Letter pubs.acs.org/NanoLett
Flash Ignition of Freestanding Porous Silicon Films: Effects of Film Thickness and Porosity Yuma Ohkura, Jeffrey M. Weisse, Lili Cai, and Xiaolin Zheng* Department of Mechanical Engineering, Stanford University, Stanford, California 94305, United States S Supporting Information *
ABSTRACT: We report the first successful xenon flash ignition of freestanding porous Si films in air. The minimum flash ignition energy (Emin) first decreases and then increases with increasing the porous Si film thickness due to the competition between light absorption and heat loss. The Emin is lower for higher porosity film because high porosity reduces both the heat capacity and the thermal conductivity, facilitating the temperature rise. These results are important for initiating controlled porous Si combustion and preventing their unwanted combustion for safety reasons. KEYWORDS: Porous silicon, energetic materials, flash ignition, minimum ignition energy, ignition the first successful flash ignition of freestanding porous Si films in ambient air and investigate the effects of film thickness and porosity on the minimum flash ignition energy (Emin) with combined experimental and numerical studies. We find that the Emin decreases with decreasing the film thickness and increasing the film porosity. In addition to energetic materials, porous Si has broad applications as high efficiency electrodes for Li ion batteries, solar cells,24 thermoelectric elements, drug delivery systems,25,26 and optical26−28 or sensor devices,29−33 so the obtained Emin values are of great safety importance when handling, storing, and applying porous Si for these diverse applications. The freestanding porous Si film is prepared by electrochemically etching Si wafers (p-type, 0.1−0.9 Ω·cm), followed by an electropolishing step to release the porous Si film from the Si wafer. Specifically, a 200 nm thick aluminum (Al) film is first deposited on the back of a Si wafer, followed by half an hour annealing at 400 °C to form an electrical contact to serve as an electrode for the following electrochemical etching step. The Si wafer containing the Al back contact is then placed in a Teflon anodization cell (Supporting Information, Figure S1) filled with an ethanolic HF electrolyte solution (mixture of 48% HF and 100% ethanol). The concentration of the solution is varied among three volumetric ratios (HF/ethanol = 1:0.5, 1:0.75, 1:1) to alter the porosity of the final porous Si film. A constant current of 50 mA/cm2 is applied between the Al electrode on the back of the Si wafer and a platinum (Pt) counter electrode submerged in the ethanolic HF solution for 2−40 min to control the porous Si film thickness.34,35 Next, the etched porous Si film is detached from the Si wafer by increasing the current to 200 mA/cm2 for 15 s due to electropolishing.35−39
Porous silicon (Si) is crystalline Si that contains nanosized pores and was accidently discovered by the Uhlirs at the Bell laboratories in the mid-1950s.1 Porous Si when filled with cryogenic oxygen was reported to spontaneously explode at temperatures as low as 4.2 K due to heterogeneous hydrogen− oxygen and silicon−oxygen branched chain reactions by Kovalev et al.2 in 2001, sparking great interest in using porous Si as nanoenergetic materials. In addition to the high reactivity, porous Si has a large volumetric energy density (80.7 kJ/cm3),3 over two times higher than that of trinitrotoluene (TNT), and porous Si can rapidly release energy with a reported burning rate up to 3050 m/s.4−7 Moreover, porous Si can be conveniently integrated into microelectromechanical systems (MEMS) as accelerometers6 and airbag initiators8 to produce heat, gas, and control delivery pressures.9 Hence, porous Si has emerged as a promising material for energetic and pyrotechnic applications, and the basic properties of porous Si for pyrotechnic applications were thoroughly reviewed by Koch,8 Clément,10 and Plessis.11 Understanding the ignition properties of porous Si is of great practical importance to reliably initiate the controlled combustion of porous Si and to prevent unwanted combustion for safety reasons. Various ignition methods, such as mechanical fracture,12 lasers,13 and hotwire14 have been applied to initiate the combustion of porous Si. However, optical ignition with a light source containing a broad band of wavelengths, such as a xenon (Xe) flash lamp, has not been reported. Flash ignition is fundamentally interesting because it couples the unique optical properties of porous Si caused by the quantum confinement effects (Ulrich Gö sele15 and Leigh Canham16) with its extraordinary chemical reactivity due to its large specific surface area and high surface coverage of Si−H bonds. Flash ignition is also practically convenient because it is nonintrusive, low-cost, and can achieve distributed ignition of energetic materials to enhance their energy release rates.17−23 In this study, we report © 2013 American Chemical Society
