Supplementary information. Polymer Derived Silicon Oxycarbide Ceramics as Promising Next Generation Sustainable. Thermoelectrics

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1 Supplementary information Polymer Derived Silicon Oxycarbide Ceramics as Promising Next Generation Sustainable Thermoelectrics Adhimoolam Bakthavachalam Kousaalya $,, Xiaoyu Zeng #, Mehmet Karakaya #, Terry Tritt #,, Srikanth Pilla $,,,*, Apparao M Rao # $ Department of Automotive Engineering, Clemson University, Greenville, SC, USA Clemson Composites Center, Greenville, SC, USA # Department of Physics & Astronomy, Clemson University, Clemson, SC, USA Department of Materials Science and Engineering, Clemson University, Clemson, SC, USA Corresponding author: spilla@clemson.edu; Tel: S-1

2 1) Experiments a. Processing of SiOC ceramic Silicon oxycarbide (SiOC) ceramic was processed via thermolysis of cross-linked polymethyl-hydrosiloxane (PMHS) (Sigma Aldrich, USA), while hexagonal boron nitride (h- BN) (Alfa Aesar, USA, particle size < 44 µm) was used as filler. h-bn was added in varying proportions of weight (0, 1 and 5 wt. %) to PMHS and stirred for 30 min to ensure uniform dispersal in the polymeric matrix. All samples were cross-linked by adding triethylenediamine (TEDA) (Sigma Aldrich, USA) at 5 wt. % of PMHS content as the catalyst, at room temperature for 24 h. Thermolysis of all cross-linked PMHS samples was conducted at 1000 C at a heating rate of 5 C/ min in Ar atmosphere, in a tubular furnace (MTIXTL Corp., USA) after flushing it with argon (Ar) gas to produce as-thermolyzed (SiOC and SiOC/BN) ceramics. Sample nomenclature is detailed in Table S1. An agate mortar and pestle was used to grind the as-thermolyzed ceramics (AT, AT-BN- 1, AT-BN-5), and subjected to pulsed electric current sintering (PECS) (Dr. Sinter SPS-515S, Fuji Electronic Co. Ltd., Japan). PECS was conducted under vacuum at a heating rate of 50 C/min and holding time of 15 min under sintering pressure of 50 MPa. PECS temperatures of 1100, 1200 and 1300 C were used for AT, while AT-BN-1 and AT-BN-5 powders were subjected to PECS at temperature of 1300 C. Sintering pressure was maintained till the samples cooled to room temperature in all cases. S-2

3 Table S1: Sample nomenclature for PMHS and SiOC samples with and without h-bn Sample Nomenclature Material Processing Conditions AT AT-BN-1 AT-BN-5 SiOC SiOC/1 wt. % BN SiOC/5 wt. % BN As-thermolyzed at 1000 C C1 SiOC PECS 1100 C C2 SiOC PECS 1200 C C3 C4 C5 SiOC SiOC/1 wt. % BN SiOC/5 wt. % BN PECS 1300 C b. Characterization i. Structural characterization X-ray diffraction (XRD) was conducted on all ceramic samples (as-thermolyzed and PECS) via Rigaku Miniflex (The Woodlands, TX, USA) using Cu K α radiation (λ = nm) with a step size of 0.05, voltage of 45 kv, and current of 30 ma. The background data was subtracted prior to analyzing XRD data. Raman spectroscopy was undertaken for all samples to determine the evolution of free carbon in ceramic samples. Raman spectra was recorded in the scanning range of cm -1 with Dilor XY triple grating (Dilor, Lille, France) and Renishaw InVia Raman microscope (Renishaw Inc., Hoffman Estates, IL, USA). The samples were subjected to an incident laser beam (λ = 514 nm) using a 50 objective lens, with the laser power kept to a minimum to avoid any heating of the samples. Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) Spectroscopy of crosslinked PMHS were S-3

4 recorded using Thermo-Nicolet Magna 550 FTIR spectrometer along with a Thermo-Spectra Tech Foundation Series Diamond ATR accessory at a fixed angle of incidence (50º). FTIR scans were obtained in the scan range of cm -1, at a resolution of 4 cm -1. ii. Density and porosity Geometrical density of all PECS samples (C1-C5) was calculated by measuring their respective mass and volume, while Archimedes principle was used to determine bulk density using distilled water as the liquid medium. Open porosity was calculated as the ratio of difference between geometrical and bulk densities to the geometrical density. iii. Thermal characterization TA Instruments TGA-2950 (New Castle, DE, USA) was used for thermo-gravimetric analysis (TGA) of crosslinked PMHS in nitrogen atmosphere at a flow rate of 25 ml/min. Approximately 5 mg of sample was taken in a Pt crucible and heated from 25 C to 1000 C at a heating rate of 5 C/min to ascertain both thermolysis temperature and ceramic yield. Thermal diffusivity measurements for sintered (PECS) samples (C1-C5) were conducted via laser flash method under argon (Ar) atmosphere using a short laser pulse. Netzsch LFA 457 (Burlington, MA, USA) was used to measure thermal diffusivity of samples (dimensions: 12.7 mm diameter, 2 mm thickness) using transient method in the temperature range: C and recorded at intervals of 100 C. A thin graphitic layer was coated on the samples prior to measurement. While the front side of the samples (i.e. the side coated with thin graphitic layer) was heated by laser pulse, InSb sensor was used to detect the increase in temperature of the other side of the sample. Thermal diffusivity was calculated by the machine using two parameters: sample thickness (~2.5 mm), and time taken by the rear side of the sample to reach 50 % (half) of its maximum temperature, as shown in Equation S1. S-4

5 =. /.. S1 where, = Thermal Diffusivity (mm 2 /s), = Sample Thickness (mm) / = Time required to reach 50 % of the maximum temperature attained by the rear surface. Using thermal diffusivity values, thermal conductivity was calculated using Equation S2. =.. S2 where, = thermal conductivity (W/m-K), = specific heat capacity at constant pressure (J/gK), = bulk density (g/cm 3 ). Specific heat capacity of samples (C3-C5) at constant pressure ( ) was measured over C under argon (Ar) atmosphere at a heating rate of 40 C/min using Netzsch (Burlington, MA, USA) Differential Scanning Calorimeter 404 Pegasus. iv. Electrical conductivity and Seebeck coefficient After measuring thermal diffusivity, sintered (PECS) samples were cut into small rectangular bars (dimensions: 10 mm 2 mm 2 mm) for measuring their electrical resistivity and Seebeck coefficient using commercially available Ulvac ZEM-3 (Boston, MA, USA) over temperatures ranging from 25 C to 150 C. Higher measurement temperatures could not be used due to limitations associated with the equipment used. S-5

6 2) Results Figure S1: XRD of as-thermolyzed ceramic Figure S2: Raman spectra of as-thermolyzed ceramic Table S2: Density of sintered ceramics Samples Geometrical Density (g/cc) Bulk density (g/cc) C C C C C S-6