Elsevier

Icarus

Volume 317, 1 January 2019, Pages 365-372
Icarus

Simple deceleration mechanism confirmed in the terminal hypervelocity impacted tracks in SiO2 aerogel

https://doi.org/10.1016/j.icarus.2018.08.017Get rights and content

Highlights

  • All kinds of A-type tracks were classified into three types based on the conditions of vapor model proposed by Dominguez (2009).

  • A simple deceleration mechanism was confirmed by the data fitted well with the regular data of the terminal A-β type track.

  • Thermal effects happened around and along track was observed by scanning electron microscopy.

Abstract

As an attractive collector medium for hypervelocity particles, SiO2 aerogel has been deployed on outer space missions. Aiming at quantifying the complicated relationship between the penetration track and the residual grains, many attempts have been made on hypervelocity experiments and models. However, models were difficult to accord strictly well with experimental data attributed to many uncertainties including thermal effects, aerogel accretions and projectile ablation during the penetration. In this paper, impact experiments were conducted at various density silica aerogels (50∼120 kg·m−3) with regular soda-lime glass beads as projectiles. Varying degrees of thermal effects happened around and along track was observed by scanning electron microscopy. That energy distribution in the track released by hypervelocity projectile has a decreasing change. The regular data of the terminal A-β type track (the track with combined features) was found according to A-type tracks classification based on the conditions of vapor model (Domínguez, 2009). Just considering for projectile overcoming the crushing strength with uniform deceleration, the simple mechanism was confirmed by the data fitted well with the snowplow model (Domínguez et al., 2004). The result after tracks classification is due to the terminal track with few thermal effects and aerogel accretions. In addition, other two types of tracks formation processes were discussed.

Introduction

Different from ordinary materials, the unique characteristics and diverse chemical compositions make the aerogel recognized as a state of matter (Du et al., 2013). Nanoporous and transparent silica aerogel had previously been proven the suitability as a capture medium for interplanetary dust in the laboratory (Tsou et al., 1988, Barrett et al., 1992, Tsou, 1995). Comet dust has been successfully brought back by SiO2 aerogels in Stardust Mission (Brownlee et al., 2003, Brownlee et al., 2006, Hörz et al., 2006). Comet dust in the aerogel formed a variety of track morphologies which could be classified into carrot-like and bulbous shapes (Brownlee et al., 2006, Burchell, 2007). The high diversity of impactor properties was demonstrated to range from highly porous and friable aggregates to consolidated monomineralic grains. The various tracks were classified into three broad types (A, B, and C) and subsequently quantified (Hörz et al., 2006, Burchell et al., 2008).

To correctly derive the characteristics of primary grains from their impact tracks, both research of the morphology and generation on the track are ongoing. To obtain stardust cometary particle size distributions, laboratory calibration experiments have provided samples as references for composition analysis teams (Hörz et al., 1998, Burchell et al., 2008, Kearsley et al., 2012).

Numerous efforts were made in experiment and theory to estimate the impact conditions in aerogel. There seems always be significant scatter of data points in penetration track length normalized to residual diameter versus density of silica aerogel in the impact experiments (Burchell and Thomson, 1996, Hörz et al., 1998, Burchell et al., 1999, Burchell et al., 2001, Domínguez et al., 2004, Burchell et al., 2008). Penetration process is accompanied by complex physical phenomena (Trucano and Grady, 1995, Hörz et al., 2009). The thermal history of the terminal particles during capture in aerogel was investigated by the theoretical calculation and the designed experiment (target and projectiles). For example, the thermal history of particles captured in several densities aerogel were calculated by Coulson (2009). Hypervelocity grains were captured by a fluorescent aerogel that passively recorded their kinetic energies (Domínguez et al., 2003). Projectiles coated with an ultrathin organic conducting polymer were fired and analyzed in-situ using Raman Microscopy after captured (Burchell et al., 2009b). Magnetic sub-micron hematite particles were selected to measure the temperatures experienced during hypervelocity capture in aerogels (Jones et al., 2013). The captured particles typically experienced some degree of thermal ablation before coming to rest (Noguchi et al., 2007). The kinetic energy of a particle is converted to thermal energy during the capture process, altering or even destroying components of the particle (Jones et al., 2013).

