MnO2 as an Environmentally Friendly Energetic

Aug 23, 2017 - Current fielded delay formulations face increased scrutiny due to their environmentally hazardous components (i.e., BaCrO4/KClO4), and ...
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Research Article pubs.acs.org/journal/ascecg

Performance of W/MnO2 as an Environmentally Friendly Energetic Time Delay Composition Joshua T. Koenig,† Anthony P. Shaw,*,‡ Jay C. Poret,‡ William S. Eck,§ and Lori J. Groven*,† †

Department of Chemical and Biological Engineering, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701, United States ‡ Armament Research, Development and Engineering Center, U.S. Army RDECOM-ARDEC, Picatinny Arsenal, New Jersey 07806, United States § Army Public Health Center, U.S. Army MEDCOM-APHC, Aberdeen Proving Ground, Maryland 21010, United States S Supporting Information *

ABSTRACT: Current fielded delay formulations face increased scrutiny due to their environmentally hazardous components (i.e., BaCrO4/KClO4), and there is an immediate need for viable replacements. In this work, the W/ MnO2 composition is explored as an environmentally benign time delay replacement. Delay performance and aging characteristics are examined at diameters (6.35 and 4.7 mm) similar to those of common delay housings. Measured maximum combustion temperatures ranged from 1466 to 1670 K as a function of mixture stoichiometry. The measured combustion velocities ranged from 0.67 to 1.68 mm/s for 6.35 mm open air pellets, and from 1.62 to 4.61 mm/s when combusted in 6.35 and 4.7 mm ID Al/SS housings. When packing density was increased and decreased from the standard 60% theoretical maximum density, combustion velocity decreased. For all W/ MnO2 formulations, the maximum measured gas production was 9.1 mL/g. Accelerated aging was performed for 8 weeks, and negligible changes in combustion characteristics were observed. Combustion products were characterized with powder X-ray diffraction and appear benign on the basis of known compound information. Hot, cold, and ambient combustion velocity experiments were performed in the M213/M228 fuze, with results within acceptable parameters. Therefore, the W/MnO2 system seems suitable as a relatively low-combustion-velocity, low-gas-producing, and low-toxicity-delay composition with acceptable longevity. KEYWORDS: Tungsten, Manganese dioxide, Pyrotechnic, Combustion, Benign, Aging, Gas production, Toxicity

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INTRODUCTION Gasless pyrotechnic delay compositions are formulated to provide a repeatable, reliable time increment for applications including the firing of munitions such as signaling flares, explosives, and hand grenades. These mixtures generally consist of a metal fuel such as W, Mn, or Zr−Ni alloy; oxidizers such as KClO4 or PbCrO4; and diluents or intermediate oxidizers such as BaCrO4. Traditional delay compositions such as W/ BaCrO4/KClO4/DE (DE = diatomaceous earth) and Mn/ BaCrO4/PbCrO4 have been used since shortly after World War II to meet the needs of increasingly complex munitions systems,1−7 and are still in use today. This is due to their relatively gasless combustion characteristics, stability under applied electrostatic discharge (ESD) and friction, long-term storage stability, low ignition sensitivity, and large range of tailorable combustion velocities (0.6−150 mm/s).1−7 Recently, the reactants of these fielded delays have been under increased scrutiny due to the environmental hazards associated with compounds such as KClO4, PbCrO4, and BaCrO4. It has been known for some time that lead, chromium(VI), and chromium(IV) compounds are toxic.8,9 However, regulations are also now © 2017 American Chemical Society

being implemented for potassium perchlorate, as it can cause water and soil contamination, primarily due to its high solubility in water and its impacts upon human development.10,11 Currently, the EPA (United States Environmental Protection Agency) has calculated a tap-water screening level of 11 μg/L for perchlorate and perchlorate salts, while many states such as California and Massachusetts are regulating perchlorate levels in drinking water at even lower levels.12 Because of a continued need for gasless pyrotechnic delays and the environmental hazards caused by many traditional delay components, a significant amount of experimental work has been done to study potential delay formulations containing environmentally benign constituents.13−21 Previous studies on environmentally benign systems have shown promising results. For example, Dixon20 and Dillehay21 reported on the chromate-free W/MnO2/KClO4 system, with combustion rates ranging from 1.7 to 9.83 mm/s in 4.7 and Received: July 31, 2017 Revised: August 17, 2017 Published: August 23, 2017 9477

