Vít Jirásek

Size and Purity Control of HPHT Nanodiamonds down to 1 nm

High-pressure high-temperature (HPHT) nanodiamonds originate from grinding of diamond microcrystals obtained by HPHT synthesis. Here we report on a simple two-step approach to obtain as small as 1.1 nm HPHT nanodiamonds of excellent purity and crystallinity, which are among the smallest artificially prepared nanodiamonds ever shown and characterized. Moreover we provide experimental evidence of diamond stability down to 1 nm. Controlled annealing at 450 °C in air leads to efficient purification from the nondiamond carbon (shells and dots), as evidenced by X-ray photoelectron spectroscopy, Raman spectroscopy, photoluminescence spectroscopy, and scanning transmission electron microscopy. Annealing at 500 °C promotes, besides of purification, also size reduction of nanodiamonds down to ∼1 nm. Comparably short (1 h) centrifugation of the nanodiamonds aqueous colloidal solution ensures separation of the sub-10 nm fraction. Calculations show that an asymmetry of Raman diamond peak of sub-10 nm HPHT nanodiamonds can be well explained by modified phonon confinement model when the actual particle size distribution is taken into account. In contrast, larger Raman peak asymmetry commonly observed in Raman spectra of detonation nanodiamonds is mainly attributed to defects rather than to the phonon confinement. Thus, the obtained characteristics reflect high material quality including nanoscale effects in sub-10 nm HPHT nanodiamonds prepared by the presented method.

Chemical oxygen-iodine laser using a new method of atomic iodine generation

The chemical oxygen-iodine laser (COIL) with a new chemical method of atomic iodine production was investigated. In this system, iodine atoms are formed in the COIL cavity by the fast chemical reaction of hydrogen iodide with chlorine atoms that are also produced chemically. It was found that, in the absence of singlet oxygen, the ground state atomic iodine can be produced with a high yield (80%-100%). In gas containing singlet oxygen, a gain on 3-4 electronic transition in iodine atom was achieved (0.35% cm/sup -1/). Both the concentration of atomic iodine and the gain depend substantially on the ratio of reacting gases and the penetration of secondary gases into the primary gas flow. In laser experiments, effects of the flow rate of reacting gases and their penetration on the laser output power were found. The output power of 310 W was attained at chlorine flow rate of 27 mmol/spl middot/s/sup -1/ corresponding to chemical efficiency of 12.7%. This was the first time the gain and laser output power were achieved in the COIL with atomic iodine generated by the proposed method.

Chemical generation of atomic iodine for chemical oxygen–iodine laser. I. Modelling of reaction systems

Abstract The mathematical modelling of reaction systems for chemical generation of atomic iodine is presented. This process is aimed to be applied in the chemical oxygen–iodine laser (COIL), where it can save a substantial part of energy of singlet oxygen and so increase the laser output power. In the suggested method, gaseous reactants for I atoms generation are admixed into the COIL primary gas flow containing singlet oxygen. Two reaction systems were proposed, based on the reaction of hydrogen iodide with chemically generated atomic fluorine or chlorine. It was found that the reaction path via Cl atoms better matches the experimental conditions of COIL with a yield of atomic iodine of up to 67%. As a result of modelling, a suitable reaction system and design of experimental arrangement for the effective production of atomic iodine in laser conditions were found.

Production of iodine atoms by RF discharge decomposition of CF3I

Generation of atomic iodine by dissociation of CF3I in a RF discharge was studied experimentally in a configuration ready for direct use of the method in an oxygen?iodine laser. The discharge was ignited between coaxial electrodes with a radial distance of 3.5?mm in a flowing mixture of 0.1?0.9?mmol?s?1 of CF3I and 0.5?6?mmol?s?1 of buffer gas (Ar, He) at a pressure of 2?3?kPa. The discharge stability was improved by different approaches so that the discharge could be operated up to a RF source limit of 500?W without sparking. The gas leaving the discharge was injected into the subsonic or supersonic flow of N2 and the concentration of generated atomic iodine and gas temperature were measured downstream of the injection. An inhomogeneous distribution of the produced iodine atoms among the injector exit holes was observed, which was attributed to a different gas residence time corresponding to each hole. The dissociation fraction was better with pure argon as a diluting gas than in the mixture of Ar?He, although the variation in the Ar flow rate had no significant effect on CF3I dissociation. The dissociation fraction calculated from the atomic iodine concentration measured several centimetres downstream of the injection was in the range 7?30% when the absorbed electric energy ranged from 200 to 4000?J per 1?mmol of CF3I. The corresponding values of the fraction of power spent on the dissociation decreased from 8% to 2% and the energy cost for one iodine atom increased from 30 to 130?eV. Due to a possible high rate of the atomic iodine loss by recombination after leaving the discharge, these values were considered as lower limits of those achieved in the discharge.

