Current voltage I V characterization was conducted

Current–voltage (I–V) characterization was conducted using a Keithley 2400 source measurement apparatus in a four-wire setup in ambient purchase (-)-p-Bromotetramisole Oxalate with an ORIEL solar simulator (class ABA) at AM 1.5G. A silicon reference cell (PV Measurements, Inc. 20mm×20mm) was used for the calibration of the illumination source to 1 Sun (100mW/cm2).
Acknowledgments The authors gratefully acknowledge the support received from European Commission 7th framework programme SMARTONICS (Grant agreement number 310229) and (Decision No 912/2009/EC). The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. We also acknowledge Dr K. D. G. Imalka Jayawardena, Dr Christopher A. Mills, and Dr Michail J. Beliatis for fruitful discussions and the mechanical workshop at Surrey University for manufacturing all parts required for the slot-die coating equipment.
Data
Experimental design, materials and methods Nanometric nickel (Ni) powder was supplied by Tekna Plasma Systems Inc. (Sherbrooke, QC, Canada). It is a monomodal powder made of spherical particles. Grain sizes were estimated by XRD using the Williamson–Hall method [1] at a value of 35nm and by TEM, with therefore a lower statistical averaging, at 48nm. Micrometer-sized nickel was supplied by Sigma-Aldrich (Saint Louis, MO, USA) and is made of agglomerates of about 1µm particles, for an agglomerate size of 5–13µm. Both initial powders were analyzed by XRD and data presented only Ni diffraction peaks, Fig. 1. Three types of samples were prepared, from nanometric powder, from micrometer-sized powder and from a mixture made of 40% nanometric – 60% micrometric powders. For the latter, mixing was performed in a 3D Turbula mixer for 12h before any further treatment. Two different compaction procedures were tried, first a direct SPS sintering [2], and second using cold isostatic pressing (CIP) before performing SPS [3]. SPS was carried out by SPS using a 515S-SYNTEX machine at the regional SPS platform facility hosted by ICMPE (Thiais, France). With this setup, the maximum operating temperature is about 2273K and the maximum pressure is about 120MPa when a graphite die is used. The process takes place under a controlled argon atmosphere. In the present case, samples were consolidated under a 100MPa uniaxial pressure and an electric direct current (1500A maximum) subdivided into trains of 12 pulses separated by 2 pulses of 3.3ms. Actually, considering instrumental limits, a uniaxial pressure of 50MPa was first applied on the graphite die containing the powder. Temperature was then increased at a minimum rate of about 90Kmin−1 while pressure increased up to 100MPa. SPS cycles were performed at a temperature of 500°C for both samples containing nanometric powders, and 850°C for the micrometric powder in 20mm graphite molds. The dwell time in all cases was 1min. A previous study using a bimodal-like Ni powder (supplied by Argonide Corporation, Sanford, FL, USA) has been published previously, to which the reader might refer for experimental details [4]. This CIP operation has been performed using a pressure of 1100MPa (11kbar), except for the pure nanometric powder, for which a higher pressure, 1900MPa (19kbar) was required to ensure machinability of the green sample to perfectly fit the SPS mold [3]. Pre-compaction might be used to insert larger powder amounts inside the SPS mold. Indeed, when loose nanometric powders were used, the resulting samples had 4–5mm in height for diameters ranging from 10 to 40mm, whereas pre-compacted powders allowed the sintering of samples with a final height of up to 20mm for a 20mm diameter (10mm height for a 10mm diameter). Fig. 2 presents the EBSD micrographs (inverse pole figures (IPF) image) obtained for the micrometric powders. Analyzing these images revealed the lack of any preferential orientation (texture). Relative densities were measured at 99% for the direct-SPS sample (a), and 98.5 for the CIP-SPS sample (b), with a relative density of the green sample of 91% after CIP at 1.1GPa. Grains sizes were measured at 12µm and 41µm, respectively. Σ3 grain boundaries, including large twins, i.e. grain boundaries with a misorientation of 60±2.5° represent a fraction number of 26.6% in the direct-SPS sample, and 63.5% in the CIP-SPS sample. Low angle grain boundaries, with a misorientation lower than 15° represent then 15.6% and 3.4%, respectively. These data are summarized in Table 1 for all different samples.