ARL-TR-7910 ● DEC 2016

US Army Research Laboratory

Investigation and Characterization of 

Water-Recrystallized Croconic Acid

by Sarah Isert and Jennifer A Ciezak-Jenkins

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ARL-TR-7910 ● DEC 2016

US Army Research Laboratory

Investigation and Characterization of 

Water-Recrystallized Croconic Acid

by Sarah Isert 

Oak Ridge Associated Universities (ORAU), Oak Ridge, TN

Jennifer A Ciezak-Jenkins

Weapons and Materials Research Directorate, ARL

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December 2016

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January–October 2016

4. TITLE AND SUBTITLE

Investigation and Characterization of Water-Recrystallized Croconic Acid

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6. AUTHOR(S)

Sarah Isert and Jennifer A Ciezak-Jenkins

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W911NF-16-2-0030

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US Army Research Laboratory

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Aberdeen Proving Ground, MD 21005-5069

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ARL-TR-7910

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14. ABSTRACT

A high-pressure phase of croconic acid is being investigated as a novel energetic material. When pressed at high pressures 

(>5.5 GPa), as-received croconic acid transforms into a β-phase that is recoverable to ambient conditions. This phase has a 

higher density and heat of combustion than the as-received material. Rectangular crystals, which are darker than the original 

powder, can be observed in the recovered material and are thought to indicate formation of the β-phase. When croconic acid is 

recrystallized from water at ambient conditions, similar appearing crystals are observed. We performed experiments to 

investigate the plausibility of recovering the β-phase croconic acid from water recrystallization as an alternative to the highpressure synthesis. Thermal analysis, bomb calorimetry, X-ray diffraction, and Raman spectroscopy were performed on waterrecrystallized croconic acid to determine whether or not it is the β-phase. We found that the water-recrystallized material is 

not the β-phase and at present high pressures is still required for β-phase formation.

15. SUBJECT TERMS

extended solid, croconic acid, high pressure, C5H2O5, water-recrystallized

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Jennifer A Ciezak-Jenkins

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Contents

List of Figures iv

List of Tables iv

Acknowledgments v

1. Introduction 1

2. Methods 2

2.1 Pressing 2

2.2 Water Recrystallization 3

2.3 Standard Analytical Tests 3

2.4 Computational 4

3. Results 4

3.1 Computational Results 4

3.2 Experimental Results 8

3.2.1 Pressing AR CA 8

3.2.2 Water Recrystallization of CA 9

3.2.3 Raman Spectroscopy and X-ray Diffraction 12

3.2.4 Bomb Calorimetry 13

4. Conclusions 15

5. References 16

List of Symbols, Abbreviations, and Acronyms 18

Distribution List 19
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List of Figures

Fig. 1 a) Molecular structure of CA. b) Extended structure of CA, showing 

the hydrogen bonding network between neighboring molecules. .........1

Fig. 2 Large-volume press use in β-CA production.........................................3

Fig. 3 Impetus of CA/JA2 propellant. For CA percentages less than 100% the 

propellant balance is JA2.......................................................................5

Fig. 4 Temperature as a function of added CA percentage in a 

JA2 propellant........................................................................................6

Fig. 5 Oxygen balance as a function of percent added CA in a JA2-based 

propellant ...............................................................................................7

Fig. 6 Pressed AR CA pellet, outer surface......................................................8

Fig. 7 Comparison between a) AR and b) WR CA. The former is clearly 

smaller and yellow-colored. The WR material is darker in color and 

larger. .....................................................................................................9

Fig. 8 Comparison between pressed AR CA (top) and WR CA (bottom). 

