity, and other printing parameters, polyimide films with good surface morphologies were printed between two conducting layers to fabricate capacitors. In this work, the highest capacitance value, 2.82 ± 0.64 nF, was achieved by capacitors (10 mm × 10 mm) with polyimide insulating layers thinner than 1 μm, suggesting that the polyimide inkjet printing approach is an efficient way for producing dielectric components of microelectronic devices. © 2016 The Authors Journal of Applied Polymer Science Published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43361.