DOI: 10.1021/acssuschemeng.7b02579 ACS Sustainable Chem. Eng. 2017, 5, 9477−9484

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device, by Motion Engineering Company, Model 120 K at 250 fps and with varying filters. Confined Formulations (Al/SS Housings). The second configuration explored formulations packed in Al/SS housings with inner diameters of 6.35 and 4.7 mm. The formulations were again pressed to a stop in a single increment at 750 or 1000 psi, with a hold time of 5 min and final compaction densities of 60 ± 2%-TMD. For consistency, a 50 mg increment of A-1A was placed at both the top and bottom of the pellets to provide ignition and end-of-combustion cues. Additionally, for exploration into the effect of density on combustion velocity and reliability, the 60/40 W/MnO2 formulation was pressed to 50%and 70%-TMD. The filled housings were then placed in a PTFE holder for combustion analysis. A SONY Handycam HDR-CX290 camera was used to record combustion. Figure 1 details the confined combustion setup.

5.94 mm ID Al channels. However, these mixtures contained up to 15 wt % KClO4. Swanepoel et al.14 studied Mn/metal oxide binary reactive systems and reported combustion velocities from 7 to 19 mm/s for Mn/MnO2, 11−21 mm/s for Mn/Bi2O3, and 4−10 mm/s for Mn/Cu2O. The work by Poret et al. 15 also reported encouraging results, with combustion velocities ranging from 0.48 to 7.70 mm/s in 4.8 mm ID Al channels for the B4C/NaIO4/PTFE system. However, because of the gassy nature of NaIO4 and PTFE, it is unlikely that this system would be appropriate for use in sealed housings. Further studies of the Mn/MnO2 system were also reported by Miklaszewski et al.,13 with combustion velocities ranging from 2.4 to 7.3 mm/s in 4.7 mm ID Al channels. The results from Miklaszewski et al.13 are especially favorable, as slow burning delays were prepared using military (MIL-spec) grade materials. Furthermore, Mn is stable at typical storage temperatures and readily oxidizes when combusted. In this effort, we extend these results by replacing the fuel component of the Mn/MnO2 mixture with W. To our knowledge, there is no significant work reported on binary mixtures of W/MnO2 in a delay-type configuration. Dixon’s20 report indicated that the W/MnO2 system failed to ignite, likely because of an inadequate igniter (not specified in his work), as well as large heat transport effects in the U.S. Army handheld signal delay housing. Therefore, the objective of this work was to perform fundamental studies such as heat generation, kinetics, aging characteristics, gas generation, and combustion product assessment on the W/MnO2 system and demonstrate the utility of the selected formulation in the M213/M228 pyrotechnic delay fuze.

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Figure 1. Configuration used for the Al/SS housing combustion experiments. Combustion Temperature Measurements. For the acquisition of temperature profiles, a small-scale combustion boat (8 × 4.5 × 2.9 cm3) was used, with 50−80 g of loose powder (∼25%-TMD) and 3.5 g of A-1A igniter mix placed into the combustion bed as depicted in Figure 2. Ignition was achieved with an electrically heated “V”-shaped

EXPERIMENTAL SECTION

Reactive Compositions. Nominal sizing and vendor information on the powders used are summarized in Table 1. The W/MnO2

Table 1. Reactant Powders and Sizing powder

nominal particle size

vendor

W MnO2 Zr Celite Hyflo Fe2O3 (III)

1−5 μm −325 mesh −325 mesh −150 mesh −325 mesh

AEE Alfa Aesar Teledyne Wah Chang Alfa Aesar Columbian Chem. Co.

