Polytetrafluoroethylene Samples Research Articles

A New Dual-Channel Measurement Method for Accurate Characterization of Low-Permittivity and Low-Loss Materials

A dual-channel attenuation measurement system has been developed to measure the dielectric properties of low-permittivity and low-loss homogenous materials. The scattering parameters are derived from a series of transmission measurements, which were performed under different reflection coefficients of the source and load ports set by a pair of automatic tuners. The reflection coefficients of the automatic tuners were calibrated with the vector network analyzer (VNA) before the measurements. The mismatch error is effectively eliminated with the measurement and calculation process. The measurement scattering parameters are of much smaller uncertainties than those with the VNA. A polyimide aerogel sample is measured at X-band frequencies, its permittivity is about 1.075, and the loss tangent is about 0.001. The corresponding standard uncertainties are 0.0014 and 0.00048, respectively. The uncertainty of the loss tangent is reduced by one order compared to the results based on VNA. For validation of the proposed method, measurements of two polytetrafluoroethylene (PTFE) samples with different thicknesses were also conducted. This paper provides a broadband method for dielectric characterization with improved accuracy for low-permittivity and low-loss materials and can easily be extended to other waveguide bands.

A dual-layer structure with record-high solar reflectance for daytime radiative cooling

We demonstrate a diffusive solar reflector with record-high reflectance when integrated over the wavelength region from 0.28 to 4.0 μm. The reflector has a dual-layer structure consisting of a polytetrafluoroethylene (PTFE) sheet on top of a silver film. The thickness of the PTFE varies from 0.24 mm to 1 mm. Spectral reflectance and transmittance of the PTFE sheets (with and without a silver film) were measured using a monochromator and a Fourier-transform infrared spectrometer, with integrating spheres, at wavelengths from 0.28 μm to 15 μm. The scattering and absorption coefficients of the PTFE samples were obtained by fitting the reflectance and transmittance spectra. Integration over the solar irradiation spectrum (AM1.5) reveals that the total solar reflectance is approximately 0.99 for the reflector. This is the highest solar reflectance reported to date. A Monte Carlo ray-tracing method and a modified two-flux model were used to calculate the reflectance and compare with the experiments to shed light on mechanisms for the high reflectance. Our measurements also suggest that PTFE has a high emittance around 0.9 in the mid-infrared region. Therefore, the proposed structure holds promise for passive daytime radiative cooling.

Study of the Secondary Electron Yield in Dielectrics Using Equivalent Circuital Models

Secondary electron emission has an important role on the triggering of the multipactor effect; therefore, its study and characterization are essential in radio-frequency waveguide applications. In this paper, we propose a theoretical model, based on equivalent circuit models, to properly understand charging and discharging processes that occur in dielectric samples under electron irradiation for secondary electron emission characterization. Experimental results obtained for Pt, Si, GaS, and Teflon samples are presented to verify the accuracy of the proposed model. Good agreement between theory and experiments has been found.

Nanoscale Water Contact Angle on Polytetrafluoroethylene Surfaces Characterized by Molecular Dynamics-Atomic Force Microscopy Imaging.

The aim of this study is to link polytetrafluoroethylene (PTFE) surface characteristics with its wetting properties in the nanoscale. To do this using molecular dynamics (MD) simulation, three series of rough PTFE surfaces were generated by annealing and compressing and next characterized by the application of the MD version of the atomic force microscopy (AFM) method. The values of specific surface areas were additionally calculated. The TIP4P/2005 water model was used to study the wetting properties of obtained PTFE samples. The simulated water contact angle (WCA) value for the most flat (but slightly rough) sample having PTFE density is equal to 106.94°, and it is close to the value suggested for a perfect PTFE surface on the basis of experimental results. Also, the changes in the WCA with PTFE compression are in the same range as experimentally reported. The obtained MD simulation results make it possible to link, for the first time, the WCA values with the surface MD-AFM root-mean-square roughness and with the PTFE density. Finally, we show that for PTFE wetting in the nanoscale, the line tension is negligible and the Bormashenko's equation reduces to the Cassie-Baxter (CB) model. In fact, our simulation results are close to the CB mechanism.

