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J. Phys. Chem. C 2010, 114, 14772–14780
Thermal and Impact Reaction Initiation in Ni/Al Heterogeneous Reactive Systems Robert V. Reeves,*,† Alexander S. Mukasyan,‡ and Steven F. Son† School of Mechanical Engineering, Purdue UniVersity, Zucrow Laboratories, 500 Allison Road, West Lafayette, Indiana 47907, and Department of Chemical and Biomolecular Engineering, UniVersity of Notre Dame, 210 Stinson-Remick Hall, Notre Dame, Indiana 46556 ReceiVed: May 21, 2010; ReVised Manuscript ReceiVed: July 12, 2010
Reaction initiation in the Ni-Al heterogeneous gasless system due to thermal and mechanical stimuli was investigated. Reactive systems with different microstructures, including micro- and nanoscaled powder mixtures, as well as composite particles formed during short-term (15 min) high-energy ball milling (HEBM) of Al/Ni clad particles were examined. Thermal and mechanical responses were tested by differential thermal analysis and shear impact testing, respectively. It was shown that nanomixtures and HEBM samples thermally selfignited at temperatures (Tig) well below eutectics for Ni-Al (Teut ) 913 K), while the ignition temperature for conventional microscale mixtures is at least Teut. Moreover, Tig for HEBM samples is typically lower than that for nanomixtures. For the HEBM system, the apparent activation energy (EHEBM ) 28 ( 2 kcal/mol) appeared to be half of the nanosystem’s measured value (Enano ) 55 ( 5 kcal/mol). Oppositely, it was shown that nanomixtures were mechanically ignitable through shear impacts of the investigated energy range, while HEBM samples were not. Thus, the HEBM samples were comparatively more sensitive to thermal initiation, while the nanomixtures were more sensitive to mechanical initiation. It is believed that the different microstructures contribute to this phenomenon; HEBM material has larger interfacial areas between active materials, which reduces its activation energy and increases thermal sensitivity. The nanomaterials consist of small, hard particles which allow for increased contact stresses during impact and increasing mechanical sensitivity. I. Introduction Gasless reactive systems potentially have different applications, including chemical energy storage, microscale energetic devices, materials synthesis, and as additives to rocket propellants. To fully exploit such a system though, a fundamental understanding of the ignition mechanisms is necessary. It is wellknown that these systems are ignitable through external heating, internal Joule heating, and mechanical means, such as high strain rate impact. While many efforts have been made for investigating the ignition of heterogeneous reactive mixtures (a suggested review was written by Barzykin1), such a fundamental understanding of the phenomenon is not complete. The behavior of exothermic systems during external preheating often can be described through thermal explosion (TE) theory, developed by Semenov,2 Todes,3 Rice and co-workers,4,5 and Frank-Kamenetskii6 and primarily focused on gas-phase reactions. Merzhanov and colleagues created the basis for such initiation in condensed homogeneous systems.7,8 These classical theories introduced several terms which require definition to continue discussion. First, one has to consider reactions initiated in static and dynamic conditions. In the former case, the reactive media are typically immersed in a volume (e.g., furnace) with constant high temperature, and under certain conditions, rapid reaction starts. In the latter case, the ambient temperature of the furnace changes with time. In both of these situations, the only driving mechanism for rapid reaction initiation is the heat released through the exothermic reaction; therefore, a strong * To whom correspondence should be addressed. Phone: 765-494-0072. Fax: 765-494-0530. E-mail:
[email protected]. † Purdue University. ‡ University of Notre Dame.
dependence of reaction rate on temperature is a required condition for rapid reaction initiation. In Arrhenius-type kinetics, this condition translates to large activation energy (Ea) of chemical reaction. This concept reveals that when the rate of heat release exceeds that for heat losses, sufficient conditions are met for the rapid reaction, or TE, to proceed. The concept of volume self-ignition or TE versus local ignition phenomena also needs to be delineated. When the characteristic thermal relaxation time of the reactive media is much greater than the chemical reaction time, the TE phenomenon, that is, the simultaneous reaction initiation in the bulk of the whole sample, prevails. In local ignition, the reaction initiates at a discrete location of the sample which meets the definition of the system’s ignition criteria. The reaction may then selfpropagate in the form of a combustion wave through the sample volume. The propagation mode (steady-state, oscillatory, etc.) and combustion front velocity are dependent on the forward transport of heat and mass, mainly through thermal conduction and mass diffusion.9 Finally, so-called adiabatic thermal explosion may occur when experimental conditions allow TE with negligible heat losses. These classical theories of reaction initiation suggest that changes to experimental conditions, like heat loss rate or system configuration, may significantly affect TE characteristics. Therefore, it is not physically sufficient to introduce parameters such as a self-ignition temperature for the TE regime or an ignition temperature for local ignition. Both parameters will vary as experimental conditions are changed. In general, the TE process is characterized by three main parameters, (i) induction time, (ii) heating of the system below the TE threshold, and (iii) the TE threshold, which is defined by characteristic times of heat
10.1021/jp104686z 2010 American Chemical Society Published on Web 08/16/2010