compositions were dry-mixed in wide-mouth 30 mL HDPE bottles using a Resodyn LabRAM instrument at 80% intensity in 2 min on/off intervals for a total of 6 min. Scanning electron microscopy (SEM) images of the tungsten and MnO2 powders can be seen in the Supporting Information. Combustion Experiments. Combustion characteristics of the W/ MnO2 system were assessed in two configurations, which included open-air pellets and mixtures packed within metal enclosures representing typical delay housings [either aluminum (Al) or stainless steel (SS)]. Pellet Configuration. The compositions were pressed to a stop to achieve the desired percentage of theoretical maximum density (%-TMD). Pellets were pressed to 60%-TMD in a 1/4 in. (6.35 mm) die for a duration of 5 min at 750 or 1000 psi (5.2 or 6.9 MPa), with a 50 mg increment of A-1A (65 wt % Zr, 25 wt % Fe2O3, 10 wt % DE) pressed at the top as an ignition increment. Typical pellet lengths were 15.24 mm. The pressed pellets were placed onto an upraised metal screw coated with vacuum grease, which acted as both a thermal inhibitor and glue for the pellet. An electrically heated “V”-shaped Nichrome (30 gauge) ignition wire was used to initiate pellet combustion. Combustion was recorded with a FASTcam Ultima APX

Figure 2. Experimental setup for the boat-type temperature profile measurements of the W/MnO2 system. The tip of the thermocouple was placed at a height of 1.6 cm above the base of the boat and inserted 1.75 cm into the mixture. Nichrome wire (28 gauge) inserted into the A-1A. Temperature profiles of the reacting compositions were measured in situ by inserting a B-type thermocouple (0.203 mm) into the reactant bed, as indicated in Figure 2. Data was recorded with an IOtech Personal DAQ 3000 instrument at a sampling rate of 100 Hz in DASYlab 13.0. Gas Generation Measurements. Dynamic gas generation was assessed using a volumetric displacement technique with a Chemglass gas analysis buret (100 mL with a precision of ±0.2 mL). For consistency, 1.0 ± 0.3 g of reactive mixture was hand-pressed into 5 mm outer diameter, 0.25 mm wall thickness SS tubes (McMaster: part 9478

DOI: 10.1021/acssuschemeng.7b02579 ACS Sustainable Chem. Eng. 2017, 5, 9477−9484

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ACS Sustainable Chemistry & Engineering number, 50415K22), with 50 mg of A-1A increment at the top. These were then inserted into a 2 mL combustion chamber, as depicted in Figure 3, and ignited with an electrically heated Nichrome wire (28

oxide compounds, with each successive compound being a step in the reaction process to form MnO. MnO2 < Mn2O3 < Mn3O4 < MnO

Once the formation of WO3 and MnO is complete, the reaction follows the path of eq 2, with the formation of manganese tungstate due to the reaction of WO3 with MnO, which results in the complete reaction path as depicted in eq 3. Using the stoichiometry from eq 3, it was found that 41.35 wt % W and 58.65 wt % MnO2 would be the stoichiometric wt % values for the system. Mixture ratios were created taking this into account, as well as the fact that quenching generally occurred at W (fuel) content below 40 wt %. Theoretical and Experimental Combustion Velocities and Temperatures. Open air pellets were combusted to determine combustion velocities of compacted formulations without heat losses associated with a housing, while experiments in Al/SS housings were conducted to determine combustion velocities of compacted mixtures in conditions similar to fielded delays. Figure 4 details the role stoichiometry

Figure 3. Combustion chamber for determining gas generated during combustion. gauge). The total volume of water displaced was recorded immediately and the gas produced determined directly from that measurement. Accelerated Aging. Accelerated aging was performed at 30% relative humidity and 70 °C on the 50/50 W/MnO2 mixture in an open 50 mL wide mouth HDPE bottle for 8 weeks. Differential scanning calorimetry and thermogravimetric analysis (DSC−TGA) and combustion velocity experiments were conducted on the mixture at 0, 2, 4, 6, and 8 weeks of aging. The thermal behavior was assessed with a TA Instruments Q600 DSC-TGA device in 90 μL alumina pans over a temperature range 50−1000 °C at a heating rate of 10 °C/min under ultrahigh-purity (UHP) argon at a flow rate of 100 mL/min. Combustion experiments were conducted as previously described. Combustion Products. X-ray diffraction spectra were gathered using a Rigaku 500 diffractometer which uses Cu Kα radiation with a tube voltage of 40 kV and tube current set at 40 mA. Samples of the reacted product (pellets combusted in 4.7 mm Al housings) were ground to a fine powder and were scanned with respect to intensity from 2θ values of 10° to 80° at a speed of 2°/min. Jade version 7.0 software was used for analysis. M213/M228 Fuze Ambient, Hot, and Cold Combustion Rates. One composition, a mixture of 50 wt % W and 50 wt % MnO2, was tested in M213/M228 fuze hardware; the details of these fuze experiments may be found in the Supporting Information.

