METHOD FOR GENERATION OF NOVEL MATERIALS USING NANOSECOND-PULSED DISCHARGE PLASMA IN LIQUID PHASE

Abstract

A method for generation of material in a liquid phase comprising a step of subjecting the liquid phase to a nanosecond-pulsed discharge plasma.

Claims

1. A method for generation of material in a liquid phase comprising a step of subjecting the liquid phase to a nanosecond-pulsed discharge plasma.

2. The method of claim 1, wherein the liquid phase comprises more than 50 wt % of liquid nitrogen.

3. The method of claim 1, wherein a solid phase is present in the liquid phase and said solid and liquid phases are subjected to the nanosecond-pulsed discharge plasma.

4. The method of claim 1, wherein the material that is generated is selected from the group consisting of neutral or ionic polymeric nitrogen, polymeric carbon monoxide and C.sub.3N.sub.4.

5. The method of claim 3, wherein the solid phase comprises an azide such as sodium azide.

6. The method of claim 1, wherein the discharge plasma is a spark discharge plasma.

7. The method of claim 1, wherein pulses are generated using a high voltage plasma source.

8. The method of claim 1, wherein pulses having an amplitude of 1-50 kV are used to ignite a spark discharge.

9. The method of claim 8, wherein the pulses are delivered from a power supply to a discharge gap between electrodes in contact with the liquid phase in a manner whereby the duration of the pulse is longer than the time it takes to propagate the pulse from the power supply to the discharge gap.

10. The method of claim 8, wherein the pulses are delivered from a power supply via a cable to a discharge chamber in a manner whereby there is an impedance mismatch between the discharge chamber and the cable and an impedance mismatch between the cable and the power supply.

11. The method of claim 2, wherein a solid phase is present in the liquid phase and said solid and liquid phases are subjected to the nanosecond-pulsed discharge plasma.

12. The method of claim 2, wherein the material that is generated is selected from the group consisting of neutral or ionic polymeric nitrogen, polymeric carbon monoxide and C.sub.3N.sub.4.

13. The method of claim 11, wherein the solid phase comprises an azide such as sodium azide.

14. The method of claim 2, wherein the discharge plasma is a spark discharge plasma.

15. The method of claim 2, wherein pulses are generated using a high voltage plasma source.

16. The method of claim 2, wherein pulses having an amplitude of 1-50 kV are used to ignite a spark discharge.

17. The method of claim 2, wherein the pulses are delivered from a power supply to a discharge gap between electrodes in contact with the liquid phase in a manner whereby the duration of the pulse is longer than the time it takes t propagate the pulse from the power supply to the discharge gap.

18. The method of claim 2, wherein the liquid phase comprises at least 90 wt % of liquid nitrogen.

19. The method of claim 2, wherein the liquid phase comprises at least about 99 wt % of liquid nitrogen.

20. The method of claim 2, wherein the liquid phase comprises less than about 1 wt. % of oxygen.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 shows a schematic diagram of an experimental setup for generation of a spark discharge in liquid nitrogen.

[0024] FIG. 2A shows current and voltage waveforms of spark discharges in liquid nitrogen ignited using the 30 m long high voltage cable for delivery of nanosecond pulses to the electrodes.

[0025] FIG. 2B shows current and voltage waveforms of spark discharges in liquid nitrogen ignited using the 3 m short high voltage cable for delivery of nanosecond pulses to the electrodes.

[0026] FIG. 3A shows an optical emission spectra of spark discharges in liquid nitrogen ignited using the 30 m long high voltage cable for delivery of nanosecond pulses to the electrodes.

[0027] FIG. 3B shows an optical emission spectra of spark discharges in liquid nitrogen ignited using the 3 m short high voltage cable for delivery of nanosecond pulses to the electrodes.

[0028] FIG. 4 shows Fourier-Transform Infrared Spectroscopy (FTIR) spectra of untreated NaN.sub.3 and NaN.sub.3 treated with nanosecond-pulsed spark plasma in liquid nitrogen.

[0029] FIG. 5A shows Raman spectra of untreated NaN.sub.3 and NaN.sub.3 treated with spark discharge in liquid nitrogen (two separate experiments are shown).

[0030] FIG. 5B shows the change of intensity ratio of the new 1660 cm.sup.1 peak to a 1369 cm.sup.1 azide peak with increasing temperature of the treated sample (two separate experiments are shown).

[0031] FIG. 6 shows X-ray diffraction spectra of liquid nitrogen treated with spark plasma and untreated NaN.sub.3.

[0032] FIG. 7 shows a schematic of an experimental setup for generation of nanosecond-pulsed discharge in liquid nitrogen.

[0033] FIG. 8 shows a typical high voltage pulse waveform generated by a FPG 120-01NM10 nanosecond-pulsed power supply and measured using high voltage probe.

[0034] FIG. 9 shows the high voltage needle and long exposure Intensified Charge-Coupled Device (ICCD) images (false colored) of the nanosecond-pulsed discharge in liquid nitrogen. Exposure time was 5 ns and the time delay between the pulses was 5 ns. The time stamp corresponds to the arrival of the high voltage pulse at the discharge gap. The white bar is a 1 mm long scale.