Reaction in Ni/Al Heterogeneous Reactive Systems relaxation and heat release. Similarly, local ignition is determined by more than a single, critical temperature. However, TE experiments conducted for heterogeneous condensed-phase systems, including the Ni-Al mixture, appear to contradict this conclusion. It is known that for many gasless reactive mixtures, thermal initiation occurs at the melting point of the less refractory compound or at system eutectics.10 These temperatures are intrinsic properties of the media and are unaffected by changing experimental conditions. The lowest melting point in the system is an important trigger to widespread reaction as the solid-to-liquid transition significantly increases the contact area between reactants and enhances mass transport, allowing the conditions necessary for TE. This observation does not preclude the use of classical theory to describe composite gasless reactive systems but instead requires that new features be introduced to capture the specifics of the heterogeneous media. Besides thermal ignition, mechanical ignition of gasless heterogeneous systems is also of interest but even less understood. This has most commonly been studied through planar shock experiments. Two distinct reaction modes have been introduced. First, shock-assisted reactions occur in the period of thermal equilibration after the shock wave and are dependent on the “shock-modified” configuration of the powder mixture.11 Second, shock-induced reactions occur during the pressure equilibration of the media and propagate with speed comparable to the mechanical impact.11 Identification of the shock-assisted reaction can be made through characterization of the sample’s chemical composition before and after experiments. However, existence of shock-induced reactions is difficult to directly witness, owing to the rate of reaction propagation, the extremely high impact energy necessary for initiation, and the use of opaque, solid materials. These characteristics lead to many practical difficulties in designing experiments to image the event in situ. However, indirect observation has suggested the occurrence of such a reaction in certain systems.12,13 For shockinduced reactions to occur during shock passage and propagate in condensed-phase reactive systems, both thermal- and masstransfer events must occur at rates far beyond what has been previously observed in thermally initiated condensed-phase systems. To increase the utility of gasless reactive systems in engineering applications, understanding of and control over the ignition characteristics of the system is required. Efforts have been focused on methods of modifying the system’s microstructure in order to enhance performance of the reactive system and control its behavior. One method of microstructural modification is reduction of the particle size of the constituent materials. In doing this, the diffusion distances that the material must travel are significantly reduced, which lowers the amount of time necessary for complete diffusion of one material into the other. However, the particles still retain a native oxide layer surrounding active material, and simply reducing particle size does nothing to eliminate the mass-transfer barrier caused by this oxide layer. The use of nanometric particles in thermite systems has been studied with increasing regularity since the early part of this decade.14-17 In contrast, much less work has been directed at using nanometric particles in other types of gasless reactive systems. Another method of reactivity enhancement is the refinement of the reactive media microstructure through high-energy ball milling (HEBM). When performed on ductile materials, HEBM results in a microstructure that is lamellar in nature18 and can be refined to layers far less than 1 µm in thickness.19 A benefit
J. Phys. Chem. C, Vol. 114, No. 35, 2010 14773 of this process is that interfacial area increases dramatically. Rather than point-to-point contact between particles, the fully dense lamellar structure creates an interfacial area greater than the entire surface of the unmilled particle.19 The extensive plastic deformation also results in the oxide barrier being stripped from interfacial areas.20 The effect of these changes on the material’s thermal sensitivity has been studied. It was seen that ball milling is capable of reducing the temperature of reaction initiation in many materials in differential scanning calorimetry (DSC) testing.21-23 This work is a first attempt to answer some fundamental issues which arise from the above discussion. First, the nature of the relationship between thermal and mechanical initiation characteristics in heterogeneous gasless system is investigated. Second, the work attempts to understand and characterize the changes in ignition characteristics in the Ni-Al composite gasless reactive system caused by HEBM and by reducing particle size to nanometric scales. In other words, is there a correlation between thermal and mechanical sensitivities to ignition in such systems and how does microstructural refinement affect this relationship? Finally, experimental methods capable of in situ investigation of impact-induced reaction are considered for systems which are initiated through mechanical impacts of modest energy. II. Experimental Methods The base material used in this work was an Al powder clad with a layer of Ni (Federal Technology Group; Bozeman, MT). The Al particle size was 30-40 µm with a Ni coating of 3.0-3.5 µm, providing a nearly equiatomic Ni/Al ratio (70/30 wt % Ni/ Al by weight). A planetary ball mill (Retsch GmbH, model PM100, Germany) was used to treat the material for milling times up to 15 min at a rotational speed of 650 rpm. Stainless steel balls (3 mm diameter) were the milling media, and the media-to-powder mixture ratio was 2:1. The milling jar was purged of air and filled with Ar gas to prevent formation of any oxides or nitrides during the milling process. No process control agent (PCA) was used during milling. Since a PCA was not present in the milling jar, cold welding of the material was prevalent during milling; therefore, the resulting particles were relatively large in size, having diameters up to 1 mm. The internal microstructure was quite refined though, showing clear lamellae with thicknesses as low as 150 nm, as seen in Figure 1. After processing, the powders were examined using powder X-ray diffraction (Siemens D500) to inspect for formation of reaction products or nonequilibrium crystalline phases. Mixtures of metal nanoparticles were also prepared for testing. Aluminum powder with a characteristic diameter of 80 nm (Novacentrix; Austin, TX) and Ni powder with diameter of 80-150 nm (Alfa-Aesar; Ward Hill, MA) were mixed at an equiatomic ratio (68.5/31.5 wt % Ni/Al). The powders were measured by mass, placed in a solvent bath of commercially available hexanes (98.5% hexane isomers, VWR; West Chester, PA), and mixed by sonication using a 400 W Branford sonifier to agitate the mixture for 30 s at 50% amplitude and 70% duty cycle. The samples were then dried to remove the hexanes and brushed through a sieve to break up clumps from the drying process. Mixtures were also created in which a portion of the Al mass was replaced with larger, micrometer-scale aluminum particles. This addition was made with the intention of easing densification of the nanomixtures and to provide a more cohesive structure. Samples of both ball-milled materials and nanomixtures were densified by cold pressing. Square plates with nominal dimensions of 20 mm ×20 mm ×2 mm were pressed