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Figure 4. Combustion velocities of the W/MnO2 system with a pellet diameter of 6.35 mm, for open air pellets and formulations pressed into SS and Al housings.

RESULTS AND DISCUSSION In this study, the following reactions were considered, where tungsten trioxide and MnO are the assumed reaction products, which then react to form manganese tungstate, with eq 3 showing the final reaction step, as predicted by FactSage 7.0. W + 3MnO2 → WO3 + 3MnO

(1)

WO3 + MnO → MnWO4

(2)

W + 3MnO2 → MnWO4 + 2MnO

(3)

(4)

and housing type has on combustion velocity for 40, 50, 60, and 70 wt % W. For open air pellets, combustion velocities ranged from 0.67 to 1.68 mm/s while for materials confined in the Al housings, combustion velocities ranged from 1.62 to 4.61 mm/ s. Only one set of data points was obtained for the SS housing, at 60 wt % W. A combustion velocity of 2.80 mm/s was obtained, which was significantly lower than that of the Al housing (3.45 mm/s at 60 wt % W). Adding a thermal conductor around the W/MnO2 pellets resulted in increased combustion velocities, which is due to the heating of the metal during the combustion event, which in turn accelerates the reaction down the housing. It should also be noted that the SS housing does not accelerate the combustion velocity as much as the Al housing. This is as expected, as SS has a much lower thermal conductivity than Al [12−45 versus 205 W/(m K)]. The peak combustion velocity was observed at the most fuel rich conditions (70 wt % W) with combustion velocities of 1.68 ± 0.04 mm/s for open air pellets and 4.61 ± 0.23 mm/s for formulations pressed into Al housings. Full propagation was observed for all W/MnO2 ratios assessed. It is possible that combustion velocities of this system could be further reduced

Equation 1 was determined on the basis of the expectation of tungsten trioxide formation when combusted, as determined by Dunn.22 Furthermore, as tungsten is exposed to oxygen, it goes through a series of spontaneous intermediate reactions until it reaches the final form of tungsten trioxide.23 This spontaneity is due to the Gibbs free energy of each oxide being lower than the next (when viewed on a per mole-compound basis), with WO3 having the lowest Gibbs free energy and WO2 the highest. Similar thermodynamic analysis has also been done on MnxOx compounds,14 with results showing MnO2 as the least thermodynamically stable of the manganese oxides, and MnO the most stable, when analyzed using Ellingham diagrams.24 Equation 4 details the thermodynamic stability of manganese 9479

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ACS Sustainable Chemistry & Engineering with additives. However, because of the likelihood of quenching events, fuel content would likely need to be reduced, especially if diluents are added at high percentages (see the Supporting Information) and in particular when mixtures are pressed into metal housings (Al). This is due to the very high thermal conductivity of tungsten [173 W/(m K)], which results in a relatively high rate of heat transfer to the housing. That being said, efforts to increase combustion velocity of this system by including stronger oxidizers or additional fuels could be highly effective. Combustion velocities were also determined for 4.7 mm diameter housings (Al and SS), and are shown in Figure 5. It

Figure 6. Combustion velocities of the 60/40 W/MnO2 system in 4.7 mm Al housings at varying %-TMD values.

while convective effects dominate combustion velocity at below 60%-TMD (further explanations in the Supporting Information). The combustion temperatures were also determined for the W/MnO2 system, as they can often give important information on the phases that occur during combustion of various pyrotechnic systems. Knowing maximum combustion temperatures allows for information on whether solid−solid, liquid− solid, or gas−solid combustion is occurring, which then relates to relevant models.28−32 Figure 7 details the maximum Figure 5. Combustion velocities of the W/MnO2 system with a pellet diameter of 4.7 mm, ranging from 40 to 70 wt % tungsten for SS and Al tube combustion.