[0035] FIG. 10 shows shadow imaging of the nanosecond-pulsed discharge in liquid nitrogen showing ignition in the liquid phase and formation of gaseous voids after the main discharge pulse.

[0036] FIG. 11A shows emission spectra of the nanosecond-pulsed discharge in liquid nitrogen over the 300-415 nm range integral (100 ns exposure time, single accumulation) emission.

[0037] FIG. 11B shows a comparison of an experimental and a Specair [29] simulated spectral line for a 100 ns exposure time.

[0038] FIG. 11C shows a time-resolved 337 nm line emission (3 ns exposure, 50 accumulations).

[0039] FIG. 11D shows a liquid nitrogen rotational temperature evolution as a function of time obtained from Specair [29] simulations.

[0040] FIG. 12A shows a high voltage electrode before and after 1-hour treatment of liquid nitrogen at 60 Hz pulse repetition frequency.

[0041] FIG. 12B shows a black powder material that was produced after a 1-hour treatment and after evaporation of excess liquid nitrogen.

[0042] FIGS. 13A-13B show Raman spectra of the produced material: overview and nitrogen vibrons in the 2300-2400 cm.sup.1 range.

[0043] FIG. 14 shows FTIR spectra of gaseous products of the plasma-produced material after evaporation in helium flow and reaction products of material heating in an air flow (spectra are shifted vertically for clarity).

DETAILED DESCRIPTION

[0044] The present disclosure is directed to the application of plasma for synthesis of polymeric nitrogen compounds, such as, for example, as neutral or ionic N.sub.6. In some embodiments, the polymeric nitrogen compounds are synthesized from a sodium azide precursor. Nanosecond-pulsed plasma ignited in liquid nitrogen is a unique tool for synthesis of unconventional materials due to the combination of energetic properties of the discharge (high densities of reactive species, pressures and radiation) with the low temperature of the surrounding dense liquid

[0045] In another aspect, the present invention relates to plasma-generated materials from liquid nitrogen, such as polynitrogen compounds. Nanosecond-pulsed discharge in liquid nitrogen ignited using a needle electrode and positive 60 kV high voltage pulses was characterized using fast and shadow imaging, as well as optical emission spectroscopy. Estimation of temperature was accomplished using molecular nitrogen emission of second positive system rotational-vibrational transition spectra, and the maximum temperature increase was estimated to be 60 K.

[0046] 1. Experimental Setup for Generation of Nanosecond-Pulsed Plasma and Treatment of NaN.sub.3 in in Liquid Nitrogen

[0047] For generation of a spark discharge in liquid nitrogen, two stainless-steel needles with 100 m tip curvature were fixed with a 0.1 mm gap in a plastic (50 ml) chamber covered with a lid (FIG. 1). High voltage pulses were generated using a 20-01NK high voltage plasma source (FID Tech Company) capable of providing pulses with a maximum amplitude of 23.7 kV and a duration (63% amplitude) of 12.5 ns. In these experiments, the spark discharge was ignited with 20 kV amplitude pulses. High voltage pulses were delivered to the electrodes via either a 30 m or 3 m long RG 393/U 50 Ohm high voltage coaxial cable. Pulse shape monitoring, and voltage measurements were done using a calibrated back current shunt mounted on the long cable and P6015A high-voltage probe (75-MHz bandwidth, Tektronix). Discharge current was measured using a current monitor (6585, Pearson Electronics Inc.) connected to a digital phosphor oscilloscope (DPO 4104B, Tektronix).

[0048] The medical grade (99% N.sub.2, O.sub.2 not more than 1.0%, CO.sub.2<0.001%) liquid nitrogen used in all experiments was purchased from Airgas, USA. Approximately 1 g of sodium azide (>99%, powder, Fisher Scientific) was treated in liquid nitrogen using the spark discharge setup. Sodium azide does not dissolve in liquid nitrogen and thus remains in the form of powder on the bottom of the holding vessel.

[0049] A discharge emission spectrum was obtained using a Princeton Instruments-Acton Research TriVista TR555 spectrometer system via a 1 m single leg fiber optic bundle with nineteen 200 m fibers (190-1100 nm, Princeton Instruments, USA) and a Princeton Instruments PIMAX ICCD camera was used for light registration. The same spectrometer was used in combination with SDM532-100SM-L 532 nm Spectrum Stabilized Laser Module (Newport) and RPB532 Raman probe (InPhotonics) for measurements of Raman spectra. Raman spectra were registered from both the treated and untreated samples directly in liquid nitrogen in a few mm thick liquid layer (in low form Dewar flask, CG-1592-03, Chemglass Life Sciences, USA). Raman spectra of heated samples were obtained in ambient airtreated azide was placed in a covered glass Petri dish (to avoid water condensation on the sample) and allowed to warm up to approximately 8 C. FTIR measurements were performed using a Nicolet 8700 FTIR spectrometer. For measurements of the infrared absorption spectra, treated samples were placed between KBr windows (254 mm, Pike Technologies, USA) that were cooled in liquid nitrogen using a cooled sample holder (Universal Sample Holder, Thermo Scientific, USA). Measurements were carried out in a nitrogen atmosphere to avoid water condensation on the windows and within a minute after placement of the sample into the measurement compartment of the spectrometer such that the corresponding temperature increase of the windows and the sample holder was less than 50 K as measured by thermocouple.

