Nickel-Graphite Composites of Variable Architecture by Graphitization-Accompanied Spark Plasma Sintering and Hot Pressing and their Response to Phase Separation

We report the formation and phase separation response of nickel-graphite composites with variable-architecture phases by graphitization-accompanied consolidation via Spark Plasma Sintering and hot pressing. It was shown that the application of pressure during consolidation is crucial for the occurrence of graphitization and formation of 3D graphite structures. We evaluated the suitability of the synthesized composites as precursors for making porous structures. Nickel behaved as a space holder with the particle size and spatial distribution changing during consolidation with the temperature and determining the structure of porous graphite formed by phase separation by dissolution in HCl. The response of the consolidated Ni-Cgr to separation of carbon by its burnout in air was studied. The result of the carbon removal was either the formation of a dense and continuous NiO film on the surface of the compacts or oxidation through the compact thickness. The choice between these two options depended on the density of the compacts and on the presence of carbon dissolved in nickel. It was found that during the burnout of graphite from Ni-Cgr composites, sintering, rather than formation of pores, dominated.


Introduction
At present, metal-assisted graphitization is widely used in the synthesis and design of materials of different microstructures and morphologies [1][2][3][4][5][6][7].The catalytic role of metals has received a lot of attention as an interesting phenomenon from the viewpoint of solid-state chemistry, and metal-catalyzed graphitization has become a process of high potential for practical applications.The interest to porous carbon materials with submicron and micron-sized pores or hierarchical structure is stimulated by the possibilities of their use as substrates for catalytic growth of carbon nanotubes [8].Recently, the preparation and microstructure design of porous materials based on the use of the phase separation approach and removable templates has gained significant attention [9][10][11][12][13][14][15][16][17][18].Porous materials can be fabricated by consolidation of composite powder mixtures by sintering [11,[13][14][17][18] or metal injection molding [18] followed by phase separation (treatment resulting in the selective removal of one of the phases -"separation" from the composite structure).Hakamada et al. [11] proposed making porous aluminium using particles of sodium chloride mixed with the aluminium powder and further removing them from the sintered compact by dissolution.This technique allowed producing pores, the size of which was controlled by the size of the space holder particles, which did not change during the processing.Finer pores can be formed by selective removal of one of the phases, if the powders are initially mixed at a smaller length scale and consolidated preserving the phase distribution.Metallic catalysts introduced in substantial quantities into the mixtures can also act as templates for porous structures.A high volume content of nickel, whose grains form a framework within the structure of a consolidated compact, is beneficial for the in situ formation of another framework -that of graphite grains.The advantages of confined graphitization have been outlined by Tao et al. [19] who used the pore space of a silica template (microreactor) to conduct graphitization by pyrolysis of polymer precursors and preserve the porous structure of the graphitic material without collapse.Graphitization of amorphous carbon can, in turn, influence sintering of the metal phase in a metal-carbon system.Sintering of the metallic particles in contact with carbon support experiencing graphitization due to the catalytic action of the metal was shown by Gan et al. [20].
The goal of the present study was to evaluate the potential of the phase separation approach for nickel-graphite composites fabricated by Spark Plasma Sintering (SPS) [21][22] in a wide range of consolidation conditions and hot pressing (HP).In this work, the results of nickel dissolution from the compacts consolidated in the solid-state at different temperatures and liquid-phase sintered are compared.The microstructure evolution accompanying the burnout of graphite from the compacts is also presented.Based on the experimental results, conclusions are drawn on the efficiency of nickel and graphite as space holders and sacrificial phases during phase separation in the Ni-C gr composites obtained by consolidation of powder mixtures.

