2B Foils in

Aug 1, 2012 - Robert V. Reeves*, Mark A. Rodriguez, Eric D. Jones Jr., and David P. Adams. Sandia National Laboratories, Albuquerque, New Mexico 87185...
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Condensed-Phase and Oxidation Reaction Behavior of Ti/2B Foils in Varied Gaseous Environments Robert V. Reeves,* Mark A. Rodriguez, Eric D. Jones, Jr., and David P. Adams Sandia National Laboratories, Albuquerque, New Mexico 87185, United States S Supporting Information *

ABSTRACT: The effect of gaseous environment on selfpropagating reactions in Ti/2B multilayer foils was studied. Multilayers with reactant layer periodicities from 50 to 3000 nm and a nominal stoichiometry of 1:2 Ti:B were grown using magnetron sputtering. Analysis through scanning transmission electron microscopy, differential scanning calorimetry, and X-ray diffraction revealed the as-deposited multilayers consisted of wellordered layers of crystalline Ti and amorphous B with the amount of intermixed material negligibly affecting the exothermicity. Propagation rate testing was performed at varied vacuum pressures and studied by high-speed imaging. Propagation rates for foils with bilayer thicknesses 666 nm or thinner did not exhibit a pressure dependency, while foils with bilayer thicknesses 857 nm and larger did. The latter, thick-bilayer foils would not propagate below a characteristic pressure. After reaction, Auger electron spectroscopy revealed O penetration in the thicker bilayer designs and incomplete mixing of Ti and B, while the thinner bilayer foils were uniformly converted to single-phase, hexagonal TiB2. Increased air pressure was found to promote the propagating reaction of the thicker bilayer designs, likely due to increased heat release from oxidation that promotes intermixing and reaction between Ti and B.

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INTRODUCTION Self-propagating, high-temperature synthesis (SHS) of intermetallic and ceramic materials through reaction of elemental species is a well-known and long studied process.1−4 Some of the chemical combinations suitable for use in SHS, such as Ti/ 2B, Co/Al, and Ni/Al, liberate significant amounts of heat and support a self-propagating reaction wave.5−8 Besides the utility that SHS-capable systems possess in producing high-value materials, the liberated heat could be useful in a variety of applications from joining and soldering to airbag initiators to micropropulsion. However, when these systems are reduced to small length scales, as is necessary in the mentioned applications, the heat losses present can prevent reaction propagation. This effect can be mitigated through reducing the constituent materials to nanometric dimensions, as in the case of vapor-deposited, multilayer thin films. Additionally, the ambient environment could be utilized to enhance the effectiveness of these materials; many metals used in SHS systems are prone to rapid oxidation in air, which could increase the energy output of the material and increase the stability of the reaction front. For example, the primal2 and one of the most exothermic SHS reactions is that of elemental Ti and B to synthesize TiB2, having a heat of formation of −109 kJ/molatoms.9 The TiO2 oxide of Ti formed through direct oxidation of Ti, however, has a heat of formation of −315 kJ/ molatoms.10 By coupling with the large heat release from oxidation, the SHS reaction might be stabilized or enhanced. While this could decrease the quality of the reaction products in © 2012 American Chemical Society

a situation where production of pure product is desired, use of the environment as reactant could be beneficial in situations where heat generation is paramount. Reactive multilayer thin films have been used to study many reactive systems.11−21 Produced through vapor-deposition methods, multilayer thin films provide a means for building reactive systems of high-purity reactants at carefully controlled length scales, generally with nanometric internal periodicity. Many studies have been performed, primarily on aluminideand titanide-forming systems and some rare-earth systems.11,13−15 Recent studies investigated the ignition16 and reaction dynamics14 of the systems, and the resulting chemical phase formation. It has been determined that these films react through forward heat transfer driving fast mixing of the nanometric material layers, promoting a self-propagating reaction wave. The design of a multilayer is critical to the reaction dynamics. In general, multilayers with thinner constituent layers (characterized as bilayer (BL) thickness, meaning the combined thickness of one layer of each reactant) have greater reactant interfacial areas per unit length and shorter diffusion distances, allowing for faster heat release. However, for extremely thin layer thicknesses, the volume of premixed material along interfaces can be significant, reducing the total exothermicity of the foil and slowing the rate of heat Received: April 19, 2012 Revised: July 3, 2012 Published: August 1, 2012 17904