[0050] The X-ray diffraction pattern was collected using a Rigaku SmartLab X-Ray diffractometer (Cu.sub.K=1.54 ). Specifically; the sample holder was cooled in liquid nitrogen and the spectra were collected in several steps to minimize sample heating (portions of the same treated sample were used).

[0051] 2. Results and Discussion

[0052] Spark Discharge in Liquid Nitrogen Ignition and Characterization

[0053] A nanosecond-pulsed spark discharge in liquid nitrogen was ignited using both of the long and short cables to deliver the high voltage pulses from the power supply to the electrodes. The longer cable delivered high voltage pulses to the electrodes with approximately the same shape (rise time and duration) and amplitude as generated by the power supply. The discharge was ignited several times as the pulses were traveling along the cable due to the mismatch of impedance between the discharge chamber and cable as well as the mismatch between the cable and the power supply. These pulse reflections were clearly seen on the oscillogram obtained using the back current shunt (FIGS. 2A-2B). The first voltage pulse was the one that traveled from the high voltage power supply to the electrode (and resulted in the discharge ignition). The second voltage pulse was the reflected one that traveled from the discharge gap back to the power supply (hence, no current peak was seen on the current waveform). In this case, the discharge ignited several times, but a portion of the energy was lost in the cable. The total energy of the the discharges ignited within one sequence was 58 mJ, with the energy of the 1.sup.st discharge being 17 mJ. In contrast, sparks generated using the shorter high voltage cable were ignited as longer (almost 1 s long) and hotter plasmas, because the duration of the high voltage pulse was longer than it takes it to propagate from the power supply to the discharge gap (it takes 14 ns for a signal to travel along a 3 m cable). In this case, the entire length of the cable was essentially at the same potential and acted as a capacitive element with a capacitance of 300 pF, which resulted in a characteristic current waveform. The total discharge energy measured was 43 mJ.

[0054] Optical emission spectra of the discharges ignited via the long and short cables showed significant differences (FIGS. 3A-3B). All spectra were obtained using 10 signal accumulations with the discharge running at a 30 Hz repetition frequency. In the case of the long cable, the spectrum of the first discharge ignition (first pulse) recorded with 100 ns exposure time, besides intense broadband background emission, showed only lines of atomic nitrogen and nitrogen ion. With a longer integration time of 1 s, the emission spectrum from the whole discharge could be recorded, including its multiple reignitions due to the pulse reflections. In this case, the appearance of additional metal lines originating from the electrode material was observed. This is an indication that the discharge may have been ignited inside of nitrogen bubbles, and that successive ignitions resulted in evaporation of the electrode material, i.e. electrode erosion and appearance of metal lines in the spectrum. This is very similar to what was observed previously for nanosecond-pulsed discharges ignited in liquid nitrogen, where the authors observed metal melting after 100-300 ns after the first discharge initiation. In contrast, the spectrum of the discharge ignited using the short cable (10 ms integration time) was densely populated with atomic metal lines, which confirmed its thermal nature due to the much longer discharge duration and, consequently, higher temperatures and concentrations of the metal material melted from the electrodes. In this case, significant electrode erosion was observed, e.g. after 30 minutes stainless steel high voltage electrode erosion caused a shortening of 300 m with a corresponding weight decrease of 0.4 mg. The grounded electrode shortened by 800 m and a 0.8 mg decrease of weight was observed.

[0055] Treatment of NaN.sub.3 in Liquid Nitrogen Using Spark Discharge

[0056] Approximately 1 g of sodium azide (>99%, powder, Fisher Scientific) was added to the liquid nitrogen before treatment. Treatments were done using both higher and lower energy discharge systems with a 200 Hz pulse repetition frequency. No significant differences in the appearance and of the treated sodium azide were observed indicating that the effects of the electrode erosion and the discharge temperature likely did not play a major role in the sodium azide transformations. The results reported below were obtained for the higher energy discharge, After 5-10 min of treatment, the NaN.sub.3 powder changes color from white to green, and if left in ambient air, treated samples turn yellow as they absorb water. The initial color change (from white to green) indicates a structural change of the sodium azide following plasma treatment in liquid nitrogen.

