Research Progress of High Energy Density Nitrogen
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摘要: 氮在常压下是非常稳定的元素，以氮气分子形式存在。研究发现，氮在高温高压下能够形成聚合结构，这种结构具有极高的能量密度，而且分解产物为无污染的氮气，从应用角度上看，它能够作为新型环保高能量密度材料。随后，人们对其进行了大量的研究，得到了氮在高压条件下的相图，并且合成出立方偏转氮、层状聚合氮等结构。然而，纯氮聚合结构的合成条件比较严苛，在常压下很难保存。人们又转向分子结构氮和惰性气体氮化物等，希望能够得到常压下稳定的高能量密度氮结构。为此，针对目前高能量密度氮的理论和实验进展进行了简要的介绍，并对未来高能量密度氮的发展方向进行了探讨。Abstract: Nitrogen is a highly stable element that exists in the form of nitrogen molecules under ambient pressure. Researchers have found that nitrogen can form polymeric structures under high temperature and pressure, which have extremely high energy density and decompose into pollution-free nitrogen. From the perspective of application, it can be used as a new type of environmentally friendly high-energy-density material. Subsequently, a large number of studies have been conducted on nitrogen, resulting in phase diagrams of nitrogen under high-pressure conditions and the synthesis of structures such as cubic gauche nitrogen and layered polymeric nitrogen. However, the synthesis conditions for pure nitrogen polymeric structures are relatively harsh, and it is also difficult to preserve them under ambient pressure. People have turned to methods such as molecular nitrogen and inert gas nitrides in the hope of obtaining stable high-energy-density nitrogen structures under normal pressure. This article briefly introduces the current theoretical and experimental progress of high-energy-density nitrogen and discusses the future development direction of high-energy-density nitrogen.
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钝感炸药（Insensitive high explosive, IHE）的安全性能是弹药设计的重点。以三氨基三硝基苯（TATB）为基的钝感炸药在国防领域有着广泛应用，PBX-3炸药为我国研制的含有少量奥克托金（HMX）的TATB基新型高能钝感塑性黏结炸药，开展其性能研究对炸药的应用和改进具有重要意义。
PBX-3炸药含有少量HMX，冲击起爆感度更高，当入射压力为6.754 GPa时，炸药样品在9.439 mm处就开始稳定爆轰，较TATB基PBX9502炸药和JB-9014炸药具有更优异的爆轰性能和冲击感度。张琪敏等[1]研究了纯TATB基钝感炸药在入射压力为11.330～14.180 GPa范围内的冲击起爆反应增长规律，结果表明，当入射压力达到11.334 GPa时，炸药样品在9.590 mm处才开始稳定爆轰。同时，PBX-3炸药的钝感性能优于HMX基PBX9501炸药和JOB-9003炸药，因此制作加工和运输使用更加安全。
