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Structural analysis of inclusions detected by x-ray inspection of preforms and parts from fine-grained graphite MPG-7

https://doi.org/10.38013/2542-0542-2019-4-54-59

Abstract

By X-ray diffraction analysis and Raman spectroscopy, we studied samples of fine-grained graphite MPG-7 with detected chemical and structural defects. We determined the effect of structural and chemical defects on the micro- and macrostructure of graphite and estimated its crystallinity depending on the type of defects detected.

For citation:


Vershinin A.V., Belyakova E.G., Vershinina M.V., Polyakov E.V., Bamburov V.G., Baklanova I.V., Vovkotrub E.G., Yazovskikh K.A. Structural analysis of inclusions detected by x-ray inspection of preforms and parts from fine-grained graphite MPG-7. Journal of «Almaz – Antey» Air and Space Defence Corporation. 2019;(4):54-59. https://doi.org/10.38013/2542-0542-2019-4-54-59

Introduction

Graphite of the MPG-7 grade belongs to the class of artificial graphite materials. Due to its resistance under conditions of thermal shock and high temperature gradient, it preserves its per­formance capacity at temperatures up to 3900 K in the medium of gas flow of erosion-aggressive products of composite propellant combustion [1], as well as gives up excessive heat due to radiation at the infrared and visible-spectrum wavelengths. For these reasons, the MPG-7 graphite is one of the most important construc­tion materials in manufacture of the throat in­serts for nozzle clusters of solid-propellant rocket motors (SPRM).

It is known that the mechanical and ther­mal-physical properties of artificial graphite materials are determined first and foremost by their structural characteristics. The MPG-7 grade graphite, which is used in the articles developed by Novator Design Bureau, is a fine-porous mate­rial produced by the method of powder sintering in the form of pressed blocks [2]. The process of graphitisation of semi-manufactured products (pressed-powder blocks) runs at a temperature above 2700 K, therefore graphite, as a rule, shall have good homogeneity and crystallinity.

A perfection criterion for such structure can be graphitisation index g, which is determined as follows:

where 0.344 nm - interlayer distance in turbo- stratic carbon [3];

d002 - interlayer distance (nm) in graphite;

0.3354 nm - interlayer distance in defect-free graphite monocrystal.

For artificial graphites of the MPG grade with high treatment temperature, the graphitisa- tion index lies within 0.8...0.9 relative units [4][5]. At the same time, graphite with more ordered structure has higher strength and performance capacity in high-temperature gas flow.

As was established earlier [6], certain regions revealed in an MPG-7 graphite ma­trix are characterised by high (up to 20 % wt.) content of impurity elements, as well as by mac- rostructural heterogeneity. Such regions are char­acterised by inhomogeneity revealed in X-ray images. One can presume that the cause of emer­gence of such macrostructure are defects in the packing of its layers and lattice bonds when a part of the carbon atoms has sp3-hybridisation [7]. However, the present-day scientific literature fails to refer to such interrelations between the observed inhomogeneity of graphite macrostruc­ture and its structural defects.

The objective of this paper is to determine the nature of macrostructural inhomogeneities during X-ray inspection of parts made of MPG-7 grade graphite and the effect of such inhomoge­neities on the graphite structure and crystallinity.

Experimental part

Based on the study of MPG-7 grade graphite pre­forms and parts by the methods of electron mi­croscopy and energy dispersive microanalysis, as well as X-ray analysis [5], the following samples were selected for carrying out structural investi­gations:

  • without determinable impurities and macrostructural inhomogeneities;
  • with chemical impurity elements;
  • with macrostructural inhomogeneities.

A detailed description of samples Nos. 2 and 3 and anomalies observed in them is given in paper [5].

The crystalline structure and phase composi­tion of the samples were studied on diffractometer Empyrean (PANalytical) in monochromated Cu- Ka radiation (graphite monochromator). Method sensitivity - up to 1% vol. of impurity phase. Determination of the crystalline structure param­eters was performed with the use of a software system for X-ray diffraction analysis [8][9]. The study samples were taken from crushed graphite material. For determining crystallographic texture, a plate cut out from a graphite preform was used.

Raman spectra (RS) were taken at room temperature using a RENISHAW-1000 spectrometer (λ = 532 nm, P = 7 mW). The sam­ples used were plates cut out from graphite pre­forms as well.

Results

For all studied X-ray diffractogram samples (Fig. 1), a characteristic feature was the presence of 2H graphite polytype basic reflexes (P63/mmc). At that, only sample No. 1 (Fig. 1, а) is a sin­gle-phase one. Reflex (*) on the diffractogram of sample No. 2 (Fig. 1, b) does not belong to graph­ite. The results of energy dispersive microanalysis performed for this sample in paper [5] demon­strated the presence of impurity elements Al, Si, and Ti in the composition. The impurity phases can be represented by TiO2 (orthorhombic) and/ or Al4C4Si (hexagonal), for which reflex (*) is the most intensive line (100 % relative intensity).