Received: August 20, 2013 Revised: October 21, 2013 Published: October 31, 2013 5528
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the film and also propagates toward the middle of the film at t = 0.66 ms (Figure 2d). Upon further propagation, the porous Si film is propelled upward from the underlying glass slide and burns violently in air (Supporting Information, Movie S1). It should be noted that the spatially nonuniform ignition for the porous Si film is due to the curvature of the Xe flash tube, which leads to slightly varying distance between different portions of the porous Si film to the Xe lamp. To determine the degree of oxidation after the flash ignition of the porous Si film, the as-synthesized porous Si film (without exposure to the Xe flash) and the Xe flash exposed sample are analyzed with X-ray diffraction (XRD, PANalytical XPert 2, Cu Kα, 45 kV, 40 mA), scanning electron microscope equipped with energy-dispersive X-ray spectroscopy (SEM-EDS, JEOL JXA-733A, JEOL USA, Inc.) and thermogravimetric analysis/ differential scanning calorimetry (TGA/DSC, LABSYS evo, Setaram). First, the XRD spectra (Figure 3a) shows that the Si
Finally, the detached porous Si film is removed from the anodization cell and dried with a critical point dryer (SamdriPVT-3B, tousimis) to prevent porous Si films from cracking. The microscopic structure of the porous Si films is characterized by transmission electron microscopy (TEM, FEI Tecnai G2 F20 X-TWIN FEG, 200 kV). The Si film (HF/ethanol = 1:1) is highly porous and formed by assembly of nanocrystalline Si with dimensions ranging from 4 to 12 nm.40 (Supporting Information, Figure S2). The experimental setup for flash ignition of porous Si is schematically illustrated in Figure 1a. The porous Si film is
Figure 1. (a) Schematic of the experimental setup for the flash ignition of freestanding porous Si films by a Xe flash lamp. (b) Optical images of a freestanding porous Si film before and during the Xe flash exposure.
placed on top of a 1 mm thick glass slide directly above the Xe flash tube of a commercial camera flash (AlienBees B1600 flash unit)22 as shown in Figure 1b (left image). Once the flash is triggered, the porous Si film ignites, propels vertically, and burns violently in air within a fraction of a second (Figure 1b, right image and Supporting Information, Movie S1). The dynamic flash ignition process of the freestanding porous Si film is recorded by a high speed camera (FASTCAM SA5, Photron USA, Inc.), equipped with a macro lens for enhanced spatial and temporal resolution (Supporting Information, Movie S2). Figure 2a shows the unreacted porous Si film as
Figure 3. X-ray diffraction pattern of the porous Si film before and after the Xe flash exposure. Inset: enlarged view of Si (400) peaks which show that the Si (400) peaks are slightly shifted after flash ignition.
(400) peaks are shifted after flash ignition (Figure 3a, inset), indicating that the strain distribution is nonuniform in the porous Si.41 The XRD spectra do not exhibit any crystalline SiO2 peaks, so only amorphous Si oxides are formed by the flash ignition. Second, to further confirm that the porous Si film is oxidized by the Xe flash, we expose one part of the porous Si film to the Xe flash and cover the other part with an aluminum plate to block the incident light. The SEM-EDS oxygen concentration elemental mapping image qualitatively shows that the oxygen concentration is increased to some extent in the region that is exposed to flash (Figure S3). Third, the TGA/ DSC is used to quantitatively assess the extent of the oxidation after the Xe flash exposure. Pieces of porous Si (either before or after flash ignition) are heated in air from room temperature to 1000 °C at a heating rate of 20 °C/min, while the mass change and heat release are simultaneously recorded (Figure 4). Both porous Si samples are oxidized to SiO1.85 after the TGA/DSC test, which is determined by the SEM-EDS analysis calibrated by the pure SiO2 reference. Based on the mass change from TGA, we can determine that the oxygen contents are SiO0.13 for the as-synthesized porous Si film and SiO0.26 for the flash-
Figure 2. High speed camera images capturing the ignition and combustion process of a freestanding porous Si film when exposed to the Xe flash. The porous Si film ignites at multiple locations, and the reaction front propagates from the ignition region into the unreacted porous Si regime.
