Nondestructive examination of epoxy adhesive-bonded structures exposed to a humid environment: A comparison of low- and high-frequency dielectric measurements

Broadband dielectric measurements (10 ?2 to 3 GHz) are reported on the effects of exposure of thick film adhesive-bonded structures to moisture. Measurement of the dielectric properties over a broad frequency range allows identification of water both in voids and as a molecular dispersion in the matrix. Changes in the low frequency region of the dielectric spectrum can be attributed to a combination of processes associated with plasticisation of the adhesive, interfacial polarisation effects, and hydration of the surface oxide layer. The data obtained are complemented by mechanical testing and failure analysis of the bond structure measured as a function of the time of exposure. This study indicates that for thick film adhesives the ageing characteristics are apparently independent of the surface treatment. In one of the joints studied an additional feature is identified which appears to correlate with the premature aging of the joint structure.     

Preparation and dielectric tunability of bismuth-based pyrochlore dielectric thick films on alumina substrates

Metal–insulator–metal capacitors structures, employing the cubic pyrochlore Bi_1.5Zn_1.0Nb_1.5O₇ (BZN) thick films, were fabricated by screen-printing techniques on alumina substrates. Chemical compatibility and microstructure between layers was studied by EDS/SEM analysis. The dielectric properties of BZN thick films have been investigated as a function of temperature and frequency. The films exhibited the dielectric constant up to 130, and the dielectric loss less than 0.005 at 1 MHz. The dielectric tunabilities of the films have been compared at different temperature. The BZN thick film showed low tunability of about 1.8% at 10 kHz under 100 kV/cm dc bias voltage in temperature from −150 to 180 °C. The temperature stability of dielectric properties for BZN thick film should be useful for designing temperature stable frequency agile.     

Biaxially oriented polypropylene film in power capacitors

The use of polymer dielectrics, particularly biaxially oriented polypropylene (BOPP), has revolutionized power distribution around the world. BOPP film of sub mil thickness has displaced impregnated Kraft paper because of superior dielectric performance, lower cost, and small volume. The polypropylene molecule has a unique set of properties which combine stable dielectric properties in the operating temperature and frequency range along with an adequate dielectric constant. High levels of orientation and a small unbalance of orientation are required to produce films of high dielectric strength over large areas of film. Control of thickness uniformities to approximately ± 5 percent are critical to the reliable operation of the capacitor under load and to control of capacitance in the product. A special fibrillated surface is required which promotes complete impregnation by the dielectric fluid. This surface is acquired by the development of a specific crystal morphology at one surface of the film. No additives are used to promote this crystalline structure since most additives effect dissipation factor and generate unwanted heat. Similarly, control of contamination (ppm and ppb) is a key factor in the manufacture of film power capacitors.     

聚酰亚胺/二氧化硅复合薄膜电学性能的研究

用均苯四甲酸二酐(PMDA)与4,4'-二氨基二苯醚(ODA)为单体,以N,N-二甲基乙酰胺(DMAC)为溶剂,通过超声机械共混法制备聚酰亚胺/二氧化硅(PI/SiO2)复合薄膜,采用红外光谱(FTIR)、扫描电镜(SEM)分别对PI复合薄膜的结构和表面形貌进行了研究表征,通过concept 80宽带介电谱测试系统对复合薄膜电学性能进行了研究。结果表明:在一定的频率下,不同无机纳米粒子含量的复合薄膜中,介电常数和介电损耗角正切随无机纳米质量分数的增加而增大。     

Time domain reflection dielectric spectroscopy for durability assessment of adhesively bonded composite structures

Abstract

Over the last ten years, the potential of high frequency dielectric techniques for the characterisation of adhesively bonded metallic structures has been extensively investigated by the co-authors. Nowadays, adhesively bonded composite materials are increasingly used in a wide range of situations, including applications in which they are exposed to high levels of moisture. In bonded composite structures, water ingress can influence not only the mechanical properties of the matrix but also those of the matrix/fibre and adhesive/adherent interfaces.