国内外研究炸药性能常用的测试方法有锰铜压力计法、PVDF压电传感器法、组合式电磁粒子速度计测试技术和激光干涉测试技术等。锰铜压力计的响应时间长，测量精度受到压阻系数、绝缘保护膜等因素影响。PVDF压电传感器的测试上限为35 GPa，当压力大于10 GPa时不能忽略温度的影响。组合式电磁粒子速度计技术和激光干涉技术是近些年国内外广泛使用的测试技术[2-4]。激光干涉测试技术只能测量冲击加载界面的粒子速度，对于普通圆柱形炸药样品，无法在一次冲击实验中得到不同深度处样品的粒子速度，为此炸药样品的形状需特殊加工（如台阶形或楔形），并且炸药到爆轰的距离和时间需要采用其他测试方法。张涛等[5]采用激光干涉测速系统得到了冲击波进入炸药特定深度位置处的波后粒子速度剖面，同时结合高速扫描相机得到了炸药样品到爆轰的距离。组合式电磁粒子速度计具有实验操作简便、响应灵敏的特点，采用的铝金属丝更加接近炸药样品的阻抗，从而减小对流场的破坏。本研究采用铝基组合式电磁粒子速度计技术测量楔形炸药样品的撞击表面及内部不同深度处的冲击波后粒子速度，利用示踪器得到前沿冲击波位置随时间的变化关系，通过一次样品冲击实验即可为研究炸药样品的Hugoniot关系、Pop关系甚至化学反应区提供丰富的实验数据。
1. 实验测试系统
实验采用火炮加载系统驱动蓝宝石飞片撞击炸药，进行一维平面冲击实验。实验装置主要包括火炮驱动系统、铝弹托、蓝宝石飞片、激光测速环等，实验测试系统如图1所示。通过改变火药装药量控制蓝宝石飞片速度，进而控制初始入射压力。
图 1 组合式电磁粒子速度计实验测试系统Figure 1. Measurement system of aluminum-based multiple electromagnetic particle velocity gauge1. Sabot; 2. Velocity-measuring ring; 3. Aluminum-based multiple electromagnetic particle velocity gauge; 4. Target buffer; 5. PBX-3 explosive sample; 6. Sapphire.PBX-3炸药样品是
$\varnothing $ 42 mm × 30 mm的圆柱形组合药柱。为测量不同深度处炸药样品的冲击波后粒子速度，炸药样品被切割成两块30°角的楔形，用环氧树脂把组合式电磁粒子速度计嵌入两块楔形炸药中。利用蓝宝石飞片通过激光测速环时遮挡3组激光束的时间差来计算飞片速度，同时在炸药表面用环氧树脂粘贴3个电磁粒子速度计，用于测量蓝宝石飞片撞击炸药表面时的瞬时界面粒子速度。在冲击波传播方向，利用铝基组合式电磁粒子速度计（8个电磁粒子速度计）测量炸药样品的波后粒子速度，其深度间隔为1 mm，3个冲击波示踪器元件的间距为0.5 mm，组合式电磁粒子速度计的安装如图2所示。
图 2 铝基组合式电磁粒子速度计的安装Figure 2. Installation of aluminum-based multiple electromagnetic particle velocity gauge2. 实验数据处理
2.1 粒子速度处理原理
在冲击波作用下，组合式电磁粒子速度计在均匀磁场中切割磁感线产生感应电动势。根据法拉第电磁感应定律，粒子速度与感应电动势的关系为
u=EBl (1) 式中：u为粒子速度，m/s；E为电动势，V；B为磁场强度，T；l为切割磁感线长度，m。
因为感应电动势由电磁粒子速度计电阻、连接的导线以及示波器自带电阻以串联形式共同分担表示，所以在计算炸药不同深度处电磁粒子速度计测量得到的粒子速度时，需要乘以补偿系数
EEoscilloscope=Roscilloscope+(R−ΔR)Roscilloscope (2) 式中：Eoscilloscope为实验中由示波器测量得到的感应电动势，Roscilloscope为示波器的电阻（50 Ω），R为实验中电磁粒子速度计和连接导线本身电阻之和（实验使用万用表测量电阻，故计算时需减去万用表本底电阻），
$\Delta R$ 为万用表本身的电阻。结合式（1）和式（2），得到粒子速度计算公式为
u=EoscilloscopeBl(1+R−ΔRRoscilloscope) (3) 2.2 入射压力计算
通过改变火炮加载系统的火药装药量，得到递增的蓝宝石飞片撞击速度。6发实验的火药装药量为500、600、700、800、900和1 100 g，分别命名为Shot 01、Shot 02、Shot 03、Shot 04、Shot 05、Shot 06。利用阻抗匹配法分析计算蓝宝石飞片撞击炸药过程产生的入射压力