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Reeves et al. wave passage in planar shock experiments is on the order of 1 µs. In shear impact experiments, the interaction time between the plunger and sample is on the order of 100 µs. As previously noted, the short time frame planar shock experiments limit the amount of time that the sample experiences high stresses,25 whereas the relatively long duration of the shear impact experiment extends this interaction time. The impact is monitored with a high-speed camera (Vision Research; Phantom v7.3; Wayne, NJ) at frame rates up to ∼90 000 frames per second. The resulting movie provides a record of the deformation caused by the impact and also allows the user to find the temporal location of the first light of reaction. The video also allows tracking of the resulting reaction wave. The high-speed IR camera was also utilized during mechanical testing to track the temperature evolution of the system during and after impact.
Figure 1. SEM images of the microstructure of HEBM Ni/Al after 15 min of milling at 50 000× magnification (top) and 12 500× magnification (bottom).
with a uniaxial pressure of 345 MPa (50,000 psi). Densities for the resulting pressings are shown in Table 1. Reaction kinetics was studied by using a differential thermal analysis (DTA) device (TA Instruments; model SDT-2960; New Castle, DE). The argon gas flow rate was controlled at 80 cc/ min and heating rates from 5 to 50 K/s across the temperature range of 300-1000 K. The adiabatic TE conditions were investigated by using a method described fully elsewhere.22 Briefly, a pressed sample was rapidly (102-103 K/s) heated through direct joule heating. The sample temperature was monitored with a high-speed infrared camera (FLIR Systems; model SC6000; Boston, MA), which allowed precise measurement of the system’s self-ignition temperature, Tig. Reaction initiation of pressed samples by mechanical impact was also performed through combined compression/shear tests, used previously with explosives.24 The monolithic samples are placed in a windowed sample holder, as shown in Figure 2. A projectile accelerated by a light gas gun travels toward the sample holder and impacts a plunger supported in the sample holder. The light gas gun used in these experiments propels a brass projectile of 9.5 mm diameter at velocities up to 400 m/s. The plunger then impinges the sample, producing strain in the material. The impacting face of the plunger can use different geometries ranging from a flat face the width of the sample, producing pure compressive strains, to a mixture of shear and compression with a radii-shape plunger to nearly pure shear with a narrow plunger. This experiment is fundamentally different from planar shock experiments performed in previous research12,13,25-29 in key aspects. First, the impact in this setup produces compressive and shear strain in the material, whereas planar shock experiments are controlled so that a planar shock passes through the sample. Second, the time frame of shock
III. Experimental Results A. Reaction Initiation by Thermal Sources. Specific features of reaction for different Ni-Al samples were performed using the DTA method under conditions described above. These conditions correspond to the dynamic regime of reaction initiation. Also, because the sample size is relatively small and preheating rates are slow, uniform temperature distribution along the sample bulk is accomplished. The results of the testing are shown in Figure 3. The baseline for comparison is the unmodified Ni-clad-Al powder. This baseline shows the behavior which is typical for a metallic gasless reactive system. At a heating rate of 50 K/s, the detectable heat release occurs at the eutectic temperature of the system (913 K). Near the eutectic, the mixtures display a small but noticeable decrease in differential temperature (∆T). This is an indication that a melt is forming in the material and absorbing some of the heat energy, thus lowering ∆T. Postmelt, the system is no longer kinetically restricted by mass transfer, and the heat release occurs suddenly, evidenced by the sharp peak that occurs just after melting. Thus, the thermal explosion phenomenon is observed. The ball-milled materials exhibit different behavior. The beginning of exothermic heat release for the ball-milled mixtures shifts to temperatures below the eutectics, as detailed previously.22 For example, for a milling time as short as 15 min, the temperature at which the exothermic reaction starts is 630 K. When the system temperature reaches the eutectic, very little additional heat release is observed, showing that the reaction has nearly been completed through a solid-state reaction. The effect of reducing particle size to nanometric scales also has been investigated. The shape of the differential temperature trace is similar to the unprocessed material’s trace in that a sudden, sharp release occurs. This indicates that the reaction occurs quickly upon initiation. However, the temperature at which the nanometric sample reacts is significantly lower than the eutectic (830 < 913 K). It is known that reducing the particle size to nanometric dimensions reduces the melting point of materials;30,31 therefore, the predicted melting point of Al, being the least refractory component of the mixture, was investigated. However, the melting point for the size of the Al nanoparticle used in these tests (80 nm) was previously reported to be at the melting temperature of bulk Al.30,31 Therefore, size-related depression of the melting temperature was deemed to be of little importance at the studied particle size. It is also important to note that the DTA trace for the nanomixture does not show any decrease in ∆T, indicative of an endothermic melt event prior to the large exothermic peak. As such, the reaction occurs solely in the solid state, like the HEBM samples.