Experimental
Carbonyl nickel (Norilsk Nickel, Russia, 99.9% purity, average particle size 5 µm) and amorphous carbon (Omsk, Russia, 95% purity, lampblack PM-15, globule size 100-200 nm) powders were taken as raw materials.Nickel-amorphous carbon mixtures (Ni-C am ) containing 81.3 wt.% of Ni were prepared to produce 50 vol.%Ni -50 vol.%C composites in the consolidated state.Powder mixtures were prepared by mixing in a horizontal ball mill and by high-energy mechanical milling in a planetary ball mill.Spark Plasma Sintering of the powders was carried out using a SPS Labox 1575 apparatus (SINTER LAND Inc., Japan).Graphite dies of 10 and 20 mm inner diameter and graphite punches were used.The die wall was lined with a carbon foil.The flat ends of the punches were also protected by circles of graphite foil.The temperature during the SPS was controlled by a pyrometer focused on the outer wall of the die at its mid-plane.The sample was heated at an average rate of 60 ºC•min -1 .The sample was held at the maximum temperature for 10 min and then cooled down to room temperature.At the beginning of the sintering cycle, a uniaxial pressure of 40 MPa was applied and kept constant through the sintering cycle.Hot-pressing was performed in argon at 1000 °C.The sample was heated with a rate of 60 ºC•min -1 , a pressure of 40 MPa was applied during the whole hot-pressing cycle.The holding time at the maximum temperature was 10 min.The temperature was measured by a pyrometer focused on the outer wall of the die at its mid-plane in a manner similar to that in the SPS.The theoretical density of nickel-graphite composites (Ni-C gr ) was estimated assuming the density of C am and C gr equal to 2 g•cm -3 .Ni-C am powder mixtures were annealed for 1 h at 900° C for a reference experiment.A quartz tube of 8 mm diameter was filled with the Ni-C am mixed powders and the assembly was held in a furnace in an atmosphere of flowing argon.The phase separation treatment was conducted on compact samples of the same size.Selective dissolution of Ni from the Spark Plasma Sintered and hot-pressed compacts was performed in 10 wt.% HCl aqueous solution.The samples were placed in solution containing an excess of HCl for 48-144 h, removed from it, washed in deionized water and dried.Burnout of carbon from the Ni-C gr compacts was carried out by annealing in air at 800 °C for 30 min.The phase composition of the consolidated materials and products of phase separation was studied by X-ray diffraction (XRD) using Cu Kα radiation.The XRD patterns were recorded using a D8 ADVANCE diffractometer (Bruker AXS).The microstructure of the consolidated materials and those obtained by phase separation was studied by Scanning Electron Microscopy (SEM) using a Hitachi-Tab.topTM-1000 and a Hitachi-3400S Scanning Electron Microscopes.Secondary electron (SE) and backscattered electron (BSE) imaging modes were used.Fractography of the compacts was performed.Differential Scanning Calorimetry (DSC) and thermogravimetric (TG) analyses were performed using a STA 449 F/1/1 JUPITER thermal analysis instrument (Netzsch) in a flow of Ar+20 vol.% O 2 .The samples were heated up to 900 °C with a heating rate of 10 °C/min.Specific surface measurements of the porous graphite samples were conducted using thermal desorption of argon by Brunauer-Emmett-Teller (BET) method.

Pressure-less sintering of Ni-C am
Graphitization in the nickel-amorphous carbon system in the powder state has been observed in the mixtures subjected to high-energy mechanical milling prior to annealing by Bokhonov&Korchagin [3]; in those mixtures, an intimate contact between the components and a well-developed interface were established.In the present work, prior to SPS and HP consolidation experiments, a reference experiment has been conducted: a quartz tube was filled with the Ni-C am mixed powders and the assembly was held for 1 h at 900° C in an atmosphere of argon.The powder was not coldpressed before annealing, as attempts to do so were unsuccessful and did not result in the formation of a compact suitable for conventional pressure-less sintering.The annealed material was able to hold shape and the compact was pushed out of the quartz tube and analyzed.The XRD phase analysis did not reveal any graphite in the compact, probably due to a lack of intimate contact between nickel and amorphous carbon powder particles.This experiment has shown that the nickel/carbon interface is crucial for the occurrence of graphitization, growth of graphite grains and formation of 3D structures composed of these grains.Therefore, in order to obtain 3D graphite structures suitable to become the basis of self-standing porous graphite, it is necessary to use a pressure-assisted consolidation technique, such as SPS or HP, to carry out graphitization-accompanied consolidation of Ni-C am materials.