p0L=ρ0LDL(uimp−up) (4) p0S=ρ0SDSup (5) 式中：p0L为蓝宝石初始入射冲击压力，GPa；p0S为炸药样品初始入射冲击压力，GPa；ρ0L为蓝宝石初始密度，g/cm3；ρ0S为炸药样品初始密度，g/cm3；DL为蓝宝石中的冲击波速度，km/s；DS为炸药样品的入射冲击波速度，km/s；uimp为蓝宝石飞片的撞击速度，km/s；up为波后粒子速度，km/s。
根据蓝宝石材料和PBX-3炸药的Hugoniot关系，将式（4）写为
p0L=ρ0L[C0L+λL(uimp−up)](uimp−up) (6) 式中：蓝宝石的初始密度ρ0L = 3.985 g/cm3；Hugoniot系数C0L = 11.19 km/s，λL = 1.0。
由界面连续性可知p0L = p0S，结合式（5）和式（6），得到未反应炸药的波后粒子速度up，并计算得出实验的初始入射冲击压力p0，结果如表1所示。
表 1 PBX-3炸药平面冲击实验参数Table 1. Parameters of plane impact experiments on PBX-3 explosiveShot No. m/g ρ0/(g·cm–3) DS/(km·s–1) up/(km·s–1) p0/GPa 01 500 1.900 3.966 0.724 5.456 02 600 1.900 4.283 0.830 6.754 03 700 1.900 4.516 1.019 8.743 04 800 1.900 4.492 1.096 9.354 05 900 1.900 4.808 1.132 10.341 06 1 100 1.900 5.281 1.292 12.964 2.3 粒子速度-时间曲线和x-t图
利用式（3）对实验数据进行处理，得到炸药样品的波后粒子速度，根据示踪器数据得到炸药样品到爆轰的位置与时间的关系，结果如图3所示。图3（a）所示为Shot 01实验，入射压力为5.456 GPa。可见，随着样品发生化学反应释放能量，粒子速度上升后缓慢增长，粒子速度达到最大值后，随着化学反应结束又逐渐减小，速度信号呈“驼峰”形状。随着冲击波强度的增加，炸药反应速率不断加快，“驼峰”形状中速度上升部分加速减小，但并没有转变为爆轰。图3（b）所示为Shot 02实验，入射压力为6.745 GPa，粒子速度达到最大值的时间间隔逐渐减小，“驼峰”形状逐渐尖锐，说明此时化学反应释放的能量使冲击波逐渐加强。当入射压力达到8.743 GPa时，如图3（c）所示的Shot 03实验，距离炸药样品8 mm处的粒子速度曲线出现迅速增长，达到最大速度后转变为逐渐减小的三角波形，说明炸药样品形成稳定的爆轰波，完成了冲击转爆轰过程（SDT）。
3. 数据分析
3.1 PBX-3炸药的Hugoniot关系
实际应用中，炸药样品的入射冲击波速度DS与粒子速度up之间近似遵循线性关系
DS=C0+λup (7) 式中：DS为炸药样品中的入射冲击波速度，km/s；C0和λ为常数；up为波后粒子速度，km/s。
由于相邻两个速度计间隔是已知的，从粒子速度曲线上读取冲击波到达相邻两个测试点的时间即可计算得到炸药样品不同深度处的入射冲击波速度 DS。如表2所示，随着冲击波在炸药样品内传播，冲击波波速逐渐增大。以图3（d）中3 mm处粒子速度曲线为例，利用图4所示的方法[6]，将粒子速度分成两部分进行线性拟合，两条拟合直线的交点则为Hugoniot关系中的波后粒子速度up。表2为6发实验所测PBX-3炸药的 DS-up实验数据，选取炸药样品达到爆轰之前的部分实验数据，绘制成如图5所示的冲击Hugoniot关系图，并得到PBX-3炸药的冲击Hugoniot关系式
表 2 PBX-3炸药的 DS-up实验数据 Table 2. Experimental results of DS-up for PBX-3 explosiveDepth/mm Shot 01 Shot 02 Shot 03 up/(km·s–1) DS/(km·s–1) up/(km·s–1) DS/(km·s–1) up/(km·s–1) DS/(km·s–1) 0 0.724 4.189 0.830 4.224 1.019 3.489 1 1.104 3.489 2 1.049 4.259 3 0.776 4.189 0.894 4.224 1.033 4.686 4 0.737 3.956 0.911 4.521 1.118 4.726 5 0.732 3.796 0.938 4.229 1.282 4.546 6 0.766 3.864 1.034 4.318 1.474 4.799 7 0.833 3.950 1.187 4.579 1.682 5.252 8 0.873 4.007 1.279 4.604 1.957 5.165 9 0.937 4.190 1.450 5.470 