Analysis of volt-current dependences discovered in thermobarically sintered detonation nanodiamond composites is covered in the paper. Composite nanodiamond materials were obtained in the course of sintering at the press-free high-pressure apparatus (BARS) under 5 GPa and at temperatures 1300, 1400 and 1500 degrees C. Volt-current dependences are non-linear and show a zero-current area in the voltage interval from -3 to +1 V. Non-linear character of the volt-current dependences is connected with the structural state of the detonation diamond non-diamond carbon shell having a graphite-like onion-shaped structure and composed of 2 - 3 graphene planes about 1 nm thick. It is just the state of the electronic sub-system of such minimum package of graphene planes that stipulates charge carrier tunneling in the current channel under voltage application between the conducting probe and the sample, as well as it defines specific non-linearity of volt-current dependences with the zero-current area.

The research was conducted to study the thermal conductivity of detonation nanodiamonds-based composites. Composite nanodiamond materials were obtained in the course of thermobaric sintering at the press-free high-pressure apparatus (BARS) under 5 GPa and at temperatures within the range of 1100 -1500 degrees C. It was ascertained that unlike diamond monocrystals with their thermal conductivity reaching up to 2100 W / (mK), the thermal conductivity of a nanodiamond composite is considerably lower and does not go beyond 18 W / (mK). Specifically, the temperature dependence of the thermal conductivity coefficient of a nanodiamond composite is anomalous as compared to a similar dependence in diamond monocrystals. The thermal conductivity coefficient in diamond monocrystals grows in compliance with the rising temperature, whereas it shows practically no changes in a nanodiamond composite in the temperature range of 50 - 300 degrees C. Such a temperature dependence of the thermal-conductivity coefficient is apparently related to the features of the phonon spectrum of diamond monocrystals. This feature is stipulated by the dependence of the phonon spectrum of nanocrystals on their size, represented by a set of phonon modes in the range of the wave vector 0 < q<1/L, i.e., the size of a diamond nanocrystal of 4.5 nm is alleged to limit the excitation of harmonics during nanodiamond composite heating, as opposed to macroscopic crystals that demonstrate the excitation of higher-frequency phonon modes during temperature growing.

Проведено исследование теплопроводности металлоалмазных композитов на основе алмазных порошков размером частиц 30-300 мкм. Композиционные металлоалмазные материалы получены в ходе термобарического спекания на беспрессовом аппарате высокого давления БАРС при температуре 1300 °С и давлении 5 ГПа. Разработанная методика проведения экспериментов позволила получить образцы объемом более 250 мм3. Установлено, что в отличие от монокристаллов алмаза, теплопроводность которых может достигать 2100 Вт/мК, теплопроводность металлоалмазного композита ниже и может достигать 490 Вт/мК. Характерно, что температурная зависимость коэффициента теплопроводности монокристаллов алмаза монотонно возрастающая, что соответствует фононному механизму теплопередачи. Теплопроводность металлоалмазных композитов является величиной эффективной и представляет собой комбинацию низкой теплопроводности металла связки и высокой теплопроводности микронных алмазных частиц с учетом весовых параметров. Очевидно, что теплопроводность зависит от наличия примесных атомов, особенно азота, в решетке алмазных монокристаллов, на которых осуществляется рассеяние фононов. Однако, существенное влияние на теплопроводность оказывают границы раздела, на которых также происходит рассеяние фононов. При наличии карбидообразующего элемента в исходной шихте теплопроводность композита возрастает в связи с образованием алмазного каркаса и хорошей смачиваемости карбида железа медью. Если в монокристаллах алмаза коэффициент теплопроводности растет при повышении температуры, то в металлоалмазном композите в интервале 50-300 °С он снижается. Такая зависимость коэффициента теплопроводности от температуры, очевидно, связана с конкурирующим вкладом фононного и электронного механизмов теплопроводности. Фононный механизм приводит к росту теплопроводности кристалла алмаза, напротив, электронный механизм теплопереноса при повышении температуры снижает теплопроводность в связи с увеличением сопротивления медной связки.