Images on the left show the outer surface of the pellet and images on 

the right, the pellet interior...................................................................10

Fig. 9 WR CA a) on the press and b) pellet interior.......................................11

Fig. 10 Raman spectra of AR CA and WR CA................................................12

Fig. 11 X-ray diffraction results for AR, WR, and pressed (mix of α- and 

β-phase) CA .........................................................................................13

Fig. 12 Heats of combustion of tested materials..............................................14

List of Tables

Table 1 JA2 reference propellant formulation ....................................................5

Table 2 Heats of combustion of various forms of CA. Heat of combustion of 

RDX is included for comparison .........................................................13
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Acknowledgments

During this project, coauthor S Isert was supported in part by an appointment to the 

Postdoctoral Research Program at the US Army Research Laboratory administered 

by the Oak Ridge Associated Universities. The authors wish to thank Dr Rose 

Pesce-Rodriguez for her assistance in use of the analytical instruments and valuable 

discussions.
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vi

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1. Introduction

Although current energetic materials provide good and reliable performance, gamechanging weapons for the future will require a new class of energetic materials. For 

example, a 40% increase in impetus is desired for the next generation of gun 

propellants to substantially increase the muzzle velocity.1 These “disruptive 

energetics” will revolutionize range and muzzle velocity of weapon systems. In 

addition to increased power, novel energetic materials should be less sensitive than 

currently used materials while being environmentally friendly and inexpensive to 

synthesize. 

A material that shows promise in the novel energetics arena is croconic acid (CA),

∗

a solid bright-yellow compound based off a 5-membered carbon ring (Fig. 1a).2

First synthesized in 1825,3 it was recently discovered that CA has the highest 

spontaneous polarization observed in organic ferroelectrics at room temperature to 

date.4 This quality is thought to be due to the unique hydrogen bonding networks; 

one network links molecules in a sheet and the other in pleated strips,2 as shown in 

Fig. 1b. 

Fig. 1 a) Molecular structure of CA. b) Extended structure of CA, showing the hydrogen 

bonding network between neighboring molecules.

Previous work done at the US Army Research Laboratory has shed some light on 

other characteristics of CA, including experimentally determining its heat of 

combustion5 and density,6 as well as further research into sensitivity,7 spectroscopy, 

and thermal properties such as melting and decomposition temperature.5 These 

studies included subjecting CA to very high pressures. At pressures between 5.5 

and 6.8 GPa, CA undergoes an irreversible phase change.6

The phase change includes density and volume changes, but is not accompanied by 

new diffraction peaks or modifications in the Raman spectrum peak distribution, 

indicating that a chemical change does not occur. The increase in density is thought 

to be due to the collapse of the hydrogen bond networks and the onset of pressureinduced polymerization. The high-pressure polymorph is recoverable to ambient 

 ∗The molecular formula for croconic acid is C5H2O5.
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conditions and is called the β-phase (the as-received [AR] material is also known 

as the α-phase).

Bomb calorimeter testing of the β-CA indicated a heat of combustion almost 80% 

higher than that of the α-CA. The high heat of combustion and low sensitivity make 

β-CA a very interesting material from the disruptive energetics point of view. 

However, at present it is difficult to make large amounts of β-CA, as the current 

synthesis method can only produce 50 mg of material per day. A proposed 

alternative synthesis method is recrystallizing CA from water, as the 

recrystallization leads to crystals similar in shape and color to those seen in the 

recovered β-CA. If the water-recrystallized (WR) material is indeed the β-phase, 

synthesis will be greatly simplified. This report describes characterization tests of 

the AR and WR CA. Computational predictions of impetus for β-phase CA are

discussed.

2. Methods

2.1 Pressing

The AR CA was pressed in a VX-5 Paris-Edinburgh cell. The press and anvils 

(tungsten embedded in stainless steel) were examined for cracks before each use. 

For a pressing cycle, the CA (about 50 mg) was measured into the disposable 

stainless steel gaskets as they were sitting on the anvil. The press was then loaded 

to 9 GPa remotely using a manually operated hydraulic pump. Ramp-up time was 

about 10 min with a 30-s rest period every minute to allow the system to equilibrate. 