Ambient aerosol composition by infrared spectroscopy and partial least squares in the chemical speciation network: Multilevel modeling for elemental carbon

ABSTRACTFourier transform infrared spectroscopy (FT-IR) has been used to predict elemental carbon (EC) on polytetrafluoroethylene (PTFE) filter samples from the United States Environmental Protection Agency's Chemical Speciation Network (CSN). This study provides a proof-of-principle demonstration of using multilevel modeling to determine thermal/optical reflectance (TOR) equivalent EC (a.k.a., FT-IR EC) on PTFE samples collected in the CSN. Initially, spectra from nine geographically disperse sites were pooled and calibrated directly to collocated TOR EC measurements. The FT-IR EC quantified in test samples was deemed substandard when judged against an earlier study, e.g., R2 = 0.760 and median absolute deviation (MAD) = 26.7%. Upon scrutinizing each sample's absolute prediction error and squared Mahalanobis distance, Elizabeth, NJ predictions were found to exhibit atypical systematic errors, motivating the development of a multilevel classification and calibration procedure. Atypical Elizabeth spectra w...

Friction characteristics of polymers applicable to small-scale devices

Abstract A review of the critical features of a published micro-tribometer design, which was intended to improve on the dynamic response of typical commercial instruments, leads to its use with a modified technique. Data post-processing is introduced to partially compensate for some potential systematic errors. This approach is demonstrated by a preliminary study of the coefficient of friction (CoF) in sub-mm length reciprocating sliding motion for samples of polytetrafluoroethylene (PTFE) and an R11 acrylic formulation made by micro-stereo-lithography, with an SiO2-coated silicon wafer used as a control sample. Testing covered normal loads in the region of 10 mN–60 mN, at scan frequencies up to 9 Hz, corresponding to sliding speeds in the broad region of 1 mm s−1. While the control samples closely adhered to Amonton's laws over all the test parameter ranges, the CoFs of the two polymers showed contrasting patterns of dependence on sliding speed and repetition rate. Such results have implications for how polymers might be used effectively in future designs for small mechanical systems. They indicate a clear need for further development of the testing methods and large-scale studies of tribological behaviour and its underlying mechanisms under the specified micro-scale conditions.

A combination dielectric and acoustic laboratory instrument for petrophysics

Laboratory testing of rock samples is the primary method for establishing the physics models which relate the rock properties (i.e. porosity, fluid permeability, pore-fluid and saturation) essential to evaluating a hydrocarbon reservoir, to the physical properties (resistivity, nuclear magnetic resonance, dielectric permittivity and acoustic properties) which can be measured with borehole logging instrumentation. Rock samples usually require machining to produce a suitable geometry for each test as well as specific sample preparation, e.g. multiple levels of saturation and chemical treatments, and this leads to discrepancies in the condition of the sample between different tests. Ideally, multiphysics testing should occur on one sample simultaneously so that useful correlations between data sets can be more firmly established. The world’s first dielectric and acoustic combination cell has been developed at CSIRO, so that a sample may be machined and prepared, then measured to determine the dielectric and acoustic properties simultaneously before atmospheric conditions in the laboratory affect the level of hydration in the sample. The dielectric measurement is performed using a conventional three-terminal parallel plate capacitor which can operate from 40 Hz up to 110 MHz, with modified electrodes incorporating a 4 MHz P-wave piezo crystal. Approximately 10 (acoustic P-) wavelengths interact with a typical (10 mm thick) sample so that the user may reliably ‘pick’ the P-wave arrival times with acceptable resolution. Experimental evidence indicates that the instrument is able to resolve 0.25 mm thickness in a Teflon sample test piece. For a number of engineering materials including Teflon and glass and also for a geological samples (Donnybrook sandstone from Western Australia) there is a perfectly linear relationship between both capacitance and P-wave arrival time with sample thickness. Donnybrook sandstone has a consistently linear increase in dielectric permittivity and P-wave velocity with saturation consistent with the Gassmann–Hill prediction. Both the dielectric permittivity and P-wave velocity are faster parallel to the bedding plane than orthogonal to the bedding plane in a shale from the Cooper Basin, Australia.

