The Pyrolytic Conversion of Perhydropolysilazane Into Si 3 N 4 : X-Ray Diffraction Analysis
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H
HSiC2 + H~iC3 + NH------
H -
1
N
+ NH 4C1
R R=NH
All manipulations of PHPS were performed under anhydrous and anaerobic conditions using either an inert atmosphere/vacuum manifold system or a dry box. To perform the pyrolysis studies, 5-10g samples of PHPS were weighed and placed in a tantalum crucible. The samples were pyrolyzed at temperatures ranging from 1000'C - 1700°C in a programmable, horizontal tube furnace (Model 54434 Tube Furnace with Eurotherm Controller 59256, Lindberg, Watertown, Wisconsin). All samples were heated at a rate of 10°C/min to the desired temperature, held at temperature for 15 minutes, and cooled to room temperature at an average rate of 15°C/min. The pyrolysis were performed under both nitrogen (N2) and NH 3 atmospheres. The heat treatment yielded a porous, ceramic char, which was transferred to a dry box, ground with an A120 3 mortar and pestle, and classified through a -325 mesh sieve. The samples were subsequently used for the XRD studies. X-Ray Diffraction The X-ray diffraction experiments were performed using a diffractometer (Phillips PW 1729 Diffractometer, Eindhoven, Netherlands) with a single crystal graphite monochromator. Each powder sample was poured and packed into a tape-backed aluminum holder to provide a randomly oriented, infinitely thick sample. Atmospheric exposure was minimized to prevent oxidation and water absorption. Step-scanned intensity data were generated using CuK,. radiation at a voltage of 40 kV and amperage of 40 mA. Data were gathered at room temperature over a 20 range of 50 to 90* using a step size of 0.05' and a 3-second count time at each step, and were recorded directly into an ASCII text file for further manipulation. Due to the significant amount of diffraction peak line-broadening observed on the XRD patterns from the lower temperature chars, crystallite sizes of each phase, ax- and P-Si 3N4 and silicon (Si), were calculated using the Scherrer equation' [12]. Corrections were made for the inherent line broadening due to the X-ray source as described in detail elsewhere [11]. The crystallite sizes were determined for each phase by calculating the thickness of each crystal for a number of peaks and averaging. To understand the evolution of the crystalline phases during pyrolysis, the relative amounts (based on weight fraction) of the three major phases present were calculated [11] with measured peak intensities using techniques based on the combined work of Gazzara and Messier [13], and Pigeon and Varma [14]. The chars pyrolyzed below a temperature of 14000 C exhibited broad maxima (characteristic of amorphous materials) that decreased with increasing pyrolysis temperature (see Figure 1). Although this phenomenon has been observed in glass-ceramics and some polymers, and used to quantify the degree of crystallinity, no calculations of this type had been previously attempted for a nitride ceramic. Therefore, a modification was developed based on the integrated area method described in the polymer literature [15], with a set of standards prep
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