The material was pressed at 9 GPa for 24 h. At that point, the press was gradually 

unloaded over a period of 10 min. The pressed CA was recovered, usually in small 

pellets. A new cycle was then initiated. Figure 2 shows the large-volume press used 

in the material synthesis.
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Fig. 2 Large-volume press used in β-CA production

2.2 Water Recrystallization

Because of concerns about the ability to mass-produce β-CA using the largevolume press, we wanted to explore alternatives to synthesizing β-CA via pressing. 

While recrystallizing CA from water for another test, the resulting crystals were 

observed to be similar in shape and color to crystals seen on the surface of the 

pressed CA. As it is thought from other experiments that the darker crystals are βphase CA, we decided to investigate water-recrystallization as a possible lowpressure route to the β-phase. 

To make the WR material, 5 g of CA were dissolved in distilled water, and the 

water was then evaporated off using a hot plate at 50 °C. Complete evaporation of 

the water took 7–10 days and produced large orthorhombic crystals. The crystals 

were stored in a desiccator until they were used. Upon crushing, the darker crystals 

took on a more yellow hue, indicating that the dark color is at least partly a particle 

size effect. 

2.3 Standard Analytical Tests

Standard analytical tests were used to characterize the CA, including differential 

scanning calorimetry (DSC), thermal gravimetric analysis (TGA), bomb 

calorimetry, X-ray diffraction, and Raman spectroscopy. A TA Instruments Q500 

and TA Instruments Q2000 operating under Universal Analysis software were used 

for the TGA and DSC measurements, respectively. The DSC and TGA tests were 

used to find melting and decomposition temperatures. The DSC samples were run 

between 40 and 400 °C in sealed aluminum pans with pinhole lids to allow gaseous 
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decomposition products to escape. The TGA samples were run in open aluminum 

pans between 40 and 350 °C. 

Bomb calorimetry (Parr Instrument Company, Model 6200 Isoperibol Calorimeter) 

was performed in oxygen. Pellets of layered CA and benzoic acid were tested in 

Inconel crucibles with 5–10 experiments performed per material to ensure 

reproducibility. Bomb calorimetry was used to find the material heat of 

combustion; from this information, heat of formation can be calculated and used in 

thermochemistry codes. 

The Raman spectroscopy was performed with a dichroic micro-Raman system in a 

backscattering geometry using a 1064-nm continuous wave Coherent diode laser. 

An IsoPlane SCT320 spectrograph with an air-cooled PIXIS 400BR eXcelon CCD 

and the LightField software (Princeton Instruments) were used to collect the 

software Raman spectra. Collection times of 1 s with a laser power of 7 mW were 

typical. The sample was not observed to react with the laser. The spectra obtained 

were compared against the library spectrum for CA. These experiments were 

performed to see if any peak shifts were visible between the as-received CA and 

the water recrystallized or pressed CA that might indicate a chemical change.

2.4 Computational

In addition to experimental studies, computational techniques were used to predict 

the effect on impetus of adding CA to gun propellants. For this study, Cheetah 8.08

was used, with data from the bomb calorimetry experiments and previously 

conducted density experiments6 supplying inputs for the code. 

3. Results

3.1 Computational Results

The heat of combustion of β-phase CA was previously found to be on average 4374 

cal/g via bomb calorimetry. For comparison, the heat of combustion for 1,3,5-

Trinitroperhydro-1,3,5-triazine (RDX) is 2285 cal/g. The heat of formation was 

calculated from the heat of combustion and along with the density (2.1 g/cc) was 

used in thermochemical calculations. We also desired to see the effect of adding 

the CA to an existing propellant as an energetic additive, and performed a series of 

computations where different percentages of JA2 were replaced by CA. For 

reference, the composition of JA2 is given in Table 1. The resulting impetus was 

calculated and the results of these computations are shown in Fig. 3. The loading 

density used in the calculations was 0.25 g/cc.
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Table 1 JA2 reference propellant formulation (from Reference 9)

Constituent wt% of total

Nitrocellulose (NC) (13.4% N) 59.5

Nitroglycerin (NG) 14.9

Diethylene glycol dinitrate 24.8

Ethyl centralite 0.7

Magnesium oxide 0.05

Graphite 0.05

Fig. 3 Impetus of CA/JA2 propellant. For CA percentages less than 100% the propellant 

balance is JA2.