The present paper reports the results of a study of the X-ray and Raman spectra of detonation nanodiamonds after high-pressure high-temperature (HPHT) annealing at different temperatures.

The paper presents the results of studies on mechanical and structural properties of detonation nanodiamonds obtained by IIPIIT (high-pressure high-temperature) annealing within a wide temperature range. The experiments were carried out using a high-pressure "split-sphere" type apparatus (BARS) under 5 GPa and at 1100-1500 degrees C. It is established that the thermobaric treatment allows the production of strong composites with the local hardness up to 15 GPa. It is shown that the average value of microhardness increases with the sintering temperature. The temperature increase from 1100 to 1500 degrees C results in an enhancement of the average value of microhardness from 8.8 to 12.2 GPa. The obtained materials are structurally inhomogeneous, regions of higher hardness are located in the central part of the samples. However, when the sintering temperature increases, dispersion of the microhardness decreases from 6.4 to 1.4 GPa which is caused by an improvement of the structural homogeneity of the composite with an increase in temperature. The thermobaric effect results in the growth of cores of diamond nanocrystals from 4.2 to 6.9 nm in samples obtained at 1500 degrees C. It is highly probable that this growth occurs as a result of embedding of non-diamond phase carbon into the diamond core lattice during thermobaric sintering. This conclusion is made on the basis of X-ray data analysis. The specific feature of transformation of detonation nanodiamond shells consists in a simultaneous occurrence of two processes - desorption of volatile impure compounds and formation of a newly-formed diamond phase or ordered graphite around diamond cores. This effect results in the formation of open fragments of diamond cores capable of contacting with neighboring nanodiamond crystals thus binding diamond grains into a composite. Sintering temperature is an essential factor that affects the nanocrystal size as well as the mechanical properties and homogeneity of the composite.

Термобарическое спекание детонационного наноалмаза при давлении 5 ГПа и температурах 1100-1500 °С позволяет получить поликристаллические агрегаты, распределенные в структуре компакта таким образом, что определяет большую дисперсию микротвердости по поверхности образца. Показано, что среднее значение микротвердости растет, а величина дисперсии уменьшается при увеличении температуры спекания. Так, например, повышение температуры от 1100 до 1500 °С сопровождается ростом среднего значения микротвердости от 8,8 до 12,2 ГПа и снижением дисперсии микротвердости от 4,9 до 1,5. Характерно, что разброс особенно велик на периферии образца, где значения микротвердости (образец, спеченный при 1200 °С) могут лежать в интервале от 3,2 до 12 ГПа. Примесный слой нанокристалла детонационного алмаза влияет на процессы консолидации частиц двояким образом. С одной стороны примесный слой препятствует контакту между смежными нанокристаллами, с другой – примесный слой, его летучая составляющая активно формирует флюидную составляющую процесса термобарического спекании. При этом во флюид, по-видимому, уходит и часть металлических примесей, таких как железо, алюминий, кальций и др. Например, частицы железа, присутствующие на поверхности наноалмазного ядра, способны активно двигаться по поверхности алмаза и агломерироваться в более крупные скопления частиц. Вовлеченные в состав флюида летучие соединения, количество которых может достигать 20 масс. %, и примесных атомов других элементов обнажают участки поверхности наноалмазного ядра, по которым могут формироваться ковалентные связи между смежными кристаллами. Ключевые слова: детонационный алмаз, термобарическое спекание, микротвердость, дисперсия микротвердости, флюидная составляющая процесса спекания.