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TABLE 1: Sample Characteristics and Shear Impact Go/No-Go Results sample type
%TMD
velocity [m/s]
muzzle energya [J]
reaction?
15 min HEBM clad powder 68.5% nNi + 31.5% nAl (wt) 68.5% nNi + 23.6% nAl/7.9% mAl (wt) 68.5% nNi + 15.75% nAl/15.75% mAl (wt) (68.5% nNi + 31.5% nAl (wt)) + 20% 15 min HEBM clad powder 68.5% mNi + 31.5% mAl (wt)
83% 62% 73% 76% 75% 80%
400 361 257 348 257 400
1360 1108 561 1029 561 1360
no yes yes occasional yes no
a
Muzzle energy calculated as the available kinetic energy of the projectile, 1/2mV2.
Figure 4. Temperature profile for thermal explosion mode in three types of Ni-Al mixtures.
Figure 2. Schematic of the Asay shear impact test.24 (A) shows the windowed sample holder, and (B) shows the plunger and sample with the retaining plates removed.
Figure 3. DTA-TGA results for baseline material, HEBM materials, and nanopowder mixtures.
Another condition for reaction initiation was used in the sonamed electrothermal explosion (ETE) experiments. Pressed cylindrical samples were extremely rapidly (103 K/s) preheated by passing a high-amperage electrical current through the sample. This method provides different conditions for preheating rate, sample mass, and induction time than the DTA experiments and also provides uniform sample preheating. In addition, because of the process’s short duration, one can neglect heat losses during ETE. Thus, the dynamic regime of adiabatic TE is accomplished. Figure 4 shows the results from the ETE experiments obtained by a high-speed thermovision (1500 frames/second) system for the baseline material, HEBM material, and nanosized Ni/Al mixtures. It can be seen that the trend obtained by DTA experiments holds, that is, the initial clad particles self-ignite at a temperature slightly above the
melting point of Al (∼990 K), while the nanomixture has an apparent self-ignition temperature of ∼630 K, significantly below the metal’s melting point. Finally, HEBM powder has the lowest apparent self-ignition temperature of ∼580 K. It was previously reported22 that the ignition temperature for ball-milled material decreases monotonically with milling time. Note that reducing the particle size to nanometric dimension (∼80 nm) does not reduce the TE temperature to the extent of reconfiguring the microstructure through HEBM. B. Reaction Initiation by Mechanical Impact. Combined compression/shear testing was performed to study the effect of reactivity enhancement methods on impact sensitivity. Plungers with the impact face having a 10 mm radius were used in all tests. Table 1 indicates the go/no-go results for reactions in the impacted materials, as well as the relative densities for the materials. The table also lists the available kinetic energy which can be transferred from the plunger to the sample. This was simply calculated as the muzzle energy available from a 17 g projectile accelerated to the velocities shown in Table 1. The results show that the initial and HEBM materials were unable to be initiated from impacts produced in this gas gun, up to 400 m/s. Indeed, even the 15 min HEBM material, which thermally exploded at 580 K, was not initiated through plastic deformation. Comparatively, mixtures containing metal nanopowders were easily ignitable. These mixtures experienced reactions after impact by a projectile with a velocity as low as 257 m/s. The nanomixtures containing a diluent portion of micrometer-scale Al were also able to be reacted through mechanical impact. These results were verified through XRD testing of the postshot materials. The next question to be addressed involves the reaction regime that occurs after the impact; does the impact cause TE in the sample or does it create local ignition conditions followed by reaction front propagation? Figure 5 displays images obtained by a high-speed video camera during an impact using a nanomixture, which reacted as a result. The most obvious
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Figure 5. Sequence of high-speed video images from a shear impact test in a nanomixture exhibiting reaction.