Graphitization-accompanied Spark Plasma Sintering and hot-pressing of Ni-C am
Fig. 1 shows a sequence of processing steps and transformations in nickelcarbon materials during the graphitization-accompanied consolidation and phase separation treatment.SPS conducted at pyrometer-measured temperatures of 800 and 1000 °C and HP were the processes of solid-state consolidation; no evidence of melting of nickel was found.SPS at a pyrometer-measured temperature of 1240 °C involved melting of a substantial fraction of nickel, an indication of which was the presence of a nickel film between the die inner walls and the punches formed as a result of the melt's partial squeezing out of the compact.Higher overall temperatures in electrically conductive samples during SPS relative to those measured on the outer wall of the graphite die have been predicted by modelling studies [23].Experimentally, local melting of a metallic component has been observed in the vicinity of the contacts between the powder agglomerates when consolidating mechanically milled composite powders at a temperature much lower than the melting point of the metal -according to the measurements performed by a thermocouple placed into a hole in the die wall [24].As can be seen from Tab. 1, while denser samples can be obtained by solid-state consolidation using mechanically milled Ni-C am mixtures than by mixed powders, the mixing state of the Ni and C am powders was not significant for densification and microstructure development in the samples, whose sintering involved melting of nickel.The temperature inside the nickel-carbon sample during the SPS run at a maximum temperature of 1000 °C measured at the outer wall of the die was higher than in the HP cycle at the same maximum temperature.This temperature difference caused a somewhat lower relative density of the hot-pressed composite (Tab.1).The amount of nickel in the Ni-C am mixtures was sufficient for achieving complete graphitization of the amorphous carbon provided consolidation was performed at sufficiently high temperatures.The enhancement of crystallinity of carbon black in its initial and graphitized forms as a result of ball milling-induced smoothing of angled surface of the particles was reported [25], however, it was not expected in our work due to mostly spherical, rather than polyhedral, morphology of the amorphous carbon powder.

Phase separation in Ni-C gr : dissolution of Ni in HCl solution
The architecture of the graphite framework depends on both the consolidation temperature and the mixing state of the Ni-C am powders; the response of the Ni-C gr material to the separation of nickel by dissolution in HCl acid is indicative of the differences in the architecture of the 3D graphite.The microstructure response of the Ni-C gr materials to treatment in HCl acid can be described using the electron microscopy characterization and phase composition analysis of graphite-based materials formed as a result of selective dissolution of nickel.XRD patterns of the Ni-C gr compacts Spark Plasma Sintered at 1000 °C and 1240 °C along with those of the porous graphite obtained by dissolving nickel from the compacts are shown in Fig. 2 (ad).In the XRD pattern of the liquid-phase sintered compact (Fig. 2 (d)), graphite (002) reflection has a significantly higher intensity and is narrower relative to the solid-state sintered compact (Fig. 2 (b)) indicating a higher graphitization and crystallisation degree of carbon.Nickel was essentially removed during the dissolution treatment in HCl for 48 h.Porous graphite obtained by selective dissolution of nickel from the Ni-C gr compacts solid-state consolidated from the mixed powders contained pores with a shape close to spherical and with a size of the initial nickel particles, whose shape was retained in the consolidated material.The microstructure of the Ni-C gr composites Spark Plasma Sintered at 1000°C from the mechanically milled powders and that of porous graphite obtained by phase separation are presented in Fig. 3 (a-b).Porous graphite obtained from the Ni-C gr compacts Spark Plasma Sintered from the mechanically milled powders does not show such large pores has a uniform fine-grained structure (Fig. 3 (b)).The grains of the porous graphite obtained from the hot-pressed compact are close in size to those revealed by selective dissolution of nickel from the compact Spark Plasma Sintered at the same temperature.In accordance with that, the specific surface values of the porous graphite obtained from hot-pressed and Spark Plasma Sintered at compacts are also close to each other (Tab.I).While 48 h of treatment in HCl solution was enough to dissolve practically all nickel from the compacts Spark Plasma Sintered at 1000 °C and hot-pressed (Fig. 2, b), this was not the case for the compacts Spark Plasma Sintered at 800 °C from the mixed powders and Spark Plasma Sintered at 1240°C (Fig. 2 (d)).The compact Spark Plasma Sintered at 800 °C from the mixed powders contained many non-deformed Ni particles retaining their shape, so their dissolution in HCl required a longer time.Interestingly, the compact Spark Plasma Sintered at 1000 °C from the mixed powders tended to lose nickel through dissolution in HCl more easily, which could have been due to more intense deformation of Ni particles at a higher sintering temperature and a reduction in their size by mixing and forming interpenetrating structures with agglomerates of the carbon particles under an applied pressure.After holding the compacts Spark Plasma Sintered at 1240 °C for 48 h in HCl solution, we found substantial quantities of nickel remaining in the compacts.The treatment in HCl acid was, therefore, continued.As is seen from the XRD pattern of the compact treated in HCl for 144 h (Fig. 2 (d)), nickel is still remaining in the material.The situation was the same for the compacts Spark Plasma Sintered at 1240 °C from mechanically milled and mixed powders.It appears that due to coalescence of molten nickel and the formation of enlarged particles, longer treatment in HCl acid was required to remove the Ni phase from the compacts.The platelets of porous graphite obtained from the compacts consolidated by liquid-phase sintering (Fig. 3 (d)) were larger than those formed during solid-state consolidation (Fig. 3 (b)), and possessed a well-defined hexagonal shape.Grain growth, however, did not coincide with densification enhancement of the compacts, coarser-grained Ni-C gr showing lower relative densities (Tab.I).

