- 摘要Abstract
- 关键词Keywords
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1 Introduction
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2 Experimental
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2.1 Materials
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2.2 Preparation of CL⁃20/H2O2
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2.3 Mechanism Experiment of Solid⁃state Phase Transition
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2.4 Characterizations
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3 Results and Discussion
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3.1 Crystallization of CL⁃20/H2O2
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3.2 Phase Transformations of CL⁃20/H2O2 with Elevated Temperature
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3.3 Insight Mechanism of CL⁃20/H2O2 Formation
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4 Conclusions
- References
1 Introduction
Energetic materials are extensively used for a variety of military purposes,industrial applications and aerospace fields.High energy density materials (HEDMs) with desired properties are already attracting wide attention in recent decade
s[1,2,3,4] .The performance of HEDMs is dominated by detonation pressure(p) and velocity(D),which are connected with density and oxygen balance (OB)[5,6] .Recent concerns about the level of environmental compatibility of energetic materials have been focused on the green energetic materials based on a good OB[7,8] . Many attributions were made to tune these materials through the physical and chemical approaches. In general,load oxidizer such as ammonium nitrate (AN),hydrazine nitroform (HNF),ammonium dinitramide (ADN) and ammonium perchlorate (AP) in energetic materials to enhance the explosive performance.However,this method would decrease the loading of explosives. Therefore,another way is to design and synthesize novel energetic compounds with a good OB,typical nitrogen⁃rich heterocycles such as tetrazole,triazole, furazan,and tetrazine derivatives[9,10] .However,the exploitation of novel energetic materials is a long⁃term challenge with remarkable obstacle. Current interest has been focused on the development of host guest energetic materials,such as hexanitrohexaazaisowurtzitane (CL⁃20)/N2 O host⁃guest energetic material and CL⁃20/H2 O2 host⁃guest energetic material(CL⁃20/H2 O2 ),which can improve the explosive performance significantly[11,12] .As is well known, CL⁃20 has been widely studied as one of the most powerful commercially available explosives.Given an insight into the lattice packing model of CL⁃20, the instinct cavities can be used to insert the specific small molecules,such as
H2 O,CO2 andN2 O[11,12,13] . As a result, the CL⁃20/H2 O2 attracted our attentions. Matzger incorporatedH2 O2 into CL⁃20 crystal by solvent crystallization, which improved the OB and the crystal density. However,the high concentrationH2 O2 (98%) was used as a solvent, which was a extremely dangerous method. Therefore this synthesis route appeared to be difficult in a large number production of CL⁃20/H2 O2 . Therefore, it is urgent to develop a convenient and safe method to prepare CL⁃20/H2 O2 .In this study, a safe and mild method was developed to prepare the CL⁃20/
H2 O2 . Especially, urea hydrogen peroxide (UHP) is adopted to replace concentratedH2 O2 . The structure determination, morphology characterization, thermal behaviour, sensitivity and phase transition of this complex were carried out. Furthermore, the mechanism was examined by PXRD and Raman spectra in detail. This study provides an effective method which encapsulating the specific molecules in the lattice cavity to design high⁃performance energetic materials.2 Experimental
2.1 Materials
Raw CL⁃20 was provided by the Institute of Chemical Materials, Chinese Academy of Engineering Physic (CAEP). Acetonitrile (C
H3 CN, 99.9%, Superdry, dried in the 4Å molecular sieve), was provided by J&K Chemical Reagent Factory. Anhydrous ether (99.5%) was purchased from Chengdu Kelong Chemical Reagent Factory. Urea Hydrogen peroxide (UHP, 97%) and molecular sieve were purchased from Aladdin industrial corporation Chemical Reagent Factory.2.2 Preparation of CL⁃20/H2O2
According to the properties of
H2 O2 ,ether was applied to extract hydrogen peroxide from UHP at low temperature,since it had a high solubility for hydrogen peroxide but not for urea and low boiling point which is easy to volatile.In addition,in this work,it was a good selection to adsorb the solution by molecular sieve.CL⁃20/H2 O2 was obtained until the solvent disappeared and there were no by⁃products.The preparation of CL⁃20/
H2 O2 was carried out at 0-5 ℃.As shown in Fig.1a,UHP(5 g)was added into anhydrous ether (20 mL) and exhaustively stirred for 3 h at 0 ℃.ThenH2 O2 was extract by filter operation,as depicted in Fig.1b,10 mLH2 O2 extract was injected to a glass vial which was loaded with the solution of ε⁃CL⁃20 (0.2 g) in dry acetonitrile (1 mL).As seen in Fig.1c, the vial was put into a vacuum dryer with molecular sieves at 0-4 ℃ and -0.06 - -0.05 MPa. CL⁃20/H2 O2 was obtained until the solution evaporated completely.2.3 Mechanism Experiment of Solid⁃state Phase Transition
To verify the formation of CL⁃20/
H2 O2 via a solid⁃state phase transition,mechanism experiments were carried out.Experiment A: As shown in Fig.2a, 0.3 g ε⁃CL⁃20 was dissolved into 600 μL acetonitrile and the solution was added into vial, then 10 mL
H2 O2 was loaded into the vial. The vial was put into a vacuum dryer with molecular sieves to volatilize (the temperature of the vacuum dryer was remained at 0-4 ℃ and the pressure of the vacuum was kept at -0.06 - -0.05 MPa).