Full four-dimensional and reciprocal Mueller matrix bidirectional reflectance distribution function of sintered polytetrafluoroethylene.

We measured the Mueller matrix bidirectional reflectance distribution function (BRDF) of a sintered polytetrafluoroethylene (PTFE) sample over the scattering hemisphere for six incident angles (0°-75° in 15° steps) and for four wavelengths (351 nm, 532 nm, 633 nm, and 1064 nm). The data for each wavelength were fit to a phenomenological description for the Mueller matrix BRDF, which is an extension of the bidirectional surface scattering modes developed by Koenderink and van Doorn [J. Opt. Soc. Am. A.15, 2903 (1998)JOAOD60740-323210.1364/JOSAA.15.002903] for unpolarized BRDF. This description is designed to be complete, to obey the appropriate reciprocity conditions, and to provide a full description of the Mueller matrix BRDF as a function of incident and scattering directions for each wavelength. The description was further extended by linearizing the surface scattering mode coefficients with wavelength. This data set and its parameterization provides a comprehensive on-demand description of the reflectance properties for this commonly used diffuse reflectance reference material over a wide range of wavelengths.

Characteristics of mass concentration, chemical composition, source apportionment of PM2.5 and PM10 and health risk assessment in the emerging megacity in China

In this study, 228 daily Particulate matter (PM) filters (57 Quartz and 57 Teflon samples for both PM2.5 and PM10, respectively) were collected from an urban site in Zhengzhou in typical months from 2014 autumn to 2015 summer representing the four seasons. PM concentrations, water-soluble inorganic ions, organic carbon, elemental carbon, and elements were determined, and positive matrix factorization was used for source apportionments. Health risks of toxic elements in PM2.5 and PM10 were also evaluated. The annual mean values of PM2.5 and PM10 were higher than the standards in China, and the highest seasonal concentrations of PM2.5 and PM10 were in winter. Secondary inorganic aerosols (SIAs) were the major component, with the ratio of SIAs/PM highest in summer. The seasonal concentrations of SO42− were high in winter and summer. Crustal elements mainly existed in PM2.5–10; however, elements from anthropogenic sources (i.e., Zn, Pb, Cu, As, Cd, and Mo) were more abundant in fine particles than in the coarse fraction. The main pollution sources were dust, SIAs, coal combustion, vehicle and road dust, and industry, accounting for 10%, 26%, 25%, 20% and 15% in PM2.5 and 32%, 14%, 24%, 18% and 8% in PM10, respectively. Dust source has the highest contribution in PM10; however, SIAs source has the highest content in fine particles. The carcinogenic risks of As to children through the daily intake pathway in PM2.5 and PM10 exceeded the acceptable level. Noncarcinogenic risks of As and Cd in PM2.5 and PM10 to children via the daily intake pathway were significant. Moreover, the sum of noncarcinogenic risks in PM10 via inhalation exposure for local residents and that via dermal absorption for children were significant. The details of the pollution characteristics and the results of source apportionments and health risks assessment of PM2.5 and PM10 in this study can play an important role for the government to formulate reasonable and effective policy to mitigate the atmospheric pollution of PM. To our knowledge, this systematic study is the first to investigate the chemical characterizations, source apportionments, and health effects of PM2.5 and PM10 in Zhengzhou.