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the entrance to the detector. Vertical aperture soller slits (0.4 deg) were configured between the antiscatter and receiving slits. Data were collected in ranges, with adjustment of the generator power in each range such that the intensity measured from the sample remained in the linear range of the detector, which is 0 to 60 000 counts per second (cps). Data were rescaled and merged to form the final XRR plot. Results of the XRR data sets were fit using the program Parratt32 (version 1.5.2). After removal from the deposition chamber, multilayers deposited on NaCl were separated from their substrates. This was achieved by washing the NaCl substrate with deionized water to loosen the foil (after removal from substrate, multilayer films are referred to as “foils”). Each foil was then again washed in deionized water to remove any residual NaCl and placed in a bath of isopropyl alcohol to remove water from the foil’s surface. Finally, the foils were air-dried and stored in antistatic containers prior to testing. Since initial microstructure can have tremendous impact on reaction behavior in heterogeneous systems, proper characterization of the structure and chemical composition of the asdeposited foils is necessary. This was performed through several methods. The microstructure and crystallinity of the foils were first characterized through scanning transmission electron microscopy (TEM). Specimens for scanning TEM were conventionally prepared using a FEI Company Helios dualbeam focused ion beam (FIB) and field emission scanning electron microscope (SEM) and placed on carbon coated TEM grids outside of the FIB system with a micromanipulator. The Ti/2B multilayer analysis was performed on the FEI Company Titan G2 80−200 aberration-corrected TEM operated at 200 kV. This system has an integral 4-sensor windowless silicon drift X-ray detector array with a combined collection solid angle 0.7 sr. The microscope has a spatial resolution of 0.08 nm and sensitivity to soft X-ray lines such as those from boron at 183 eV. Chemical phases, both in the as-deposited foils and in postreaction foils, were identified through X-ray diffraction (XRD) analysis performed using a custom-built microdiffractometer employing a Rigaku 12 kW rotating Cu anode X-ray source. Auger electron spectroscopy (AES) was performed to examine the elemental distributions throughout the sample. The instrument used for AES was a Physical Electronics Auger 690 system with a base pressure less than 2 × 10−9 Torr. Xenon was used as the sputter gas for depth profiling. AES data were acquired with a 10 kV, 10 nA electron beam. The first step in characterizing the reaction behavior of the foils was to determine the total heat release. The heat release characteristics for the foils were quantified through differential scanning calorimetry (DSC) using a DSC 404F3 (Netzsch). Milligram quantities of the foils were placed in alumina-lined platinum crucibles and placed in the DSC furnace. The furnace was initially evacuated of gas and then backfilled with N2. The samples were heated at a rate of 50 K/min. The resulting traces were integrated to determine the total heat release of the foils. High-speed photography was utilized to investigate the effects of both bilayer thickness and the surrounding gaseous environmental pressure on the reaction propagation behavior of the foils. The environmental pressure effects were evaluated by reacting the foils in a chamber that had been evacuated to a desired vacuum level. The pressure in the chamber was controlled by the use of an oil-free scroll vacuum pump and a turbomolecular vacuum pump with chamber pressure moni-

release. Thicker bilayers, having longer diffusion distances, slow heat release and eventually prevent propagating reactions from occurring. Since such foils typically consist of metals that oxidize readily, it is thought that by coupling the solid-state reactions to combustion reaction with environmental gases that the stability of reaction in these foils can be extended to thicker bilayer designs. This study serves to investigate the design-dependent reaction characteristics of Ti/2B reactive multilayer foils. This highly energetic reactive system has been characterized extensively as powder mixtures22−27 or composite particles28,29 but only basically as a reactive multilayer having nanometer periodicity.30−32 The effects of system geometry are considered, as are the effects of the gaseous environment on the reaction dynamics. The goal of this work is to determine if higher air pressures will enhance the reaction propagation rate in the foils by increasing the exothermicity of the system. The contribution of oxidation reactions are characterized by examining the reaction propagation rates, the chemical phases of the reaction products, and the elemental distributions within the reacted material.