a. experiment A
b. experiment B
c. experiment C
Fig. 2 Mechanism experiments of CL⁃20/
H2 O2 solid⁃state phase transitionExperiment B: As shown in Fig.2b,0.3 g CL⁃20/C
H3 CN was loaded in dense gauze and was suspended in the mouth of vial. And 10 mLH2 O2 extract was loaded in vial, then the vial was put in a vacuum dryer in the same condition as experiment A.Experiment C: As shown in Fig.2c, a vial loaded with 10 ml
H2 O2 extract was placed into a vacuum dryer to volatilize at 0-4 ℃ and -0.05- -0.06 MPa. WhenH2 O2 extract was less than half in vial, the vial was taken out, 0.3 g CL⁃20/CH3 CN was placed in a dense gauze and suspended into the mouth, and then put back. Keep evaporating until the disappearance of the solution.2.4 Characterizations
PXRD were recorded on a Bruker D8 Advance X⁃ray diffractometer equipped with Cu K α radiation source (40 kV, 40 mA). Raman were conducted on a Renishaw (UK, model InVia) with the 532 nm laser excitation. The thermal behavior was investigated at 25- 300 ℃ through the TG⁃DSC, the sample was heated with a heating rate of 10 ℃·mi
n-1 under the nitrogen flow. Microstructures of CL⁃20/H2 O2 were collected on a scanning electron microscope on an Apollo 300 operating at an acceleration of 3 kV. The crystal morphologies were observed via ZEISS 2000⁃C optical microscope in reflection mode. The polymorph transitions were analyzed by in situ high temperature XRD with a temperature programming. The scanning data were collected during temperature from 30- 185 ℃ in an interval of 5 ℃ at a fixed heating rate of 5 ℃·min-1 . At last, the temperature was reduced to 30 ℃ and the final scanning process was carried out. Moreover, the polymorphic contents of γ⁃CL⁃20 were quantified by Topas software[15] . The impact sensitivity was according to the GJB772A-1997 standard method 601.2. Conducted by small⁃scale impact drop testing with a 2 kg drop mass on approximately 30 mg samples, which was determined statistically with the drop height of 50% explosion probability (H50 ). The friction sensitivity was determined with a WM⁃1 type friction sensitivity instrument according to GJB⁃772A-1997 standard method 602.1. Measured with 1.5 kg pendulum mass on 20 mg sample. For comparison, the sensitivity of raw material ε⁃CL⁃20 was also tested.3 Results and Discussion
3.1 Crystallization of CL⁃20/H2O2
The PXRD pattern of the experimental CL⁃20/
H2 O2 is shown in Fig.3a, which mathches with the simulated pattern of CL⁃20/H2 O2 (CCDC: 1495520). Additionally, the experimental data of CL⁃20/H2 O2 are in accordance with those of α⁃CL⁃20 (CCDC: 1495519) which indicate that experimental sample of CL⁃20/H2 O2 and α⁃CL⁃20 have the same space group of Pbca and exhibit in a rhombic packing[11] . Moreover, Raman spectra of the CL⁃20/H2 O2 , α⁃CL⁃20 and raw materials (UHP and ε⁃CL⁃20) are displayed in Fig.3b and Fig.3c. The spectrum of the experimental CL⁃20/H2 O2 is similar to that of α⁃CL⁃20 and is different from that of ε⁃CL⁃20. The strong peak of experimental CL⁃20/H2 O2 at 3557 cm-1 could be attributed to the stretching vibration of O—H bond assigned toH2 O2 . In contrast with α⁃CL⁃20, O—H vibration at 3610 cm-1 is assigned toH2 O. Moreover, the characteristic peak of O—O bond stretching vibration for sample could be seen at 866 cm-1 in the experimental CL⁃20/H2 O2 . Compared with that of UHP (located at 871 cm-1 ), the peak decreases approximately at 866 cm-1 , which is caused by the hydrogen bond betweenH2 O2 and CL⁃20. The results of Raman spectra are in good agreement with that of CL⁃20/H2 O2 in reference[11].a. PXRD patterns