characteristic of this sequence of images is the significant delay which occurs between impact and first light, which indicates reaction initiation. Consistently, a delay of around 10 ms passes between the completion of sample deformation and the moment when the camera witnesses the initiation of reaction near the face of the plunger. For every occurrence of reaction, a propagating wave was observed that consumed the entirety of the sample. From the information that the visible light camera provided, the nature of the delay is difficult to understand. Therefore, a high-speed IR camera was also utilized to gather more specific information about the system during this delay period. The temperature-time history of the system is also important to note. The impact heating of the nanometric sample is seen occurring in the initial frame of Figure 6A. At the impact, the rapid densification of material causes high local heating, with the highest recorded temperatures being in excess of 371 °C (644 K). Over the next ∼10 ms, the small heated zone appears at the impact face and remains static. This local zone remains steady, while not being intense enough to produce light visible for the high-speed movie. After 10 ms, this heated zone begins to expand in a propagating wave that travels through the rest of the material. In comparison, a nonreacting case is shown in Figure 6B. Again, the first frame displays the impact, and subsequent frames are time-stamped from that event. The impact occurs similarly to the reactive case in that an initial area is strongly heated. However, this high-temperature area quickly dissipates. Thus, an impact-assisted combustion propagation regime takes place for the reactive mixtures of nano-Ni and -Al particles at relatively low energies of mechanical stimulation. For the HEBM samples, impact also leads to the formation of a hot zone near the plunger surface; however, this local heating does not lead to a propagating reaction.
Figure 6. IR images from shear impact tests that result in a reaction (A) and that do not result in a reaction (B). The reacting sample in A exhibits a locally heated spot that remains heated due to reaction progression, while the nonreacting sample in B quickly dissipates heat from the initial impact and does not produce a reaction front.
IV. Discussion A. Thermal Reaction Initiation. Several issues should be discussed in comparing thermal initiation of the reaction in HEBM and nanosized gasless systems. The HEBM and nanosized systems are similar in that they are both characterized by self-ignition temperatures that are well below the Ni-Al eutectics. However, they differ in that the heat release profiles obtained in DTA experiments are qualitatively dissimilar for HEBM and nanosized mixtures. Figure 4 demonstrates these differences. The extended heat release is observed during DTA testing for the ball-milled sample over a wide range of temperatures, while the DTA testing of the nanosystem exhibits a sudden heat release, indicating that a fast reaction occurs at a
specific temperature. The mechanism for this difference seems to be related to the interfacial characteristics of the systems that provide a larger apparent activation energy for the reaction in the nanomixture compared to that for the HEBM powders. While there are many system characteristics that can affect ignition in heterogeneous mixtures, including composition of the bulk system, particle packing, extent of mixing, stoichiometry of the system, and others, these experiments focus on the effects of the difference in microstructures between HEBM materials and nanomixtures and the effect of particle size on ignition behavior. To verify this observation, the apparent activation energy of the Ni/Al nanomixture was calculated. By heating samples in the DTA device at different heating rates and through use of
Reaction in Ni/Al Heterogeneous Reactive Systems
Figure 7. Kissinger analysis for chemical kinetics in the nanometric Ni/Al system.
the Kissinger method,32 the apparent activation energy of the nanomixture was found to be 55 ( 5.2 kcal/mol. Figure 7 presents the Arrhenius-type dependency for the nanomixture and the resultant apparent activation energy. Previously, this method was used to calculate the apparent activation energies in the unmilled Ni-clad-Al powder and the 15 min HEBM powder,22 which were 84 ( 2 and 28 ( 1 kcal/mol, respectively. The apparent activation energy for the HEBM system has also been obtained previously by the ETE method and was reported to be 25 ( 2 kcal/mol,33 which is in a good agreement with the value obtained by the Kissinger method. Therefore, it is seen that the apparent activation energy for the nanomixture is much less than that of the clad powder but significantly higher than that for the HEBM material. The significantly higher activation energy for nanomixtures compared to that for the HEBM material is consistent with the observed behavior in the DTA experiments and could have many possible causes. The HEBM process, by creating clean interface surfaces and increasing interfacial area, allows the materials to interact with a small energy barrier, inhibiting reaction kinetics. It has been suggested that HEBM also enhances diffusion by creating a high defect density in the interfacial region.34 Molecular dynamics simulations also suggest that the repeated shearing of the bimetallic layers can create atomic diffusion similar to that occurring at elevated temperatures.35 These enhancements to the diffusion process, along with the low-energy-barrier interface typical for this HEBM system, could provide explanation for the rate of mass transfer necessary in the solid state to create the heat release shown in Figure 3. In contrast, the nanomixtures have unmolested surfaces, which still retain thin oxide layers, preventing direct interaction of the active materials. This produces an energy-barrier-inhibiting reaction kinetics that is comparatively higher than that in HEBM systems. The oxide layers require degradation or fracture before reaction can occur at high rates. Although the temperatures reached in the DTA-TGA were far less than that required to melt alumina (Tm ) 2350 K), it is possible that the temperature elevation caused expansion in the active Al, resulting in fracture of the alumina coating, as suggested previously.17,36 Another recently suggested mechanism is that an induced electric field across the oxide layers caused by heating of the particle forces diffusion of the Al from the center of the particle.37 This sort of mechanism, allowing interaction between Al and Ni without removal of the native oxide layer, may explain the enhanced reactivity of the nanomixtures to thermal ignition. However, a
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Figure 8. Effect of particle size on heat generation and TE temperature (inset).