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EXPERIMENTAL METHODS Multilayers of Ti and B were deposited using direct-current planar magnetron sputtering.33 All of the multilayers were deposited with a nominal stoichiometry of Ti:2B. The Ti target was formed by vacuum melting, with a purity of 99.995% (VEM, Inc.), and the B target was consolidated through hotisostatic pressing, with a purity of 99.97% (Umicore). The deposition chamber was evacuated to 10−7 Torr and then backfilled with a sputter gas of ultrahigh purity Ar at 25 mTorr for deposition. The stacking sequence was Ti/B/Ti for all foils, leaving Ti as the outermost layers so that heat transfer and other interactions with the environment are the same on both sides of the foil. The bilayer thickness (the combined thickness of a layer of B and a layer of Ti) was fixed for each film, and films of different bilayer thicknesses ranging from 50 to 3000 nm were grown. Since Ti was the outermost layer for all foils, the thickness of each Ti layer was adjusted to account for the additional Ti layer. For the 3000 nm BL thickness film, the Ti capping layers each provided half of the necessary Ti to ensure the proper elemental ratio. All multilayers had a total film thickness of 3.0 μm. This means that as bilayer thickness changed, the number of bilayers, the number of interfaces between Ti and B, and the diffusion distance also changed. Films to be analyzed by electron microscopy were deposited on Si substrate attached to a rotating platen and films to be tested as free-standing foils were deposited onto 1” NaCl substrates attached to a rotating platen. The sample temperature remained at ∼50 °C or less during each deposition. Prior to deposition of the experimental multilayers, the density of the as-deposited reactants was verified through X-ray reflectivity (XRR) analysis of witness multilayer coatings on Si. The XRR technique has been described fully elsewhere.34 XRR was performed using a PAD X1 diffractometer (Scintag), equipped with a sealed-tube source employing Cu Kα radiation (λ = 1.5406 Å), and a Peltier-cooled Ge solid-state detector. The incident beam was focused using a curved-mirror optic to form a parallel beam. The exit slit assembly of the mirror optic was adjusted to a beam thickness of 50 μm. This assured that the beam footprint would not exceed the film sample size at low angles (e.g., 0.3 deg 2θ). Antiscatter and receiving slits with apertures of 0.5 deg and 0.2 deg, respectively, were employed at 17905

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with an effective pixel size of 17.8 μm. The reactions were imaged at frame rates between 1 × 104 frames per second (fps) and 9 × 104 fps, depending on the reaction front velocity. The movies were taken without additional illumination, that is, the reactions being imaged self-illuminated sufficiently for the movies to be recorded without additional lighting. However, the exposure time necessary for proper imaging differed between samples, so the exposure time used in each movie is noted. The reaction wave speed was tracked by temporally locating the position of the reaction front and plotting this data. Using a linear regression of the position/time data, reaction propagation rates were calculated.

tored using a convectron gauge (Granville Phillips). The gas in the chamber was residual air from the pump down; no backfill gases were utilized. The residual gas in the chamber was analyzed at reduced pressures using a Transpector XPR2 (Inficon) residual gas analyzer (RGA). At the upper pressure limit of the RGA (5 mTorr), the predominant oxygen carrying gases were found to be H2O and CO2, with N2 being a predominant inert species. Gaseous O2 was present at a partial pressure 3 orders of magnitude less than the other O carrying species. This indicates that during pump down, the removal of oxygen occurs at a preferred rate compared to the other gases. Since O2 is the strongest oxidizer, the preferential removal of it will enhance any pressure dependence of the foils to the oxidizing environment as the pressure decreases. The foils were reacted using a test setup illustrated schematically in Figure 1. In this configuration, a piece of Ti/

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RESULTS AND DISCUSSION Initial Characterization. The as-deposited multilayers were first characterized using TEM, as described above. Figures 2 and 3 display results of the TEM imaging. The first image of

Figure 1. Schematic representing burn rate test setup. Ti/2B test foil and Ni/Al foil attached to tape so that they are lightly touching. The Ni/Al foil is ignited which, in turn, thermally ignites the Ti/2B foil.

Figure 2. The TEM image on the left shows the structure of a Ti/2B multilayer and reveals some roughness at the layer interfaces. The compositional map presents the elemental distribution of Ti, B, Si, and trace O. The Si is the substrate on which this multilayer was grown.

2B foil, roughly 5−10 mm wide × 10−15 mm long, is supported at its edges by two pieces of polyimide tape that are attached to a glass slide. A strip of 5−10 mm wide × 25−30 mm long, commercially available Ni/Al reactive foil (Indium Corporation) is also attached to the tape so that it is lightly touching the Ti/2B test foil at one end. An electrical impulse is applied to the Ni/Al from a 4 J capacitance discharge unit (Research Energy of Ohio, model CD45-4J), igniting a reaction wave in the Ni/Al. This autowave travels through the Ni/Al foil and thermally ignites the Ti/2B foil. A benefit of using this Ni/ Al intermediate foil for ignition, rather than using a direct electrical impulse, is that the Ti/2B foil does not exhibit a zone near the ignition point where excess enthalpy from the rapid input of energy from ignition source is stored. Since the energy input from a spark discharge can enter the material more quickly than it can be dissipated, the reaction propagation velocity near the ignition point would be artificially elevated, causing spurious results. Using the Ni/Al foil as a primer though does not preheat a bulk region in the test foil and results in constant velocity reaction fronts in the Ti/2B foil, ensuring the ignition conditions are consistent and the burn rate data are comparable between tests. Another benefit is that direct ignition from spark discharge can destroy the test foil, while an igniter foil initiates the reaction without damaging the test specimen. The reaction wave in the Ti/2B foil was monitored using a Phantom V12.1 high-speed camera (Vision Research) and macroscopic optics (Zeiss Makroplanar) with 68 mm of lens tubes. This optical configuration produced images

