Study of Laboratory-Scale Burning of Composite Solid Propellant for Novel Methods of Nanoparticle Synthesis

Transcription

1 Paper # 070HE-0091 Topic: Heterogeneous Combustion 8 th U. S. National Combustion Meeting Organized by the Western States Section of the Combustion Institute and hosted by the University of Utah May 19-22, 2013 Study of Laboratory-Scale Burning of Composite Solid Propellant for Novel Methods of Nanoparticle Synthesis Tyler W. Allen, 1 Andrew R. Demko, 1 Mitch Johnson, 1 Thomas Sammet, 1 Eric L. Petersen 1 David L. Reid, 2 Robert Draper, 2 and Sudipta Seal 2 1 Mechanical Engineering, Texas A&M University, College Station, TX, 77843, USA 2 Materials Science & Engineering, Nanoscience Technology Center, Advanced Materials Processing and Analysis Center, University of Central Florida, Orlando, FL, 32816, USA Advancements in chemical synthesis techniques allow for the production of improved solid rocket propellant nano-scale additives. These additives show larger burning rate increases in composite propellants compared to previous additive generations. Previous methods involved the mixing of dry powders by way of grinding or applying an ultrasonic bath to pulverize the larger agglomerates in the powder additive. Dry titania nanoparticles were mixed into hydroxyl terminated polybutadiene (HTPB) and ammonium perchlorate (AP) composite solid propellants. Relative dispersion of the nano particles was indicated in the burning rate of the propellants synthesized, but with dry powders the burning rate enhancement was around 9% with the use of 0.5% by weight of the additive. In bimodal propellants, the use of a dry titania additive showed almost no improvement on the burning rate. In addition to improving additive effectiveness, novel synthesis methods helped improve manufacturability, reduced safety risks, and maximized energy efficiency of nano-scale burning rate enhancers. Burning rate increases as high as 69% from additive mass loadings of less than 0.5% were seen in non-aluminized, ammonium perchlorate-based propellants over the pressure spectrum of 500 psi (3.5 MPa) to 2250 psi (15.5 MPa). Increases in burning rate up to 73% were seen in similarly formulated aluminized propellants. 1. Introduction Utilizing catalytic additives to enhance the burning rate of solid composite propellants has been the primary method used to tailor the burning characteristic of propellants for various purposes. A new method for synthesizing TiO 2 (titania) directly in the hydroxyl-terminated polybutadiene (HTPB) binder of an ammonium perchlorate composite propellant (APCP) has been developed recently in our laboratory. The process of a nano-assembly has shown to increase the efficiency of titania as a burning rate catalyst in APCPs, as described in more detail below. This novel method presents three main benefits compared to traditional nanoparticle additives. First, by minimizing particle/agglomerate diameter, the surface area-to-mass ratio is increased. High surface area-to-mass ratio nanoparticle additives have been shown to increase catalytic activity during ammonium perchlorate decomposition combustion (Kreitz 2010 and Li 2007). Second, by assembling the particles directly into the propellant binder the surface chemistry, dispersion, and particle topography can be controlled in such a way as to tailor performance in a propellant (Reid 2007). Finally, by synthesizing the particles directly in the propellant binder mitigates the health and safety risks associated with the handling of dry nanoparticles (Reid 2007). The combustion rate of APCPs can be significantly increased with the addition of nano-scale metals and metal oxides (Krishnan 19992). Recent research by Ramamurthy and Shrotri (1996) has shown that materials previously thought to either be ineffective or even detrimental to the combustion of solid propellants have profound effects when average particle size is reduced to the nano-scale and when such additives are synthesized to produce specific surface chemistry and morphological characteristics. Recent investigations by Reid et al. in 2007 and Fujimura and Miyake (2010) have indicated that titania acts as a catalyst and affects AP decomposition rate. The product of the new method of synthesizing titania nanoparticle has been termed a nano-assembly to describe the process by which the particles are assembled molecule by molecule in the propellant binder (Reid 2007). This new method is the latest incarnation of an increasingly effective series of procedures for introducing catalytic nanoparticles into a solid propellant matrix. The initial method of additive introduction was termed dry mixing. Dry-mixing is the process by which dry, powdered nanoparticles are mixed into the propellant slurry similar to other ingredients, i.e. without introducing any solvents or