Figure 3 clearly shows that the impetus of a JA2/β-CA propellant increases as more 

β-CA is present in the propellant. From these results it would appear (due to the 

high heat of combustion of β-CA) that the ideal propellant formulation would be 

close to 100% CA with only enough binder present to ensure good mechanical 

properties. However, impetus is not the only consideration for a propellant. Two 

other important characteristics of a propellant are flame temperature and oxygen 

balance. Other parameters like carbon monoxide/carbon dioxide (CO/CO2) ratios 

and chemical species present in the propellant flame gases are also important, but 

were not investigated in this study.

Gun barrel erosion can, in general, be reduced by using propellants with low-flame 

temperatures.10 The high-temperature fluctuations that the gun steel is subjected to 

with every firing can cause the microstructure of the surface to change, and the steel 

becomes harder and more brittle. Chemical species from the combustion gases can 

% -Croconic Acid

0 20 40 60 80 100

Impetus (J/g)

1000

1200

1400

1600

1800

2000
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diffuse into the steel and alter its chemical properties11 and are another major cause 

of gun barrel erosion.12 An increase in temperature is not exactly proportional to an 

increase in gun barrel erosion; the chemical composition of the propellant and 

associated phenomena like heat conduction and chemistry effects on the gun barrel 

come into play, and in some propellants erosion is more dependent on chemical 

species than temperature.

Figure 4 shows calculated flame temperatures of β-CA/JA2 propellants as a 

function of composition. Predicted temperatures for these propellants, even at 

moderate CA inclusion percentages, are high and problematic for traditional gun 

barrel designs. Flame temperatures as low as possible to minimize gun barrel 

erosion are required, but adding CA produces the opposite effect. Additionally, it 

is known that propellants producing more CO/CO2 are more erosive than those that 

produce large amounts of nitrogen gas (N2), unless flame temperatures are low. The 

combination of high flame temperatures and large amount of CO/CO2 production 

raises the specter of a CA-based propellant being highly erosive.

Fig. 4 Temperature as a function of added CA percentage in a JA2 propellant

% -Croconic Acid

0 20 40 60 80 100

Temperature (K)

3500

4000

4500

5000

5500
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Another factor that limits the amount of CA that can be added to a propellant is 

oxygen balance. Oxygen balance is the ratio of the number of oxygen atoms a 

molecule contains compared to the number of oxygen atoms it needs to completely 

oxidize the carbon to CO or CO2, the hydrogen to water, and any metal to metal 

oxide. A positive oxygen balance indicates a molecule contains excess oxygen, and 

a negative oxygen balance indicates a molecule requires more oxygen to be in the 

balanced state. Many propellants and explosives operate in the negative oxygen 

balance or CO-balanced regime. However, if the oxygen balance is too negative, 

the propellant will not burn cleanly and will leave behind solid carbon.13

The char-free oxygen balance threshold varies with propellant type, composition, 

and operating pressure; for example, the threshold for an NC/NG-based propellant 

was found to be around –60% depending on exact propellant formulation and 

combustion pressure.14 Another report stated that condensed phases, typically solid 

carbon, tend to appear when the oxygen/carbon atomic ratio is less than 0.9.1

Croconic acid itself has an oxygen balance of about –67%. The addition of JA2 

(oxygen balance ~ –27%) will increase the oxygen balance somewhat (Fig. 5). 

Assuming the oxygen balance requirements for noncharring combustion are similar 

to that of a typical double-base propellant, the maximum allowable percentage of 

CA in a JA2-based propellant that would limit char formation would be about 82%. 