b. Raman spectra
c. zoomed in (500 - 1000 c
m-1 ) Raman spectraFig. 3 Identification of obtained CL⁃20/
H2 O2 The thermal behavior of experimental CL⁃20/
H2 O2 and raw material ε⁃CL⁃20 are shown in Fig.4a and Fig.4b. Compared to ε⁃CL⁃20,the TG analysis of experimental CL⁃20/H2 O2 shows a remarkable mass loss about 3.4% at 160-167.3 ℃. Corresponding to the DSC profile,there are two exothermic peaks (162.5,252.0 ℃) and a sharp endothermic peak at 167.3 ℃ for CL⁃20/H2 O2 . In contrast with ε⁃CL⁃20, there is no mass loss before 200 ℃ in TG curve,and its DSC profile displays an endothermic peak at 166.9 ℃ and one exothermic peak at 238.7℃[16] . The mass loss is confirmed to beH2 O2 and the stoichiometric ratio of CL⁃20 toH2 O2 is 2∶1 (calculated value:3.7%). TheH2 O2 molecules embedded in voids of CL⁃20 crystal lattice firstly decomposes at 165 ℃, followed by a phase transition to the γ⁃CL⁃20 at 167.3 ℃ (supported by in situ PXRD below).H2 O2 is unstable and easily broken down in the air. However, the embedding ofH2 O2 into the capsule⁃shaped voids of CL⁃20 could delay the decomposition, in other words, theH2 O2 in the voids is more stable than that in air. Additionally, the structure stability and the decomposition temperature of CL⁃20/H2 O2 are higher than ε⁃CL⁃20, due to the incorporation of guest molecule and the change of lattice packing.The SEM images of CL⁃20/
H2 O2 are showed in Fig.5, which revealed the three⁃dimensional porous distribution on the surface and the inner of experimental CL⁃20/H2 O2 randomly. This may result from the embedding ofH2 O2 molecule. In addition, there are some cracks on the surface of the sample. The thermal phenomenon of CL⁃20/H2 O2 have a higher decomposition temperature which may be related to its porous microstructure of CL⁃20/H2 O2 . In addition, the appearance of small inhomogeneous holes and cracks may be due to its growth mechanism.Table 1 shows the impact and friction sensitivity of ε⁃CL⁃20 and CL⁃20/
H2 O2 . The 50% impact height (H50 ) of CL⁃20/H2 O2 is 13.8 cm,close to the raw material ε⁃CL⁃20 with 14 cm. The friction sensitivity of CL⁃20/H2 O2 is consistent with that of ε⁃CL⁃20 which all 100% ignited. The results reveal that CL⁃20/H2 O2 and ε⁃CL⁃20 exhibited similar sensitivity.3.2 Phase Transformations of CL⁃20/H2O2 with Elevated Temperature
The phase transitions of CL⁃20/
H2 O2 with increasing temperature were investigated by in situ high temperature XRD and the results are presented in Fig.6a. From ambient temperature to 140 ℃, the results of PXRD of CL⁃20/H2 O2 agree well with the simulated results of CL⁃20/H2 O2 [11] . When the temperature is up to 155 ℃, the intensity of the weak peaks at 12.8° (triplet) and 15.5° increases. With the increase of temperature, the intensity of 12.8° (triplet) and 15.5° becomes stronger which is attributed to the emergence of the γ⁃CL⁃20 phase gradually. In addition, the characteristic peaks of CL⁃20/H2 O2 (12°,15.1°,17.9° and 18.8°) become weak and disappear. And the rate of transition becomes large during 155-165 ℃. Until to 185 ℃,CL⁃20/H2 O2 pattern is in accordance with the simulated pattern of γ⁃CL⁃20 completely. For ε⁃CL⁃20, the result is identical to the previous work (Fig.6b)[15] . The phase contents of γ⁃CL⁃20 of CL⁃20/H2 O2 and ε⁃CL⁃20 with elevated temperature are depicted in Fig.6c. Compared to ε⁃CL⁃20,CL⁃20/H2 O2 has a higher phase transition temperature, indicating that CL⁃20/H2 O2 is more stable than ε⁃CL⁃20. Furthermore, it would entirely transform into γ⁃CL⁃20 at 185 ℃, because theH2 O2 resolves intoO2 andH2 O when temperature is over 140 ℃, and produce interspace in the crystal structure to make the transition easier.[12,17] 3.3 Insight Mechanism of CL⁃20/H2O2 Formation