Figure 2 reveals the regularity of the deposited layers. The Ti and B layers are seen to be clearly defined and with controlled thicknesses. The image also shows some surface roughness between layers, but it is not well-defined at this magnification. Elemental mapping through energy-dispersive spectroscopy (EDS) was also performed during image acquisition, as shown in Figure 2. In this image, the Si substrate and the constituent layers are clearly delineated and seen to have high purity. There is trace O within the specimen (indicated by yellow in the image), but this appears to be minimal; potentially this could have formed during FIB sample preparation. Figure 3 includes a high-resolution image of an interface between Ti and B. This image reveals the microstructure and interfacial details. The Ti layer exhibits a defined crystalline structure, as seen in the bottom image of Figure 3. The B, seen more clearly in the inset, appears to be amorphous. The Fourier transform of the B region reveals broad, diffuse rings, further indicating the lack of a periodic lattice structure. The interfacial region shows a 3 nmthick volume where both Ti and B are present. It is not clear if this region has intermixed reactants or if it is the result of surface roughness. This region is thin compared to the pure elemental regions, though, so the effect on reactivity is expected to be minimal for the multilayer designs studied here. The chemical phase composition of the as-deposited foils was studied using XRD analysis. A representative diffractogram obtained from an as-deposited multilayer is shown at the bottom of Figure 4. As seen in this trace, the only identifiable 17906

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Figure 4. XRD traces for an as-deposited foil (bottom) and postreaction products of Ti/2B foils with different BL thicknesses reacted at atmospheric pressure (∼630 Torr) and 5.0 Torr.

between Ti and B is not absolute. To verify that intermixing is not diminishing the total heat release, DSC analysis is performed. Typical DSC results are shown in Figure 5. The trace presented in this figure represents the heat release from a Ti/

Figure 3. The high-resolution dark field image (bottom) shows the interface between the Si substrate and the first Ti layer. The bright field image (top) presents an interface between Ti and B. The Ti exhibits lattice fringes in both images, indicating crystallinity, while the Fourier transform of B results in only broad, diffuse rings consistent with amorphous material. Figure 5. DSC trace for a Ti/2B foil with a 50 nm bilayer thickness using a heating rate of 50 K/min.

crystal peaks are for oriented Ti. No B peaks are present, due to the transparency of B to X-rays and to the amorphous nature of the deposited B, as observed in Figure 3. While the TEM imaging already revealed the nature of the reactants, the most important aspect of the XRD trace is that no product phases are detectable in the as-deposited state. This implies that the heat of reaction should be based solely on the reaction between elemental Ti and B without any reduction that would be caused by the formation of intermediate product phases during deposition. However, XRD analysis is not able to resolve nanocrystalline phases or solid solutions, so by this analysis alone, determination of the chemical phases in the interface

2B foil with bilayer thicknesses of 50 nm. The DSC trace reveals that the onset of reaction occurs at ∼100 °C, with the peak of the exotherm located at about 500 °C. Through integration of this curve with respect to time, the total heat release was determined. The total heat of reaction was −110 kJ/molatoms for the 50 nm bilayer foil. When compared to literature values of −109 kJ/molatoms9 and −127.8 kJ/molatoms,35 the value for the 50 nm bilayer foil fits well. Since the 50 nm BL foil would have the greatest diminishment of exothermicity 17907