2 dispersants. The dry titania nanoparticles were synthesized via a modified sol-gel method developed by the UCF authors at the Advanced Materials Processing and Analysis Center (AMPAC). The sol-gel method is a common chemical technique for synthesizing a variety of ceramics and metal-oxides (Hench 1990). The fundamental process involves growing a metal-oxide solid network (gel) in a colloidal solution (sol) from a chemical precursor. Spray-drying of the titania sol forms the dry titania nanoparticles. Dry-mixing was seen to cause significant agglomeration of the particles, reducing their effectiveness (Kreitz 2011a, Kreitz 2011b, and Tingming 2009). The successor to the dry mixing method is known as pre-mixing or wet-mixing, and involves first mixing the HTPB binder in a low-viscosity solvent, and then adding the nanoparticle sol. Once the solvent is evaporated off, the HTPB-additive solution is then used to form the propellant slurry similarly to unadulterated HTPB. Additives dispersed with the pre-mixing method exhibited better dispersion properties and increased performance compared to standard dry mixing (Kreitz 2011b)..Nano-assembly is the latest and most effective generation of these nanoparticle synthesis techniques. By synthesizing the particles in-situ with the HTPB binder, agglomeration is minimized and a high-quality nanoparticle dispersion in the binder, without settling or segregation, is achieved. The nano-assembly method has been seen to consistently produce nanoparticles with diameters ranging from 5 to 25 nm. The fundamental nano-assembly process involves using a titania molecular precursor which reacts with the HTPB binder in solution. Hydrolyzing the precursor then grows titania nanoparticles which remain encapsulated and stabilized by the HTPB. The solvent is then removed, and the HTPB with suspended titania nanoparticles is mixed into the propellant slurry. The additive particles are continually encased in a protective layer of HTPB and retain their small size and uniform dispersion during formation of the end-stage propellant matrix. A more thorough analysis of the nanoassembly process is described in a paper in progress by the authors at UCF. These novel additives have been evaluated in the authors propellant mixing and testing laboratory at TAMU. Laboratory-scale testing allows researchers to continuously produce new propellant formulations while maintaining an intimate knowledge of the propellant properties during each stage of mixing. Over the past several years, the authors have refined and optimized propellant mixing and testing procedures. This paper describes the propellants tested, results from the burning rate tests of the new additives, and a discussion on the importance of the findings. 2. Methods The primary focus of this paper is on low-smoke, non-aluminized propellants; however, to provide evidence of the effectiveness in aluminized propellants, several aluminized propellants were tested and produced similar results to their non-aluminized counterparts. Propellants investigated in this paper are based on a baseline formulation containing five key components: oxidizer, fuel-binder, plasticizer, bonding agent, and curative (Thakre 2010 and Kuboto). Ammonium perchlorate (AP) is used as the oxidizer. R45-M hydroxyl-terminated polybutadiene (HTPB) acts as both fuel source and binder; however, AP can act as its own fuel source burning as a monopropellant. Dioctyl adipate (C 22 H 42 O 4 ) is the plasticizer used to decrease the viscosity of the propellant slurry, HX-752 Dynamar acts as a bonding agent helping strengthen the propellants through cross-linking, and isophorone diisocynate (IPDI) is the curative. Propellants formulated in the author s lab were by techniques developed by Stephens et al. in Two classes of mixtures were studied herein: non-aluminized APCPs and aluminized APCPs. Non-aluminized propellants contained an 85% by mass solids loading with pure AP as the solid to mimic the formulation used for practical applications. Aluminized APCPs contained 83% by mass solids loading with both AP and Al. A bimodal AP distribution with peaks at 20 μm and 250 μm and a ratio of 30/70 fine AP to coarse AP was used for both the aluminized and non-aluminized propellants. The mean and median of the coarse AP were measured to be μm and μm, respectively, with a standard deviation of 108 μm, a right skewness of 1.42, and 5.83 leptokurtic. The mean and median of the fine AP were measured to be μm and μm, respectively, with a distribution standard deviation of 23 μm, a right skewness of 2.06, and 5.05 leptokurtic. It is important to record the distribution of AP particle sizes, because consistent sizing helps to ensure repeatability over multiple batches of propellant. Figure 1 depicts the particle size distribution for the AP particles used and was provided by Fluid Energy. 2