In order to make the formation of the CL⁃20/
H2 O2 energetic material clear, Raman spectra were utilized to analysis the crystallization process of samples. Crystallization process can be divided into 3 stages. When theH2 O2 extract was added into the CL⁃20 acetonitrile solution,crystal precipitated from the solvent, and its spectrum is shown in Fig.7a. In comparison with the literature[18] ,the crystal is found to be CL⁃20/CH3 CN and this process is defined to be stage 1.The crystal was exposed to gaseous environment gradually as solvent volatilize.The spectral analysis indicates that the composition of the solid phases starts to change and the critical state is called state 2. Peak at 2255 cm-1 dominated by CL⁃20/CH3 CN became weaker,simultaneously,two characteristic peaks at 866 cm-1 and 3557 cm-1 appeared which was attributed to the formation of CL⁃20/H2 O2 [11] . This indicates that the stage 2 is the state in which CL⁃20/CH3 CN gradually transformed to CL⁃20/H2 O2 . With the disappearance of solvent,stage 3 emerges, the spectral analysis shows that the characteristic peak at 2245 cm-1 of CL⁃20/CH3 CN has disappeared totally,peaks at 866 cm-1 and 3557 cm-1 ascribed to CL⁃20/H2 O2 has developed further. These changes suggest that the complete emergence of CL⁃20/H2 O2 .a. Raman spectra of samples from crystallization process
b. PXRD patterns of samples from crystallization process
c. morphology of CL⁃20/C
H3 CNd. morphology of CL⁃20/
H2 O2 Fig. 7 The formation process of CL⁃20/
H2 O2 PXRD was employed to detect the formation process accurately. As seen in Fig.7b,the results show a good agreement between the PXRD pattern of the samples from solvent (stage 1) and the simulated CL⁃20/C
H3 CN (CCDC: 1569059).Once the crystal exposed to gas environment (stage 2),the PXRD pattern begins to change. Diffraction peak at 8.94° of CL⁃20/CH3 CN becomes increasingly weaker as the solvent volatilizes,and the diffraction peaks referred to CL⁃20/H2 O2 (a triplet around 13.7° and a peak of 17.62°) can be observed clearly.As the disappearance of the solvent (stage 3),diffraction peaks of CL⁃20/H2 O2 can be observed merely[18] . The changes of PXRD are in accordance with Raman results,revealing that the CL⁃20/CH3 CN is a key intermediate for the formation of CL⁃20/H2 O2 .Along with morphological change of the sample in preparation process, CL⁃20/C
H3 CN as an intermediate and preparation process can be concluded as a solid⁃state phase transition. The image of sample at the end of stage 1 is shown in Fig.7c. It is a transparent sample with a colourless cubic crystal. By contrast, the crystal from stage 3 (Fig.7d) retained original shape, however, it becomes an opaque crystal[19] . It can be implied that the CL⁃20/H2 O2 is obtained based on CL⁃20/CH3 CN without the dissolution⁃recrystallization process.To explore mechanism further,the crystal structure of CL⁃20/C