[0057] The IR spectrum of both the treated and untreated samples shown in FIG. 4. Typical NaN.sub.3 IR lines at around 800, 1630 and 2050 cm.sup.1 were detected for the untreated sample. After the plasma treatment, a number of new peaks appear, and the new peaks at 2350 cm.sup.1 and 1030 cm.sup.1 are especially strong. The 2350 cm.sup.1 peak (together with the much weaker 2281 cm.sup.1 and 662 cm.sup.1 peaks) probably originate from solid. CO.sub.2 [20] which is present in liquid nitrogen as an impurity. The peak at 1030 cm.sup.1 is close to the absorption band v.sub.3 of ozone. However, no other vibrational ozone bands were registered at around 1100 and 700 cm.sup.1 (v.sub.1 and v.sub.2 respectively) [1-24], The experimental IR absorption spectrum was compared to the IR vibrational frequencies of polynitrogen molecules predicted by R. Bartlett and colleagues [25]. A strong band at around 1030 cm.sup.1 was predicted for a number of polynitrogen molecules. However, the combination of the observed lines matches best with the predictions for N.sub.6.sup.+ ions (Table 1) [26, 27].

TABLE-US-00001 TABLE 1 List of predicted IR vibrational frequencies of N.sub.6 molecules [24] compared to observed lines in this study Molecule IR frequency Intensity IR frequency structure calculation, cm.sup.1 calculation observed, cm.sup.1 Notes N.sub.6.sup.+ C.sub.2h planar .sup.2B.sub.g 448.2 3.5 447 Weak 584 12.6 533 weak 1036 112 1030 Strong 2191.8 206 2185 Weak 2146 Broad N.sub.6.sup.+ C.sub.2v planar .sup.2B.sub.g 530.7 0.5 533 Weak 769.1 46.4 853 Broad 840.9 35 844.6 12.8 1281.6 0.7 1295 Weak 2138.7 54.1 2146 Broad 2189.5 35.3 2184 Weak N.sub.6.sup.- C.sub.s .sup.2A 415.2 4.8 412 511.3 5.6 511 Weak 661.4 109 662 Strong 882.4 32.9 853 Broad 1016.3 192.9 1030 Strong 1283.2 11 1298 Weak 1636.8 390.5 1599 Broad 2145.5 936 2146 Broad

[0058] Raman spectra of untreated azide, treated azide and treated azide heated to 8 C. show characteristic NaN.sub.3 peaks at 1273 cm.sup.1 and 1369 cm.sup.1 (FIGS. 5A-5B). After treatment, additional peaks appear in the Raman spectrum similar to the spectrum of azide treated with X-ray and UV at elevated pressures [15]. No Raman peaks for ozone were registered in these experiments. Numerical calculations for Raman vibrational frequencies for N.sub.6.sup. ions predict strong Raman peaks at around 1283 cm.sup.1 and 1636 cm.sup.1, as well as other vibrational bands of N.sub.6 molecules close to the observed ones (although predicted intensities are low). Following analysis in [17]: [0059] Peak at 1660 cm.sup.1 can probably be assigned to v.sub.s(NN) [0060] The intensity enhancement of the v.sub.b(N.sub.3) modes at around 610 cm.sup.1 (at higher pressures in the cited study 640 cm.sup.1) could indicate the change in linear NN=N to bent NNN.

[0061] We have followed the peak at 1660 cm.sup.1 as a function of the temperature of the treated azide. The result (compared to the relatively constant intensity of 1369 cm.sup.1 peak) showed disappearance of the 1660 cm.sup.1 peak at around 55 C., which could indicate that the obtained material is stable up to this temperature at ambient pressure conditions.

[0062] The X-ray diffraction pattern (FIG. 6) is compared to the XRD data from [27] obtained using the same 1.54 (and compared to that for =0.41686 employed by Eremets et al. [14]). The lines indicated by peaks at 37, 66 and 77 2 degrees correspond closely to the (110), (211) and (220) reflections of the cubic gauche structure of polymeric nitrogen reported by Eremets and co-workers. The strong lines correspond to unreacted NaN.sub.3. Our results also show peaks at 38 (new compared to untreated), 68 (new or shifted 0.5 degree compared to untreated) and 76 2 degrees. In addition, the peak at 41.3 splits into two new peaks at 40.9 and 41.7; a new peak appears at 73.4; the peaks at 37.1, 68.7, 78.8 appear shifted to negatively by 0.5 to 36.9, 68.2 and 78.2 and, correspondingly; peaks at 50.1, 56.6, 60, 63.9, 74.7 and 81.8 are positively shifted to 50.3, 57, 60.2, 64.6, 75.1 and 85.3.

[0063] Overall, the experimental observations support that liquid nitrogen spark discharge plasma induces transformations in sodium azide, and likely results in formation of polynitrogen materials, most probably neutral or ionic N.sub.6. The produced material is probably stable up to a temperature of about 55 C. at ambient pressure. The mechanism behind the reaction products could be related to the effects of plasma radiation (for example, UV radiolysis and UV absorption). Indeed, it was suggested that two-photon absorption could produce azide radicals and ultimately N.sub.6.sup. ions in reactions like:

N.sub.3.sup.+hv.fwdarw.N*.sub.3+e.sup. and N.sub.3.sup.+N.sub.3*.fwdarw.N.sub.6.sup.[15].