more thorough study of each of these phenomena is necessary to verify these possibilities and is beyond the focus of the current work. It is interesting to note that the qualitative differences between the DTA traces of the HEBM material and the traces of the nanomixtures are similar to those observed in previous work comparing thermite systems before and after ball milling.38-40 In these studies, the ball-milled thermites exhibited a slow heat release similar to the HEBM Ni/Al materials’ traces, while the unmilled thermites showed a sudden heat release, similar to that of Ni/Al nanomixtures. These studies also suggest that ball milling induces changes in the thermite systems that cause the mixture to release heat through a series of exothermic reactions rather than a single, fast-acting reaction. However, a previously reported time-resolved X-ray diffraction (TRXRD) analysis of the thermal explosion in the HEBM Ni-Al system23 shows that HEBM Ni/Al does not proceed through a series of reactions, as was suggested for the ball-milled thermites. The TRXRD analysis provided in situ evidence that Ni-Al systems react via a single stage, forming the final NiAl phase, rather than in a sequence of reactions. The next topic to be addressed is the effect of particle size on the self-ignition temperature. Reducing the particle size in the range from 100 to 1 µm does not change the self-ignition temperature in the system. The self-ignition temperature remains equal to or greater than the eutectic in the Ni-Al system.10 However, when 80 nm metal particles are used, TE occurs at a temperature well below the melting point of Al (Tm ) 933 K). This result can be explained by using Semenov’s classical diagram, similar to those suggested in our previous work.10 Figure 8 illustrates this concept. As mentioned in the Introduction, the required conditions for TE are a strong dependence of reaction rate on temperature. However, it is also known that reaction kinetics is dependent on the particle size. From these experiments, it appears that if the metal particle size in the system is greater than some critical diameter (Dcr), the rate of heat release at temperatures below the melting point of Al is not great enough to overcome the system’s heat losses and initiate TE in the material. In this case, when the reagents do reach the melting temperature, a significant and sudden increase of reaction rate occurs that leads to TE. This is identified in curve D1 of Figure 8. As D decreases, the heat release rate increases, as seen in curve D2 of Figure 8. At the critical diameter, Dcr, the heat release rate at the melting temperature
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is equal to the heat loss rate, and TE is initiated. However, as particle size continues to decrease, that is, when D < Dcr, the reaction rate may be sufficiently large to initiate TE at temperatures below the melting point. This case is shown in curve D3 of Figure 8. The experimental determination of this critical value, Dcr, is an important task and one that will require a significant effort beyond the focus of this current work. The comparison of results obtained under the different test protocols must be considered next. In review, these test conditions include the slow heating (5-50 K/min) of small (∼mg) samples during DTA experiments, rapid (102-103 K/s) Joule preheating of large (∼grams) samples by ETE, as well as relatively slow (50-100 K/min) heating of large (∼grams) samples in the furnace.23 As mentioned, uniform heating of the sample volume was achieved in all of the above cases. It was shown in furnace experiments that if the preheating rate is less than 50 K/min, the TE phenomenon cannot be observed for the HEBM system, which was explained by extensive reaction completion at relatively low temperatures, as seen in the DTA traces of Figure 3. A higher heating rate in the furnace leads to TE, and the general trend Tig,initial > Tig,nano > Tig,HEBM obtained in ETE and DTA experiments holds. B. Impact Reaction Initiation. In the experiments performed on the nanomixtures, reactions were witnessed during mechanical impact by both visible light (Figure 5) and IR cameras (Figure 6). The reaction was seen to begin at discrete locations where heating is localized. From this spot, it propagates in a coherent wave which consumes the entirety of the sample. Due to the aforementioned difficulties in designing and monitoring impact experiments, this is believed to be the first time that this sort of impact-initiated reaction has been captured for Ni/ Al systems. The first and most basic observation which should be made about this event is the nature of the reaction regime to which it corresponds. By exhibiting a single initiation point, the reaction is started by a localized ignition event, rather than initiating TE in the sample. However, the source of the ignition event itself must also be considered. The energy available to be transferred from the plunger, as seen in Table 1, is relatively small when compared to the activation energy of the total sample material (4102 J for the nanomixture and 8064 J for the HEBM material). Since the maximum available input energy is less than the activation energy for the entire sample, a TE of the bulk of the sample simultaneously is not possible. Instead, a propagating reaction wave passing released chemical energy forward in the form of heat is necessary for complete combustion of the sample. This requires the kinetic energy transferred during impact to be localized in a region that is sufficiently small, so that ignition conditions can be met locally. By examining the events occurring in Figure 6A, observations can be made about the heat source’s origin for the reactive case. Since ignition is seen to occur consistently after a ∼10 ms delay after impact, an additional heat source besides the impact must be present. If the impact itself was sufficient to cause ignition, the delay would not be present. Therefore, heat must be generated locally at a rate exceeding the ability of the surroundings to dissipate heat. The only source available is the chemical reaction between Ni and Al; therefore, it must be concluded that a reaction between these materials provides the additional heat energy. Such system behavior during impact test can be understood when the events leading to it are placed in context. First, the plunger impacts the material. This compresses the material, with the main change in porosity taking place only near the face of