H3 CN and CL⁃20/H2 O2 were investigated (Fig.8).The formation of CL⁃20/CH3 CN depends on hydrogen bonds between CL⁃20 molecules and acetonitrile molecules, with intermolecular distances of 2.362 Å,2.573 Å and 2.411 Å,respectively.For CL⁃20/H2 O2 ,theH2 O2 molecule interaction with two CL⁃20 molecules are via hydrogen bonds, with bond lengths of 2.224， 2.294，2.224 Å and 2.259 Å,respectively. Manifestly,each hydrogen bond in CL⁃20/H2 O2 is stronger than that of CL⁃20/CH3 CN.Therefore, the transformation from CL⁃20/CH3 CN (metastable phase) to CL⁃20/H2 O2 (stable phase) is a spontaneous process thermodynamically[20] . It was a convincing evidence that the formation of CL⁃20/H2 O2 via an intermediate of CL⁃20/CH3 CN.a. hydrogen bonding interaction between CL⁃20 and C
H3 CNb. crystal packing diagram in the unit cell of CL⁃20/C
H3 CN along the b⁃axisc. hydrogen bonding interaction between CL⁃20 and
H2 O2 d. crystal packing diagram in the unit cell of CL⁃20/
H2 O2 along the a⁃axisFig. 8 Hydrogen bonding and crystal packing diagrams of CL⁃20/
H2 O2 and CL⁃20/CH3 CNThe morphology change has implied that there is a solid⁃state phase transition between the CL⁃20/
H2 O2 and CL⁃20/CH3 CN. To make the transformation process clearer,the mechanism experiments were carried out. The samples from experiment A,B and C were tested by PXRD and Raman spectra.As shown in Fig.9a,the PXRD patterns of the samples of experiment A and C were assigned to CL⁃20/H2 O2 .However,the peaks of sample in experiment B can be well identified to β⁃CL⁃20.The results of mechanism experiments were further confirmed by Raman spectra shown in Fig.9b. In experiment A,CL⁃20/CH3 CN was soaked in solution, as the ether evaporated,phase transition occurred until it is exposed to the concentratedH2 O2 gas molecules. In experiment B,however,when the CL⁃20/CH3 CN was suspended in vial,the gas molecules were almost ether vapor,fewH2 O2 molecules exist. And in experiment C, CL⁃20/CH3 CN was surrounded by the gas moleculesH2 O2 and ether.Therefore, it can be concluded that the preparation of CL⁃20/H2 O2 is a solid⁃state phase transition process which is induced byH2 O2 gas molecules.4 Conclusions
A mild method is utilized to prepare CL⁃20/
H2 O2 by evaporation of the solution containedH2 O2 and CL⁃20 without by⁃product. It is found that CL⁃20/H2 O2 would have cause a mass loss about 3.4% around 162.5 ℃ caused by the deformation ofH2 O2 ，and the phase transitions was detected with in situ high temperature XRD. It is found that the CL⁃20/H2 O2 was stable up to 140 ℃, then converted to γ⁃CL⁃20 quickly during the temperature range of 140⁃170 ℃. And the SEM show that CL⁃20/H2 O2 by this method would have a three⁃dimensional porous structure, and it may lead to a similar impact sensitivity and friction sensitivity to ε⁃CL⁃20. Moreover, a convincing evidence given by PXRD patterns, Raman spectra and optical images pointed out that the formation of CL⁃20/H2 O2 via a metastable phase CL⁃20/CH3 CN solvate. The conditions of the phase transition was studied by the mechanism experiments, which reveal that CL⁃20/CH3 CN transformed into CL-20/H2 O2 is a process induced by theH2 O2 molecules. The simple preparation strategy of CL⁃20/H2 O2 host⁃guest energetic material makes it possible for a large⁃scale production. Furthermore, the preparation method of stable phase via a metastable phase can be a promising approach to construct a novel energetic materials.(责编： 张 琪)
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Abstract
Hexanitrohexaazaisowurtzitane (CL⁃20) with a negative oxygen balance, is currently the most powerful commercially available explosive. In this work, the CL⁃20/
摘要
六硝基六氮杂异伍兹烷（CL⁃20）是目前能量最高的单质炸药，为了进一步提高其爆轰性能，以过氧化脲作为
Graphic Abstract
图文摘要
The Pbca CL⁃20/