Tab
No difference was detected in the size of graphite platelets formed in the liquidphase sintered Ni-C gr compacts obtained from the milled and mixed powders.The growth of graphite platelets can occur by dissolution of carbon in liquid nickel, as the solubility of carbon in liquid nickel is greater than in the solid state, and its precipitation on the already existing grains.The specific surface of the porous graphite-based material obtained by phase separation from the compacts Spark Plasma Sintered at 1240 °C was one order of magnitude lower than that of the porous graphite produced from the solid-state consolidated Ni-C gr compacts.

Phase separation in Ni-C gr : burnout of carbon in air
In many cases, phase separation is a simple process of void formation; however, chemical reactions and structural transformations may accompany the removal of the space holder phase.An example of the influence of the phase separation process on the state of the target phase was reported in our previous work [14], in which porous silver was obtained from mechanically milled Ag-Fe composites by dissolving Fe in HCl solution.Upon dissolution of the space holder phase, recrystallisation of silver -the porous structure-forming phase -was observed.The microstructure response of the nickel-graphite composites to separation of graphite by its oxidation presents an interesting and new subject, as, to our best knowledge, the oxidation behaviour of nickel-carbon composites has not been studied until now.It can be assumed that burning the carbon phase out of the Ni-C gr compacts in air can be associated with the formation of NiO.It was shown that there exists a complex relationship between the microstructure of nickel oxide and the purity of the bulk metal, the volume change upon oxidation playing an important role and determining the grain morphology of NiO [26][27].TG and DSC data obtained in the Ar+O 2 flow (Fig. 4) showed that between 600 and 800 °C, the oxidation of graphite (leading to weight losses) and nickel (leading to weight gain) proceed simultaneously.Indeed, the Ni-C gr compacts annealed in air at 600 °C, showed the presence of NiO, Ni and graphite reflections in their XRD patterns.A temperature of 800 °C was, therefore, selected to carry out the phase separation in the Ni-C gr compacts.The compacts Spark Plasma Sintered at 800 °C get oxidized through the thickness of the sample (Fig. 5 (a)) while the compacts Spark Plasma Sintered at 1000 °C form a dense layer of NiO on the surface (Fig. 5 (b)), the inner parts remaining intact.The compact Spark Plasma Sintered at 800 °C from the mechanically milled mixture and that Spark Plasma Sintered at 1000 °C from mixed powders possess the same relative density (Tab.1); however, their microstructure responses to phase separation by burnout of carbon are very different.The morphological outcome of the oxidation treatment appears to be dependent on the relative density of the Ni-C gr compacts as well as on the presence of Ni(C) solid solutions formed as a result of high-energy ball milling.During oxidation, the presence of carbon in the solid solution may result in the evolution of CO thus preventing the formation of a continuous NiO film.So, while denser Ni-C gr compacts tend to form a continuous NiO film on the surface, the dissolved carbon can cause the disruption of this film.A longer annealing of the compacts in air leads to the formation of NiO structures shown in Fig. 5 (c).The NiO-based compacts did contain porosity; however, that porosity was inherited from the Ni-C gr compact that was not fully densified (78% dense), also, the NiO structures can contain closed porosity.The specific surface of this porous NiO-based compact was low -1.06 m 2 /g.Hence, it can be concluded that the microstructure response of Ni-C gr compacts to separation of carbon by burning in air is the formation of a NiO-based compact with porosity inherited from the porosity of the compact and closed porosity in the grown NiO hollow structures.
The NiO framework formed as a result of nickel oxidation simultaneous with oxidation of carbon did not structurally repeat the Ni framework that existed in the compact.The NiO grains tended to sinter forming either a dense continuous layer on the surface of the Ni-C gr compacts or structures with non-uniformly distributed porosity and low specific surface.Large pores in the synthesized NiO originated from the pores of the Ni-C gr compacts that did not reach full density.During the burnout of graphite from Ni-C gr composites, a sintering process, rather than formation of porous structures, dominated.