was observed that as the diameter of the housings was reduced to 4.7 mm, combustion velocities were slightly higher than those of the 6.35 mm diameter housings in general, with significantly larger standard deviations at 40 and 50 wt % W. The standard deviation results indicate that consistent combustion velocities are harder to manage in smaller diameter tubes, likely because of the larger relative amount of heat conducted to the housings and inconsistent contact between the housing and the packed formulation. This can result in unsteady and inconsistent combustion for mixtures that do not propagate as easily. Combustion velocities were also determined as a function of packing density, which is a measure of the void spaces in a packed system, and thus determines how easily gas flow occurs during combustion, which then determines how quickly the unreacted powder will preheat (if a significant amount of gas is produced).25,26 Furthermore, changes in packing density modify the amount of contact occurring between particles in a system, which then affects the thermal conductivity of that system.27 Consequently, as packing density decreases, thermal conductivity of the system decreases and thermal convection increases, indicating that the magnitude of these phenomena relative to each other control the combustion velocity of the system. Figure 6 details the influence of porosity on combustion velocity. It was found that a packing density of 60%-TMD is ideal for maximizing combustion velocity, with both higher and lower packing densities showing a reduction in combustion velocity. These results indicate that conductive effects dominate combustion velocity at above 60%-TMD,

Figure 7. Maximum temperature plot of the W/MnO2 system (○) with FactSage 7.0 results superimposed ().

combustion temperatures as a function of varying wt % W, obtained experimentally and with the FactSage 7.0 simulation for W and MnO2 as the reactants. The experimental maximum combustion temperatures were significantly lower than the predicted FactSage 7.0 adiabatic temperatures, and were 1530 (40 wt % W), 1670 (50 wt % W), 1525 (60 wt % W), and 1466 (70 wt % W) K. This is as expected, as FactSage 7.0 assumes an ideal situation with no heat losses, which is not the case in a real system. Miklaszewski et al.16 also reported similar results for the Ni/Al−Ti/C formulation, with theoretical adiabatic temperatures being higher than experimental maximum temperatures by ∼600 K when combusted in 4.8 mm ID Al housings. 9480

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ACS Sustainable Chemistry & Engineering Furthermore, the experimental results indicate that, for this system, combustion temperature plays a minimal role in controlling combustion velocity. Combustion velocity results from Figure 4 agree with this assessment, as maximum combustion velocity occurs at maximum fuel content. Impact of Stoichiometry on Gas Generation. Knowledge of the amount of gas produced by the W/MnO2 system is critical, as low-gas-production systems are required if they are to be used in sealed housings. Consequently, gas-production measurements were performed to see if this mixture could be considered a “gasless” delay mixture. Figure 8 details the

Figure 9. XRD of the combusted W/MnO2 products at varying wt % W. Combustion products determined to be manganese tungstate− MnWO4 (1), trimanganese tungstate−Mn3WO6 (2), tungsten−W (3), and manganese (II, III) oxide β phase−Mn3O4 (4).

middle of the pellet, with incomplete reactions occurring at the edges. However, overall the XRD results indicate that, at this size scale, eq 3 is not the reaction path that the W/MnO2 system follows, as the reaction does not have the chance to reach thermodynamic equilibrium. This conclusion is a result of the formation of Mn3O4 instead of MnO, probably due to rapid cooling. Lastly, there is no thermodynamic data on Mn3WO6, so it is unknown if this compound is thermodynamically favored over the more well-known MnWO4. That being said, eq 5 can be considered.