Evolution in the dielectric characteristics of such structures as a function of exposure time can be used to monitor the changes occurring in the joint structure. The present paper discusses the application of high frequency dielectric spectroscopy to the study of carbon fibre reinforced plastics (CFRP) adhesively bonded composite structures. The effects of the carbon fibre orientation in the bulk and at the surface of the adherent on the dielectric signal propagation are investigated. Measurements of the ingress of moisture in the adherent and the joint structure are presented. The high frequency time domain reflection (TDR) analysis allows the integrity of the structure to be explored and a good correlation is shown between TDR analysis and gravimetric results.     

High Dielectric Breakdown Strength Nanoplatelet-Based Multilayer Thin Films

Dielectric materials that can withstand high voltages are of great interest due to the growing need for high-performance insulation systems in electronics. Polymer nanocomposites have gained popularity as electrical insulators due to their processability, high operating voltage, and tortuous paths for current flow created by the nanoparticles in the polymer matrix. The dielectric breakdown strength of a relatively thick multilayer thin film containing polyethylenimine (PEI) and vermiculite clay (VMT), thickened with tris(hydroxymethyl)aminomethane (tris), is evaluated as a function of bilayers (BL) deposited. The resulting nanobrick wall structure of this clay-based assembly is ideal for protective insulation. An 8 BL PEI+tris/VMT film achieves a dielectric breakdown strength of 245 kV mm⁻¹, with a thickness of 5 µm. With increasing bilayers, the breakdown strength gradually decreases, but 20 BL of PEI+tris/VMT achieves a breakdown voltage of 2.36 kV. This nanoplatelet-based system is the first “thick growing” layer-by-layer deposited film to be used as an insulating layer. Its unusually high breakdown strength can be useful for the protection of various high voltage electronics.     

Crystallization of Bismuth Borosilicate-Based Dielectric Thick Films on a LiZn Ferrite Substrate

The compatibility and crystallization of dielectric thick films, consisting of a bismuth borosilicate glass and crystalline cordierite, on a LiZn ferrite substrate were investigated by focusing on phase development and microstructural changes. Significant diffusion of Li and Fe from the substrate to the dielectric was confirmed as unexpected crystalline phases such as Li₂Al₂Si₃O₁₀ and Fe₂O₃ were found in the thick films fired at 850°C. The crystallization was believed to be initiated from the film interface and developed further toward the film surface as evidenced from cross-sectional microstructures of the films with additional firings. The degree of crystallization and the relative contents of the observed phases were dependent on the ratio between the glass and cordierite and the number of refirings.     

EIS study of photo-induced modifications of nano-columnar TiO2 films

Long-lasting UV-induced physical and/or physico-chemical modifications of nano-structured TiO₂ (anatase) films are highlighted in this work by electrochemical methods. These changes occurring in TiO₂ film upon UV exposure are analysed through electrochemical impedance spectroscopy (EIS) measurements. Interfacial capacity measurements show significant and long-lasting modifications of the semiconducting and dielectric properties of TiO₂ due to UV exposure. Furthermore, UV exposure significantly modifies the energetic distribution of surface states in the gap of TiO₂. Based on Mott-Schottky analysis, the relative dielectric constant is found to increase (from 440 to 870) after UV exposure. An explanation for such an effect is that UV exposure enables hydrogen insertion into the nano-columnar TiO₂ film and thereby increases the dielectric constant of the film.     

Temperature-induced changes of the electrical and mechanical properties of aerosol-deposited BaTiO3 thick films for energy storage applications

Aerosol deposition (AD) is a room-temperature film deposition method for the fabrication of scalable ferroelectric ceramic films on different substrates, which is particularly appealing for thick film energy storage applications. However, the electrical and mechanical properties of AD ferroelectric films are not yet satisfactorily understood. Here, we report the dielectric, energy storage, and mechanical properties of aerosol-deposited BaTiO₃ (AD-BT) thick films with nanosized grains by combining macroscopic electric measurements with indentation tests. We find that thermal annealing is an effective way to improve dielectric permittivity and polarization of the AD-BT film, as well as to increase the hardness and Young's modulus of the film. However, crack formation in the annealed AD-BT film is promoted in comparison to the as-processed sample, suggesting that the interplay between the nanosized grains and release of the internal stress during annealing may have major consequences for the mechanical properties and hence should be taken into consideration in application.     