[0064] As no differences in the azide transformation between the plasma regimes (high energy vs low energy), the mechanism is likely related to the effects of plasma radiation in the UV range and possibly excited nitrogen and is not related to the electrode erosion and the discharge temperature. It is possible that liquid nitrogen spark discharge also results in generation of iron nitride compounds (for example, FeN.sub.2) that are linked to formation of double bonded Na species as well [28].

[0065] Using different lengths of the high voltage cable, it is possible to generate spark discharges with different durations and energies (and expected temperatures). These discharges were used for treatment of sodium azide in liquid nitrogen. Experimental characterization techniques showed that plasma treatment of NaN.sub.3 results in production of colored material with spectral characteristics close to N.sub.6 polynitrogen compounds, although it is most likely is a mixture of different compounds. The obtained material appears to be stable at ambient pressure at temperatures up to around 55 C.

REFERENCES

[0066] The following references may be useful in understanding some of the principles discussed herein: [0067] 1. W G Graham and K R Stalder 2011 Plasmas in liquids and some of their applications in nanoscience J. Phys. D: Appl. Phys. 44 174037 [0068] 2. T Belmonte et al 2014 Interaction of discharges with electrode surfaces in dielectric liquids: application to nanoparticle synthesis J. Phys. D: Appl. Phys. 47 224016 [0069] 3. N I Kuskova et al 2007 Obtaining nanocarbon using the electric-discharge treatment method of organic liquids. Surface Engineering and Applied Electrochemistry, 43(4), 269-275. [0070] 4. T Belmonte et al 2018 Analysis of Zn I emission lines observed during a spark discharge in liquid nitrogen for zinc nanosheet synthesis Plasma Sources Sci. Technol. 27 074004 [0071] 5. A Hamdan et al 2018 Synthesis of two-dimensional lead sheets by spark discharge in liquid nitrogen Particuology 40 pp 152-159 [0072] 6. D V Schur et al. 2007 Production of carbon nanostructures by arc synthesis in the liquid phase Carbon, 45 (6) pp. 1322-1329 [0073] 7. G Touya et al 2006 Development of subsonic electrical discharges in water and measurements of the associated pressure waves J. Phys. D: Appl. Phys. 39 5236 [0074] 8. imek M, Pongrc B, Babick V, lupek M and Luke P 2017 Luminous phase of nanosecond discharge in deionized water: morphology, propagation velocity and optical emission Plasma Sources Sci. Technol. 26 07LT01 [0075] 9. Marinov I, Starikovskaia S and Rousseau A 2014 Dynamics of plasma evolution in a nanosecond underwater discharge J. Phys. Appl. Phys. 47 224017 [0076] 10. Pongrc B, imek M, lupek M Babick V and Luke P 2018 Spectroscopic characteristics of H /OI atomic lines generated by nanosecond pulsed corona-like discharge in deionized water J. Phys. D: Appl. Phys. 51 124001 [0077] 11. Marinov I, Guaitella O, Rousseau A and Starikovskaia S M 2013 Modes of underwater discharge propagation in a series of nanosecond successive pulses J. Phys. D: Appl. Phys. 46 464013 [0078] 12. Dobrynin D, Seepersad Y, Pekker M, Shneider M, Friedman G and Fridman A 2013 Non-equilibrium nanosecond-pulsed plasma generation in the liquid phase (water, PDMS) without bubbles: fast imaging, spectroscopy and leader-type model J. Phys. D: Appl. Phys. 46 105201 [0079] 13. K Christe 2007 Recent Advances in the Chemistry of N5+, N5 and High-Oxygen Compounds Propellants, Explosives, Pyrotechnics 32 (3) pp 194-204 [0080] 14. Eremets M I et al 2004 Single-bonded cubic form of nitrogen Nat. Mater., 3, 558-563 [0081] 15. N Holtgrewe et al 2016 Photochemistry within Compressed Sodium Azide J. Phys. Chem. C, 120, 28176-28185 [0082] 16. S M Peiris et al 2003 Photolysis of Compressed Sodium Azide (NaN.sub.3) as a Synthetic Pathway to Nitrogen Materials J. Phys. Chem. A, 107, 944-947 [0083] 17. S Duwal et al 2018 Transformation of hydrazinium azide to molecular N8 at 40 GPa J. Chem. Phys. 148, 134310 [0084] 18. J Jiang et al 2016 High pressure studies of trimethyltin azide by Raman scattering, IR absorption, and synchrotron X-ray diffraction RSC Adv., 6, 98921 [0085] 19. H Zhu et al 2013 Pressure-induced series of phase transitions in sodium azide J. Appl. Phys. 113, 033511 [0086] 20. Isokoski K, Poteet C A and Linnartz H, 2013, Highly resolved infrared spectra of pure CO2 ice (15-75 K), Astronomy and Astrophysics 555 A85 [0087] 21. L Brewer 1972 Infrared Absorption Spectra of Isotopic Ozone Isolated in Rare-Gas Matrices J. Chem. Phys. 56, 759 [0088] 22. L. Schriver-Mazzuoli et al 1995 Ozone generation through photolysis of an oxygen matrix at 11 K: Fourier transform infrared spectroscopy identification of the O . . . O3 complex and isotopic studies J. Chem. Phys. 102, 690 [0089] 23. A Lakhlifi et al 1993 Interpretation of the infrared spectrum of ozone trapped in inert matrices Chem Phys 177 31-44 [0090] 24. B D Teolis et al 2007 Low density solid ozone J. Chem. Phys. 127, 074507 [0091] 25. R D Bartlett et al Stable polynitrogen molecules from N.sub.2 to N.sub.10 and their anions and cations IR vibrational frequencies and intensities, http://www.qtp.ufl.edu/bartlett/downloads/polynitrogen.pdf [0092] 26. M Tobita and R J Bartlett 2001 Structure and Stability of N.sub.6 Isomers and Their Spectroscopic Characteristics J. Phys. Chem. A 105 16 4107-4113 [0093] 27. Benchafia et al 2017 Cubic gauche polymeric nitrogen under ambient conditions, Nature Communications, vol 8, Article number: 930 [0094] 28. Laniel D, Dewaele A and Garbarino G 2018 High Pressure and High Temperature Synthesis of the Iron Pernitride FeN2, Inorg. Chem. 57 6245-6251