In addition, there have also been several attempts to model the capture process when fine, hypervelocity particles are decelerated and stopped in low density media. The energy loss of projectiles captured in organic foams was investigated by Anderson and Ahrens (1994). Based on previous experimental results, the vaporization model for impact cratering in low density foams was proposed by Kadono (1999). The snowplow model, a kinetic model of impact cratering in aerogels were developed and tested was discussed by Domínguez et al. (2004). Energy loss in aerogel with carrot shaped tracks was researched. After then, a general model of track formation was proposed by Domínguez (2009). This vapor model includes the presence of partially and completely vaporized aerogel material that can reduce to the kinetic “snowplow” model. Analyzing experimental data from in-situ observation, deceleration mechanism of projectiles was provided by Niimi et al. (2011), Kadono et al. (2012). In this simplified penetration model, the deceleration of hard sphere projectiles in aerogel is described by hydrodynamic and crushing regimes for higher and lower velocities, respectively.

In this paper, we focus on thermal effect and formation process in terminal type A tracks. Hypervelocity impactor experiments were conducted in silica aerogel (50∼120 kg·m−3) @5 km·s−1 with glass bead projectiles. The landing stage was designed to place the two samples in the same impact experiment. Some tracks created on the cylinder surface were observed by scanning electron microscope (SEM). The thermal effects were studied by various degrees of damaged microstructure in aerogel. The dispersion of track classification results was greatly reduced by vapor model. Based on the classified track results, the track formation process was briefly discussed. Among them, the regular data of the terminal A-β type was investigated. The result was fitted well with sample interaction model, which just considering for projectile overcoming the crushing strength with uniform deceleration.

Section snippets

Experimental samples

A series of SiO2 aerogel samples (50∼120 kg·m−3) were used for hypervelocity impact experiments. Combined with ethanol supercritical drying process, they were prepared by acid-base catalyzed method similar to our previous work (Liu and Zhou, 2013, Liu et al., 2013). All aerogel samples were monolithic obtained in the shape of 1.5∼2.5-cm-long cylinder, 3.2∼4.9 cm in diameter. The apparent density is determined by weighing method. Corresponding number of shots for different density samples in

Results

As provided in Fig. 3, the impacted track in the sample was observed morphologically by SEM. As a morphological contrast picture, the typical structure of silica aerogel is presented in Fig. 3b, like stacked silica spheres and the porous morphology. The opening (entrance hole size ∼60 µm) into the interior of the track is observed vertically (Fig. 3a). The track wall is surrounded by depressed and spalled region. The entrance hole (point c) shows obvious porous structure that was ablated and

Data correlation

As shown in Fig. 5b, the T/Dp data are widely scattered in the low-density range, there is no clear trend or relevance. In the high-density range, the T/Dp data tends to decrease with increasing density. The inhomogeneities of aerogels, rotation and shock pressure of projectile may contribute to the wide variation of T/Dp in the low-density range (Kitazawa et al., 1999). As shown in Fig. 5c, the double-exponential fitting relationship of the normalized maximum impact diameter (M/Dp) versus

Conclusion

Impact experiments were performed to investigate the interaction between spherical hypervelocity glass particles and silica aerogel. We found the destruction of porous morphology by SEM and varying degrees of thermal effects happened around and along the track. The results imply that the kinetic energy lost from the particle goes into heating the aerogel.

The dispersion of experimental data means that the tracks were created with many uncertain factors, including different impact velocities,

Acknowledgement

We are thankful for financial support from National Key Research and Development Program of China (2017YFA0204600), Science and Technology Innovation Fund of Shanghai Aerospace, China (SAST201469) and the Natural Science Foundation of China (No. 11404213).

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