Journal of Alloys and Compounds 509 (2011) 1790–1795 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Neck growth kinetics during microwave sintering of nickel powder D. Demirskyi a,b,∗ , D. Agrawal a , A. Ragulya b a Materials Research Institute, 107 MRL Bldg., The Pennsylvania State University, University Park, PA 10802, USA b Frantsevich Institute for Problems in Material Science, 3 Krzhizhanovsky str., 03680 Kyiv-142, Ukraine article info Article history: Received 7 June 2010 Received in revised form 16 September 2010 Accepted 10 October 2010 Available online 21 October 2010 Keywords: Microwave heating Sintering Diffusion Metals Microstructure abstract Model experiments on initial stage of microwave sintering of nickel powder showed anomalous neck- growth rate during isothermal soaking, which is not the case for conventional sintering. Neck growth was determined as a function of time. Values for the neck growth exponent in the neck growth equation, (x/a) n = Bt, of 5.2, 5.4, 5.8, and 5.9 were found for within the temperature range 700–950 ◦ C, respectively. The evidences of formation of liquid phase during microwave sintering have been revealed, that may support enhancement of mass transfer during sintering process. The activation energy of 48 kJ mol -1 was found for microwave sintering of nickel, according to sphere-to-sphere model. Value revealed is signif- icantly lower then values for conventional sintering (136 kJ mol -1 ), and is on same level with activation energy for diffusion of metals in liquid state. An explanation and analysis of this phenomenon has been attempted. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Microwave sintering is relatively new method for processing of ceramics and metals and is generally associated with high heat- ing rates and volumetric heating [1]. Microwave sintering ensures considerable time and energy saving [2], and therefore is viewed as one of most perspective sintering techniques in material processing [3]. Thus, it is of great importance to establish relationships those are acting during microwave sintering process of powder systems. Therefore metal powders were used for various scientific research on modeling of heating [4], shrinkage kinetics [5], microstructure evolution during sintering of metal powders in single-mode cavi- ties [6]. However, most of these works were dealing with either heat- ing of metals [7] or later stages of sintering [8], those are usually similar to those for conventional sintering [9]. The results that have been recently revealed on initial stage of microwave sintering [10], confirmed that during microwave heat treatment of metals the neck formation process may be started during heating, or soak- ing for few seconds in case of processing in pure E or H field [6]. Later is generally attributed to intense interaction of eddy cur- rents on particles’ surface with surface layer [6]. This may lead ∗ Corresponding author at: Frantsevich Institute for Problems in Material Science, 3 Krzhizhanovsky str., 03680 Kyiv-142, Ukraine. Tel.: +380 44 424 7435; fax: +380 44 424 21 31. E-mail addresses: dmytro.demirskyi@gmail.com (D. Demirskyi), dxa4@psu.edu (D. Agrawal), ragulya@ipms.kiev.ua (A. Ragulya). to ignition of microplasma discharges [11], or even to local melt- ing [12]. For ceramics enhancement in diffusion was described for ionic conductors in terms of pondermotorive force model [13]. But from the point of view of sintering process, the origin of overall enhancements during microwave sintering processes is therefore still questionable, in this terms works that deal with exploration of sintering mechanisms are generally required. Therefore work using initial sintering stage models were suc- cessfully used for research on novel sintering processes like Spark-Plasma Sintering [14], as well as microwave sintering [10], revealing possible mechanism behind enhancement of sintering processes. In the present work, study of neck growth kinetics by exploit- ing classical sphere-to-sphere model [10,15–17] was proposed as model experiment for studying initial stage of microwave sintering of spherical shaped nickel powder. Secondly, investigations were undertaken to formulate hypotheses about the mechanisms con- trolling the initial stage of microwave sintering and evaluate the effective diffusion coefficient on the basis of experimental data obtained. 2. Materials and methods For model experiments in microwave sintering spherical shaped gas-atomized nickel powder was used (IPMS, NAS Ukraine). Particle’s size distribution (Master- sizer) and EDX techniques were applied to characterize initial powder. A number of neck growth kinetics experiments were conducted using sphere-to- sphere approach [15–17], in order to investigate diffusion mechanism that control initial stage of microwave sintering. Thus a monolayer of free packed nickel particles was placed on microwave transparent mulite substrate, which was then placed into mulite insulator which is named as sample in Fig. 1, number of spheres on the plate 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.10.042