Fig. 1. X-ray diffractograms of samples: а – No. 1; b – No. 2; c – No. 3; d – crystalline phase identifier diagram; (*) – impurity phase reflex

The observed increase of the background level in the diffractogram small-angle region (17-23°) indicates the presence of an amorphous com­ponent in the sample along with the impurity phases. In sample No. 3 no impurity phases are observed; however, same as in sample No. 2, there is an amorphous component in it (Fig. 1, c). Broadening of graphite reflexes can be caused by defects in its structure. Characterised by the narrowest reflexes of lines 00l and hk0 is sample No. 1, which proves the presence of large-size co­herent scattering regions (CSR) as compared with the other samples (see Fig. 1).

The table for graphite samples shows lattice parameters a and c, CSR size <L>, and graphitisation index g calculated by equation (1). Given for comparison are also the values of а, с, and g, taken from literature [4][10] for the MPG-7 grade graphite. As seen from the table, sample No. 1, having the minimum value of lattice parameter c and the maximum graphitisation index g amongst the samples considered, has a more perfect crys­talline structure. The obtained parameter values for this sample well correlate with the data of [4][10], whereas the values of parameter c in sam­ples Nos. 2 and 3 are more characteristic for the turbostratic rather than for the ordered 3D struc­ture of graphite.

Lattice parameters а and c, CSR size and graphitisation parameter g for considered samples of MPG-7 graphite

Sample

a, А (±0,002)

с, А (±0,002)

<L>, nm (±1)

g, rel. un.

No. 1 (a)

2,466

6,745

15

0,8

No. 2 (b)

2,459

6,775

10

0,6

No. 3 (c)

2,462

6,778

8

0,6

Graphite МPG-7 (data from [4][10])

2,460

6,716-6,754

29

0,8-0,9

Along with determination of lattice param­eters, crystallographic texture in the considered samples was analysed. To that end, the diffraction spectra obtained on powders and from plate sur­face were compared. The intensities and shapes of spectrum lines showed absolute matching in both cases, which testifies to the absence in them of any preferential crystallographic orientation.

For estimation of defect structure and the amount of amorphous phase in the samples, fur­ther investigation into their structure was per­formed with the use of Raman spectroscopy, which enables to make a quantitative evaluation of graphite’s crystalline structure perfection de­gree and get an insight into the nature and amount of structural defects. This method is sensitive to defects in the layers, presence of interstitial atoms, ordering in the packing. Besides, it helps to determine the nature of hybridisation in the layer fragments of graphite.

Fig. 2 shows RS of the graphite samples.

The RS of all samples include lines char­acteristic of polycrystalline graphite. The line at 1572 cm-1 corresponds to its ideal vibrational mode with symmetry E2g (line G) [11]. The po­sition and intensity of line G, by means of which it is possible to determine carbon graphitisation degree, correspond to carbon atoms oscilla­tions in sp2-hybridisation. The line at 1343 cm-1 is induced by disordered carbon atoms; it relates to lattice oscillations with symmetry A1g (line D). This line is conditioned by C-C bonds with sp3-type hybridisation. Line D represents a characteristic of carbon material defectiveness degree and is the cause of structural disorder. It is absent in single-crystal graphite, and an in­crease of its intensity is conventionally regard­ed as the result of an increase in the amount of disordered carbon in a sample. The line at 2693 cm-1 (line G' ) is an overtone of line D.

All three samples are characterised by well-resolved and intensive line G (see Fig. 2), which means presence of a large number of car­bon atoms in sp2-hybridisation in all the samples. In the spectrum for sample No. 1 line D is not observed. This goes to show that the sample is characterised by good crystallinity. In samples Nos. 2 and 3 line D manifests itself. Its consider­able intensity, as compared with line G, indicates the presence of defect regions in the samples’ structure.

The known interpretation of the intensity ratio of lines D and G allows us to estimate at the semi-quantitative level the dimensions of the or­dered regions of an amorphocrystalline substance and distinguish graphite, by its RS, from other carbon forms [12]. A high value of the ratios of lines D/G intensity indicates considerable defec­tiveness of the samples. The relationships (ID /Ig) are linked by semi-quantitative expression (2) to the size of graphene crystallites (La) [13] present in the base plane:

Here, λ - excitation laser wavelength in nm (in the considered case - 532 nm).

Sample No. 1, for which line D is absent (see Fig. 2), should be categorised as crystalline graphite, whereas samples Nos. 2 and 3 are amor­phous graphites, size La for which is equal to 5 and 7 nm, respectively.

