Our experiments were conducted both under terrestrial conditions and aboard the MIR Space Station in 1997–98. Experiments were performed in Optizon-1 apparatus primarily designed for zone melting  but then adapted to SHS.  Sealed reaction chamber was equipped with three halogen lamps that ensured sample heating up to 1600K. Combustion was initiated by focused (to a spot of 1.5–2.0 mm in diameter) radiation from three halogen lamps located around the sample (Fig. 1).
In our experiments, we used spherical particles of Al cladded with Ni (d = 100–150 mm). The thickness of Ni coating corresponded to the ratio of Ni/Al=1/1 for each cladded particle. Cladded particles are convenient for these experiments because of reliable contact between reactants (Ni and Al), irrespective of separation between the particles. Two types of samples were used: (a) cylindrical pellets of cladded powder with a density of 3.45 g/cm3 and porosity of 32 % and (b) loose powders with the bulk density of 2.37 g/cm3 and porosity of 53 % (under normal g-conditions). In case (b), powders were placed in evacuated sealed quartz ampoules, the ratio of free space (pores + unoccupied ampoule volume) to powder volume being 70/30. The samples used in these experiments are shown in Fig. 2, a. Combustion was initiated at the bottom of the quartz ampoule.

The effect of gravity on SHS in pressed samples
The pressed samples burnt in space and on the ground retained their cylindrical shape and size. The surface of the space-burnt sample is smoother, exhibits a metallic glitter, and has no cracks and cavities found in the ground-burnt samples. At the top (initiation side), we found the drops of melt formed due to excessively high intensity of initiating light flux. Several rounded drops (1–3 mm in diameter) were found to run away from the top of the space-burnt sample. According to the data of electron probe microanalysis and X-ray diffraction, the chemical and phase compositions are identical (NiAl). Narrower (by a factor of 1.5) peaks were observed in the diffraction pattern of the space-burnt sample indicating more perfect crystal structure. As follows from Fig. 4, b  the NiAl grains are larger in the space-produced material. For this material, the intercrystalline fracture is prevailing. Meanwhile, the ground-produced material exhibits marked transcrystalline fracture, which is evidenced by structures in the fractograms. The structure of the space-produced material is more uniform with lower fraction of large anisotropy pores. The open porosity is higher in the ground-produced material, while the closed porosity is higher in the space-produced material. Therefore, larger and more perfect crystals were found to form in microgravity.

The effect of gravity on SHS in loose powder and  particle clouds
As we can see from Fig. 2, a, b,  burning of the loose powder at normal gravity proceeds without volume change. The space-produced material (NiAl) exhibits higher porosity (density 1.51 g/cm3, porosity 70%) and acquires the shape of the ampoule (Fig. 2, c). Combustion velocity in the loose powder (under terrestrial condition) is about 1.5 cm/s. At microgravity, for the first time we observed the propagation of gasless SHS wave in the particle cloud in vacuum. Some video-frames of this process are presented in Fig. 3. The combustion front propagates along the quartz ampoule with the average combustion velocity about 1.0 cm/s.
The microstructures of the samples are presented in Fig. 4, c, d. Prevailing are the globules of irregular shape forming a porous skeleton. In the space-produced material, this skeleton is more delicate and weaker linked: under pressure, the material easily disintegrates into powder. Since the expansion is absent, it can be assumed that the charge comprised of a powder suspension in vacuum, with the particles slowly moving in the cloud.
 The space-produced material exhibits a high-porosity skeleton (bound) structure. This can be explained by the specific character of combustion of the cladded Ni–Al particles. Since the melting point of Ni is higher than that of Al, the latter tears up the Ni shell, thus forming shaggy pieces which, upon overlapping, form a skeleton structure.

The experimental results shown that space-produced pressed samples of NiAl possess more perfect microstructure with larger crystal grains.
Gasless combustion of the vacuum particle clouds is the most interesting result of the present work. The observations and video recording of the initial powder inside ampoules aboard the MIR Space Station shown that the powder particles were uniformly distributed over the ampoule volume. After ignition, heat transfer from burnt to unburnet particles in microgravity and vacuum may occur by radiation and/or upon collisions. These mechanisms of the reaction front propagation differ significantly from the common mechanisms of combustion based on thermal conductivity and convection. High-porous continuous structure is formed due to change in the shape and size of particles during combustion.

This work was supported by the ISMAN-RSC “Energy” Project (grant no. 6/97).

                                                                                                                        (a)          (b)        (c)
Fig. 1.  Astronaut A.Solov’ev is performing SHS                                         Fig. 2.  Overall view of the starting
experiments in the Optizon-1 facility aboard                                                 ampoule (a) and the samples  burnt under
the MIR Space Station.                                                                                normal (b) and microgravity (c) conditions.

Fig. 3. Video-frame sequence showing gasless SHS in the Ni–Al particle cloud in microgravity.

             (a)                           (b)                              (c)                         (d)
Fig. 4. Structure surfaces of the NiAl synthesized under normal ((a), (c)) and microgravity ((b), (d)) conditions:
(a), (b) – pressed samples,  (c) – loose powder, (d) – particle cloud.

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