III. RESULTS AND DISCUSSION
4. Electrical Conductivity
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Figure 49: Lithium silicate sintered part profile four, surface (left, 100 μm scale bar) and layer (right, 150 μm scale bar).
Figure 50: Lithium silicate sintered part profile five, surface (left, 50 μm scale bar) and layer (right, 100 μm scale bar).
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shown in Figure 5. The three parts at the bottom of Figure 51 are the "surface" samples where the layer orientation is parallel to the electrodes also shown in Figure 5.
Figure 51: Gold-coated lithium silicate (left), Tosoh TZ-8Y (middle), and Tosoh TZ-8YS (right) samples after impedance testing.
Samples were placed into a furnace programed to follow a heating cycle of, 100°C to 800°C at 50°C increments for YSZ and 100°C to 300°C at 25°C increments for lithium silicate. Platinum leads were placed on the top and bottom of the sample disk, contacting the electrodes on both the top and bottom. The platinum leads were connected to the impedance analyzer. Figure 52 shows interior set up of the impedance test rig before being closed. An alumina rod was used as a weight to hold the platinum leads against the electrodes on the samples.
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Figure 52: Lithium silicate test sample in furnace with impedance test leads.
Once the samples had undergone impedance testing the bulk resistance was determined using a semicircle fit. The data collected was plotted in ZVeiw with half circles fitted to the first curve which was representative of the bulk resistance. Figure 53 shows an example of data fitted from a lithium silicate sample. The resulting bulk resistance (Rb) was used to find the bulk conductivity of the material calculated using (2)12, 22.
𝑅𝑏 = 1
𝜎𝑏 𝑑
𝐴 → 𝜎𝑏 = 𝑑
𝐴𝑅𝑏 (2) where d is the thickness of the disk tested (distance between the electrodes), A is the surface area of the electrode contacting the sample or face of the disk, and Rb is the bulk resistance. Evident of contributions from grain boundaries and the electrode interface seen in Figure 54. The first fitted curve being the bulk resistance and the rest of the curves represent the grain boundary and electrode interface resistance. Ideally, an additional contribution from the layered structures to the impedance data is expected. In order to confirm such a contribution, more systematic study varying layer thickness and detailed impedance data with equivalent circuit analysis will be needed.
The bulk conductivity was then used to generate Arrhenius plots, which were used to find the activation energy of the printed parts. The log of the bulk conductivity was calculated and plotted as a function of the inverse of the absolute temperature. The
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slope of a linear best fit line was used to find the activation energy. The Arrhenius equation (3)22, 23 was used to find the activation energy for the two materials.
𝑘 = 𝐴𝑒−𝐸𝑎/𝑅𝑇→ ln 𝑘 = −𝐸𝑎
𝑅𝑇+ ln 𝐴 → 𝑦 = 𝑚𝑥 + 𝑏 (3)
Figure 53: Lithium silicate surface at 200°C with bulk and grain boundary resistance curve fit.
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Figure 54: YSZ surface at 400 °C bulk and grain boundary resistance curve fit.
Key features tested were layer orientation to determine if there were any effects on the conductivity and activation energy of the material. AM YSZ samples resulted in a bulk conductivity average of 1.33 10-3Ω-cm at 300 °C. AM lithium silicate samples resulted in an average bulk conductivity value of 5.14 10-5 S cm-1 at 300 °C compared to Adnan et al.12 who reported conductivity value of 1.16 10-4 S cm-1 at 100 °C. The conductivity values were less for the AM parts compared to monolithic parts.
TABLE VIII. is a comparison of the bulk experimental averages found in the literature. The bulk activation energy for both lithium silicate layer orientations resulted
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in 0.22 eV. However, the surface orientation had slightly higher bulk resistance values compared to the layered orientation. The same bulk resistance variation was also
observed in the YSZ samples with slightly higher resistance values for surface orientation samples compared to layer orientation samples. The bulk activation energy for the two YSZ compositions had a 0.1 eV difference. With the samples grained TZ-8Y having the higher activation energy. Both YSZ and lithium silicate materials demonstrated no
significant direct effect on electrical properties caused by varying layer orientation during printing and electrode placement.
TABLE VIII: Bulk Activation energy experimental values vs literature values.
Experimental Average
Experimental Range
Literature Average
Literature Range
Lithium Silicate 0.22 eV 0 0.19 eV 0
YSZ (TZ-8Y) 0.39 eV 0.01 1.16 eV 0.18
YSZ (TZ-8YS) 0.29 eV 0.01 1.16 eV 0.18
Figures 55 and 56 show the Arrhenius plots for both lithium silicate and YSZ.
The average calculated bulk activation energy for YSZ was 0.34 eV and the average for lithium silicate was 0.22 eV. Figures 55 and 56 also show the Arrhenius plots for average grain boundary activation energy for both YSZ and lithium silicate. The grain boundary activation energy for the YSZ was 0.48 eV and 0.40 eV for lithium silicate. Both
materials had consistently higher grain boundary activation energies then bulk activation energy.
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Figure 55: Arrhenius plot for Tosoh YSZ samples bulk and grain boundary activation energy.
Figure 56: Arrhenius plot for lithium silicate samples bulk and grain boundary activation energy.
1.3 1.4 1.5 1.6 1.7 1.8
-6 -5 -4 -3 -2 -1
Log(s) S-cm-1
1000/T(K-1)
Bulk
Grain Boundary
1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 -8
-7 -6 -5 -4 -3 -2 -1
Log(s) S-cm-1
1000/T(K-1)
Bulk
Grain Boundary
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Lithium silicate experimental activation energy was determined for a temperature range of 125-300 °C with a printed layer thickness of 100 μm. A 100 μm layer thickness is relatively high compared to typical LCM layers. The experimental average bulk activation energy for lithium silicate was 0.22 eV. The lowest recorded bulk activation energy found was from Adnan et al.12, which was 0.19 eV for samples prepared by sol- gel and pressed into pellets and sintered. The 3D-printed lithium silicate samples had high porosity with an average density of roughly 85% as well as large outgassing from unreacted precursors creating large scale dysconnectivity and defects in the materials, thus increasing the activation energy.
YSZ experimental activation energy was determined for a temperature range from 300°C to 500°C with a printed layer thickness of 25 μm. The experimental average bulk activation energy was 0.34 eV with Kwon et al.24, report bulk activation energies of 1.16 eV. The lower activation energy for the experimental 3D-printed and sintered material may be attributed to electrons potentially finding a path of least resistance not found in traditionally formed parts. The layering may lead to micro cracks in the bulk material as well as uneven sintering leading to stresses in individual layers also causing
delamination. The thermal profile does not separate debinding from sintering combining both processes into one. Debinding may be causing tunneling in the microstructure from the escaping binder gases with sintering following leading to a nontraditional
microstructure.
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