[0095] Experimental Setup and Methods

[0096] For generation of discharge in liquid nitrogen, a sharp (75 m radius of curvature) a steel electrode was placed in liquid nitrogen contained in a 450 ml double-walled glass vacuum flask (FIG. 7). The flask was fixed inside of an evacuated metal chamber to decrease the liquid nitrogen evaporation rate and to screen electromagnetic noise. The vacuum flask was also closed at the top by a plastic lid provided with a 3 mm diameter venting hole to minimize liquid nitrogen contamination by the surrounding air, especially oxygen. Medical grade (99% N.sub.2, O.sub.2 not more than 1.0%, CO.sub.2<0.001%) liquid nitrogen was used in all experiments and was purchased from Airgas, USA. The high voltage electrode was powered via a 10 m long 50 Ohm coaxial cable by an FPG 120-01NM10 nanosecond-pulsed power supply (FID Tech., Germany) capable of providing positive high voltage pulses with a maximum pulse amplitude 120 kV, a rise time 10% to 90% amplitude in less than 1 ns, and a pulse duration at 90% amplitude of 8-10 ns. The pulse waveform recorded using the high voltage probe (P6015A, Tektronix, OR, USA) is shown in FIG. 8. However the shape of the pulse is distorted due to the low bandwidth of the high voltage probe and circuit ringing. In most of the experiments the discharge was ignited using HV pulses with an amplitude of 60 kV. The discharge energy was not measured due to the practical difficulty of installing a back current shunt in the high voltage cable system. The voltage is estimated to be about 100-130 mJ based on the energies of similar nanosecond-pulsed discharges in water measured at lower voltages using the back current shunt [37].

[0097] Discharge imaging was performed using a 4Picos ICCD camera (Stanford Computer Optics, USA) equipped with a UV lens and synchronized with the power supply using an AFG-3252 function generator (Tektronix, USA). Shadow imaging was carried out using a 30 W/mm Deuterium arc lamp (Newport, USA) as a source of back light. The discharge emission was recorded using a Princeton Instruments-Acton Research, TriVista TR555 spectrometer system via a 1 m single leg fiber optic bundle with nineteen 200 m fibers (190-1100 nm, Princeton Instruments, USA) and a 4Picos ICCD camera. FTIR measurements were performed using a Nicolet 8700 FTIR spectrometer equipped with a 2 m gas cell with KBr windows and having a 200 ml internal volume (Thermo Fisher Scientific, USA). Raman spectra were obtained using a SDM532-100SM-L 532 nm Spectrum Stabilized Laser Module (Newport, USA) and a TriVista spectrometer system. For that, the excitation fiber of a RPB532 Raman probe (InPhotonics, USA) was connected to the laser source and the emission fiber was connected to the entrance slit of the spectrometer. The Raman probe was positioned at 7.5 mm (focal length of the probe) above the examined samples. At the focal point, the probe spot size was approximately 160 m and depth of field was 2.2 mm Spectra were typically recorded with a 1 s exposure time and 10 accumulations. The spectrometer was calibrated using a 6035 Hg(Ar) calibration lamp (Newport).

[0098] Results and Discussion

[0099] Discharge Imaging

[0100] FIG. 9 shows long (5 ns) exposure images of the discharge in liquid nitrogen taken with a 5 ns time step. The discharge was visible using ICCD camera for about 30 ns, and high voltage pulse reflections were absorbed by the power supply. This is in contrast to other studies (see, for example, [32, 37, 55, 56]) where reflected pulses resulted in successive discharge ignitions in the gaseous voids (bubbles).

