Elsevier

Vacuum

Volume 191, September 2021, 110368
Vacuum

High-vacuum setup for permeability and diffusivity measurements by membrane techniques

https://doi.org/10.1016/j.vacuum.2021.110368Get rights and content

Highlights

  • Versatile high vacuum set up for gas transport measurement properties.

  • Accurate permeability and diffusivity measurements on several order of magnitude.

  • A single apparatus that combine two measurement methods by membrane technique.

Abstract

The knowledge of the permeability and diffusivity (the diffusion coefficient) of materials to different gases is essential for a wide range of applications, such as food packaging, gas separation techniques, and vacuum technology. In this paper, we describe a versatile high-vacuum apparatus for permeation measurements, based on a membrane technique, with the aim of increasing the reliability of results and allowing measurements of permeability of different materials whose permeability spanning several orders of magnitude. The apparatus is equipped with a residual gas analyzer for measurements of partial and total pressure, a spinning rotor gauge for accurate determination of pressure, a calibrated leak to determine pumping speed and capacitance diaphragm gauges for accurate pressure measurements greater than 10−2 Pa. The apparatus works in a static or a dynamic mode. Its performance is tested on polydimethylsiloxane with N2 and Viton® with He as tracer gases. The lower measurement limit of the apparatus depends mainly on the degassing of the inner walls of the chamber. With the procedure described in this paper, it is possible measured permeability from 10−5 cm2/s to 10−10 cm2/s.

Introduction

Precise knowledge of the gas permeation properties of materials is required in a wide range of applications. Gas separation technology [1] exploits the permselectivity characteristics of polymeric materials to produce pure gases, for CO2 capture in the fuel gas [2] of power plants, and for sensing applications [3]. Permeability P and diffusivity D are the primary quantities that characterize sealing materials for applications in food packaging, cryogenic storage, and hermetic packaging for micro- and nano-electronic systems. In vacuum technology, knowledge of P and D is essential to describe the sealing and outgassing properties of vacuum materials [4,5].

Although data on the permeability and the diffusivity of materials are widely available in the scientific literature and in technical reports, there are disagreements among the results that are not within the range of experimental error. As reported in Ref. [6], interlaboratory testing has revealed that permeance (the ratio of permeability to the material thickness) measured by the most common procedures exhibit a strong dependence on the procedure being used, as well as the laboratory performing the test. The methods commonly used to measure permeability can be classified into two main categories: infusion/outgassing and membrane techniques. In the first category, the sample under investigation is degassed in a vacuum chamber, and it is subsequently exposed to a controlled atmosphere of tracer gas for the time necessary to completely reabsorb it. The saturated sample is then again mounted in a vacuum chamber to carry out permeation measurements by throughput, rate of pressure rises, or gravimetric methods [7,8]. The second category encompasses techniques in which the permeability and diffusivity of a thin sheet sample are usually measured in a variable-volume or variable-pressure apparatus according to the standard test method (ASTM D1434 − 82, Reapproved 2015) [6], where they are reported respectively as volumetric and manometric procedures. Various laboratories have implemented these techniques with many slight variations and using different instrumentation depending on the specific scope of the investigation. The International Organization for Standardization (ISO) also standardize the measurements method of gas permeability as ISO 15105-1 (differential pressure methods) and ISO 15105-2 (equal-pressure method).

Tremblay et al. [9] developed an ultrahigh-vacuum (UHV) apparatus based on membrane techniques for selective online measurements of gas fluxes. It uses a Residual Gas Analyzer (RGA) to measure transient fluxes directly, rapidly, and selectively. Dong et al. [10] presented a new permeation apparatus allowing the measurement by membrane techniques of the helium gas permeability of polymers. It is based on the difference pressure method, where concurrent measurements in a reference chamber exclude the outgassing background effect. Sebok et al. [11] constructed a novel instrument for the investigation of small permeation fluxes with static and dynamic methods. They also developed a new type of support for the membrane. It is based on a fine stainless-steel mesh and allows more accurate measurements to be obtained, thereby enabling the whole surface of the membrane to be “active” in the permeation process. Jannot et al. [12] reported a quasi-steady method for the determination of the apparent permeability of porous materials, Ranade et al. [13] assembled a new system for quantitative and high-sensitivity measurements of gas permeation through thin flexible substrates, and Graff et al. [14] developed a methodology for calculating the diffusivity and solubility of ultrathin barrier films.