Fabrication and characterization of aluminum nitride thick film coated on aluminum substrate for heat dissipation

For effective heat dissipation in high-power LED applications, aluminum nitride (AlN) thick films as thermally conductive dielectric layers were developed, which were deposited on an Al substrate by aerosol deposition (AD). The aerosol-deposited AlN thick films on Al substrates have advantages over conventional polymer-based dielectric substrates or ceramic substrate mounted heatsink systems including an epoxy adhesive, such as excellent heat dissipation capacity and low thermal resistance. AD is an effective method to fabricate high-quality AlN thick film without the Al₂O₃ phase because the film is formed at room temperature. Highly dense and well-adhered, pure AlN thick films with thicknesses up to 30 µm were deposited on an Al substrate. AlN-Al₂O₃ and AlN-polyvinylidene fluoride (PVDF) composite films were also deposited on an Al substrate in order to investigate the effect of Al₂O₃ and polymer on the microstructure and thermal properties. Among the films, pure AlN thick film exhibited the highest dielectric strength, the highest thermal conductivity, and the lowest thermal resistance. Therefore, it can be expected that the aerosol-deposited AlN thick film on Al substrate could be used as a powerful heatsink.     

Effects of Mn doping on dielectric properties and energy-storage performance of Na0.5Bi0.5TiO3 thick films

Lead-free Na_0.5Bi_0.5Ti_1−xMn_xO₃ (NBTMn_x, x=0, 0.01, 0.03 and 0.05) ferroelectric thick films have been fabricated on LaNiO₃/Si(100) substrate by using a polyvinylpyrrolidone-modified sol-gel method and the effects of Mn content on their microstructure, dielectric properties and energy-storage performance were investigated. Compared with the pure Na_0.5Bi_0.5TiO₃ (NBT) thick films, NBTMn_x thick films exhibited a large enhancement in dielectric properties and energy-storage performance. Particularly, a giant recoverable energy-storage density (W) of 30.2 J/cm³ and the corresponding efficiency (η) of 47.7% were obtained in NBTMn_0.01 thick film at 2310 kV/cm. Moreover, the NBTMn_0.01 thick film displayed good energy-storage stability over a large temperature range at different frequency.     

Environmental Ageing of Adhesively-Bonded Joints. I. Dielectric Studies

High and low frequency dielectric measurements are reported on adhesively bonded aluminum structures aged by exposure to moisture over an extended period of time. The changes observed are interpreted in terms of initially water diffusing into the adhesive layer and there is little evidence of changes occurring to the surface oxide layer. Differences in the form and rate of change of the dielectric signature reflect the effects of the pretreatment and matrix resin on the water diffusion process. A good correlation is observed between the time domain data and the increased dielectric permittivity resulting from ingress of moisture into the adhesive bond. The paper illustrates the use of high frequency dielectric measurements in non-destructively probing adhesive bonds and indicates the potential for in-situ applications.     

Combustion synthesis and electromagnetic properties of nanocrystalline Pb(Zr0.52Ti0.48)O3

The properties of nanocrystalline lead zirconate titanate [PZT, Pb(Zr_0.52Ti_0.48)O₃] thick films and pellets synthesized by self-propagating auto combustion route using sucrose as a fuel are reported. The X-ray diffraction reveals the formation of nanocrystalline tetragonal PZT having a particle size of 5.52 nm in thick films and 7.08 nm in bulk material. SEM images show the grain size of 300–400 nm with some agglomeration, as well as modified open grain boundaries and grain growth in the PZT thick film as compared to the bulk material. The DC electric resistivity of PZT shows semiconducting behavior. We also characterized the 8–18 GHz microwave insertion loss, absorption, complex permittivity, and microwave conductivity of synthesized materials. The real part of permittivity and dielectric loss were found to decrease with increasing frequency.     