Figure 8. Measured gas volume produced as a function of stoichiometry for the W/MnO2 system.

measured gas production of the W/MnO2 system as a function of wt % W. Error bars are shown, which were generated from the standard deviation of each measurement. As the wt % W increases, one can see a decrease in gas production. This is due to the fuel-rich nature of the mixtures, which results in less gas leaving the system as the majority of the oxygen oxidizes the tungsten. Of particular note is how the gas production of the W/MnO2 system compares to that of the traditional tungsten delay. Miklaszewski et al.13 found that various W/BaCrO4/ KClO4/diatomaceous earth systems exhibited gas production ranging from 15.7 ± 2.0 to 29.4 ± 2.2 mL/g. In comparison, the maximum gas production of the W/MnO2 system is 9.1 ± 0.47 mL/g, which shows that the W/MnO2 system could be suitable for use in sealed housings. Assessment of Combustion Products. Combustion product analysis is a critical aspect of this work, as one of the major goals is to formulate an environmentally benign delay. Therefore, XRD was performed on the combustion products of the 40, 50, 60, and 70 wt % W mixtures, as detailed in Figure 9. Crystalline products formed include MnWO4, Mn3WO6, and Mn3O4, with the amount of tungsten detected in the products drastically decreasing as the wt % W approaches stoichiometry. This result is expected from the stoichiometry shown in eq 3, as tungsten is readily present in all ratios of W/MnO2 except for the 40 wt % W, 60 wt % MnO2 system. This is due to complete oxidation of tungsten, as tungsten is in excess in all but the lowest-fuel-content systems. Beyond those differences, combustion products vary minutely in magnitude and not at all with products formed across the varying wt % W. It should be noted that since pellets of reacted product were ground up, it is possible that complete reaction of this system did occur in the

3W + 7MnO2 → W + MnWO4 + Mn3WO6 + Mn3O4 (5)

Significantly different wt % ratios are obtained as compared to that of eq 3, with the stoichiometric wt % ratios being 52.6 wt % W and 47.4 wt % MnO2. If we assume complete oxidation of tungsten, eq 6 could also be considered. 2W + 7MnO2 → MnWO4 + Mn3WO6 + Mn3O4

(6)

This agrees quite well with the wt % ratios obtained in eq 3, with the stoichiometric wt % ratios being 37.7 wt % W and 62.3 wt % MnO2. However, due to the fact that quenching generally occurs at wt % W ratios below 40%, the reaction path from eq 6 is not considered feasible. Environmental Assessment. Reactants and products in this formulation are all assessed to have low environmental and human health impact.33,34 Furthermore, these compounds do not have the stringent regulations associated with hexavalent chromium, barium compounds, perchlorates, and lead compounds.8,9,12,35 The design of the gasless delay fuze essentially eliminates exposure to the components via inhalation, which is the route of exposure with the greatest resulting toxicity. Manganese compounds, if released to soil as a result of weathering of dud or expended munitions, are not mobile in the environment because of their insolubility, and are not bioavailable to organisms for the same reason.36 For tungsten, tungstates WO42− and WO66− are the primary naturally occurring forms, occurring mainly in insoluble minerals.37 As with the manganese compounds, tungstates are insoluble, and therefore pose no inhalation exposure hazard; are immobile in the environment, are not bioavailable, and 9481

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this system, because of the similarity of results across time, the differences in combustion velocity from 0 to 8 weeks are considered negligible. Both weight loss and heat flow curves were obtained with DSC−TGA for 0, 2, 4, 6, and 8 weeks of aging. For all results, there were negligible impacts on weight loss and heat flow, with nearly identical DSC−TGA curves for aging weeks 0−8. The temperature at peak heat flow was found to occur at 578.4 ± 3 °C. See the Supporting Information for further details. Combustion in the M213/M228 Fuze. Understanding the role that hot and cold temperatures have on the reliable combustion of pyrotechnics is critical in determining mixture suitability for use in the field. Figure 11 details the hot, cold,