Термобарическое спекание детонационного наноалмаза при давлении 5 ГПа и температурах 1100-1500 С позволяет получить поликристаллические агрегаты, распределенные в структуре компакта таким образом, что определяет большую дисперсию микротвердости по поверхности образца. Показано, что среднее значение микротвердости растет, а величина дисперсии уменьшается при увеличении температуры спекания. Так, например, повышение температуры от 1100 до 1500 С сопровождается ростом среднего значения микротвердости от 8,8 до 12,2 ГПа и снижением дисперсии микротвердости от 4,9 до 1,5. Характерно, что разброс особенно велик на периферии образца, где значения микротвердости (образец, спеченный при 1200 С) могут лежать в интервале от 3,2 до 12 ГПа. Примесный слой нанокристалла детонационного алмаза влияет на процессы консолидации частиц двояким образом. С одной стороны примесный слой препятствует контакту между смежными нанокристаллами, с другой примесный слой, его летучая составляющая активно формирует флюидную составляющую процесса термобарического спекании. При этом во флюид, по-видимому, уходит и часть металлических примесей, таких как железо, алюминий, кальций и др. Например, частицы железа, присутствующие на поверхности наноалмазного ядра, способны активно двигаться по поверхности алмаза и агломерироваться в более крупные скопления частиц. Вовлеченные в состав флюида летучие соединения, количество которых может достигать 20масс. , и примесных атомов других элементов обнажают участки поверхности наноалмазного ядра, по которым могут формироваться ковалентные связи между смежными кристаллами.

The results of modification of the impurity subsystem of detonation nanodiamonds (DND) (from the “Altai” Federal Research and Production Center (Biysk)) with active metals and mixtures of nickel and aluminum metals are presented. Samples made from three different metal-diamond mixtures were investigated: 1) 70 wt % of DND and 30 wt % of Al, 2) 50 wt % of DND and 50 wt % of mixture of nickel and aluminum in NiAl stoichiometry, 3) 70 wt % of DND and 30 wt % of mixture of nickel and aluminum in Ni3Al stoichiometry. The heating of samples of nanodiamonds in a vacuum volume (VUP-5) is accompanied by thermal desorption of volatile compounds. Analysis of the samples using differential scanning calorimetry and mass-spectrometry (DSC - on STA 409 PC Luxx NETZSCH, MS - on QMS 403 D Aeolos NETZSCH) allowed us to study the kinetics of desorption and the molecular composition of volatile compounds desorbed by heating nanodiamonds. Annealing of the charge with aluminum is accompanied by a nonmonotonic change in the weight of the sample during heating. Heating mixtures of nanodiamond with nickel and aluminum is accompanied by an almost monotonic decrease in weight. Weight loss at up to 950 °C annealing can reach 20%. When samples are heated to 950 °C, H2O (up to 200 °С), O2 (up to 60 °С), H2S (up to 700 °С), CO2 (up to 600 °С), SO2 (up to 450 °С), N2 (up to 60 °С) evaporate from the surface of the samples. Endo- and exo-effects indicate the occurrence of chemical reactions between impurities and metals. The cleaning process most actively takes place during the annealing of the metal-diamond mixture containing Ni and Al in Ni3Al stoichiometry.

Abstract—The structural and physicomechanical properties of composite materials obtained by sintering detonation nanodiamonds under high pressures and temperatures (P = 5 GPa, T = 1200°C) have been investigated. There is a slight growth of diamond crystals after sintering from 4.5 to 5.2 nm. It is shown that the strength of the samples locally reaches 14 GPa. The high microhardness of the material is due to the consolidation of diamond nanocrystals into strong polycrystalline aggregates owing to the formation of covalent bonds between crystals under high pressure and temperature conditions.

The process of consolidation of nanocrystals of detonation nanodiamond (DND) into polycrystalline aggregates during their annealing at high pressures and temperatures (HPHT) was studied. The experiments on HPHT annealing of DND powder were carried out using a high pressure apparatus BARS at 5GPa and 1100, 1200°С. It was established that after HPHT annealing the microhardness of sintered DND samples had a value of 14GPa. It was proposed that HPHT annealing of DNDs leads to deep purification of nanoparticle surface, resulting in formation of strong nanodiamond polycrystalline aggregates. The observed growth can be a result of interaction of neighboring DND nanocrystals along the contact areas that are purified of impurities during the HPHT annealing.