In this paper, we present a new apparatus for permeation measurements based on membrane techniques. Compared with the approaches described above, it incorporates several novel characteristics, employing an RGA for selectivity measurements and taking account of background outgassing. Its main advantage is the possibility of working with both static and dynamic methods. The different procedures of the two methods allow, with a single apparatus, measurements of the permeability in a range spanning more than five orders of magnitude. Allowing to characterize materials with very different gas transport properties. Another benefit is to provide traceability of measurements by in-situ calibration of effective pumping speed and sensitivity of the RGA. The apparatus incorporates a new freestanding membrane assembly that guarantees the same area for both sides of the membrane, thus avoiding miscalculation of the permeation area and unwanted lateral diffusion, which are artifacts often observed in measurements of the permeability of composite membranes [15,16].

We used PolyDiMethylSiloxane (PDMS) and Viton® membranes to test the new setup with N2 and He, respectively, as pure tracer gases. PDMS is a polymer with a high permeability, and it can be employed as an outgassing reference. Viton®, characterized by a far lower permeability, is one of the most widely used materials for sealing. Measurements of P and D were carried out by applying a differential pressure Δp across the samples equal to 105 Pa, i.e., the pressure difference to which vacuum materials are generally subjected. Static and dynamic modes of operation give the same permeation values within experimental errors.

Section snippets

Theory

The basic theory relating permeability and diffusivity is well known [17]. Assuming the validity of Fick's and Henry's laws for the permeability coefficient P, we haveP=JLΔp=DSwhere J is the rate of transfer per unit area through a sample cross section, L is the membrane thickness, D is the diffusivity, and S is the solubility. Here we consider gas diffusion through a membrane, assumed as an infinite plate, due to a pressure difference Δp = pu − pd across it, where pu and pd are the upstream

Materials and methods

Fig. 1 shows the high-vacuum experimental setup for gas permeation measurements.

It is composed of a high-vacuum chamber that, through a high-vacuum gate valve V1, is pumped with a turbo pump V70LP (Varian, now Agilent) with a CF 40 inlet flange. The primary pump is a Scroll-type pump nXDS10i (Edwards) with ultimate pressure 7 × 10−1 Pa. An orifice with a conductance approximately 8.5 l/s for nitrogen is mounted between V1 and the chamber. This orifice was made by drilling a hole with a diameter

Data evaluation and uncertainty estimation

Using the dynamic method, we calculated ISS and I0 as averages of at least 50 values with time interval of 2 s for each measurement. After the introduction of tracer gas, we recorded continuously the measured ion current I(t) and used the values for calculation of D according to Eq. (4).

In the static method, we performed a linear fit by the least squares method for the last part of the recorded curve of pressure increase in the downstream chamber, pd(t). OriginLab Software was used for curve

Results and discussion

The results of the degassing rate measurement described in step 7 of Sec. 3.2.1 were all in the region of 10−8 Pa/s. Taking account of the chamber volume and the active area of the membrane, the MDS for P in static method was in the region of 10−10 cm2/s.

Table 2 shows the values of P and D obtained for both materials and with the two method.

These results agree with literature data. The data of P and D for N2 through PDMS agree with reference [24], those of P for He through Viton® agree with

Conclusions

The high-vacuum setup that we have presented in this paper is simple and versatile. It allows to work with two different procedures in a single apparatus (static and dynamic) allowing to measure the gas transport properties with values spanning on different order of magnitude. The static method, in comparison with dynamic one, has the advantageous of a lower measurement limit as well as lower uncertainty. On the other hand, the dynamic method, respect to the static one, thanks to the use of an

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by grants from the Italian Ministry of Education, University and Research, Flagship Project Nanomax.

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