Dependence of triboelectric charging behavior on material microstructure

We demonstrate that differences in the microstructure of chemically identical materials can lead to distinct triboelectric charging behavior. Contact charging experiments are carried out between strained and unstrained polytetrafluoroethylene samples. Whereas charge transfer is random between samples of identical strain, when one of the samples is strained, systematic charge transfer occurs. No significant changes in the molecular-level structure of the polymer are observed by XRD and micro-Raman spectroscopy after deformation. However, the strained surfaces are found to exhibit void and craze formation spanning the nano- to micrometer length scales by molecular dynamics simulations, SEM, UV-vis spectroscopy, and naked-eye observations. This suggests that material microstructure (voids and crazes) can govern the triboelectric charging behavior of materials.

Low pressure micro-Joule picosecond laser-induced breakdown spectroscopy and its prospective applications to minimally destructive and high resolution analysis

A time-resolved spectroscopic study is performed by using 125–500 micro-Joule (µJ) ps laser focused directly without the aid of microscope on a Cu plate sample in a variety of low-pressure ambient gases including air, helium and argon. It is shown that the ultrashort µJ laser-induced low-pressure plasma in Ar ambient gas exhibits the typical characteristics of shock wave plasma responsible for the thermal excitation and sharp emission of the analyte atoms. It is found that the highest signal to background (S/B) ratio of about 100 is achieved in 1.3 kPa argon ambient gas and detected with optical multichannel analyzer (OMA) gate delay of 1 ns and gate width of 50 µs. The emission spectra obtained from pure Zn sample show the effective suppression of the ionic emission with ablation energy around and below 500 µJ. The experimental setup is successfully applied to Cr analysis with low detection limit in steel. In particular, its application to C analysis in steel is demonstrated to resolve the long standing problem of overlapping contributions from the neutral and ionic Fe emission. It is further found that an element of high excitation energy such as fluorine (F) can be clearly detected from a non metal teflon sample. Further, its application to alluminum sample containing various concentrations of Mg, Ca, Fe, and Si impurity elements clearly displays the existence of linear calibration lines promising for quantitative analyses in certain dynamical ranges. Finally, in view of the tiny crater sizes of less than 10 µm diameter created by the very low ps laser energy, this technique is promising for micrometer resolution mapping of elemental distribution on the sample surface and its depth profiling.

Investigation of Snow Milling Mechanics to Optimize Winter Tire Traction

ABSTRACT A detailed understanding of effects occurring in the contact patch between tire tread and snow surface is needed to maximize tire grip in winter conditions. The main focus of this study is quantifying the snow milling effects of individual tire tread block elements during sliding. Tests are carried out using the high-speed linear friction tester (HiLiTe), located at the Institute of Dynamics and Vibration Research at Leibniz University of Hannover, Germany. Test tracks are prepared using artificially produced snow. To solely investigate snow milling effects and exclude material properties of rubber, in a first instance the tread block samples are made of polytetrafluoroethylene (PTFE). Because PTFE is at the same time rigid and hydrophobic, known friction mechanisms such as adhesion and hysteresis can be neglected, leaving only the tread pattern milling mechanics to transmit frictional forces to the snow track. The PTFE samples are shaped in such a way that they mimic the geometry of different siped rubber tread blocks under load, varying the sipes' number, shape, and tilt angle. Results show the benefit of multiple sipes and give information on the evolution of transmittable forces with respect to sliding distance. It is found that the block element shape and tilt angle are directly linked to the frictional force, showing a distinct optimum for specific angle and shape combinations. In addition, forces are not depending on sliding speed, but on sliding distance. The snow milling results of PTFE block elements are then compared to siped rubber block samples. Corresponding high-speed videos show that PTFE sample snow milling mechanics can be directly applied to rubber samples.