Chemical generation of atomic iodine for the chemical oxygen-iodine laser. II. Experimental results

Abstract A new method for the chemical generation of atomic iodine intended for use in a chemical oxygen–iodine laser (COIL) was investigated experimentally. The method is based on the fast reaction of hydrogen iodide with chemically produced chlorine atoms. Effects of the initial ratio of reactants and their mixing in a flow of nitrogen were investigated experimentally and interpreted by means of a computational model for the reaction system. The yield of iodine atoms in the nitrogen flow reached 70–100% under optimum experimental conditions. Gain was observed in preliminary experiments on the chemical generation of atomic iodine in a flow of singlet oxygen.

Chemical generation of atomic iodine for COIL

A method of the chemical production of atomic iodine aimed for application in COIL was studied experimentally. The method is based on chemical generation of chlorine atoms and their subsequent reaction with hydrogen iodide. Effects of initial ratio of reactants and the way of their mixing were investigated and interpreted by means of the developed model of the reaction system. In optimum conditions, the yield of iodine atoms, related to HI, attained 70 - 100 percent.

Preliminary experimental results on chemical generation of atomic iodine for a COIL

A chemical method of atomic iodine generation with a potential application in chemical oxygen-iodine laser (COIL) was investigated experimentally. The process consists in a fast reaction of gaseous hydrogen iodide with chlorine atoms produced in reaction of gaseous chlorine dioxide with nitrogen oxide. In conditions characteristic for a subsonic mixing region of COIL, atomic iodine was produced with a yield of 20-50 %. This is in a fair agreement with results ofmathematical modeling ofthis complex reaction system.

Filamentation of diamond nanoparticles treated in underwater corona discharge

Diamond nanoparticles (DNPs), also known as nanodiamonds, have attracted significant interest in recent years due to a number of potential applications. Their particular usage requires proper surface engineering. In this work, DNPs with a nominal diameter of 5 nm were treated using underwater pulsed streamer corona discharge. A reactor with a needle-to-plate electrode system was employed. The electrolytic conductivity of aqueous DNPs suspensions (0.37 g l−1) was adjusted by NaCl to 100 and 500 μS cm−1. The discharge-treated particles predominantly formed several mm long filaments consisting of agglomerates with submicron diameter, independent of the solution conductivity and the treatment time. The treatment of DNPs decreased the sp2-bonded carbon atoms, as evaluated by XPS for more conductive solution. For both solutions, oxidation of the DNP surface was observed. FTIR measurements showed evolution of new bands at 800–950 cm−1 and 1261 cm−1, which were attributed to the formation of epoxides via the attack of HO2˙ radicals on surface CC double bonds.

Influence of Diamond CVD Growth Conditions and Interlayer Material on Diamond/GaN Interface

In this study we present the diamond deposition on AlGaN/GaN substrates focusing on the quality of the diamond/GaN interface. The growth of diamond films was performed using microwave chemical vapour deposition system in different gas mixtures: standard CH4/H2 (at low and high ratio of CH4 to H2) and addition of CO2 to CH4/H2 gas chemistry. The diamond films were grown directly on GaN films either without or with thin interlayer. As interlayer, 100 nm thick Si3N4 was used. Surprisingly, in the case of standard CH4/H2 gas mixture, no diamond film was observed on the GaN with SiN interlayer, while adding of CO2 resulted in diamond film formation of both samples with and without SiN interlayer. Moreover, adding of CO2 led to higher growth rate. The morphology of diamond films and the quality of the diamond/GaN interface was investigated from the cross-section images by scanning electron microscopy and the chemical character (i.e. sp3 versus sp2 carbon bonds) was measured by Raman spectroscopy.

RF discharge generation of I atoms in CH3I and CF3I for COIL/DOIL

A cw/pulsed radiofrequency discharge coupled by electrodes in coaxial arrangement was used to dissociate iodine atoms from CH3I or CF3I molecules diluted in a carrier gas (a mixture of Ar and He). The discharge chamber was arranged directly inside an iodine injector (made of aluminum) to minimize the recombination of generated atomic iodine and enabling an increased assistance of UV light for a photo-dissociation enhancement of I atoms production. The effluent of the discharge chamber/iodine injector was injected into the flow of N2 downstream the nozzle throat. Measurements of I atoms concentration distribution at different distances from the injection and in two directions across cavity were done by means of absorption measurements at the wavelength of 1315 nm. Dependences of atomic iodine concentration on main RF discharge parameters and flow mixing conditions were measured. This novel method could be an alternative to the chemical generation of atomic iodine and also an efficient alternative to other electric discharge methods of I atoms generation for chemical oxygen-iodine laser (COIL) and discharge oxygen-iodine laser (DOIL).