10 0.983 4.280 1.598 4.960 Depth/mm Shot 04 Shot 05 Shot 06 up/(km·s–1) DS/(km·s–1) up/(km·s–1) DS /(km·s–1) up/(km·s–1) DS/(km·s–1) 0 1.096 4.164 1.131 4.955 1.292 5.152 1 1.113 4.164 2 1.122 4.119 3 1.137 4.680 1.347 4.955 1.634 5.152 4 1.232 4.682 1.545 4.699 1.998 5.774 5 1.407 4.664 1.654 4.947 2.230 6.187 6 1.637 4.990 1.867 5.144 2.066 7.353 7 1.962 5.213 2.104 5.656 2.282 7.082 8 2.510 5.868 2.137 7.083 2.120 7.043 9 2.308 6.746 10 2.318 6.699 2.283 7.374 
图 5 未反应 PBX-3炸药的Hugoniot关系Figure 5. Hugoniot relationship of uncreated PBX-3 explosiveDS=(3.102±0.140)+(1.234±0.115)up (8) 3.2 PBX-3炸药的Pop关系
炸药的初始入射压力与到爆轰距离xD（到爆轰时间tD）的关系称为Pop关系，反映了炸药的冲击起爆性能。组合式电磁粒子速度计的冲击波示踪器呈“梯子锯齿”状，示踪器元件切割磁感线产生的电压会发生正负周期性变化。在冲击波传播方向，冲击波示踪器中每个测量元件的间距为0.5 mm，结合正负电压信号转变的时间间隔，可以得到冲击波速度。根据示踪器信号，可以绘制出显示前沿冲击波位置随时间的变化关系（x-t图），如图3所示。由图3中的实验数据拟合出两条直线，其斜率分别表示未反应炸药的平均冲击速度和爆轰速度，两条拟合线的交点表示到爆轰的位置和时间，结果如表3所示。根据表3数据可以得到初始入射压力在5.456 ～ 12.964 GPa范围内的PBX-3炸药的Pop关系拟合线，如图6所示。Pop关系表达式为
表 3 PBX-3炸药Pop关系相关参数Table 3. Related parameters of PBX-3 explosive’s Pop-plotShot No. ρ0/(g·cm–3) p0/GPa xD/mm tD/μs 01 1.900 5.456 02 1.900 6.754 9.439 2.224 03 1.900 8.743 7.329 1.674 04 1.900 9.354 6.257 1.421 05 1.900 10.341 6.234 1.292 06 1.900 12.964 4.143 0.785 lgxD=(2.013±0.133)−(1.236±0.136)lgp0 (8) 为更好地研究PBX-3炸药的冲击起爆性能，将纯TATB基炸药TATB-1的实验数据和PBX-3进行对比，如图7所示。由图7可知，PBX-3的Pop关系曲线在TATB-1的下方，在相同入射压力条件下，PBX-3到爆轰的时间更短。PBX-3是少量HMX和TATB混合的钝感炸药，由于HMX的爆轰压力和爆轰速度比TATB高，故PBX-3炸药的冲击起爆性能比纯TATB基炸药TATB-1更敏感。
3.3 PBX-3炸药反应区评估
ZND（Zeldovich-von Neumann-Doring）模型是一种描述炸药爆轰波剖面的经典模型。模型假设爆轰波阵面是由前导冲击波和具有一定厚度的化学反应区组成。对于稳定传播的爆轰波，其终态点为Chapman-Jouguet（C-J）点，化学反应终止时压力急剧下降至C-J点，C-J面后为等熵膨胀流动区，即泰勒波稀疏区，如图8所示。炸药样品发生稳定爆轰时，冲击波后粒子速度突跃至最大速度，随着化学反应的进行，从粒子速度曲线斜率的变化可知，样品粒子加速度由开始的快速下降逐渐变成缓慢下降。化学反应结束后，样品粒子受到泰勒波稀疏影响，在终态C-J点处发生分散，最终粒子加速度趋于零。
如图3（f）所示，组合式电磁粒子速度计测得的粒子速度曲线存在信号噪声，但PBX-3炸药界面粒子速度有较明显的拐点，可以通过读取炸药界面粒子速度的拐点（即受到泰勒波稀疏影响的粒子速度曲线分散点）粗略地确定炸药的C-J点，如图9所示。
为了更准确地测量反应区时间，选择了爆轰波形一致性较好的实验数据。在Shot 06实验（入射压力为12.964 GPa）中，炸药样品在4.143 mm处开始发生稳定爆轰，将达到爆轰的6条速度曲线（即5～10 mm深度处的粒子速度数据）修整成相同零点。在0.226 μs处6条实验曲线分离，分离点即为反应区末端相应的C-J点，由此可得化学反应时间为0.226 μs。确定炸药化学反应区宽度的关系式[7]为