Propellants with larger percentages of CA would theoretically have a high impetus 

(impetus at 55% β-CA is about 1500 J/g), but residual char and temperatures well 

above 4000 K (according to the calculations) could limit their usability in guns.

Fig. 5 Oxygen balance as a function of percent added CA in a JA2-based propellant

% -Croconic Acid

0 20 40 60 80 100

Oxygen Balance(%)

-65

-60

-55

-50

-45

-40

-35

-30
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3.2 Experimental Results

3.2.1 Pressing AR CA

As previously reported, when CA is pressed it undergoes a density change from 

1.9 g/cc to 2.1 g/cc,6 and the heat of combustion increases by about 80%. This new 

high-pressure phase is known as the β-phase, while the original material is known 

as AR CA or the α-phase. Both the α- and β-phases are insensitive to friction, 

impact, and electrostatic discharge (ESD) stimuli, and do not display any peculiar 

sensitivities.7 It was found in diamond anvil cell experiments that the CA transitions 

from a yellow to a yellow-red color under the influence of pressure.6 Similar results 

were seen in the large-volume press. Figure 6 shows the outer surface of a pressed 

AR CA pellet. There are many small, dark crystals visible on the pellet surface. 

Crystals of similar sizes, shapes, and colors are visible on the interior surface of the 

pellet.

Fig. 6 Pressed AR CA pellet, outer surface

Previous experiments showed that pressures between 5.5 and 6.8 GPa are required 

to induce polymerization.6 The high pressures involved limit what equipment can 

be used for the synthesis. The polymerization pressures were found in a diamond 

anvil cell; however, due to the kinetic factors controlling the transition time at larger 

volume, it is necessary to overdrive the pressure in the large-volume cell in an effort 

to in push the transition to completion in a reasonable timeframe. Hence we press 

the CA at 9 GPa. The high pressures required can typically only be achieved using 

small anvil diameters and therefore small sample sizes. The average amount of 

material produced in the large-volume cell is 50 mg/cycle; since a cycle is 24 h it 
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takes 20 working days to produce a gram of material, and a year’s output (assuming 

a 5-day work week and no breaks) would be 13 g. Obviously other methods would 

be needed to produce the material in production quantities.

The long press cycle time is disadvantageous; however, the amount of time needed 

to press the material before it is converted to the β-phase has not yet been 

characterized. The long press time may not be necessary, and in that case, the output 

could be increased. However, if 24 h is close to the optimum time, or if an even 

longer time would be better, there would be an even more pressing need to move 

to another synthesis method.

3.2.2 Water Recrystallization of CA

A proposed non-pressure method of synthesizing β-CA was recrystallizing α-CA 

in water. When CA is recrystallized in water, crystals similar in color and shape to 

those visible on the surface of the pressed AR CA are observed. The WR CA was 

tested using bomb calorimetry to determine if it was a higher-energy form of CA 

or even the β-phase. Raman spectroscopy on the WR CA was performed to see if it 

was still CA or if it had undergone a chemical change (to a hydrate, for example)

as part of the recrystallization process. Finally, we pressed the WR CA in the large 

volume press and used bomb calorimetry to investigate the difference in heat of 

combustion between the pressed WR CA and the β-CA.

Recrystallization changes the CA morphology. The AR powder (Fig. 7a) is 

composed of small, irregularly shaped yellow particles, while the WR material 

(Fig. 7b) is darker in color and visibly crystalline. Yellow powder considered to be 

CA dust is visible on the WR material. Throughout this study we noticed that the 

color may be in part a particle size characteristic. When the CA was in larger 

crystals it was a dark red-orange in color, compared to the yellow of the small 

particle size CA.