Fast Determination of Monocyclic Aromatic Hydrocarbons in Ambient Air Using a Portable Gas Chromatography–Photoionization Detector

Many air sampling methods are time consuming and require complex pre-treatment steps. Gas chromatography–photoionization detector (GC–PID) is a rapid method for sampling and analysis. However, although it has been used in a number of studies, its operating conditions and performance parameters have not been optimized systematically. In this study, a GC–PID method for analysis of monocyclic aromatic hydrocarbons in gas samples without pre-concentration or enrichment was developed and optimized. This GC–PID can perform both online and off-line analysis. In online analysis, the sample was pumped directly into a Teflon sample loop (pumped online injection), which resulted in minimal loss of sample. The optimum parameters were as follows: 30-s pumping time, 10 mL min−1 of carrier gas flow rate, and 40 °C oven temperature. GC–PID was applied to analysis of benzene, toluene and xylene. The calibration curves showed good linearity for online analysis. The results obtained by GC–PID were accurate and reliable, with all the correlation coefficients ≥0.9972 and all the relative standard deviations <3%. A mixture of benzene, toluene, and o-, m-, and p-xylenes was separated satisfactorily in 10 min, except for m- and p-xylene. The performance of the portable GC–PID was compared with that of an ATD–GC–FID for quantification of benzene, toluene and xylene in calibration gas samples, and benzene, toluene, ethylbenzene, and the o-, m-, and p-xylenes in outdoor ambient air. The results indicated that GC–PID with pumped online injection was stable and accurate for analysis of these monocyclic aromatic hydrocarbons.

Observation of asphalt binder microstructure with ESEM.

The observation of asphalt binder with the environmental scanning electron microscope (ESEM) has shown the potential to observe asphalt binder microstructure and its evolution with binder aging. A procedure for the induction and identification of the microstructure in asphalt binder was established in this study and included sample preparation and observation parameters. A suitable heat-sampling asphalt binder sample preparation method was determined for the test and several stainless steel and Teflon sample moulds developed, finding that stainless steel was the preferable material. The magnification and ESEM settings conducive to observing the 3D microstructure were determined through a number of observations to be 1000×, although other magnifications could be considered. Both straight run binder (PG 58-28) and an air blown oxidised binder were analysed; their structures being compared for their relative size, abundance and other characteristics, showing a clear evolution in the fibril microstructure. The microstructure took longer to appear for the oxidised binder. It was confirmed that the fibril microstructure corresponded to actual characteristics in the asphalt binder. Additionally, a 'bee' micelle structure was found as a transitional structure in ESEM observation. The test methods in this study will be used for more comprehensive analysis of asphalt binder microstructure.

Studies of surface and bulk 210Po in metals using an ultra-low background large surface alpha spectrometer

First measurements of natural surface and bulk 210Po specific activities for metals are reported. If covered with protective foils, the surfaces did not show indications of 210Po and the obtained upper limits are in the range of single mBqm−2. Weak bulk activities, in the range of 50 – 280mBqkg−1, were registered for Stainless Steel and Copper, while significant amounts of 210Po, ∼1.5Bqkg−1, were detected in Titanium. One special Teflon sample was investigated with respect to its bulk 210Po.

Reflectance dependence of polytetrafluoroethylene on thickness for xenon scintillation light

Many rare event searches including dark matter direct detection and neutrinoless double beta decay experiments take advantage of the high VUV reflective surfaces made from polytetrafluoroethylene (PTFE) reflector materials to achieve high light collection efficiency in their detectors. As the detectors have grown in size over the past decade, there has also been an increased need for ever thinner detector walls without significant loss in reflectance to reduce dead volumes around active noble liquids, outgassing, and potential backgrounds. We report on the experimental results to measure the dependence of the reflectance on thickness of two PTFE samples at wavelengths near 178nm. No change in reflectance was observed as the wall thickness of a cylindrically shaped PTFE vessel immersed in liquid xenon was varied between 1mm to 9.5mm.

Plasma-Textured Teflon: Repulsion in Air of Water Droplets and Drag Reduction Underwater.