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from intermixing and still exhibited a heat of reaction typical for the bulk material, it is shown that the level of intermixing between Ti and B at interfaces does not significantly affect the heat release for the studied bilayer designs. The DSC was performed in a N2 atmosphere; however, no evidence of nitriding was present. Titanium combustion with gaseous reactants has been shown to occur with the α to β phase transition that occurs in Ti at 882 °C.36−38 At this temperature, the heat release was nearly completed and no new exotherm was noted, so it is not believed that any nitriding of the material occurred during DSC testing that would influence the results. The initial characterization of the Ti/2B multilayers after deposition revealed that, through DC planar magnetron sputtering of these materials, high-quality multilayers can be produced. The internal structure of the foil was found to be a regular layered structure with controlled layer thicknesses. It also showed that the amount of intermixing between reactants was minimal, with crystalline Ti and amorphous B as the predominant phases. The interfaces between reactants are potentially intermixed and certainly nonplanar, but these regions are small compared to the bulk reactant volumes, so the effects on exothermicity are not considered to be significant. This was confirmed through DSC testing that provided evidence that the heat released during reaction was consistent with literature values. The interfaces were also found to be devoid of contamination, providing a clean interaction surface. Pressure Dependent Reaction Rate Testing. The rate of reaction propagation was tested at vacuum levels ranging from 10−4 Torr to atmospheric pressure of ∼630 Torr at local altitude. The propagation rate results are plotted in Figure 6A for foils having bilayer thicknesses ranging from 50 to 3000 nm. It is important to notice the extremely broad range of stable reaction propagation speeds in the Ti/2B system. Stable propagation speeds range across more than two decades, from about 15 to 0.05 m/s. These propagation rates also seem very low, when compared to other exothermic thin films, like the Al/ Pt system, that can propagate reactions at rates up to 90 m/s on substrate.39 The volumetric heat release of Ti/2B is roughly twice that of the Al/Pt system (−17.4 kJ/cm3 vs −10.5 kJ/ cm3),35 but the reaction propagation rate of Ti/2B is much lower. The difference in speed is likely due to the tremendous disparity in the thermal conductivity between the Ti/2B and the Al/Pt multilayers. Thermophysical properties are tabulated in Table 1, and from this, thermal conductivity is estimated to be many times lower for Ti/2B foils than for Al/Pt multilayers. The values for the bulk thermophysical properties of the multilayers were estimated by volumetric averaging of the constituent materials’ properties. The rate of interatomic mixing is also higher in Al/Pt films than in Ti/2B films and contributes to the greater reaction propagation rate. This is inferred from comparing exothermic peak positions from DSC testing. As shown above, the 50 nm BL thickness Ti/2B foil exhibits an exothermic peak at ∼500 °C, while a 160 nm BL thickness Al/Pt has an exothermic peak at ∼290 °C, suggesting that Al/Pt interdiffusion is greater for a given temperature. Since heat transfer rates are higher, Al/Pt is better able to utilize the freed energy by raising temperature in the unreacted material and this more significant temperature elevation has a comparably greater effect on atomic mixing than in Ti/2B foils, resulting in much greater reaction propagation rates. The reaction rates for foils with bilayer thicknesses of 666 nm and smaller show little pressure dependence. Each design in this thickness range exhibited a characteristic propagation rate,

Figure 6. (A) Propagation rates for reactions in Ti/2B foils with BL thicknesses ranging from 50 to 3000 nm as a function of air pressure. (B) Reaction propagation rates at different pressures as a function of bilayer thickness. Dotted lines indicate that foils with bilayers thicker than those shown did not propagate a reaction at that pressure. Error bars smaller than the point markers are not visible.

Table 1. List of Thermophysical Properties of Elements, Foils, and Reaction Products

material Ti B Al Pt Ti/2B multilayer Al/Pt multilayer

thermal conductivity at 298.15 K, [W m−1 K−1]

heat capacity at 298.15 K, [J mol−1 K−1]

heat of reaction, [kJ molatoms−1]

heat of reaction, [kJ cm−3]

21.9a 27.0a 237.0a 71.6a 24.3b

25.0a 11.4a 24.4a 25.9a 20.6b

−112.1c

−17.44c

158.2b

25.7b

−100.2d

−10.50d

a

From ref 39. bValue is estimated as a volume average of constituent material properties. cFrom DSC. dFrom ref 31.

and each rate remained nearly constant across the entire range of tested environmental pressures. Very much the opposite, foils with thicker bilayers have strong pressure dependence and even have critical pressures below which a reaction is not able to propagate. This is also made clear by presenting the data in a slightly different manner, as seen in Figure 6B. In this plot, the propagation rate is shown for different environmental pressures as a function of bilayer thickness. The propagation rates for 17908