As a result of structural analysis of macro- structural and chemical inhomogeneities of the scrutinised articles [5], their belonging to objects with poor crystallinity was established [4][14]. Samples with such defects can be categorised as amorphous graphite because of the minimum size of the ordered regions and the maximum content of amorphous phase with sр3-type hybridisation.

In that case impurity elements form indi­vidual phases, characterised by their symmetry type and lattice parameters, and increase total defectiveness. It can be stated that the observed impurity phases in graphite are conditioned by the quality of initial stock material [15], when its low homogeneity and crystallinity is a consequence of insufficient thermal treatment of preforms. Such structural defects may become the cause of unwanted variability in physical and mechanical properties of the end product [2]. Subsequently, graphite preforms with structural peculiarities (chemical and macrostructural inhomogeneities) detected by the X-ray inspection method, as ap­plied in the R&D activities of “Novator” De­sign Bureau, require additional evaluation and screening in order to improve their performance capacity as throat inserts of SPRM.

Conclusions

  1. Macrostructural inhomogeneities observed during X-ray inspection of parts manufactured from graphite of the MPG-7 grade represent amorphous graphite with the size of ordered car­bon regions of the order of 5-10 nm.
  2. Impurity elements observed in graphite form their own phases characterised by their in­dividual symmetry type and lattice parameters.
  3. The studied graphite samples, with their inherent chemical and structural defects, are char­acterised by turbostratic rather than the ordered 3D structure, which is characteristic of the MPG-7 grade graphite.
  4. Based on the obtained data, to carry out incoming inspection by the X-ray inspection method, quality references for preforms and parts of the throat inserts for nozzle clusters of SPRM were created.

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About the Authors

A. V. Vershinin
Experimental Design Bureau “Novator”, Joint Stock Company
Russian Federation

Vershinin Aleksandr Vadimovich ‒ Candidate of Physical and Mathematical Sciences, Leading Design Engineer. Science research interests: structure and properties of carbon materials.

Ekaterinburg



E. G. Belyakova
Experimental Design Bureau “Novator”, Joint Stock Company
Russian Federation

Belyakova Elena Germanovna ‒ Doctor of Engineering Sciences, Head. Science research interests: chemistry and technology of non-metallic materials.

Ekaterinburg



M. V. Vershinina
Experimental Design Bureau “Novator”, Joint Stock Company
Russian Federation

Vershinina Marina Vadimovna ‒ Candidate of Engineering Sciences, Head of Department. Science research interests: nondestructive control.

Ekaterinburg



E. V. Polyakov
Institute of Solid State Chemistry of the Ural Branch of the Russian Academy of Sciences
Russian Federation

Polyakov Evgeniy Valentinovich – Doctor of Chemical Sciences, Chief Research Fellow. Science research interests: sorption, thermodynamics, kinetics, trace elements, radionuclides, state forms.

Ekaterinburg



V. G. Bamburov
Institute of Solid State Chemistry of the Ural Branch of the Russian Academy of Sciences
Russian Federation

Bamburov Vitaliy Grigorievich – Doctor of Chemical Sciences, Professor, Chief Research Fellow. Science research interests: physical chemistry of rare and rare earth elements, processes and apparatus for producing chemical compounds.

Ekaterinburg



I. V. Baklanova
Institute of Solid State Chemistry of the Ural Branch of the Russian Academy of Sciences
Russian Federation

Baklanova Inna Viktorovna – Candidate of Chemical Sciences, Senior Research Fellow. Science research interests: study of structural features of inorganic compounds by vibrational spectroscopy (Raman spectroscopy, infrared spectroscopy), determination of their optical and emission characteristics, electronic spectra of absorption of ultraviolet, visible and near infrared regions, luminescence.

Ekaterinburg



E. G. Vovkotrub
Institute of High Temperature Electrochemistry of the Ural Branch of the Russian Academy of Sciences
Russian Federation

Vovkotrub Emma Gavrilovna – Candidate of Engineering Sciences, Senior Research Fellow. Science research interests: Raman spectroscopy of inorganic compounds.

Ekaterinburg



K. A. Yazovskikh
Federal State Budgetary Center of the Ural Branch of the Russian Academy of Sciences
Russian Federation

Yazovskikh Kseniya Aleksandrovna – Candidate of Physical and Mathematical Sciences, Senior Research Fellow, Physical-Technical Institute. Science research interests: X-ray diffraction, structural phase analysis, crystal structure.

Izhevsk



For citation:


Vershinin A.V., Belyakova E.G., Vershinina M.V., Polyakov E.V., Bamburov V.G., Baklanova I.V., Vovkotrub E.G., Yazovskikh K.A. Structural analysis of inclusions detected by x-ray inspection of preforms and parts from fine-grained graphite MPG-7. Journal of «Almaz – Antey» Air and Space Defence Corporation. 2019;(4):54-59. https://doi.org/10.38013/2542-0542-2019-4-54-59

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