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Research on Adsorption Performance of Porous Materials Based on Ceramics to Organic Molecular Contamination

  • Xiao-xiao YuanEmail author
  • Na Li
  • Shou-cheng Pang
  • Dong-sheng Yang
  • Wei-guo Zang
Review
  • 43 Downloads

Abstract

In the space environment, spacecraft on orbit released a series of organic gas molecules by means of material outgassing, vacuum outgassing, eliminating propellant, and so on. These gas molecules can deposit on sensitive surface by direct transportation, reflection, atmospheric-scattering transportation, self-scattering transportation, and thermalization atmospheric-scattering transportation, such as optical elements, thermal control coating, and solar cell, which can result in decrease of the property even out of work. Contamination control was the trend of long-life and high-reliability spacecraft. As a new type of contamination control technology, molecular adsorber based on ceramics with convenient measuring, lightweight, low power consumption is a useful way of controlling contamination of spacecraft-sensitive device. There is not any study on molecular adsorber, although the molecular contamination test and simulation analysis have been researched by many people in China. This article analyses the adsorption mechanism study of porous material to organic molecular contamination in vacuum and studies the adsorption property study of porous material to organ-molecular contamination in vacuum. Through the method study of molecular adsorber absorption capability, the development of molecular adsorber is suggested.

Keywords

Porous material Absorption Contaminant Contamination control 

1 Introduction

Non-metal materials of spacecraft released a series of organic gas molecules by means of material analytic which are phthalates and silicones when spacecrafts work on orbit [1, 2]. These gas molecules can deposit on sensitive surfaces [3] by direct transportation, reflection, and atmospheric-scattering transportation, such as optical elements [4], thermal control coating, and solar cell, which can result in decrease of the property even out of work. To effectively control contamination levels and improve reliability, contamination control study on the ground and on orbit is in-phase developed. Therefore, the goal the contamination control is a new and effective contamination control method for spacecraft.

As a new type of contamination control technology, molecular adsorber based on ceramics [5] with convenient measuring, lightweight, low power consumption is a useful way of controlling contamination of spacecraft-sensitive device. There is not any study on molecular adsorber, although the molecular contamination test and simulation analysis have been researched by many people in China.

This article analyses the adsorption mechanism study of porous material to organ-molecular contamination in vacuum and studies the adsorption property study of porous material to organ-molecular contamination in vacuum. Through the method study of molecular adsorber absorption capability, the development of molecular adsorber is suggested.

2 The Source and Characteristic of Organic Molecular Contaminant

The source of spacecraft contaminant includes the following: (1) the contaminant of assembly and test; (2) the contaminant of emission; and (3) the contaminant of non-metal materials outgassing on orbit.

Based on the outgassing test result on the ground, the contaminant is mostly due to the outgassing of non-metal materials. The typical test result is summed up in Table 1.
Table 1

Result of component analysis

No.

Name

Molecular weight

Boiling point (°C)

Frequency

1

Dioctyl phthalate

390.56

386.9

4

2

Dibutyl phthalate

278.35

340

1

3

Di-2-ethylhexyl phthalate

390.56

370

1

4

Dioctyl sebacate

426

539

1

5

Methylphenyl siloxane

1

6

3-Methyl 5-phenyl 3-siloxane

541

245 (0.5 Torr)

1

7

18-Methyl 5-pheny l-siloxane

666

4

In Table 1, the contaminant components are usually phthalate and siloxane. The contaminants are non- volatile and possess mighty adsorption capability. In addition, the contaminants can denature in the space environment, such as atom oxygen and ultraviolet, causing more serious contamination effects.

The outgassing component of grey cable in vacuum is mainly phthalate at 65 °C, and phthalate and siloxane at 85 °C and 105 °C. The volatilization temperature of phthalate is relatively low, so it is indicated that phthalate is unstable, volatile, and exudative. The volatilization temperature of siloxane, with big molecular weight, is relatively high, so it is indicated that siloxane is stable, non-volatile, and non-exudative.

3 The Characteristic of Porous Materials Based on Ceramics

The porous materials based on ceramics include adsorbent and substrate. The choice of adsorbent material is according to the adsorption performance to organic molecular contaminants, and the choice of substrate is according to the binding force and effect with adsorbent.

Zeolites are very good adsorbent materials, with high surface typically more than 200 m2/g. Zeolites are molecular sieves by synthesized, which is the crystalline of aluminum silicates. The molecular sieves lose crystal water by heating. The lattice zeolite has specific, controlled physical parameters. This feature offers advantages: the adsorbent characteristics depend on the crystalline framework, uniform throughout the zeolite; the selectivity for and retention of sorbates unique to specific, known crystalline dimensions; and the properties within the lattice confer selective adsorption for different functional organic molecules. The “cages” provide volumes with high available, where multiple layers of sorbates may be adsorbed. The pore size and connections (between the “cages”) of the frameworks determine the transport dimensionality and adsorption rates unique to different zeolites (Fig. 1).
Fig. 1

Crystal structure of zeolite

Zeolites are the crystalloid aluminum silicates, consisting of nature or manual alkali and alkaline earth (e.g., Na, K, and Ca) oxide, which is micropore material. The basic structure is tetrahedron with three-dimensional cavity. The basic unit is TO4 (T = Si, Al), consisting of SiO2 and A12O3 tetrahedron sharing atomic oxygen. The chemical structure is M2/nO, Al2O3, xSiO2, yH2O, in which M is the cation with valence n and x ≥ 2. Due to its open aperture, it can be synthesized to different zeolites with different pore sizes according to the sorbates.