Fig. 7 Comparison between a) AR and b) WR CA. The former is clearly smaller and yellowcolored. The WR material is darker in color and larger.
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3.2.2.1 Pressed WR CA Compared to Pressed AR CA

When subjected to standard pressing conditions, the pressed WR CA pellets looked 

different than the pressed AR CA pellets (Fig. 8). The exterior of the WR CA pellets 

was more uniformly dark in color, but the darker color is generally not attributable 

to clearly defined crystals, unlike the pressed AR CA pellets. The pellet interiors 

also differed. While the AR CA pellet interiors were typically lemon yellow in color 

with some pockets of darker crystals, the WR CA pellet interiors showed large 

amounts of dark marbling. A thick dark band was also visible around the perimeter 

of the WR CA pellet interior; though a similar band is visible in the AR CA pellets, 

it is not as pronounced. 

Fig. 8 Comparison between pressed AR CA (top) and WR CA (bottom). Images on the left 

show the outer surface of the pellet and images on the right, the pellet interior.
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3.2.2.2 WR Pellet Reaction

Under the same unloading conditions as the AR CA pellet, the WR CA pellets 

tended to react when pressure was released. A loud report was followed by a puff 

of CA emanating from the press. When the press was opened the pellet appeared to 

be partially reacted to a greater or lesser degree. An example of a partially reacted 

pellet on the press is shown in Fig. 9a, and the interior of a different partially reacted 

pellet is shown in Fig. 9b. 

Fig. 9 WR CA a) on the press and b) pellet interior

In Fig. 9a one can see a reacted core surrounded by unreacted material. The inner 

gasket is distended, with the distension pointing to about the 1 o’clock position. 

The outer gasket is broken at about the 12:30 position; however, such breaks 

commonly occur regardless of the material being pressed. The center of the pellet 

is almost completely charred. Such complete reactions were rare; most reactions 

were partial as seen in Fig. 9b. For partial reactions, while there are black sections 

of char, the pellet remains mostly unchanged. Microscopic examination of the pellet 

interior showed that the char appeared in areas with high concentrations of dark 

crystals. In some cases one could see where the char was extending into the dark 

areas indicating that the latter are a precursor to the former. 

As the WR CA reacted on the press when the AR CA did not, we also wondered 

whether or not the percentage of β-CA was higher or if the WR CA, when pressed, 

had transformed into a different material. Raman spectroscopy, X-ray diffraction, 

bomb calorimetry, and thermal tests were performed to investigate chemical 

transformations, heat of combustion, and decomposition onset temperature, 

respectively.
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3.2.3 Raman Spectroscopy and X-ray Diffraction

Because the pressing was not expected to cause any chemical changes, the reaction 

of the WR CA on the press encouraged testing to see if recrystallization in water 

produced CA or promoted the formation of another material such as a hydrate. The 

Raman spectrum of the WR CA showed several slightly shifted peaks and changes 

in peak magnitude compared to the AR CA, as shown in Fig. 10. No indications of 

added water were seen in the WR CA Raman spectrum indicating that the WR 

material is not CA hydrate or an inclusion compound. X-ray diffraction on the AR 

CA and WR CA was collected at the X-ray Crystallography Center at the University 

of Maryland and is shown in Fig. 11. Although subtle changes are observed in the 

patterns such as peak splitting and intensity changes, these suggest a change in the 

local molecular ordering (such as a modification to the hydrogen bonding 

sublattice) rather than a distinct phase change. This is supported by the Raman data. 

Fig. 10 Raman spectra of AR CA and WR CA
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Fig. 11 X-ray diffraction results for AR, WR, and pressed (mix of α- and β-phase) CA

3.2.4 Bomb Calorimetry

Bomb calorimetry was performed on both the pressed and unpressed WR CA. The 

results were compared with those previously found for RDX and both α- and 

β-CA. No difference was found in the heat of combustion between WR CA powder 

and α-CA powder, and while the pressed WR CA had a higher heat of combustion 

than the unpressed WR CA powder, the difference (while statistically significant) 

was about 6% between the heats of combustion compared to an 80% difference 

between the α- and β-CA heats of combustion. Heats of combustion for all materials 

tested are shown in table form in Table 2 and graphically in Fig. 12.