3 Figure 1. Particle size distribution for the AP used in the study. The image on the left reflects the distribution of the 200 micron AP, and the image on the right depicts the distribution of the 20-micron particles. AP was purchased from FireFox Enterprise and milled by Fluid Energy to the 20-micron size. Aluminized formulations contained 16% by mass aluminum with a mean particle diameter of 24 μm. Titania effectiveness was tested at 0.3% by mass in the non-aluminized propellants and 0.4% by mass in the aluminized propellants. As titania itself is most likely acting only as a catalyst and not contributing energy to the combustion reaction, but rather increasing the molecular weight of the exhaust, it is important to maximize its effectiveness while minimizing its mass footprint in the propellant mixture. Table 1 summarizes the mixtures evaluated in this paper. Table 1. APCP mixture summary and burning rate correlation coefficients a and n. Non-Aluminized APCPs Aluminized APCPs Mix Percent Additive Additive Type Additional Information Base % Baseline Ti-DP % Dry Powder Ti-PM % Pre-Mix Ti-NA % Nano-Assy No heat treatment Ti-NA % Nano-Assy 100C for 12 hours Ti-NA % Nano-Assy 75C for 12 hours Ti-NA % Nano-Assy 80% Monomodal Base-A01 0.0% Baseline Ti-DP-A01 0.4% Dry Powder Ti-PM-A01 0.4% Pre-Mix Ti-NA-A01 0.4% Nano-Assy a n AP size distribution is an occasionally overlooked parameter but has a dramatic effect on the effectiveness of titania nanoparticles. Previous research has shown that overall AP size distribution, mass-loading, and modality all play roles in how well titania can act as a burning rate enhancer (Kreitz 2011a and Kreitz 2011b). To determine the appropriate formulation for an aluminized propellant, the ProPep (Propellant Performance Evaluation Program) software was used to determine the best ratio of aluminum to AP. As a result, the propellant with the highest adiabatic flame temperature for various solids loadings was chosen to optimize performance, but the limitation of the maximum percentage of solid material restricts the capability to manufacture a viable propellant. Thus, APCPs with aluminum containing 83% solids loading were studied as they provide an effective propellant formulation in terms of solids loading and approach the fueloxidizer ratio producing the maximum adiabatic flame temperature. A trend for flame temperature calculations as a function of solids mass loading was generated using the data taken from the ProPep code and shows good agreement with published data (Kubota). A new batch of AP was used for this study; therefore it was important to test new baselines as past experience has shown the effectiveness of catalytic nano-metal oxides to be significantly affected by 3