[0101] The typical discharge size was on the order of few mm and appeared to be significantly larger than was reported previously for slower but lower voltage (30 kV) pulses applied for generation of a streamer in liquid nitrogen, although in these experiments the electrode size was quite large compared to, for example, the 1 m needle used in [40]). From these images, streamer propagation velocity was estimated to be at least 0.70.810.sup.3 km/s, using the relatively long exposure time of 5 ns. Previously, similar propagation velocities were reported for discharges in water (see, for example, [29, 33, 56]), however in [40] and [41] streamer propagation velocities in liquid nitrogen were an order of magnitude lower.

[0102] In order to examine whether the discharge is ignited in preexisting gaseous bubbles which could be present from, for example, previous discharge ignitions or evaporation of nitrogen on the needle, shadow imaging of the discharge was carried out. The results (FIG. 10) indicated that no large-scale irregularities existed in the liquid before the discharge, and only after about 15 ns, when the main plasma event started to decay, visible gas voids started to appear at the location of the streamers. This corresponds well with the previous observations for the nanosecond discharge in water [37].

[0103] Optical Emission Spectra of the Nanosecond-Pulsed Discharge in Liquid Nitrogen

[0104] Emissions from the discharge in the 300-415 nm range were recorded using the 4Picos ICCD camera with either a 100 ns exposure time and a single accumulation or a 3 ns exposure time and 50 accumulations. Obtained spectra are shown in FIGS. 11A-11D, where the short exposure time spectra are shown with the time corrected for the light propagation delay in the optical fiber and spectrometer, as compared to the discharge imaging system. The emission spectrum in this range (300-415 nm) is mostly from molecular nitrogen emission lines second positive system C .sup.3.sub.u-B .sup.3.sub.g (SPS). The first positive system B .sup.3.sub.g-A .sup.3.sub.u.sup.+ emission was also observed in the 700-900 nm range, as well as some atomic nitrogen emission lines, but due to high noise the data are not shown here.

[0105] Using the ro-vibrational emission spectrum of the 0-0 C .sup.3.sub.u-B .sup.3.sub.g transition (SPS) at around 337 nm and assuming equilibrium of the rotational temperature T.sub.r(C) of the C state and T.sub.r(X) of the ground state of nitrogen, the temperature of the discharge were estimated (FIGS. 11A-11D). The rotational temperature of nitrogen was estimated by fitting a synthetic spectrum to the experimental spectrum of the 337 nm transition emission band of the second positive system of nitrogen with the program Specair 3.0 [29] (FIGS. 11A-11D). Simulation of the long exposure (100 ns) spectrum shown in FIGS. 12A-12B resulted in the estimated temperature value of 110 K, and the maximum discharge temperature obtained from the time resolved spectra was about 140 Kabout 60 K above the liquid nitrogen temperature. Previously, spectroscopic measurements of nanosecond-pulsed streamers in liquid nitrogen were done for slower, but probably more energetic, pulses and the estimated temperature was 500 K [40]. No significant broadening of the emission lines was observed, in contrast to water experiments, which could be interpreted to indicate that the emission originated from the low-density regions.

[0106] Nitrogen Material Production by the Nanosecond-Pulsed Discharge in Liquid Nitrogen

[0107] Nanosecond-pulsed discharge was used for treatment of liquid nitrogen. The treatment duration was 30-60 minutes at a pulse repetition frequency of 60 Hz. After 60 minutes of treatment, no significant erosion of the high voltage electrode was observed (FIGS. 12A-12B). Liquid nitrogen after 1 hour of treatment (about 50 mL) was heated in room air and in a helium atmosphere (closed vessel) on a hot plate having a temperature of about 400 C.) in order to increase the concentration of the produced material. With evaporation of the liquid nitrogen the liquid darkened leaving a black powder-like material (see FIGS. 12A-12B), and within a second exploded with generation of both light and sound. No residue was left after the explosive material decomposition. The observed material appeared to be very similar to the one described in [50]a solid amorphous non-molecular form of nitrogen.

[0108] We attempted to measure the Raman spectrum of the obtained material. The Raman spectrum of the liquid nitrogen changes after treatment (FIGS. 13A-13B) but is difficult to interpret. A nitrogen vibron is present at around 2330 cm.sup.1 in both the treated and untreated samples. Similar to [50-54], vibron excitation was observed in the vicinity of the main 2330 cm.sup.1 peak which could indicate appearance of a new phase. Broad features around 800 cm.sup.1, 930 cm.sup.1 and 1050 cm.sup.1 probably could be assigned to NN modes (e.g., compared to calculations and measurements from [49] where the line at 1060 cm.sup.1 corresponds to the N.sub.8.sup. vibrational frequency) likely broadened due to structural disordering (amorphization). No characteristic lines from azide groups at 1360 cm.sup.1 were observed, nor were any Raman peaks associated with ozone observed, which further supports the liquid nitrogen-based plasma production of energetic nonmolecular form of a nitrogen-rich material.