The experimental results of investigation of nanostructured diamond materials obtained by high-pressure high-temperature sintering of detonation nanodiamonds are presented. High-pressure high-temperature sintering of diamond nanoparticles provides solid polycrystalline aggregates. The value of polycrystalline aggregate microhardness was 9,1 GPa. Sintering of detonation nanodiamonds at the pressure of 5 GPa and the temperature of 1100-1200°C reduces the concentration of impurity atoms. Sintering is accompanied by a small growth of nanoparticles from 4,5 nm to 5,2 nm. It is demonstrated that the decrease in a concentration of impurity atoms is a result of temperature and pressure effects on detonation nanodiamonds. In conclusion, the assumption is made that this effect is associated with an increase of impurity atoms diffusion mobility and the formation of areas of impurity atoms excessive concentration. The formation of such areas reduces the impurity concentration on the borders of the diamond cores.

The impurity component of the detonation nanodiamond is studied. Detonation nanodiamonds were obtained by detonation decomposition of carbon-containing explosives in the production process at the Altai enterprise. According to technical specifications TU 84-112-87, the detonation products have passed the purification stage with mixtures of acids H2SO4 and HNO3. The content of the diamond component was brought to 95%. Using X-ray energy-dispersive microanalysis (the measurements were performed with a Quanta-200-3D scanning electron microscope), the main impurity elements on the surface of diamond nanocrystals are: oxygen (4.93 wt %), iron (4.52 wt %), sulfur (4.33 wt %), calcium (1.92 wt %) and aluminum (1.27 wt %). In a small amount, there may be other elements, the number of which depends on the technology of obtaining detonation nanodiamonds. The heating of samples of nanodiamonds in a vacuum volume (VUP-5) is accompanied by thermal desorption of volatile compounds, which is well documented by a decrease in the vacuum quality in the vacuum system. The mass loss during annealing can reach 20%. Purified during thermal desorption annealing, nanodiamonds have high sorption properties and practically restore the impurity shell, absorbing volatile compounds when placed in the atmosphere. Analysis of the samples using differential scanning calorimetry and mass-spectrometry (DSC -on STA 409 PC Luxx NETZSCH, MS - on QMS 403 D Aeolos NETZSCH) allowed us to study the kinetics of desorption and the molecular composition of volatile compounds desorbed by heating nanodiamonds. When nanodiamonds are heated to 950 degrees C, water (up to 200 degrees C), hydrocarbons (up to 300 degrees C), hydrogen sulfide (up to 480 degrees C), sulfur dioxide (up to 580 degrees C), carbon dioxide (up to 820 degrees C), hydrogen (up to 900 degrees C) evaporate from the surface of the nanodiamond. The obtained results testify to the complex hierarchical structure of the impurity subsystem of nanodiamond, consisting of metal atoms bound to carbon atoms of nanonuclear covalent bonds, and a layer of adsorbed volatile compounds.

A model of particles of detonation nanodiamonds is discussed. The model is based on the analyses of experimental data obtained from transmission electron microscopy, X-ray diffraction, smallangle X-ray scattering and simultaneous thermal analysis. This model considers the diamond particle as a crystalline diamond nanokernel coated by an impurity shell.

The molecular composition of volatile compounds belonging to the impurity subsystem of detonation nanodiamond particles has been analyzed. It is established that the main volatile impurities are water, hydrogen, nitrogen, methane, carbon dioxide, and sulfur dioxide. In the course of annealing, the concentration of these volatile impurities exhibits a significant decrease, the sample weight loss reaching up to 20%. This process is accompanied by both endo- and exothermal effects.

The element composition of admixture detonation nanodiamond is analyzed. Is established, that the basic admixture are oxygen, sulfur, iron. In a course annealing concentration of oxygen grows, concentration of sulfur decreases, decreases also concentration of carbon.