The advantage of zeolites includes:
  1. (1)

    High surface. Granularity range from 1 to 10 μm and the average size of the particle is about l–5 μm. As payload of the spacecrafts, the zeolites featured as low mass, small volume, and good facility to use.

     
  2. (2)

    Good selectivity for the adsorption. The zeolites present sufficient apertures for the contaminant molecules to enter and be adsorbed and the aperture is well-proportioned. Only the diameter of the molecule is smaller than aperture of the zeolites, the molecule can get into the aperture. The zeolites belong to micro absorbent, so the aperture and diameter of the molecule on the same level without lag adsorption.

     
  3. (3)

    Large capability at low pressure. Zeolite is polar absorbent and Si cation present strong polarity, so it still has strong adsorption capacity and good selectivity even under lower pressure and room temperature. Molecular adsorbers is mainly applicated in the spacecraft vacuum thermal test and clean the contamination out of the spacecraft on orbit, so we need material which can absorb polar organic gas, and the zeolite satisfy all requires.

     
  4. (4)

    Large capability at low concentration. Due to phenomenon of the force field superposition produced by the wall of aperture in the vent of the zeolites, it makes the adsorption potential increased and the zeolites have a large capability at low concentration of the contaminant molecules.

     

The form of the porous materials is micro-particle (1–10 μm), and it can bring such serious problems as non-reclaimed, adsorbent loss, and secondary contamination. Therefore, the porous materials must be immobilized effectively.

Cordierite is a good substrate material [6], and its density is 400 cells/inch2, and it is belonged to ceramics. The component of cordierite is Mg2Al4Si5O18, with stead structure, cheap cost, little flow resistance, and dilatation coefficient. Figure 2 shows the shape of cordierite.
Fig. 2

Shape of cordierite

In-situ hydrothermal synthesis is a simple and effective method for porous materials immobilized on porous cordierite ceramics substrate [7]. In the course of hydrothermal synthesis, porous cordierite ceramics are immerged thoroughly in the porous materials gels. When the synthesis conditions are towardly, porous materials crystal can grow selectively on the substrate, not in the liquid phase. Therefore, porous materials can shape on the substrate. The in-situ hydrothermal synthesis method not only does not need cementing material, but also makes the growth of porous materials in order. The order can decrease the diffusion resistance of the organic molecular contaminant in the porous materials’ adsorbent coating, so it can decrease the dead volume and increase the efficiency of the adsorbent.

4 Research on Adsorption Performance of Porous Materials to Porous Materials

Research on adsorption performance of porous materials based on ceramics to porous materials includes study of adsorption mechanism and adsorption model.

4.1 Study of Adsorption Mechanism

Based on the outgassing test of non-metal materials and adsorption capability test to porous materials, the movements, interactions, and collisions to boundary of molecules in vacuum are simulated by direct simulation Monte Carlo method [8]. Through the above analysis, the study of adsorption mechanism to organic molecular contamination of porous materials is developed.

First, the non-metal materials outgassing test project is instituted, making use of the mass, the location, and the environment condition of the spacecraft non-metal materials. The project can be used to study the outgassing mass of non-metal materials and analyze the outgassing components by gas chromatography–mass spectrometry (GC/MS). This is the first input.

And then, porous materials as the contamination depositing surface, non-metal materials as the outgassing source, the transmission and depositing of contamination in vacuum test project is instituted, analyzing the proportion of the contamination to porous materials, and developing the sticking coefficient model. This is the second input.

Finally, the total adsorption process, including the outgassing of organic molecules, the transmission of the contamination and the sticking of the contamination to the porous materials, is analyzing by direct simulation Monte Carlo method, making use of the above two inputs. In addition, the adsorption model of the porous materials is established.

4.2 The Establishment of the Adsorption Interaction Model

On the basis of the above adsorption model, study the adsorption method of the porous materials to phthalate. Through the test of the absorptivity and sticking coefficient, the adsorption law is mastered.

First, choose typical porous materials as the study objects, through analyzing the actuality home and aboard.

And then, non-metal materials as the outgassing source, porous materials as the contamination depositing surface, the absorptivity and sticking coefficient of the porous materials test project is instituted, analyzing the effecting factors, such as outgassing time, outgassing temperature, environment temperature and pressure, making use of Thermogravimetric analysis (TGA) and Quartz Crystal Microbalance (QCM).

Finally, the model of interaction between adsorbent and contaminant is established, taking the adsorption capability, practicability, and convenience into account. Choose appropriate porous materials as the adsorbent mainly adsorbing phthalate.

5 Conclusion

It is a new type of contamination cleaning technology on orbit [9], making use of porous materials to reduce the location contamination of the spacecrafts. It can protect the sensitive equipments and the safety of the cosmonauts, and it is small, light, and no-power. It can be used on the contamination-sensitive surfaces and in manned module, improving the quality and life.

References

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Copyright information

© Chinese Society of Astronautics 2019

Authors and Affiliations

  • Xiao-xiao Yuan
    • 1
    Email author
  • Na Li
    • 1
  • Shou-cheng Pang
    • 2
  • Dong-sheng Yang
    • 1
  • Wei-guo Zang
    • 1
  1. 1.Beijing Institute of Spacecraft Environment EngineeringBeijingChina
  2. 2.Beijing Institute of Space Mechanics and ElectricityBeijingChina

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