Table 2 Heats of combustion of various forms of CA. Heat of combustion of RDX is included 

for comparison

Material Average heat of combustion

(cal/g)

RDX 2243a

α-CA powder 2416a

Water-recrystallized CA powder 2374

Pressed water-recrystallized CA 2514

β-phase CA 4374 a

See Reference 5.
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Fig. 12 Heats of combustion of tested materials

The similar heats of combustion between the α-CA powder and the pressed WR 

CA pellets were a surprise, because the WR CA pellet reacted on the press where 

the AR CA pellet did not, and an increased percentage of β-CA in the pressed WR 

CA had been thought to be the cause. Thermal tests (DSC and TGA) did not indicate 

a lower decomposition temperature for the pressed WR CA compared to the pressed 

AR CA. So why would the pressed WR CA pellet react where the pressed AR CA 

did not? And why did pressing the WR CA not result in the high heat of combustion 

that was achieved by pressing the AR CA?

A possible answer to both questions could be due to powder grain morphology. The 

AR CA powder, as shown in Fig. 7, contains very small, nonuniform particles. The 

WR CA powder, on the other hand, has large particle sizes. It is thought that the 

transition between the α-phase and the β-phase requires the formation of a transient 

“amorphous phase”, similar to a solid melt. Indications of an amorphous phase is 

shown in Fig. 11 where β-phase material (red trace) shows a higher background 

from low angle to about 27° in 2-Theta. While it is difficult to quantify because of 

the offset patterns, this broad background is typical of an amorphous sample. The 

small particle size of the AR CA powder means less energy is required for the 

transient stage to form. On the other hand, the larger crystals of the WR CA powder 

hinder the formation of the transient phase as the particles are large and require 

large amounts of energy for the transition. It is possible that the slight heat of 

combustion increase in the pressed WR CA powder is due to the transformation of 

the loose powder to the β-phase.

A similar theory can also start to explain why the WR CA pellets react upon 

depressurization. The large WR CA crystals begin to elastically deform and 
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plastically flow resulting in the formation of voids and stress concentrators between 

the crystals if they do not powderize. If the pressure is released rapidly enough, the 

elastically deformed crystal “springs” back toward its initial shape, releasing 

energy from the deformed bonds in the process. This may cause hotspots and 

charring in the area surrounding the deformed crystal, which fits in with the 

observation that the black areas are often near observed crystal structures. If enough 

energy is released to partially react the area, gas formed from this reaction may be 

the cause of the distended inner gaskets. 

A large number of alternative mechanisms have been suggested in the literature for 

hot spot formation, many of which apply to materials that are being compressed. 

While the reaction of the WR CA occurs on decompression, it may be that 

compression is occurring somewhere in the pellet, such as on crystal boundaries. 

Hot spot formation methods that could apply in these cases include adiabatic 

compression of trapped gas spaces, cavity collapse, viscous or plastic heating of the 

surrounding matrix material, friction between impacting and sliding crystal 

surfaces, heating at crack tips, or heating at dislocation pileups.15

After several WR CA pellets had reacted the press unloading time was increased, 

and the pellets did not react. In this case, as pressure is slowly released, the crystals 

more gradually release the energy from the distortion and there is no appreciable 

hot spot formation. The reduced incidence of hot spots means the CA crystals are 

less likely to decompose into carbon and gas, and there is therefore no charring and 

no reaction on the press. At this point it is difficult to know what exactly is causing 

the hot spot formation, if hot spots are indeed what are causing the partial reaction 

of the CA pellets. However, the main point of interest for the current study is that 

there are other possible reasons besides the presence of β-CA for why the WR CA 

is more reactive.