[0109] FTIR analysis of the gaseous products of sample evaporation and decomposition in air (explosion) was done using a Nicolet 8700 FTIR spectrometer equipped with a 2 m gas cell. For evaporation product measurements, the samples were placed in a tightly closed reaction vessel with an outlet connected to the spectrometer gas cell; in order to prevent possible reactions with oxygen in the ambient air. Additional helium flow at rate of 1 slpm was supplied into the system. The reaction products of the sample decomposition were examined in the presence of ambient air. For that, the treated sample was placed into a reaction vessel heated using a hot plate, and ambient air was pumped into the reaction vessel at a flow rate of 1 slpm. The representative spectra are shown in FIG. 14.

[0110] FTIR spectra of the gaseous products from heated samples show peaks of ozone, N.sub.2O, water and CO.sub.2. Samples evaporated in helium show significantly lower concentrations of ozone and CO.sub.2. The presence of carbon dioxide in the evaporated (unheated) sample is due to its presence in liquid nitrogen and contamination from ambient air. Ozone can be generated in liquid nitrogen during the discharge from the 1% oxygen that is present in the untreated liquid nitrogen, though its concentration is relatively low and is estimated to be only a few ppm. It is, however, unlikely that the presence of ozone and nitrous oxide is the result of their direct generation by the discharge in liquid nitrogen since no other NO.sub.x species (e.g., NO, NO.sub.2, N.sub.2O.sub.5) were detected that would also be expected to be produced in air plasmas [58]. Moreover, the production of atomic nitrogen in the presence of molecular oxygen and atomic oxygen in the presence of nitrogen immediately leads to generation of NO.sub.x species. See for example [58]:

N+O.sub.2.fwdarw.NO+O

N+O.sub.3.fwdarw.NO+O.sub.2

O+N(.sup.2P).fwdarw.NO.sup.++e

O+NO+M.fwdarw.NO.sub.2+M,M=N.sub.2,O.sub.2,NO,NO.sub.2,N.sub.2O, and others

[0111] In contrast, N.sub.2O can be produced in the following reaction [30]:

N.sub.2(A)+O.fwdarw.N.sub.2O+O.(1)

that does not require the availability of NO.sub.x species. This also results in simultaneous production of ozone:

[00001]$\begin{array}{cc}\{\begin{array}{c}{N}_2\left(A\right)+O_2.fwdarw.N_2\left(X\right)+O+O\\ O+O_2+M.fwdarw.O_3+M\end{array}& \left(2\right)\end{array}$

[0112] During heating in the presence of air, the sample rapidly decomposes with generation of large amounts of both ozone and N.sub.2O. In this case, ozone concentrations of up to several percent and N.sub.2O concentrations of 0.1-0.5% were observed. This significant increase in both O.sub.3 and N.sub.2O can be explained by a significant energy release during sample decomposition. Due to the absence of NO and other similar species, it appears that one possible mechanism of such rapid production of both nitrous oxide and ozone during the sample decomposition is related to energy release and production of excited nitrogen via reactions (1) and (2). This is somewhat surprising since production of electronically excited nitrogen (triplet sigma nitrogen, N.sub.2 (A.sup.3.sub.u.sup.+)) requires energies on the order of 6.2 eV and this type of nitrogen is not typically produced during explosions. On the other hand, the NN triple bond energy is characterized by a value of 229 kcal/mol (9.9 eV), while the NN double and NN single bond energies are only 100 kcal/mol (4.3 eV) and 38 kcal/mol (1.6 eV), respectively. Back conversion to diatomic molecular nitrogen is, therefore, highly exothermic and the corresponding energy release could be the source of production of electronically excited N.sub.2 (A.sup.3.sub.u.sup.+) which leads to generation of N.sub.2O. Multiple N.sub.x all-nitrogen compounds could be formed in the non-thermal plasma in liquid nitrogen. Ions like N.sub.3.sup.+ could be produced that can further polymerize in reactions like:

N.sub.3.sup.+hv.fwdarw.N.sub.3*e.sup. and N.sub.3.sup.+N*.sub.3.fwdarw.N.sub.6.sup.[59].

[0113] The results can be summarized as follows: [0114] 1. Nanosecond-pulsed discharge in liquid nitrogen, much like in water, initially ignites directly in the liquid phase, and the energy release eventually results in generation of gaseous voids. [0115] 2. First temperature estimations from the molecular nitrogen emission showed a maximum temperature increase on the order of 60 K, which is advantageous for non-thermal material synthesis in the liquid phase. [0116] 3. Production of nitrogen-based material from this discharge was observed, which appears to be a form of an energetic non-molecular nitrogen compound due to the following reasons: [0117] a) no electrode erosion was observed while the amount of produced material was significant; [0118] b) material decomposition, accompanied by light and sound wave generation, was triggered by heating; [0119] c) no residue was left after the material decomposition; and [0120] d) the absence of NO.sub.x species, except N.sub.2O, in the reaction products as determined by FTIR indicates a low-temperature decomposition mechanism.

[0121] The multitude of species that can be formed in plasma, as well as the structural disorientation of the produced material results in complicated Raman spectra that cannot be interpreted with a high degree of certainty or be compared with the large pool of previous data on polynitrogen material production at elevated pressures.

REFERENCES

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