Conclusions
This work has shown that Spark Plasma Sintering and hot-pressing of nickelamorphous carbon powder mixtures containing 50 vol.% of Ni result in the graphitization-accompanied consolidation and the formation of Ni-C gr composites with architecture of the phases sensitive to the consolidation temperature and initial structure of the powder mixtures.As a next step, we evaluated the suitability of these composites as precursors for developing porous materials through the removal of one of the phases.Nickel served as a graphitization catalyst and as an efficient space holder for producing porous graphite by phase separation in Ni-C gr composites.The growth of graphite grains in the presence of molten nickel was more pronounced resulting in a reduced specific surface of the porous graphite relative to that obtained from solid-state consolidated compacts.We have brought the phase separation method of making porous structures to a new level by developing materials, in which the architectures of both phases -the sacrificial phase and the target phase -can be controlled by changing the consolidation conditions.The Ni-C gr composites responded to the removal of carbon by its burnout in air via two different routes.One was the formation of dense and continuous NiO film on the surface of the compacts, and the other was oxidation through the compact thickness.Which of the two routes was preferred was influenced by the density of the compacts and possibly by the presence of carbon dissolved in nickel.It was found that during the burnout of graphite from Ni-C gr composites, a sintering process, rather than formation of porous structures, dominated.This study has provided examples of the influence of the architecture of the phases on the results of the phase separation treatment of composite materials.

Fig. 1 .
Fig. 1.Processing steps and transformations in nickel-carbon materials during graphitization-accompanied consolidation and phase separation treatment.

Fig. 2 .
Fig. 2. XRD patterns of the consolidated Ni-C gr composites and porous graphite obtained by phase separation: (a) Ni-C gr Spark Plasma Sintered from the mixed powders at 1000 °C; (b) porous graphite obtained by phase separation from (a), treatment in HCl acid for 48 h; (c) Ni-C gr Spark Plasma Sintered from the mixed powders at 1240°C; (d) porous graphite obtained by phase separation from (c), treatment in HCl acid for 144 h.

Fig. 2 .
Fig. 2. XRD patterns of the consolidated Ni-C gr composites and porous graphite obtained by phase separation: (a) Ni-C gr Spark Plasma Sintered from the mixed powders at 1000 °C; (b) porous graphite obtained by phase separation from (a), treatment in HCl acid for 48 h; (c) Ni-C gr Spark Plasma Sintered from the mixed powders at 1240°C; (d) porous graphite obtained by phase separation from (c), treatment in HCl acid for 144 h.Nickel was essentially removed during the dissolution treatment in HCl for 48 h.Porous graphite obtained by selective dissolution of nickel from the Ni-C gr compacts solid-state consolidated from the mixed powders contained pores with a shape close to spherical and with a size of the initial nickel particles, whose shape was retained in the consolidated material.The microstructure of the Ni-C gr composites Spark Plasma Sintered at 1000°C from the mechanically milled powders and that of porous graphite obtained by phase separation are presented in Fig.3 (a-b).Porous graphite obtained from the Ni-C gr compacts Spark Plasma Sintered from the mechanically milled powders does not show such large pores has a uniform fine-grained structure (Fig.3

Fig. 3 .
Fig. 3. Microstructure of the Ni-C gr composites and porous graphite: (a) Ni-C gr Spark Plasma Sintered at 1000°C from the mechanically milled powders, SPS-temperature (BSE image); (b) porous graphite obtained by phase separation from (a), treatment in HCl acid for 48 h (SE image); (c) Ni-C gr Spark Plasma Sintered at 1240°C from the mixed powders (BSE image); (d) porous graphite obtained by phase separation from (c), treatment in HCl acid for 144 h.

SPS or HP: consolidation and Ni-catalyzed graphitization
I. Consolidation parameters, relative density of Ni-C gr compacts and specific surface of porous graphite obtained by selective dissolution of nickel.