Microstructure and conductivity of blacklight-sintered TiO 2 , YSZ, and Li 0.33 La 0.57 TiO 3

Rapid densification of ceramics has been realized and its merits were demonstrated through multiple approaches out of which UHS and flash sintering attract recent attention. So far, however, scalability remains difficult. A rise in through-put and scalability is enabled by the introduction of blacklight sintering powered by novel light source technology. Intense illumination with photon energy above the bandgap (blacklight) allows high absorption efficiency and, hence, very rapid, contactless heating for all ceramics. While heating the ceramic directly with light without any furnace promises scalability, it simultaneously offers highly accurate process control. For the technology transfer to industry, attainable material quality needs to be assured. Here, we demonstrate the excellent microstructure quality of blacklight-sintered ceramics observed with ultrahigh voltage electron microscopy revealing an option to tune nanoporosity. Moreover, we confirm that electronic, electron, oxygen, and lithium-ion conductivities are equal to conventionally sintered ceramics. This gives the prospect of transmitting the merits of rapid densification to the scale of industrial kilns.

compete with conventional sintering, 9 such fast processing approaches must become more practical and industrially scalable.
Blacklight as a power transmitting medium is suggested to be a very straightforward approach to realizing rapid sintering. 10 Green bodies are heated within seconds merely through intense illumination with photon energies above the bandgap, allowing a contactless and containerless process. This minimizes process and sample change times, opening the door to immediate industrial scaling. Moreover, it holds the potential to beat the energy efficiency of industrial-scale furnaces independent of batch size while replacing natural gas with intermittently available renewable electricity. [10][11][12] Rapid sintering can alter the defect structure within the produced ceramics. The modified defect chemistry ultimately translates to an impact of the processing technique on the behavior of the final material. Alongside the high heating and cooling rates, 13 electric fields within the ceramic and/or reducing atmosphere are key contributors to defect creation in established rapid sintering techniques. 14 Avoiding currents in the sample or a reducing atmosphere will allow more control over the densification process and material properties. Therefore, blacklight sintering has great potential to provide material qualities that match or even exceed large-scale furnace sintering.
Here, electronically conducting TiO 2 , oxygen ion conducting YSZ, and Li-ion conducting Li 0.33 La 0.57 TiO 3 are chosen as model materials to study the impact of blacklight sintering, 10 on the electrical conductivity. The ceramics are characterized in detail with XRD and ultrahigh voltage electron microscopy (UHVEM). The electrical conductivity is reported for all three materials and compared to conventionally sintered references. Moreover, all results are cross-validated using blacklight sintering with a 450-nm laser and an Xe-flash lamp.

EXPERIMENTAL PROCEDURES
Synthesis of the blacklight-sintered samples is described in great detail in an accompanying publication. 10 A Xe-flash lamp pulsed at 60 Hz was used in the test labs of Heraeus Noblelight, Cambridge, UK, whereas a 450-nm laser was used in the test labs of Laserline, Mülheim-Kärlich, Germany. TiO 2 was sintered on ceramic wool insulation, whereas YSZ and Li 0.33 La 0.57 TiO 3 were sintered on expandable graphite. Parameters were reported in Table  1, temperature curves in Figure 5, and SEM micrographs in supplementary materials 9 and 10 of the accompanying publication. 10 Electrical conductivity data was recorded with impedance spectroscopy using an impedance analyzer (Novocontrol, Montabaur, Germany). Temperaturedependent conductivity was recorded during cooling after a 2-h holding time at 800 • C. Oxygen partial pressure (pO 2 )-dependent data was recorded in the range of 10 −1 -10 −5 bar at 700 • C after 40 min of holding time for each data point. Such procedure, the temperature range, and the rationale behind it were established elsewhere. 15 Electrodes from Pt-paste fired at 930 • C for 5 min were used for all measurements. No sample post-processing after blacklight sintering, such as grinding off surfaces, was required. UHVEM samples were thinned by conventional polishing and ion milling (Fischione Model 1051). The acceleration voltage was set to 1 MV in TEM or STEM mode using the JEOL JEM-1000k RS at Nagoya University. Here, the UHVEM is used to better analyze internal structures with various types of pores in the samples.