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each pressure line are similar at bilayer thicknesses up to 300 nm. Beyond this bilayer thickness, the results diverge. At the lowest shown vacuum pressure of 0.1 Torr, the rate decreases more significantly as a function of bilayer thickness than for the higher pressures. Foils with bilayers thicker than 666 nm did not propagate at this pressure, as shown by the dashed line. For the 10 Torr pressure line, the 666 nm foil exhibits a slightly lower rate than the sample at air pressure, but this difference is exacerbated when the bilayer thickness is increased to 857 and 1225 nm. Bilayer thicknesses larger than 1225 nm did not support self-propagating reactions at 10 Torr, while at atmospheric pressure, 3000 nm bilayer foils were able to propagate. So, from this presentation of the data, it is again clear that atmospheric pressure plays little role in the reaction dynamics for small bilayer foils, but increased atmospheric pressure increases the propagation rate for thick bilayer foils. It also allows thick foils to propagate, when they are unable to do so at decreased pressures. In order to further clarify the pressure dependence of the reactions, the data from each foil design were fit to a power law, r = r0 + aPn, where n describes the magnitude of the foil’s pressure dependence. This is a typical treatment for propellants and pyrotechnics40−42 to describe pressure-dependent behavior over a limited pressure range. The magnitude of n for each foil design is shown in Figure 7. Again, the reaction propagation

Figure 8. Images of reaction fronts in Ti/2B foils. Reaction propagates from bottom of image to top. In (A) the bright flecks ahead of the reaction front are flakes of reacted material, while in (B) the flakes are seen behind the reaction front. Reactions in (C) and (D) did not produce flakes visible in these images. Figure 7. Plot of pressure exponent as a function of bilayer thickness. As the pressure exponent increases, the dependence of reaction propagation rate on the surrounding air pressure increases. The pressure exponent is roughly constant for bilayers thinner than 666 nm and increases slightly for the 666 nm design. Designs with bilayers thicker than 666 nm have a rapid increase in pressure exponent. Error bars smaller than the point markers are not visible.

atmospheric pressure and at the investigated vacuum levels. The image in Figure 8A shows the reaction front for the 50 nm bilayer thickness foil at 4.5 Torr. The exposure times for Figure 8A,B are roughly equivalent, as are the luminosities of the two fronts, so the change in air pressure has not strongly affected the self-illumination (and temperature) of the reaction in this design. At both pressures, the reaction products do not remain as a foil. The products break apart, as seen in Figure 8. Movies of the reaction propagation can be found in the Supporting Information. For the thickest bilayers, represented in Figure 8 by images of a 3000 nm bilayer design, the reaction front exhibits significant differences from the other reaction fronts. First, the actual front is much broader and the luminosity in the front increases gradually. This is typical for self-propagating reactions between a gas and a solid.44 It is also important to note the difference in exposure time for the 3000 nm foil across the vacuum range. At 630 Torr, the required exposure was only 2 μs, while 6 μs was needed to provide a bright image at 24.9 Torr. Since the reaction at 24.9 Torr was so much dimmer, it is appropriate to

rate dependence on pressure increases significantly when the bilayer is larger than 666 nm. Since the pressure dependence for the thick bilayers is large, combustion reactions are the prevalent heat source for these designs. Oppositely, the solidstate reaction between Ti and B dominates in thinner bilayer designs, as evidenced by the minimal dependence on oxidizer availability, which varies with surrounding air pressure. Besides affecting the reaction front speed, the variation in air pressure has an effect on the characteristics of the reaction front. The variation in the front can be seen in the images of Figure 8. In a 50 nm foil the reaction front is narrow, without any gradual change in luminosity. This is indicative of a reaction that exhibits very strong temperature dependence.43 The thin, bright flame front is characteristic for this foil both at 17909

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Figure 9. AES traces for 666 nm bilayer Ti/2B films reacted at (A) atmosphere and (B) 4.99 Torr. AES trace for 857 nm bilayer Ti/2B foils reacted at (C) atmosphere and (D) 5.00 Torr are also shown.

intermediate phases were found, nor were any oxides. The same chemical phase was the sole component found in the 666 nm bilayer and the 857 nm bilayer designs reacted in normal atmosphere. The XRD traces for a 3000 nm bilayer film reacted at atmospheric pressure revealed the presence of many chemical phases, including TiB2, TiB, and possibly anatase TiO2. The measured TiO2 peak was very small, but the quantity of TiO2 could appear diminished due to flaking of the outer Ti layers or formation of amorphous TiO2 phases. No evidence of TiN formation was found. It was noted during testing that the 3000 nm bilayer foil produced a bright shower of white sparks that was not exhibited by thinner bilayer foils, so the ejection of oxidizing Ti seems logical. No elemental Ti was present in this reacted foil, indicating that the Ti capping layers either reacted with O2, B, or were ejected during combustion. The presence of oxide in the reaction products provides clear evidence that, in the thickest bilayer design, the two Ti capping layers interact with the surrounding gaseous environment during the reaction. X-ray diffraction patterns were also collected for the 666 and 857 nm bilayer designs reacted at 5.0 Torr. This pressure was selected as it was the lowest pressure at which reactions in the 857 nm bilayer design would self-propagate. In both cases, only TiB2 crystal peaks were clearly present, despite the differing reaction characteristics of the foils. Even though clear pressure dependence and a defined quenching pressure were exhibited,