Microwave dielectric properties of barium substituted screen printed CaBi2Nb2O9 ceramic thick films

In the present study, Aurivillius-structured Ba²⁺ substituted CaBi₂Nb₂O₉ (CBNO) ceramic powder was synthesized by co-precipitation method. The CBNO thick films were delineated by screen printing method on alumina substrates using co-precipitated ceramic powder. The overlay method was adopted to measure the microwave dielectric properties of prepared thick films. Single phase layered perovskite structure of the prepared thick films was confirmed by X-ray Diffraction. The effects of Ba²⁺ substitution on the surface morphology, bonding, and microwave dielectric properties of thick films were systematically presented. The maximum value of microwave dielectric constant for the CBNO thick films at 11.8 GHz is 15.6 for Ba²⁺=0.8 substitution. The shift in the stretching vibration modes of the Nb-O bond of NbO₆ octahedron in the Raman spectra with a substitution of Ba²⁺ in CBNO was observed. The substitution of Ba²⁺ on A-site of CBNO improves the microwave dielectric properties of prepared thick films. This work may provide a new approach to enhance the microwave dielectric performance of Aurivillius-structured ceramic thick films.     

Bismuth Zinc Niobate Thin Film Multilayer Capacitors with Cu Electrodes Fabricated at Low Temperature by RF Magnetron Sputtering

Thin film multilayer capacitors (MLCs) composed of amorphous Bi_1.5Zn_1.0Nb_1.5O₇ (BZN) dielectric layers with Cu internal electrodes were fabricated by radio‐frequency magnetron sputtering at a temperature below 150°C. Both BZN thin films and Cu internal electrodes were deposited in situ through a set of steel shadow masks at room temperature and postannealed at 150°C. The BZN dielectric layers used in the MLCs are amorphous and the thickness for each BZN layer is approximately 220 nm. Metallic Cu layer used as the internal electrode is about 50 nm thick. Auger electron spectroscopy analysis indicates that there are no diffusion between BZN films and Cu electrodes, as well as no oxidation of Cu electrodes during the fabrication process owing to room‐temperature deposition and low‐temperature postannealing (150°C). The thin film MLCs with different number of BZN layers were fabricated. The thin film MLCs with five BZN layers exhibit promising properties with dielectric constant of 72, capacitance density of 1600 nF/cm², and loss tangent of 5.4% at 10 kHz. These results suggest that the BZN thin film MLCs have potential applications for the embedded PCBs.     

Observation of Cell Structures Developed by Marangoni Convection in Glass Thick Films

Cell structures developed by convection in glass thick films have been studied with special emphasis on the effects of material parameters and heating conditions on the cell structure. Borosilicate glass films from 10 to 30 μm thick were prepared on alumina substrates by printing and firing at temperatures between 750° and 950°C up to 2 h, and surface flow patterns were observed with an optical microscope under dark-field illumination. In most cases a flow pattern developed and changed with time, finally reaching a steady state. Cell structures with regular polygons of four to seven sides from 15 to 150 μm across developed, depending on the glass composition. The effects of heating time, temperature, film thickness, and glass chemistry on the cell structure have been examined. From observations of cell structures, the driving force for the convection in glass thick films has been identified to be the gradient in surface tension due to small temperature fluctuations on the surface. Hence, it is concluded that the convection in glass thick films is of Marangoni type.     