4. Conclusions

Several conclusions can be drawn from the results of this study. First, our attempt 

to create β-CA at ambient pressure by recrystallizing the material from water was 

not successful. Additionally, pressing crystals of WR CA to form the β-phase was 

not successful, as shown by the bomb calorimeter data. The β-CA has a higher heat 

of combustion than the α-CA, whether the α-CA is AR CA, WR CA, or pressed 

WR CA. Further research into synthesis of β-CA is needed, but the fact that we can 

make a material with a very high heat of combustion from an inert, commercially 

available material is promising.
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5. References

1. Simmons RL. Guidelines to higher energy gun propellants. In: Energetic 

materials—technology, manufacturing, and processing. 27th International 

Annual Conference of ICT; 1996 June 25–28; Karlsruhe, Federal Republic of 

Germany. 22.21-22.16. 

2. Braga D, Maini L, Grepioni F. Crystallization from hydrochloric acid affords 

the solid-state structure of croconic acid (175 years after its discovery) and a 

novel hydrogen-bonded network. CrystEngComm. 2001;(3):27–27.

3. Gmelin L. Ueber einige merkwurdige, bei der Darstellung des Kaliums nach 

der Brunner’schen Methode, erhaltene Substanzen. Ann Phys Chem. 

1825;80:31–62.

4. Horiuchi S, Tokunaga Y, Giovannetti G, Picozzi S, Itoh H, Shimano R, 

Kumai R, Tokura Y. Above-room-temperature ferroelectricity in a singlecomponent molecular crystal. Nature. 2010;463:789–792.

5. Ciezak-Jenkins JA, Gottfried JL, Pesce-Rodriguez RA. Thermal and laserinduced shock characterization of oxocarbon acids. Aberdeen Proving Ground 

(MD): Army Research Laboratory (US); 2014. Report No.: ARL-TR-6924.

6. Ciezak-Jenkins JA, Jenkins TA. The high-pressure characterization of carbon 

oxides: croconic acid. Aberdeen Proving Ground (MD): Army Research 

Laboratory (US); 2014. Report No.: ARL-TR-6957.

7. Bukowsi EJ, Johnson EC, Ciezak-Jenkins JA. Drop weight, friction, and ESD 

test report. Aberdeen Proving Ground (MD): Army Research Laboratory (US); 

2015. Unpublished work.

8. Bastea S, Fried LE, Glaesemann KR, Howard WM, Kuo I.-FW, Souers PC, 

Vitello PA. Cheetah 8.0. Livermore (CA): Lawrence Livermore National 

Laboratory; 2016.

9. Reese DA, Groven LJ, Son SF. Formulation and characterization of a new 

nitroglycerin-free double base propellant. Propellants Explos Pyrotech. 

2014;39:205–210.

10. Morphy CC, Fisher EB. Gas chemistry effects on gun barrel erosion: a shock 

tube gun investigation. Aberdeen Proving Ground (MD): Ballistic Research 

Laboratory (US); 1982. Report No.: ARBRL-CR-00481.

11. Lawton B. Thermo-chemical erosion in gun barrels. Wear. 2001;251:827–838.
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12. Johnston IA. Understanding and predicting gun barrel erosion. Edinburgh

(Australia): Defence Science and Technology Organization; 2005. DSTO-TR1757.

13. Babuk VA. Properties of the surface layer and combustion behavior of 

metallized solid propellants. Combust Explos Shock Waves. 2009;45:486–

494.

14. Zheng W, Chen H, Li Q, Pan R, Lin X. Effect of oxygen balance on the 

formation of gun propellant residue. Int J Energ Mater Chem Propul. 

2014;13:421–434.

15. Field JE. Hot spot ignition mechanisms for explosives. Acc Chem Res. 

1992;25:489–496.
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List of Symbols, Abbreviations, and Acronyms

AR as-received

CA croconic acid

CO carbon monoxide

CO2 carbon dioxide

DSC differential scanning calorimetry

ESD electrostatic discharge

N2 nitrogen gas

NC nitrocellulose

NG nitroglycerine

RDX 1,3,5-Trinitroperhydro-1,3,5-triazine

TGA thermal gravimetric analysis

WR water-recrystallized
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