infer the temperature was lower as well. The reaction front exhibits traits similar to previous gas−solid reactions and the luminosity and propagation rate of the reaction front are dependent on pressure for the 3000 nm bilayer thickness foil, so it is reasonable to assume that the dominant reaction is between the gaseous environment and the foil for this design. The thick BL films remain intact postreaction, without significant breakup. The reactions are depicted in movies found in the Supporting Information. Post-Reaction Chemical Characterization. X-ray Diffraction. After concluding the reaction propagation testing, the reaction products were analyzed to determine their chemical composition, as well as the distribution of elements throughout the material. First, XRD analysis of the products was performed, the results of which are seen in Figure 4. The tested designs include foils with bilayer thicknesses of 50 nm, 666 nm, 857 nm, and 3000 nm. The 50 and 3000 nm bilayer designs were chosen, since they represent the experimental limits for this study. The 666 and 857 nm bilayer thickness foils were selected, because they are the thickest bilayer design that propagated at all investigated vacuum levels and the thinnest bilayer design that failed to propagate at all vacuum levels, respectively. For a 50 nm bilayer foil reacted at atmospheric pressure, the only identifiable chemical phase was the hcp TiB2 phase, indicating complete conversion of the reactants. No 17910

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m/s) is due to the reduction in heat release in the reaction front caused by the elimination of the Ti oxidation reaction. Similar effects are expected for the other coarse multilayer design described in Figure 6A having a bilayer thickness of 1225 nm. A small internal O peak is seen in Figure 9D as well. Since the purity of the as-deposited samples were verified, the origin of this peak is not clear at this time. It is thought that the flaking of Ti that occurs during reaction could expose underlying B. The heated B could react with environmental gases to form oxides or oxyhydrides that would not be discernible on XRD but would present an O trace resolvable through AES. The AES data provide several insights into the reaction dynamics of the Ti/2B system. First, it appears that a critical bilayer thickness exists, which defines both the interfacial area for reaction and the diffusion distance, below which the condensed phase reaction between Ti and B is able to selfpropagate due to a short time to completion and to high volumetric heat release. This is exhibited by the 666 nm bilayer material’s production of a thoroughly mixed TiB2 phase at all tested vacuum pressures. In comparison, the 857 nm bilayer design showed O penetration during reactions at atmospheric pressure and incomplete mixing when reacted at 5.0 Torr. This indicates that the critical bilayer thickness to ensure completion of the solid-state reaction (for all pressures tested) is between 666 and 857 nm for foils of this total thickness and chemistry. Foils with a greater overall thickness would likely be able to steadily propagate the solid-state reaction with thicker bilayers. Increasing the thickness of the films provides greater total heat release, while keeping heat losses to the environment roughly the same. As such, the more favorable energy balance should allow thicker bilayer designs to propagate steadily. Other reactive foil systems have been known to exhibit either a propagating oxidation wave that trails the condensed phase reaction and consumes a portion of the intermetallic product phase or bulk oxidation where a discrete wave is not present. Examples of these systems are Ni/Ti11 and Sc/Cu.15 Oxidation is also noted in experiments involving Ti/B nanocomposite particles; however, those experiments do not make clear whether the oxidation occurred as secondary oxidation of the intermetallic reaction product or as oxidation of the elemental Ti and B.28 Secondary oxidation behavior is not readily apparent from the data for the Ti/2B foils presented here. In comparison, Ti/2B foils with thin bilayer designs exhibit no discernible secondary oxidation. The thicker Ti/2B foils, though, are affected by oxidation of the Ti. Since thicker bilayer designs were unable to propagate a reaction at low air pressures, the oxidation reaction is more important for these configurations. As mentioned above, the thicker bilayer design de-emphasized the solid-state reaction by decreasing the rate of heat output through reduced reactive surface area and increased diffusion distances. In doing so, oxidation of Ti was allowed to present itself as a dominant reaction mode over the solid-state reaction between Ti and B for the thick bilayer designs.

