Nanocomposites and nanomaterials

Low frequency dependence of AC resistivity of green compacts of spark eroded particles
G.E. Monastyrsky
National technical university of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”, 37, Peremogy Av., UA-03506 Kyiv, Ukraine 
E-mail: monastyrsky@yahoo.com

Frequency dependence of the real and reactive part of the resistivity of green compacts compressed from the micron-sized and nano-sized particles of Ti-Ni-Hf, Ni-Mn-Ga and Cu-Al-Ni obtained by spark erosion method in liquid argon are considered. The surface of particles, which are metallic in nature, is usually oxidized during the handling. Nano-sized oxide layer meanwhile has strongly impact on the processes of sintering. Its influence is especially important for the spark plasma sintering (SPS) method using vast current for the materials consolidation although its role is discussing until now. 
The AC resistivity of green compacts was measured by E7-11 RLC meter. Both resistivity and capacitance was decreased exponentially with the increasing of frequency form 0.1 to 10kHz. The typical value of resistivity alternates between several hundred and several kOhmm. The dielectric losses were also decreased. Such behavior as well as typical value of resistivity can be attributed to the oxides layers on the surface of metallic particles, namely NiO, TiO2, CuO and alumina. Such kind of oxides were found before in Ti-Ni-Hf and Cu-Al-Ni spark eroded particles while no significant quantities of oxides in Ni-Mn-Ga were observed.
The constant bias of 100V didn’t influence significantly on the AC resistivity. At the same time the preliminary treatment of compacts by the high voltage discharge during 3 min decreased the resistivity over four order. The tension of discharge were 8kV being the amperage was several dozens of nA. After treatment the resistivity didn’t dependent on the frequency and dielectric losses was so small that were unmeasurable. These peculiarities confirms the metallic conductance of the green compact after treatment. It points out that the electrical breakdown of the oxides layers in places of the particles contacts appears. Although the tension was used in experiment is several order higher than that used in SPS methods the local electrical field strength can be comparable due to additional factor of the pressure up to 100MPa, which applied to the samples during the SPS treatment. Thus metallic conductance which appear in compacts under simultaneously action of high electrical field strength and pressure provides effective action of vast current on the sintering processes.