A superhydrophobic behavior can be obtained by properly modifying the surface topography of Teflon or other fluorinated polymers having an inherent hydrophobic character. According to this strategy, we have micro/nanotextured Teflon both as plane material (sheets) and as three-dimensional (3D) object (spheres) with a single step plasma process. The obtained textured Teflon samples were compared with those made of pristine Teflon in air, in terms of repulsion of impacting water droplets, and underwater, in terms of air layer behavior under static and dynamic conditions. The latter case was investigated by subjecting the spheres to a vertical fall in water. Modified surfaces present nanofilaments on the top of micrometric vertical structures, which can increase the air retaining capacity, resulting in a biomimicry effect due to a similarity with the Salvinia molesta leaf. On this surface, repulsion of impacting water droplets can be as fast as previously reached only on heated solids. Also, the air layer over the modified spheres underwater is shown to play a role in the observed reduction of hydrodynamic drag onto the moving object.

0.05–3 GHz VNA characterization of soil dielectric properties based on the multiline TRL calibration

We present a methodology for characterization of soil relative dielectric permittivity in the frequency range 0.05–3 GHz. Soil samples are placed in a measurement cell constructed out of a EIA coaxial transmission line, and then measured with a calibrated vector-network-analyzer. From these measurements the relative dielectric permittivity is obtained by use of a modified Boughriet algorithm. In order to calibrate the vector-network-analyzer directly at the EIA coaxial-transmission-line measurement planes, we use the multiline through-reflect-line method. This method, while providing superior vector-network-analyzer calibration accuracy, is also easy to implement since it uses only transmission lines with known lengths and a single unknown highly-reflective termination. The implemented calibration method was compared to a simplified approach that uses the standard SOLT calibration in Type-N reference planes, and then accounts for the Type-N/EIA adapters by removing their electrical delay. Experimental results for teflon and soil samples with different moisture content and salinity confirmed the validity of our approach.

Measurement of the absolute reflectance of polytetrafluoroethylene (PTFE) immersed in liquid xenon

The performance of a detector using liquid xenon (LXe) as a scintillator is strongly dependent on the collection efficiency for xenon scintillation light, which in turn is critically dependent on the reflectance of the surfaces that surround the active volume. To improve the light collection in such detectors the active volume is usually surrounded by polytetrafluoroethylene (PTFE) reflector panels, used due to its very high reflectance—even at the short wavelength of scintillation light of LXe (peaked at 178 nm). In this work, which contributed to the overall R&D effort towards the LUX-ZEPLIN (LZ) experiment, we present experimental results for the absolute reflectance measurements of three different PTFE samples (including the material used in the LUX detector) immersed in LXe for its scintillation light. The obtained results show that very high bi-hemispherical reflectance values (⩾ 97%) can be achieved, enabling very low energy thresholds in liquid xenon scintillator-based detectors.

A high viscous polymer processed by conventional and unconventional techniques: evaluation of mechanical behaviour and physical characteristics

The physical and mechanical properties of polytetrafluoroethylene (PTFE) processed by conventional and unconventional techniques are reported. Whereas, manufacturing PTFE is problematic due to its high melt viscosity, a new process namely Spark Plasma Sintering (SPS) is performed to elaborate PTFE samples from the particles powder. Thanks to this new sintering process the series of PTFE samples are easily obtained in a remarkably short time. The properties of PTFE-SPS samples are compared to those measured in commercial samples processed by two classical methods: the extrusion and moulding. The tensile mechanical properties carried out at room temperature are found to be superior for the first set of samples (PTFE-SPS). The difference in these macro-behaviours including stiffness, toughness is related to some physical properties of the three series of samples such as their density or crystallinity. The stress–strain curves are then confronted to the Mooney–Rivlin model in order to capture the physics of the large deformations observed in these materials. It is concluded that the SPS technique appears to be a promising way to elaborate the PTFE for various reasons: (i) resolving the high molecular weight problem, (ii) reducing the processing time and cost and (iii) without compromising their mechanical or physical properties.