Microdielectric Sensors Research Articles

Microdielectric analysis and curing kinetics of an epoxy resin system

AbstractMicrodielectric analysis (DEA) was carried out to investigate the cure behavior of a bisphenol F epoxy/aromatic amine resin system using an online dielectric cure monitoring technique. Ionic conductivity measured by a microdielectric sensor under isothermal conditions was correlated to the degree of cure and glass‐transition temperature, which are determined by differential scanning calorimetry (DSC). Results obtained by isothermal DSC measurement were used to establish a cure kinetic model for the epoxy resin. Experimental results show that the ratio of the ion conductivity to the initial ion conductivity, Logσ/Logσ0, has a linear relation with the glass‐transition temperature. Furthermore, correlations between ion conductivity and degree of cure and cure rate are established using the best fit of the measured data. Cure behavior of the epoxy resin obtained by DEA is compared with that predicted by the cure kinetics model. Good agreement was observed. POLYM. ENG. SCI., 47:150–158, 2007. © 2007 Society of Plastics Engineers

Development of a Lab-on-a-Chip for the Characterization of Human Cells

AbstractMicrofabricated biochips are developed to continuously monitor cellular phenotype dynamics in a non-invasive manner. In the presented work we describe the novel combination of contact-less micro-dielectric sensors and microfluidics for quantitative cell analysis. The cell chip consists of a polymeric fluidic (PDMS) system bonded to a glass wafer containing the electrodes while temperature and fluid flow are controlled by external heating and pumping stations. Additionally, the cell chip contains an integrated reference arm providing a low-noise detection environment by eliminating background signals and interferences. The high-density interdigitated capacitors (µIDC) are designed to monitor living cells in a space of approximately 10 nL volume by controlling critical electrode characteristics, such as size, shape and passivation composition as well as thickness. The integrated µIDCs are isolated by a 300 nm multi-passivation layer of defined dielectric property and provide non-invasive, stable, robust and non-drifting measurement conditions. The performance of this detector is evaluated using various bacterial, yeast and mammalian cells.

Processing monitoring of carbon/phenolic composites using smart sensors

High void content and delaminations are observed during the fabrication of thick carbon/phenolic composites due to the high percentage of volatile release which occurs during polymerization. In order to eliminate these imperfections and enhance product quality for thick carbon/phenolic composites, smart sensors are used to monitor the property changes during the processing and control of the component to minimize this effect. This paper describes a technique to monitor and collect sensor data during the curing process of a general material system. Data obtained from sensors are used to generate an expert processing knowledge base which automatically controls the composite cure state based on direct sensor response, in lieu of classical time/temperature techniques. Microdielectric, ultrasonic, thermopile, thermal couple and extrinsic Fabry-Perot interferometer (EFPI) sensors are investigated as potential candidates to monitor and subsequently control the manufacturing process of a composite material. Microdielectric sensors are used to monitor resin property changes. Ultrasonic sensors are used to measure stiffness changes and porosity development in the composite material. Thermopile and thermal couples are used to monitor temperature variance and heat reaction rates within the material. EFPI sensors, historically used for monitoring structural integrity during service applications, are used to detect local strain changes caused by temperature, pressure, or process induced residual stresses. In addition to classical optical sensors, a modified EFPI thermal sensor is employed to monitor temperature variations.

Rheological and dielectric changes during isothermal epoxy‐amine cure

AbstractDynamic viscosity and ionic conductivity were measured simultaneously during the cure of a digylcidyl ether of bisphenol‐A (DGEBA) epoxy resin with diamino‐diphenyl sulfone (DDS) by mounting a microdielectric sensor into the plates of a rheometer. Two different cure temperatures were examined. Periodically, throughout the cure, samples were removed from the plates of the rheometer, quenched, and analyzed for the glass transition temperature and epoxide conversion. The relationship between conductivity and viscosity appeared to be independent of cure temperature. A linear relation with a slope of −1 was observed between the natural logarithms of conductivity and viscosity during the cure up to approximately 85% cure conversion. It was hypothesized that the reaction rate was hindered by diffusion at this stage in the polymerization. A free volume relationship was used to successfully correlate conductivity with viscosity up to the diffusion limited region. ©1995 John Wiley & Sons, Inc.