indicated by the dashed line, which is placed on top of the Xe flash tube. At t = 0.46 ms after exposing the porous Si film to the Xe flash, ignition is observed along the front edge of the film (Figure 2b). At t = 0.56 ms (Figure 2c), the ignition region has propagated from the front edge toward the middle part of the film, as evidenced by the observed fragmented film. At the same time, another ignition is initiated near the back edge of 5529
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Figure 4. Comparison of TGA/DSC traces of porous Si films (a) before and (b) after the Xe flash exposure. All of the traces are recorded in air at a heating rate of 20 K/min.
exposed porous Si film. Finally, the DSC heat release profiles show representative thermal oxidation characteristics of porous Si for both as-synthesized and flash-exposed porous Si. Both exhibit a dominant exothermic peak around 523 K that is related to the backbond oxidation and a minor secondary exothermic peak around 750 K due to the oxidation of Si−H to Si−O or Si−OH, which is consistent with previous studies.42,43 All of these characterizations indicate that porous Si is only partially oxidized after flash ignition in air. An important parameter for practical applications of flash ignition of porous Si is the minimum flash ignition energy (Emin). The mechanism of flash ignition is through the photothermal effect. When the porous Si film absorbs enough light to raise the temperature beyond its ignition temperature, flash ignition occurs. The ignition temperature of the porous Si in air is in the range of 500−580 K in air,10 at which the surface Si−H bonds of the porous Si break and the exposed Si atoms react with oxygen, leading to ignition.2 To measure the Emin, the areal energy density of the flash at each power setting is measured using an optical power detector (XLP12-3S-H2, Gentec-EO USA, Inc.), and the Emin of each sample is determined by increasing the power of the flash until ignition occurs. The typical porous Si sample dimension for the Emin experiment is about a couple of millimeters, which is much smaller than the diameter of the Xe lamp tube (15 mm), so that the incident light intensity on the entire sample can be assumed to be uniform. Here, we investigate the dependence of Emin on controlling two parameters: (1) film thickness and (2) porosity, because both parameters affect the optical absorption,16 thermal conductivity,44−46 and combustion properties5 of porous Si. Freestanding porous Si films with different thicknesses and porosity are prepared by varying the electrochemical etching time and the HF/ethanol volume ratio. The film thickness is increased by increasing the etch time. The porosity is increased by decreasing the HF/ethanol volume ratio34 of the etchant and the porous Si films with low/ intermediate/high porosities are respectively prepared by the HF/ethanol volume ratios of 1:0.5, 1:0.75, and 1:1. It should be noted that the porosity also increases with increasing the etch time, correspondingly with increasing the film thickness.34 Hence, it is necessary to study the effect of porosity by comparing films of the same thickness. The Emin of freestanding porous Si films are plotted as functions of film thickness and porosity in Figure 5. It is clearly seen that the Emin increases with increasing the film thickness and decreasing the film porosity.
Figure 5. Experimentally measured minimum flash ignition energies of freestanding porous Si films as functions of film thickness and porosity. The error bar represents the standard deviation, and the lines are the fitted results of the data with an allometric function that satisfies the power law .
To understand the observed dependence of the Emin on the porous Si film thickness and porosity, we calculate the dynamic temperature profiles within the porous Si film due to a single flash exposure using the COMSOL Multiphysics software. Specifically, the numerical simulation schematic is set up on the basis of our experimental configuration (Figure 1a), and it consists of a porous Si film on top of a 1 mm thick glass slide with a pulse of heat supplied from the bottom of the glass slide (Figure 6a). We assume that this is a one-dimensional time dependent heat transfer problem in solids. The heat provided by the Xe flash is mainly absorbed by the porous Si according to the Beer−Lambert law, not by the glass slide due to its high transparency. The porous Si simultaneously loses heat to the bottom glass slide due to heat conduction and air due to natural convection. The incident Xe flash is simplified into a single wavelength light source at 450 nm, where the Xe flash intensity peaks. The output power density−time profile of the single flash pulse is modeled with a Gaussian function as I0(t ) =
E ⎡ −(t − τ )2 ⎤ exp⎢ ⎥ τ ⎦ ⎣ 2τ 2
(1)
where Io(t) (W/m ) is the output power density of the flash, E is the incident energy density (J/m2), and τ is set to be 0.5 ms to simulate the Xe lamp pulse time. The spatial and temporal 2