4 mean AP particle size and distribution (Kreitz 2011b). Frazier s dissertation was on the development of a model that will help predict the influence of nanoparticles on the burning rate of composite propellants. The propellants tested in this paper exhibit the highest concentration of nano-assembly titania to date at 0.3% to 0.4% by mass in the propellant matrix. Previous generations of the nano-assembly method exhibited significant changes to the propellant mechanical properties at higher concentrations, but modifications to the method made recently by the UCF authors have eliminated these negative effects. In previous attempts to increase the additive concentration, the binder viscosity began to increase to a point where mixing became unfeasible, whereas in other cases the cured propellant was either too malleable or too brittle to be effectively tested. The latest nano-assembly method results in a binder-additive complex with nearly identical mixing properties to unadulterated binder and produces propellants nearly identical to baselines with regard to mechanical properties. Details on the mixing and burning rate results are presented in the sections to follow. 3. Results and Discussion Propellant burning rates plotted against baseline formulations showed a marked increase in average burning rate over the pressure spectrum tested. According to St. Robert s law the propellants will have a linear trend when plotted on a log-log plot and can be found in the work done by Sutton and Biblarz. Nano-assembly additive-containing propellants presented significantly higher burning rates at equal or lower concentrations when compared to propellants containing pre-mixed or dry powder nanoparticles. Figure presents burning rate data for 85% bimodal propellants containing no aluminum. The nano-assembly propellant showed an average increase in burning rate of 66%, whereas the dry-powder and pre-mixed propellants only presented increases of 9% and 41%, respectively, at 0.3% mass loading. It is interesting to note that while the pre-mixed propellant s burning rate reaches that of the nano-assembly s at high pressures, it is much more dependent on pressure than the other propellants. A high pressure dependency, proportional to the pressure exponent n, is not always desirable as small chamber pressure fluctuations can result in thrust instabilities. The considerable change in slope is interesting as previous research by the authors with pre-mixed titania resulted in a significant upward translation of the burning rate trend without the change in slope observed here. Figure 2. Comparison of the effects of nano-scale titania synthesis method on APCP burning rate. Dashed lines represent 90% confidence intervals. Figure 3. Evaluation of the effects of heat-treating (HT) on the effectiveness of nano-assembly titania as a burning rate enhancer in APCPs. Dashed lines represent 90% confidence intervals. Previous investigations by the authors showed that seemingly insignificant changes in the additive synthesis process, such as heat treating of additives post-synthesis, can result in marked changes in additive effectiveness (Kreitz 2011b and Kreitz 2012). To evaluate the sensitivity of nano-assembly additives to heat-treating, several additive batches with different degrees of heat treating were mixed into propellants and tested. These propellants are plotted against a baseline in Figure. The three additive batches produced propellants with similar burning rates trends over the pressure spectrum tested. The average increase in burning rate ranged from 64% to 69% compared to the baseline. This relative 4

5 insensitivity to heat-treating may indicate that the effects of nano-assembly additives are somewhat less influenced by the specific propellant mixing process, compared to previous additives. The effectiveness of titania nanoparticles as a burning rate enhancer in aluminized propellants was examined as well. Aluminized propellant burning rates plotted as a function of pressure in Figure 4 show similar trends to their nonaluminized counterparts, albeit with an overall downward shift in burning rate. Comparing burning rate increases shows that the nano-assembly increases the absolute burning rate by 73%, whereas dry powder and pre-mixed aluminized propellants increase the burning rate by only 19% and 35%, respectively. Increasing the burning rate of the aluminized propellants is of interest to improve the performance of high thrust propellants and the data indicates that nano-titania can still be an effective propellant catalyst in aluminized APCPs. Figure 4. Comparison of the effects of nano-scale titania synthesis method on aluminized-apcp burning rate. Dashed lines represent 90% confidence intervals. 4. Conclusions Overall, nano-assembly titania was seen to enhance the burning rate of non-aluminized and aluminized APCPs by upwards of 60%-70% over the pressure range of 500 psi (3.5 MPa) to 2250 psi (15.5 MPa). These increases were seen at extremely low mass loadings of 0.3% by mass in non-aluminized propellants and 0.4% by mass in aluminized propellants. At equivalent mass loadings, nano-assembled titania was seen to perform significantly better than titania introduced via pre-mixing or the traditional dry mixing, which showed increases of only 35%-41% and 9%-19%, respectively. Increases of this magnitude better allow titania to enhance the performance of a solid propellant while minimizing the detrimental effects. A growing body of research supports the use of titania as a practical, effective burning rate enhancer in solid composite propellants. Titania nanoparticles exhibit a consistently upward trend in performance as average particle and agglomerate size is decreased, and the nano-assembly method is able to produce the smallest particles with the least agglomeration in APCPs to date. Future research will involve testing titania-containing propellants with a wider range of oxidizer mass-loading and propellants manufactured in large-scale settings, as well as a more thorough investigation into the effects of nanoassembled titania in aluminized propellants. Additionally, there exists the potential for future research into alternative metal-oxide additives or atomically doped titania synthesized via the nano-assembly method. Acknowledgements The authors would like to thank Fluid Energy for the particle processing and sizing performed on the ammonium perchlorate use in the research. Funding for this research came in part from the National Science Foundation, Grant Number EEC and from the Texas A&M Engineering Experiment Station. 5

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