oxide phases were not identified in the 857 nm bilayer foil, even when reacted at atmosphere. In order to further investigate the behavior of the pressure dependent and nonpressure dependent reactions, analysis of the chemical composition was performed through spectroscopy. Auger Electron Spectroscopy. AES was performed on recovered samples of the thickest bilayer design that did not exhibit a significant pressure dependency (666 nm) and the thinnest bilayer design that did exhibit a pressure dependency (857 nm). Samples for each design were recovered after reaction at atmospheric pressure and at 5.0 Torr, the quench pressure of the 857 nm bilayer foil. The samples were chosen because these designs represent the transition between the dominance of the condensed phase reaction and the oxidation reaction. The pressures were chosen to maximize the differences between the samples. These data are presented in Figure 9. In Figure 9A,B, the results from the 666 nm bilayer thickness foil are shown. After reaction at both 5.0 Torr and atmospheric pressure, the indicated atomic ratio of Ti:B is roughly constant at 40/60, indicating thorough mixing of the materials. Note, the 40/60 atomic ratio of Ti:B is a slight underestimation of the amount of B that should be present in stoichiometric TiB2 formation. The error is believed to be due to preferential sputtering of B during AES depth profiling, which is an artifact of the analysis method and not an error in deposition or mass loss through reaction. It is known that, in alloys, lighter elements and more volatile elements are preferentially sputtered.45 While the heat of volatilization of Ti (467.35 kJ/kmol)9 and B (480 kJ/kmol)46 are reasonably close, B has a much lower atomic number than Ti and is more likely to be removed through sputtering. Besides the Ti:B ratio being fairly constant, the amount of O present in the 666 nmbilayer samples is minimal. This indicates that the reaction occurring in the material was only the TiB2 synthesis reaction and that oxidation reactions were minimized. This is consistent with the lack of a pressure dependency for the self-propagating reaction for this material design. The foil with 857 nm bilayers, though, exhibited different behavior, as seen in Figure 9C,D. The sample reacted at atmospheric pressure displays a significant O impurity at the surface of the reacted foil as indicated by the red line in the plot. Titanium is the prevalent metal in this region, and the amount of B present is comparable to the O impurity. This implies that the Ti capping layer has undergone some oxidation. Further into the sample, a layer that is rich in B appears. This B-rich zone is likely a B layer that has not fully diffused into Ti (i.e., a remnant structure of the original multilayer). This structure is very similar to that seen in a 3000 nm bilayer foil reacted at 24.9 Torr (not shown), which exhibited O in the near-surface volume, a layer of mixed Ti and B and unmixed B in its center. The lack of mixing can be due to a couple of reasons. First, the O has bound to the Ti in the outermost layer, preventing the B from reacting with the Ti. Second, the rate of the TiB2 synthesis reaction has been reduced enough to prevent it from self-propagating. This would imply that the heat released by the Ti oxidation has driven the limited mixing between Ti and B. AES was performed on a sample with 857 nm bilayers, reacted at 5.0 Torr. The AES trace for that sample (shown in Figure 9D) revealed little O, indicating Ti oxidation was minimal. This is expected for a sample reacted in a reduced O partial pressure. The accompanying decline in propagation rate in this particular sample (ratms = 0.277 ± 0.032 m/s vs rP=5Torr = 0.118 ± 0.064

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CONCLUSIONS High-purity Ti/2B reactive multilayers were created through sputter deposition, and their reaction behavior in varied gaseous environments was studied. While in foils with bilayer thicknesses of 666 nm or smaller the average reaction propagation velocity increased with decreasing bilayer design over the tested range, the average propagation velocity exhibited little dependence on the surrounding air pressure. Conversely, foils with bilayer thicknesses of 857 nm or greater 17911

dx.doi.org/10.1021/jp303785r | J. Phys. Chem. C 2012, 116, 17904−17912

The Journal of Physical Chemistry C

Article

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exhibited a marked pressure dependency, with reactions failing at pressures between 1 and 10 Torr, depending on the design. The oxidation of these thicker foils was able to provide energy to augment intermixing between the Ti and B layers. By utilizing the reactivity of environmental gas with Ti layers to augment heat release, the stable design range of Ti/2B multilayer foils was extended.

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ASSOCIATED CONTENT

S Supporting Information *

Movies, from which the stills in Figure 8 were taken, depicting typical reaction progress for 50 and 3000 nm BL foils of Ti+2B nanolaminates reacted at atmospheric and reduced pressures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

Paul Kotula is thanked for performing the electron microscopy. Tony Ohlhausen is thanked for the AES analysis. Chris DiAntonio is thanked for the high-temperature DSC analysis. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

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