Structure of metal-carbon nanocomposite based on pyrolysed derivatives of diphthalocyanines for immobilization of radioactive waste
In order to solve the problems of processing, disposal and transmutation of radioactive waste (RW), it is necessary to create chemically and thermally stable matrices of primary immobilization that are resistant to ionizing radiation and capable of reliably binding (immobilizing) long-lived isotopes, including minor actinides. Nowadays, the matrices based on borosilicate and phosphate glasses [1–2] and SYNROC-type materials  are used to immobilize the highly radioactive waste (HRW) and spent nuclear fuel (SNF) on an industrial scale. The mineral-like composites [4–6] are also being developed. The carbon based matrices do not concede to these traditional materials by functionality. It is well-known that carbon is chemically stable and capable of withstanding high temperature, and both its isotopes (12C and 13C) have a low neutron capture cross-section (3.4 mb and 1.3 mb respectively), that is important at transmutation processes of nuclides enclosed in such matrices.
Thus, the actual problems are to search for nanocomposites, where metal atom is imbedded into the carbon matrix and may be fixed in it, to study such structures and their physical and chemical properties and to develop simple methods of synthesis ensuring high yield of the product. This work is aimed at obtaining metal-carbon nanocomposites using pyrolysis of diphthalocyanine molecules in oxygen-free media and investigations of their structure. The diphthalocyanine molecule is a precursor and may be formed by almost all f and some 4d-elements as a "sandwich" where a metal atom fixes two organic ligands (molecules of phthalonitrile) . A high level of ligands was kept as the tests for chemical, thermal and radioactive stability  at the pyrolysis of diphthalocyanines where the metal as a complex former was represented by radioactive isotopes prepared from fractioned radioactive waste (RW) and highly radioactive fuel (HRF) of Novovoronezh nuclear power plant (its activity was equal to 5 Ci, approximately). Because of high level of the activity a study of such compounds has not been possible. In this work yttrium as a metal for the complex formation was chosen as the object of study, which is an analogue of 4f- elements of group III. The peculiarities of the modeling object structuring will be characteristic of radioactive isotopes of lanthanides and actinides, and studying of the object with modern methods, such as transmission electron microscopy (TEM) and atomic force microscopic (AFM) allows of forming the scientific basis of metal-carbon nanocomposites usage in the different fields of nuclear power engineering.
METHODS OF RESEARCH
Diphthalocyanines were obtained by a well-known method  by alloying of yttrium acetate with o-phthalonitrile in a quartz reactor (weight ratio 1:6, inert atmosphere) at a temperature of 280–290 °C for 25–30 minutes, after which the temperature was increased to 400 °C for distilling the unreacted o-phthalonitrile. Pyrolysis was carried out at 1270 °С in a vacuum furnace for 1 hour where temperature was controlled by a pyrometer. Transmission electron microscopy measurements were carried out with TITAN 80–300 (FEI, USA). The yttrium diphthalocyanine pyrolysate powder was pre-ground in an agate mortar, mixed with alcohol and subjected to ultrasound for 25 minutes, then a droplet of the suspension was applied to a carbon net (Lacey Carbon, USA) using a dispenser. The substrate was placed into a sample cleaning installation for TEM measurements in argon-oxygen plasma, Plasma Cleaner-1020 (Fischione, USA).
AFM-measurements were made in a semi-contact scanning in air mode with Solver microscope (NT-MDT, Russia). Cantilevers NSG03 (NT-MDT) with rigidity constant 1.74 N/m, were used, the probe tip radius was 10 nm and the scanning speed 1 Hz.
The sapphire glass (Lighten Optics, China) was used as a substrate in ACM measurements after vacuum evaporation of diphthalocyanine onto its surface. After this the sapphire glass was placed in a vacuum furnace and the pyrolysis of diphthalocyanine was carried out at 1,300 °С.
RESULTS AND DISCUSSION
Transmission electron microscopy images for different parts of the sample (yttrium pyrolisate diphthalocyanine YCx) have been obtained. In all cases the crystal particles of 6 to 30 nm distributed in the carbon matrix were observed (see Fig.1).
The carbon matrix structure consists of parallel carbon crystal layers oriented chaotically towards each other at different angles, and interplanar spacing is slightly enlarged to 0.345 nm as compared with the polycrystalline graphite (0.3354 nm). This structure corresponds to the turbostratic carbon , but carbon matrix does not almost transform to a graphite one, and turbostratic carbon layers cover the crystal particles; amorphous carbon has been presented in a volume of the matrix. Fig.2 shows a magnified image of a crystal particle. It was detected that the parameters of a crystal lattice correspond to α-yttrium one (interplane distance is 0.31 nm) which refers to magnesium structural type peculiar of the majority of lanthanides (Gd, Tb, Dy, Ho, Er, Tm, Lu) and actinides – Am, Cm. Hence, taking into account the general type of a metal crystal lattice, we can expect that the structuring processes for various diphthalocyanine metal complex formers, which are responsible for pyrolysate structure at the high-temperature pyrolyse, account for high fixing of radioactive nuclides of lanthanides and actinides observed in previous experiments .
Crystal metal particles covered by a carbon shell made of turbostratic carbon are formed at high-temperature pyrolyse of diphthalocyanines and reliably protect the metal against environmental influence. According to , the content of metal in the samples varies from 16 to 30% by weight. On the contrary, adding of metal to well-known metal-carbon structures, such as carbon nanotubes, graphite compounds, metallocarbons and nano-onions lead to formation of the carbides, but current arc synthesis of endofullerens produces a low-level yield of the product (0.4 %) . Thus, the pyrolyse of diphthalocyanines is the simple and effective method to obtain the metal-carbon structures with metal complex-formers consisting of lanthanides and actinides.
When a scale of the research changes from nano- to micro objects, the AFM-measurements of YCx surface morphology detect the nanoclusters across all scanned field (see Fig.3). In this case the nanoclusters form the long-scale branched structure with obviously high-porous structure inside the carbon matrix, which was confirmed in previous experiments at small angle scattering of neutrons on pyrolysate powders of Y, La, Sm and U . It is well known that carbon has sorption properties because of high-porous structure . However, in case of turbostratic carbon, where a binding energy of planes is slightly low than for polycrystal graphite (4.2–8.4 and 4.2–18.2 kJ/mol correspondingly) , a smaller temperature stability can be observed.
At that, the high-porous structure of pyrolysate of diphthalocyanine will be an additional barrier to fix the radioactive nuclides at the thermal destruction of turbostratic layers at temperatures higher ~1,600 °С .
Structures of metal-carbon nanocomposites based on yttrium pyrolysate of diphthalocyanine in nano- and micro-scales were studied using TEM and AFM methods. TEM-measurements data confirm that metal crystals inside the graphitized matrix are being formed at pyrolysis of diphthalocyanine. The structure of graphitized layers consists of the turbostratic carbon. It was determined that parameters of metal particles correspond to α-yttrium crystal lattice, i.e., to magnesium structure type which is characteristic of the majority of lanthanides and actinides, such as americium and curium. AFM-data indicates the aggregation of crystals to nanoclusters and formation of highly porous structure. The data obtained in this work lay the scientific basis of metal-carbon nanocomposite usage in the different fields of nuclear power industry, in particular, as the immobilizing matrices for the highly radioactive waste (HRW) and the spent nuclear fuel (SNF) on an industry scale
The study was carried out with the financial support of the Russian Foundation for Basic Research in the framework of the research project No. 18-32-00500.