A microdielectric analysis of moisture diffusion in thin epoxy/amine films of varying cure state and mix ratio

AbstractRecent work has shown that moisture diffusion coefficients can be measured in thin polymer films through monitoring changes in permittivity with microdielectric sensors. The sensor is constructed in silicon and consists of an interdigitated electrode and two depletion mode field effect transistors. When operated in conjunction with appropriate external circuitry, the sensor measures dielectric constant and loss factor of any material placed on the SiO2 insulator and aluminum electrodes. The dielectric properties are measured locally within the first 10 microns of the film in contact with the sensor. As a thin polymer film on the sensor is exposed to a high relative humidity, the dynamically measured dielectric constant can be used to determine exactly when the moisture arrives at the polymer‐SiO2 interface and the rate at which the concentration increases. The magnitude of the change in permittivity after equilibration has also been related to total amount of moisture uptake. In this investigation, several epoxy/amine films of varying mix ratios and cure state are cured on microdielectric sensors. The films, approximately 100 microns thick, are alternately exposed to wet and dry environments. Diffusion coefficients for both absorption and desorption are determined from the dielectric data as a function of cure state and epoxy/amine mix ratio. In addition, relative changes in amount of moisture uptake after equilibration is monitored.

Effect of advancement on epoxy prepreg processing—a dielectric analysis

AbstractWith the relatively recent trend toward adaptive process control during processing of structural composites, there is need for an understanding of the nature of variations during cure that are encountered. Part‐to‐part variations can arise because of differences in thermal exposure arising from part placement or part thickness, or from chemical differences due to formulation or advancement. Microdielectric sensors are particularly well suited for monitoring thermoset cures from initial point of flow, through the liquid region, and on into the solid state. In this study, microdielectric sensors are used to systematically study the influence of resin advancement on cure behavior. Relationships between advancement and critical cure phenomena such as flow temperature, point of viscosity minimum, and reaction end point are discussed.

Simultaneous dielectric and dynamic mechanical analysis of thermosetting polymers

AbstractThis work presents a new method to measure simultaneously the dielectric loss factor and viscosity of thermosetting polymers during various cure cycles. A microdielectric sensor was mounted in the bottom plate of a parallel plate rheometer. Three types of high performance laminating resins were investigated. During non‐isothermal curing, dipole peaks were found to correspond to softening/devitrification, the maximum in the loss factor followed the same heating rate dependence as the minimum in the viscosity, and dipole peaks were observed to correlate with vitrification when the cure temperature was below the ultimate glass transition temperature of the resin.

Effects of Stoichiometric Mixing Ratio on Epoxy Cure —A Dielectric Analysis

The goal of this study was to investigate the effects of epoxy/amine mix ratio on cure through the use of real time dielectric response. Microdielectric sensors were used to monitor molar mix ratios of EPON 828/DDS ranging from 3/1 to 1/3. Microdielectric results show that dielectric constant varies from about 9 to 30 with increasing DDS content in uncured resin. Times to the 10 kHz dipole loss peak during cures at 177°C are found to decrease with increasing DDS content. Above a ratio of 2/1 and below 1/2, due to the low molecular weight of the reaction products, loss peaks do not occur. Ionic conduction is shown to have a strong correlation with calorimetric kinetic cure data. The influence of both excess amine and excess DDS is to produce cured material with high ionic conduc tion levels. Deviations from ideal stoichiometery as small as 1.1/1 produce noticeable dif ferences in the finally cured network.

Dynamic Cure and Diffusion Monitoring in Thin Encapsulant Films

AbstractRecent developments in the area of microelectronics now enable the fabrication of microdielectric sensors that can follow the drying, curing, and diffusion phenomena in thin films. This paper first investigates the response of the rricrodielectric sensor to cure in a thin epoxy film. The multi-frequency loss factor data are reduced to a single “ion-viscosity” response curve representative of the change in ion mobility during the cure reaction. The dynanic dielectric data are compared to Tg as a function of time as determined by differential scanning calorimetry (DSC) on several partially cured samples. The dielectric response is shown to be extremely sensitive to the entire cure process. A similar epoxy film is exposed to alternately wet and dry environments while the dielectric response is monitored. The diffusion coefficient of water in the cured resin is estimated through the dynamic change in permittivity using a simple Fickian model and the results are shown to be in good agreement with the literature values. Finally, changes in dielectric properties of a photoresist during UV exposure are presented.

Microdielectric monitoring of prepreg cure

AbstractThe feasibility of using a microdielectric sensor to characterize the cure behavior of a commercial prepreg roving was investigated. Chemical and physical characterization methods were first used to determine the uniformity of the prepreg material. Thermal, dynamic mechanical, and dielectric methods were then utilized to monitor the cure cycle. The dielectric response was found to be independent of the glass fibers of the prepreg and to show reproducible changes in the early part of cure over a temperature range of 80 to 135°C. The manufacturer's recommended cure temperature of 121°C (250°F) was found to correspond to the onset of the ultimate glass transition of the material. Variations in the dielectric response in the latter part of cure at this temperature and above were attributed in part to this event.