therefore do not pose an environmental or human health hazard via the routes of exposure that are relevant to the gasless delay fuzes.38 In contrast to traditional delays, the reactants and products of the W/MnO2 system exhibit significantly lower toxicity in all cases. Combustion Mechanisms. The results of this work allow for a clear picture of the mechanisms behind the combustion of W/MnO2. The results from the DSC−TGA runs (see the Supporting Information) show the first exotherm at 578.4 °C, similar to the temperature of decomposition of MnO2. The Hao−Tanaka model28 was used to further study this system, and a diffusivity of 10−9 cm2/s (on the order of gas-phase diffusion) was calculated when experimental (boat combustion fraction reacted results) and property parameters were placed into the model. An explanation of the model may be found in the Supporting Information. The maximum temperature (Figure 7) and combustion velocity results (Figure 4) indicate that combustion velocity is not a function of combustion temperature. Therefore, one can conclude that for the W/ MnO2 system the reaction proceeds because of initial gas-phase diffusion (gas−solid reaction) as a result of the decomposition of the MnO2, followed by solid−solid reactions, which also have a significant impact on system combustion (Figure 6) due to particle contact. Furthermore, one can conclude that limitations of maximum combustion velocity will primarily be due to availability of tungsten. Impacts of Accelerated Aging. Accelerated aging was conducted on the 50/50 W/MnO 2 system using the NAVSEAINST 8020.5C39 standard to determine if the W/ MnO2 mixture would be stable if stored over long periods of time. Figure 10 details combustion velocities of the aged 50/50

Figure 11. Each fuze contained 1.89 g of 50/50 W/MnO2 delay, loaded in four increments. The delay columns were consolidated to 64%-TMD, and were approximately 18.5 mm long. A proprietary titanium-based igniter was used as an input/output charge. At each temperature, 10−12 fuzes were tested.

and room-temperature combustion velocities of the 50/50 W/ MnO2 system within M213/M228 fuze hardware. The M213 fuze is used in the M67 fragmentation grenade, while the M228 is used in the M69 practice grenade. Both fuzes contain the same delay element, the only distinction being the detonator or black powder charge that is subsequently attached. These tests involved the delay housing only, in which the delay column was ignited by the action of a percussion primer and a layer of titanium-based igniter composition (the input charge). Another layer of igniter composition served as an output charge and as an indication of complete functioning. Combustion velocities of 3.01 ± 0.07, 3.58 ± 0.12, and 3.84 ± 0.12 mm/s were obtained at temperatures of −51.1, 20, and 62.8 °C, respectively. These rates correspond to ambient temperature functioning times that are within the desired 4.0−5.5 s interval. The W/MnO2 system therefore appears to be a viable candidate for use in handgrenade fuzes.

Figure 10. Combustion velocity as a function of aging time for the 50/ 50 W/MnO2 mixture.

W/MnO2 material. Combustion velocities were impacted minimally upon aging. For weeks 0−8, the largest standard deviation was 0.11 mm/s. Combustion velocities were found to vary from 2.35 mm/s at 2 weeks of aging to 2.42 mm/s at 6 weeks of aging. These results are interesting, as Dillehay21 reported that an initial decrease in combustion velocity occurs during aging of formulations with tungsten fuel, followed by minimal changes due to aging after that fact. It is likely that the tungsten already underwent a passivation reaction prior to being formulated in the W/MnO2 mixture. It may be worthwhile to further study this phenomenon. However, for

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02579. SEM images, FactSage 7.0 simulation results; DSC− TGA graphs; combustion velocity curves; explanations of experimental procedures; descriptions of the Hao− 9482

DOI: 10.1021/acssuschemeng.7b02579 ACS Sustainable Chem. Eng. 2017, 5, 9477−9484

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Tanaka model; and a brief account of the theory behind conduction, convection, and %-TMD in pyrotechnic pellets as it pertains to this work (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Joshua T. Koenig: 0000-0001-9248-7757 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this work was provided by the Strategic Environmental Research and Development Program (SERDP), Project WP-2518, “Environmentally Sustainable Gasless Delay Compositions for Fuzes.” Rajendra K. Sadangi (ARDEC) is thanked for acquiring the SEM images.

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DOI: 10.1021/acssuschemeng.7b02579 ACS Sustainable Chem. Eng. 2017, 5, 9477−9484

Research Article

ACS Sustainable Chemistry & Engineering (39) Peletski, C. NAVSEAINST 8020.5C: Qualification and Final (TYPE) Qualification Procedures for Navy Explosives; DTIC Accession Number ADA509544, Technical Report for Naval Sea Systems Command: Arlington, VA, 2000.

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DOI: 10.1021/acssuschemeng.7b02579 ACS Sustainable Chem. Eng. 2017, 5, 9477−9484