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The density of the porous Si is calculated by ρp‑Si = ρbulk‑Si(1 − porosity).46 The initial and boundary conditions are expressed as T (z , t = 0) = 300 K; T(z = −1 mm, t ) = 300 K; ∂T k = h(300 K − T (d , t )) ∂z z = d (3)
where d (μm) is the total thickness of the porous Si film and h is the convective heat transfer coefficient of ambient air 5 W/ (m2K). In COMSOL, the temperature profiles of porous Si films are simulated using the Heat Transfer in Solids Module. The material parameters for the silica glass, bulk silicon, and air are obtained from the COMSOL material library. The physical properties of porous Si are referred from literature values (Supporting Information, Table S1). A representative temperature history of the porous Si film is shown in Figure 6b for which the film is specified with a thickness d of 20 μm, porosity of 70%, and incident energy density E (eq 1) of 700 mJ/cm2. Initially at t = 0 ms, the entire porous Si film is at room temperature (300 K). At t = 0.5 ms immediately after the flash pulse, the temperature at the bottom of the porous Si layer rises above 600 K, which is higher than the ignition temperature (500−580 K) of porous Si in air.10 The temperature is lower at higher z values (further away from the flash) since the amount of light absorption drops exponentially with distance. Once the flash pulse is over (t = 1.0−2.0 ms), the temperature of the porous Si film gradually returns to the room temperature due to heat losses to the glass slide and ambient air. The calculated maximum temperature of the porous Si film due to the flash heating is plotted as functions of the porous Si film thickness and porosity (Figure 7a). Here, the thickness and porosity are varied between 1 and 50 μm, and 50 and 70%, while the incident energy density is fixed at 700 mJ/cm2. First, the largest temperature rise is observed for porous Si film about 5 μm in thickness. Below 5 μm, the porous Si is too thin to absorb all the incident light, resulting in a lower temperature rise. Above 5 μm, the additional porous Si thickness serves as a heat sink since the first ∼5 μm absorbs the majority of the light, again lowering the temperature rise. Second, the maximum temperature rise is slightly higher for higher porosity Si since it loses heat slower due to its lower thermal conductivity (Supporting Information, Table S1). The optimal thickness
Figure 6. (a) Schematic of the numerical setup for calculating the temperature rise of the freestanding porous Si film by the Xe flash exposure. (b) Calculated time-dependent temperature profiles within the freestanding porous Si film.
temperature profiles within the porous Si and glass are described by a one-dimensional unsteady heat transfer equation ρcp
∂T ∂ ⎛⎜ ∂T ⎞⎟ = α(1 − R )I0(t )e−αz + k ∂t ∂z ⎝ ∂z ⎠
(2)
where T is the temperature (K), and ρ, cp, k, and α are the density (kg/m3), specific heat (J/(kg·K)), thermal conductivity (W/(m·K)), and absorption coefficient (m−1) at 450 nm of the porous Si for z ≥ 0 and glass slide for z < 0, respectively. Since the glass is almost transparent, its α value is set to be zero. R is the reflectivity of porous Si at 450 nm and is set to be 0.1.47,48
Figure 7. (a) Calculated maximum temperature rise and (b) minimum flash ignition energies for freestanding porous Si films by the Xe flash exposure as functions of film thickness and porosity. 5531
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ACKNOWLEDGMENTS This work was supported by the Army Research Office under the grant W911NF-10-1-0106. Y.O. acknowledges support from the Japan Student Services Organization Fellowship.
for the highest temperature rise is slightly higher for higher porosity Si because it has smaller light absorption coefficient (Supporting Table S1). Third, the effect of porosity becomes more prominent for thicker porous Si films. As seen in Figure 7a, the temperature rise is similar at smaller thickness but differ significantly at larger thickness for the three Si films with different porosities. For thicker porous Si films, high porous film has much smaller heat capacity (ρcp), leading to a larger temperature rise. Finally, the minimum ignition energy Emin is plotted as functions of thickness and porosity of the porous Si films in Figure 7b to compare with our experimental results. For qualitative comparison purposes, we numerically define ignition when the peak temperature of the porous Si exceeds 600 K. The plot is in Figure 7b generated by calculating the threshold film thickness, at which the maximum temperature reaches 600 K, for each fixed incident energy density level and porosity. Similar to the observations made from Figure 7a, the Emin increases with increasing the film thickness and decreasing porosity, which is consistent with our experimental measurement (Figure 5). The Emin is more sensitive to the film thickness when it is less than 25 μm (larger spacings between different curves in Figure 7b), and the Emin is more sensitive to the film porosity for thicker porous Si films (steeper slopes in Figure 7b). To summarize, we have demonstrated that freestanding porous Si films can be optically ignited in ambient air by a low power Xe flash (