x=∫τ0(DCJ−ˉuP)dt (9) 式中：τ为化学反应区持续时间，为0.226 μs；DCJ为PBX-3炸药样品的C-J爆速，为7.86 km/s；
${\bar u}_{\rm{P}}$ 为化学反应区持续时间内反应产物粒子平均速度（利用软件将速度曲线积分，得到反应区持续时间内反应产物粒子运动的位移，结合化学反应区持续时间，得到反应产物的粒子平均速度）。表4所列数据为计算得到的化学反应区宽度，均值为1.449 mm。国内外学者[8-10]主要采用速度和时间分辨率较高的激光干涉技术，结合状态方程、数值模拟和粒子速度求导等方法进行TATB基炸药反应区的研究。表5列出了国内外学者得到的TATB基炸药的反应区宽度和时间。由于PBX-3炸药含有少量HMX，因而其化学反应区的持续时间和反应区宽度较纯TATB基炸药[11]更短、更窄。表 4 PBX-3炸药化学反应区实验参数Table 4. Parameters of chemical reaction zone of PBX-3 explosiveDepth/mm up/(km·s–1) t/μs x/mm Depth/mm up/(km·s–1) t/μs x/mm 5 1.539 0.226 1.429 8 1.425 0.226 1.454 6 1.540 0.226 1.428 9 1.370 0.226 1.467 7 1.436 0.226 1.452 10 1.386 0.226 1.463 表 5 TATB基炸药反应区相关参数Table 5. Related parameters of reaction zone of TATB-based explosive4. 结 论
采用火炮驱动蓝宝石飞片方法和铝基组合式电磁粒子速度计技术，对高能钝感炸药PBX-3进行了一维平面冲击实验。利用11个电磁粒子速度计测量得到炸药撞击表面及内部不同深度处的冲击波后粒子速度，通过3个冲击波示踪器绘制了炸药到爆轰距离与时间的x-t关系曲线。通过实验撞击表面及内部不同深度处冲击波后粒子速度up和入射冲击波速度DS，得到PBX-3炸药的Hugoniot关系为
DS =(3.102 ± 0.140) + (1.234 ± 0.115)up。根据炸药到爆轰时间与距离的x-t关系曲线和初始入射压力，得到反映炸药冲击起爆性能的Pop关系为lg xD = (2.013 ± 0.133)－(1.236 ± 0.136)lg p0。将入射压力为12.964 GPa时达到爆轰的6条速度曲线修整成相同零点，通过读取6条曲线的分离点，得到化学反应区时间为(0.226 ± 0.030) μs，化学反应区宽度为(1.449 ± 0.200) mm。 -
图 2 (a) Pba2和P212121的原胞和延伸结构[15]，(b) N10的结构及ELF[20]，(c) Pnnm相的能带结构和各种结构的带隙随压强的变化[21]，(d) 100～400 GPa压强区间聚合氮的相图[15]，(e) 100～500 GPa压强范围内聚合氮的相图[20]，(f) 0～14 TPa压强范围内聚合氮的相图[22]（H、HBP、HPba2分别为聚合氮、BP相和Pba2相的焓，ΔH为聚合氮与P4/nbm相的焓差）
Figure 2. (a) Primitive cell and extended structures of Pba2 and P212121[15]; (b) structure and ELF of N10[20]; (c) band structure of Pnnm phase and the band gap variation with pressure for various structures[21]; (d) phase diagram of polynitrogen from 100 GPa to 400 GPa[15]; (e) phase diagram of polynitrogen from 100 GPa to 500 GPa[20]; (f) phase diagram of polynitrogen from 0 TPato 14 TPa[22] (H, HBP, HPba2 represent the enthalpies of nitrogen polymeric phases, BP phase and Pba2 phase, respectively,and ΔH represents the enthalpy difference between various pure nitrogen polymeric phases and the P4/nbm phase.)