Reeves et al. the plunger, as seen in Figure 6A. Figure 6A also shows that this region receives the largest temperature increase from the rapid densification of the material. At this point, the reaction begins to occur in the local region, although at a sufficiently low rate that it is not apparent in the visible camera images. However, heat losses to the surroundings are substantial, which was not the case in the DTA and ETE experiments which provided TE conditions. The heat losses maintain the reduced reaction until, but during the ∼10 ms delay, the accumulated energy caused by the chemical heat release raises local temperature to the point that full ignition occurs. Thus, the ignition regime with ignition delay has been accomplished due to shear impact. In comparison, the HEBM samples experienced similar local heating from the impact but did not proceed to reaction. Although the IR images reveal local heating in excess of 371 °C (644 K), no reaction wave is formed, and no measurable formation of reaction products is shown in postshot XRD traces. The heat release profile from the DTA experiments provides an explanation as to why these samples did not react when impacted. The extended heat release in the DTA trace reveals a tendency for slow reaction rates at temperatures below the eutectic. The heat losses to the surroundings dominated any small scale reactions that may have taken place in the heated region, quickly reducing the local temperature and preventing reaction in the HEBM samples. The characteristics of the densified samples prior to impact should also be considered when delineating between the nanomixture’s behavior and the HEBM sample’s behavior. As previously noted, performing HEBM without the use of PCA creates large, full density particles with refined microstructure. Oppositely, the nanomixtures consist of extremely small, hard particles with little aptitude for plastic flow. As a result, higher %TMD’s are possible with the HEBM materials than those for the nanomixtures, as seen in Table 1. This variation in initial relative density has an effect on the amount of energy from the impact which can be stored in the sample through rapid densification. The shock energy which can be deposited in a porous medium has been defined as41
1 E ) P(V - V0) 2
(1)
where P is the shock pressure, V is the specific volume of the powder compact, and V0 is the specific volume of the fully dense material. Therefore, the ratio of shock energy which can be absorbed in mixtures of the same materials with different densities can be written in terms of densities and, more usefully, in terms of %TMD
ES1 ES2
)
()
(V1 - V0) F2 (F1 - F0) ) ) (V2 - V0) F1 (F2 - F0)
(
)
%TMD2 (%TMD1 - 100) %TMD1 (%TMD2 - 100)
Using this relation to compare the storable energy through densification for the higher-density HEBM case and the lowerdensity nanomixture, it is seen that ESnano/ESHEBM ) 3.0. This is a significant difference and likely plays a role in the increased impact sensitivity of the nanomixtures. This is similar to the case of hotspot ignition in crystalline explosives. It is known that increased gas inclusions in conventional explosives greatly
Reaction in Ni/Al Heterogeneous Reactive Systems affect the impact sensitivity of that explosive due to localized heating associated with the collapse of those voids.42,43 In this case, the nanomixtures have a much greater overall volume and, due to the smaller particle size, frequency of porosity that allows the subsequent heating from pore collapse to be greater and more equally mixed with reactants than that in the HEBM materials. Ideally, testing of the materials would be confined to directly comparable densities; however, for these materials, practical considerations prevented it. Pressing the nanomixtures to higher densities was not possible, and at lower densities, the HEBM materials shattered at the particle boundaries after impact. In addition to density variation, the microstructure of the samples affects the stress distribution in the samples. In previous work, differential velocities between particles of constituent materials caused by shock wave passage were considered as a limiting property for reaction threshold.25 This seems to be a reasonable assumption for the nanomixtures since they consist of particles with very limited potential for plastic deformation. The nanomixtures have interparticle contact only at discrete points; therefore, when impact occurs and the particles, stress is concentrated at these interaction points. Eventually, stresses will increase to the point that the harder Ni particles will penetrate the softer Al particles. This forces mass diffusion in the system at areas of high-energy concentration. In comparison, the HEBM materials have few discrete interparticle interfaces. Instead, stress in HEBM systems is dissipated through deformation of the many layers typical of the HEBM structure. Since the Ni-Al interfacial area is very high in HEBM systems, the stress state at the interface is comparably low. Also, as previously noted in shock consolidation