Electrophoretic Deposition and Characterization of Micrometer-Scale BaTiO3 Based X7R Dielectric Thick Films

Uniform and relatively dense BaTiO₃ thick films of 1–5 μm were prepared by an electrophoretic deposition process using submicrometer BaTiO₃ powders (mean particle size: ∼0.2 μm). Two different BaTiO₃ powders and solvent media were used to investigate the film quality and thickness control. The surface charge mechanism of BaTiO₃ particles was explained according to the observed results. The microstructures were examined by means of SEM. The experimental results show that the thickness could be controlled independently of suspension concentration by keeping a constant applied voltage and a constant current drop in a given suspension. BaTiO₃ thick films have good insulation resistance and dielectric properties such as a dielectric constant and a dissipation factor that are compatible with the data from conventional tape-cast BaTiO₃ thin layers.     

Effects of Annealing Temperatures on the Structural and Dielectric Properties of ZnO Nanoparticles

ZnO nanoparticles with hexagonal wurtzite structure were synthesized by using microwave irradiation. The effects of annealing temperatures on the structural, surface morphology and dielectric properties of annealed ZnO nanoparticles were investigated by XRD, FTIR spectra, and dielectric measurements. The XRD reveals the presence of hexagonal wurtzite structure of ZnO average grain size 25.7–36.4 nm. The spectra obtained from FTIR containing two intense absorption bands are attributed to bending and stretching vibrations absorption of Zn–O bond for all annealed ZnO nanoparticles. Dielectric properties of annealed ZnO nanoparticles at different temperatures were studied with respect to frequency. The dielectric measurements reveal that the dielectric response of ZnO nanoparticles is significantly enhanced, especially in the low-frequency range. In conclusion, both the rotation direction polarization and space charge polarization due to a large amount of oxygen vacancies and nano-size effects are responsible for the enhancement of the dielectric properties of ZnO nanoparticles.     

Influence of the exposure of AISI 304L stainless steel at moderately high temperature on the susceptibility to crevice corrosion

Stainless steels passivate in air by forming a very thin (a few nanometre thick) protective film of Fe and Cr mixed oxides. The nature and the protective properties of this film change if the stainless steel surface is previously exposed to elevated temperatures. Thermal treatment, chemical processes and mechanical manufacture to which stainless steel surfaces are normally subject, result in changes to the surface conditions, and therefore to modification of the corrosion resistance. Among the various forms of corrosion of stainless steels, crevice corrosion is undoubtedly the most common in industrial environments and one of the most insidious from an engineering point of view. In this work the effects of surface conditions, in particular as a consequence of exposure at moderately high temperature ranging between 150 and 300°C, on the corrosion behaviour of type AISI 304L stainless steel have been investigated. The aim has been to verify if common treatments causing local heating, such as welding, even in remote regions, thermal stretching, grinding, exposure to superheated steam, or sterilisation processes can constitute aggravating circumstances in respect of crevice corrosion susceptibility.     

Low-Temperature Synthesis and Characterization of Lead Zinc Niobate Thick Films

Perovskite Pb(Zn_1/3Nb_2/3)O₃ (PZN) thick films were prepared by a hydrothermal route at a low temperature (150°C) on Ti metal, where the titanium (Ti) metal served as both the substrate and bottom electrode for the PZN films. The thickness of the PZN film fabricated on the Ti substrate was about 20 μm. We have demonstrated that the concentration of KOH plays a key role in obtaining the perovskite structure in the PZN film. The dielectric relaxation was studied as a function of temperature and frequency. A dispersion of the maximum dielectric permittivity (ɛ_max) appears around the temperature of T _m, which shifts toward higher temperatures with increasing frequency. The variation of T _m with frequency follows the Vogel–Fulcher relationship. The variation of 1/ɛ with temperature above T _m deviates from the Curie–Weiss (CW) law but satisfies the modified CW law. The relaxation indication coefficient (γ) and broadening parameter (Δ) were estimated from a quadratic fit of the modified Curie–Weiss law and were found to be 2.00 and 42 K, respectively, indicating strong relaxor behavior. The samples showed excellent reproducibility in the measurements of leakage current, a remnant polarization of 14 μC/cm², and a coercive field of 300 kV/m.