RESULTS AND DISCUSSION
Produced densities were beyond 99% for Li 0.33 La 0.57 TiO 3 , 94%-98% for TiO 2 , and 87% for YSZ. The microstructure was found to be homogenous across the entire sample thickness with next to no cracks observed; see also supplementary materials 4, 9, and 10 elsewhere. 10 Here, UHVEM reveals the microstructure and pore distribution in much greater detail. Basically, no pores were observed in the >99% dense Li 0.33 La 0.57 TiO 3 , as displayed in Figure 1C. This shows that rapid densification with blacklight is capable of producing high densities approaching 100%. In contrast, connected pores in the 87% dense YSZ (see Figure 1D) hint toward incomplete sintering and too low process temperatures. Higher illumination power or a longer illumination application is needed for full densification. The respective sintering process of the ceramics depends on the bandgap and its temperature dependence, the related absorption efficiency, light exposure time, and even the properties of the substrate on which the ceramic is positioned. 10 This gives a wide range of optimization opportunities. For example, porosity or grain size gradients can be generated. Concerning the grain sizes, no significant differences from regularly sintered material were found for TiO 2 or YSZ samples in this work. 10,16 However, the Li 0.33 La 0.57 TiO 3 is not only denser but seems to exhibit grain sizes from the lower end of the spectrum. 17,18 No significant difference is found when contrasting the microstructure on TEM images of TiO 2 synthesized with 450-nm laser and Xe-flash lamp in Figure 1A,B. This shows the generality of the approach and its applicability with different kinds of suitable light sources. The grain size was determined to be 5.8 ± 0.6 µm for samples produced with Moreover, UHVEM reveals nanopores at grain boundaries, within the grains, and on triple points of TiO 2 . A pore density of 30 pores per 100 µm 2 with a pore diameter of 0.35 ± 0.13 µm was found for the sample sintered with a 450-nm laser. The majority of the pores were found inside grains with a fraction of 10%-15% at grain boundaries and another 10%-15% at triple points. A slightly lower pore density of eight pores per 100 µm 2 was found in the sample sintered using an Xe-flash lamp, however, with a comparable diameter of 0.38 ± 0.11 µm. Pore diameters are notably small with respect to the microstructure as they are significantly less than 1/10th of the grain size (refer to Figure 1B). Such porosities are very unusual for conventional sintering. However, similar porosity was found after UHS sintering. 19 Apparently, the sintering trajectories for high heating rates differ from conventional sintering. 20,21 To confirm proper synthesis, XRD patterns were recorded, as shown in Figure 2. No or negligible indications for secondary phases were observed, confirming a synthesis outcome at par with conventional sintering.
The electrical conductivities of TiO 2 of the conventionally sintered specimen and the ones produced with an Xe-flash lamp and a 450-nm laser are illustrated in Figure 3A. In the comparison, bulk conductivity and total conductivity, including grain boundary resistance, are separated. Taking the error bar of the measurements into account, the depicted values are very similar, which documents the negligible differences between synthesis methods. Additionally, oxygen partial pressure-dependent conductivity measurement conducted for a sample sintered with 450-nm laser reveals that the conductivity agrees well with what is expected for undoped TiO 2 and is in the conductivity minimum between electron and hole conductivity, 15,22,23 see Figure 3B. The nanoporosity observed in the microstructures appears not noteworthily affect the conductivity here.
Bulk conductivity for conventionally sintered YSZ and blacklight-sintered YSZ samples produced with 450-nm laser and Xe-flash lamp YSZ samples, again, lie within the error bar of the respective values, see Figure 3C. These are also in accordance with literature values. 24 The total conductivity of the blacklight-sintered materials is slightly lower than the reference, which is likely associated with Bulk resistance and interface resistance can be identified clearly, followed by a Warburg diffusion response. 17,28 The bulk resistivity of the laser fabricated sample is 0.15 mS/cm demonstrating an outstanding sintering outcome for this material. 17 the low density. 25 As described in the previous work, the density and microstructure can be controlled, and a low density or gradient in density can also be beneficial for YSZ, for example, for sensor applications to obtain a high surface area. 10 The impedance spectrum of the Li 0.33 La 0.57 TiO 3 , produced with 450-nm laser and Xe-flash lamp, as shown in Figure 3D, reveals a bulk and grain boundary resistance and an electrode polarization tail at low frequencies.
Resistances for the Xe-flash lamp-sintered samples are higher than for the 450-nm laser in this case. The bulk conductivity of Li 0.33 La 0.57 TiO 3 produced with a 450-nm laser is 0.15 mS/cm, whereas the total conductivity is 4 × 10 −4 S/cm. This is a high value for such a composition, 17,26,27 which illustrates its suitability for synthesizing battery materials. Moreover, we note the high density exceeding 99% and the strong control over the grain boundary conductivity enabled here, which are key parameters for the application of this battery material. 25

CONCLUSION
Synthesis results of blacklight sintering appear comparable to conventional sintering in all regards. The investigated benchmark ceramics TiO 2 , YSZ, and LLTO all exhibit the same or similar electrical conductivity compared to regularly sintered reference material. UHVEM confirms defect-free microstructure aside from nanopores. Hence, blacklight sintering exploits all the merits of rapid sintering while producing excellent electrical properties for the electronically and ionically conductive materials. Its potential for upscaling and competitive energy efficiency makes it a very promising densification technique for the 21st century. Atsutomo Nakamura https://orcid.org/0000-0002-4324-1512 Wolfgang Rheinheimer https://orcid.org/0000-0002-2906-4265 Till Frömling https://orcid.org/0000-0002-8827-1926