experiments,20,44,45 deformation preferentially occurs in the Al layers. This implies that the regions of greatest strain will be in the Al channels themselves, rather than at their edges. Compared to the nanomixture case, this causes the Ni and Al to have less forceful interaction. As such, the forced diffusion possible in impacted mixtures of hard particles, such as that in a nanometric mixture, will be less likely in the lamellar HEBM material. Finally, it is worth noting that differences in thermal diffusivity of the reaction media could also play a role in favoring impact initiation of the nanomixtures over the HEBM materials. Since the samples pressed from nanomixtures have a higher porosity than those pressed from HEBM materials, one may assume that the nanomixture samples have comparably lower effective thermal diffusivity. It was experimentally shown that for both materials, densified regions near the plunger tip reached high temperatures following the impact. However, only the nanomixtures maintained these high local temperatures and eventually produced a propagating reaction wave. The HEBM samples dissipated the heat energy. Two options may be considered to explain these individual effects. First, as discussed above, the specific properties of the pressed nanomixtures can allow the impacts to produce higher local temperatures as compared to HEBM media, supporting ignition in the nanomixtures. Second, the higher thermal diffusivity of the HEBM sample leads to faster energy dissipation, reducing the likelihood of viable local ignition points forming in the HEBM samples. V. Conclusions It was found that reducing the size of Ni and Al powders to the nanometric scale increases sensitivity to thermal stimulation. DTA and ETE results showed that the ignition temperature decreased by about 80 K compared to that for the microscaled system. The energy was seen to release quickly and thoroughly
J. Phys. Chem. C, Vol. 114, No. 35, 2010 14779 in a mode similar to the baseline material. This is different from the energy release from the HEBM system, which gradually releases energy solely through the solid-state reactions. The two reactivity enhancement methods considered here also have different effects on shear impact ignition. With the shear impact testing performed, only mixtures containing nanopowders of Ni and Al were able to be ignited. These experiments produced the first direct IR and visible images of ignition in a shear impact of these materials. The HEBM materials were not able to be initiated, even at the highest velocity impacts available with the testing setup. Refinement of the material’s microstructure through HEBM creates an environment conducive to thermal initiation, as demonstrated in DTA-TGA testing, but does not appear to result in sufficient interaction between Ni and Al to initiate reaction through impact of the considered energy range. Future experiments will utilize higher impact energies to determine reaction initiation thresholds for this material. The results of these tests indicate that the reactivity enhancement methods of particle size reduction and HEBM have differing effects on the sensitivity of the Ni/Al system. This is a fairly basic, yet important observation to be made. Although the underlying details of the changes in reactivity are not yet known, it has been shown that HEBM is capable to producing greater sensitivity to thermal reaction initiation, while reducing the particle size to nanometric dimensions has a more profound effect on the system’s mechanical impact sensitivity. Acknowledgment. This work was funded by the Office of Naval Research under Contract Number N0014-07-1-0969 with Dr. Clifford Bedford as the Program Manager. References and Notes (1) Barzykin, V. V. Pure Appl. Chem. 1992, 64, 909–918. (2) Semenov, N. N. Zh. Fiz. B 1929, 42, 571. (3) Todes, O. M. Acta Physicochim. URRS 1936, 5. (4) Allen, A. O.; Rice, O. K. J. Am. Chem. Soc. 1935, 57, 310–317. (5) Rice, O. K.; Allen, A. O.; Campbell, H. C. J. Am. Chem. Soc. 1935, 57, 2212–2222. (6) Frank-Kamenetskii, D. Diffusion and Heat Exchange in Chemical Kinetics; Princeton University Press: Princeton, NJ, 1955. (7) Merzhanov, A. G. Combust. Flame 1966, 10, 341. (8) Merzhanov, A. G.; Barzykin, V. V.; Abramov, V. G. Khim. Fiz. 1996, 15, 3–44. (9) Mukasyan, A. S.; Rogachev, A. S. Prog. Energy Combust. Sci. 2008, 34, 377–416. (10) Thiers, L.; Mukasyan, A. S.; Varma, A. Combust. Flame 2002, 131, 198–209. (11) Thadhani, N. N. J. Appl. Phys. 1994, 76, 2129–2138. (12) Lee, J. J.; Zhang, F. Shock-induced reactions in cylindrical charges of titanium-silicon powder mixtures; Conference of the American Physical Society-Topical-Group on Shock Compression of Condensed Matter, Waikoloa, HI, 2007. (13) Gur’ev, D. L.; Gordopolov, Y. A.; Batsanov, S. S.; Merzhanov, A. G.; Fortov, V. E. Appl. Phys. Lett. 2006, 88. (14) Son, S. F.; Asay, B. W.; Foley, T. J.; Yetter, R. A.; Wu, M. H.; Risha, G. A. J. Propul. Power 2007, 23, 715–721. (15) Son, S. F.; Yetter, R. A.; Yang, V. J. Propul. Power 2007, 23, 643–644. (16) Yetter, R. A.; Risha, G. A.; Son, S. F. Proc. Combust. Inst. 2009, 32, 1819–1838. (17) Levitas, V. I.; Asay, B. W.; Son, S. F.; Pantoya, M. J. Appl. Phys. 2007, 101, 20. (18) Koch, C. C.; Whittenberger, J. D. Intermetallics 1996, 4, 339–355. (19) Dreizin, E. L. Prog. Energy Combust. Sci. 2009, 35, 141–167. (20) Eakins, D. E.; Thadhani, N. N. Acta Mater. 2008, 56, 1496–1510. (21) Cardellini, F.; Mazzone, G.; Antisari, M. V. Acta Mater. 1996, 44, 1511–1517. (22) White, J. D. E.; Reeves, R. V.; Son, S. F.; Mukasyan, A. S. J. Phys. Chem. A 2009, 113, 13441–13447. (23) Mukasyan, A. S.; White, J. D. E.; Kovalev, D. Y.; Kochetov, N. A.; Ponomarev, V. I.; Son, S. F. Physica B 2010, 405, 778–784.
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