图 3 (a) 室温150 GPa下激光加热氮气的拉曼光谱、LP-N的2个特征振动频移随压力的变化与cg-N的对比、层状Pba2的晶体结构与三维结构的对比（左侧插图中，A、B、C分别代表黑色非晶态、LP-N、cg-N）[23]，(b) HLP-N的X射线散射谱以及晶体结构[25]，(c) BP-N的晶体结构[26]
Figure 3. (a) Raman spectra of laser-heated nitrogen at 150 GPa and ambient temperature, the pressure-dependent shifts of two characteristic vibration of LP-N shown in comparison with that of cg-N, and the comparison of the crystal structure and three-dimensional structure of layered Pba2 (A, B and C represent black amorphous, LP-N, and cg-N, respectively)[23]; (b) X-ray scattering spectrum and crystal structure of HLP-N[25]; (c) the crystal structure of BP-N[26]
图 4 (a) N8的分子结构和晶体结构[34]，(b) 0～60 GPa压强范围内各结构的焓[34]，(c) N6链的电子结构[35]，(d) N6分子组成的晶体结构及其焓[35]
Figure 4. (a) Structure and crystal structure of N8 molecule[34]; (b) enthalpy values of various structures from 0 GPa to 60 GPa[34];(c) electronic structure of N6 chain[35]; (d) crystal structure and enthalpy values of N6 molecule[35]
图 5 (a) V型、P型以及I型N10的晶体结构[38]，(b) N10分子的可能合成路径[39]，(c) 0～80 GPa压强区间的焓值[38]，(d) N16的分子结构[40]，(e) N16分子的分解路径[40]
Figure 5. (a) Crystal structures of N10 for V-type, P-type, and I-type[38]; (b) possible synthesis path of N10 molecule[39]; (c) enthalpy values from 0 GPa to 80 GPa[38]; (d) structure of N16 molecule[40]; (e) decomposition path of N16 molecule[40]
图 6 (a) P4/nmm-BeN4的成键模式[54]，(b) P21/m-CuN5的可能合成路径[51]，(c) Ga-N化合物的晶体结构[53]，(d) 85 GPa下Ga-N化合物的XRD谱[53]
Figure 6. (a) Chemical bonding pattern of P4/nmm-BeN4[54]; (b) possible synthesis route of P21/m-CuN5[51]; (c) crystal structures of Ga-N compounds[53]; (d) XRD patterns of Ga-N compounds at 85 GPa[53]
图 7 (a) XeN6的晶体结构和成键性质[55]，(b) HeN4的晶体结构和成键性质[56]，(c) He-N体系的晶体结构[57]，(d) Ne-N体系的晶体结构以及纯氮框架[58]
Figure 7. (a) Crystal structure and bonding properties of XeN6[55]; (b) crystal structure and bonding properties of HeN4[56]; (c) crystal structure of He-N compounds[57]; (d) crystal structures of Ne-N compounds and pure nitrogen[58]
-
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