ELECTROCHEMICAL CELL STACK

Abstract

Stack (10) of electrochemical cells (100) containing: a first separator (200); a second separator (200); and an electrochemical cell (100) arranged between the first separator (200) and the second separator (200), which electrochemical cell (100) contains an anode (110), a cathode (130) and a solid electrolyte layer (120), which solid electrolyte layer (120) is arranged between the anode (110) and the cathode (130) and contains a zirconium dioxide-based material as a main component, wherein the solid electrolyte layer (120) has an upstream part (120a) and a downstream part (120b), wherein the upstream part (120a) is positioned on an upstream side in a flow direction (FG) of the fuel gas flowing in a fuel flow channel (110S) between the anode (110) and the first separator (200), wherein the downstream part (120b) is positioned on a downstream side in the flow direction (FG), wherein the upstream part (120a) includes a first region (a1) within 3 µm of an anode-side surface and a second region (a2) provided between the first region (a1) and the cathode (130), wherein an intensity ratio (R1) of tetragonal zirconia to cubic zirconia in a Raman spectrum of the first region (a1) is greater than an intensity ratio (R2) of tetragonal zirconia to cubic zirconia in a Raman spectrum of the second region (a2), and wherein the intensity ratio (R1) of tetragonal zirconia to cubic zirconia in a Raman spectrum of the first region (a1) is greater than the intensity ratio of tetragonal zirconia to cubic zirconia in a Raman spectrum of the downstream part (120b).

Description

Technical area

[0001] The present invention relates to a stack of electrochemical cells. State of the art

[0002] A fuel cell stack having a so-called flat plate stack structure in which the electrochemical cell stack has a plurality of fuel cells and a plurality of separators alternately stacked is known as a conventional type of electrochemical cell stack (for example, refer to Patent Literature 1).

[0003] Each fuel cell comprises an anode, a cathode, and a solid electrolyte layer disposed between the anode and the cathode. A space between an anode and a separator located on one side of a fuel cell forms a fuel flow channel. During operation of the fuel cell, fuel gas (e.g., hydrogen gas) flows into the fuel flow channel.

[0004] Further prior art is known from patent literature 2. List of printed publicationsPatent literature [Patent Literature 1] Japanese Patent Application Laid-Open No. JP 2015-115181 A [Patent Literature 2] German patent application laid open specification DE 11 2015 002 517 T5 Brief description of the inventionTechnical problem

[0005] If an upstream part in the direction of the fuel gas flow in the solid electrolyte layer is subjected to a thermal load as a result of insufficient preheating of the fuel gas in the upstream part of the fuel flow channel, this is associated with the risk of damage to the solid electrolyte layer.

[0006] This type of damage to the solid electrolyte layer can occur not only when the electrochemical cell stack is started up for the first time, but also when it is resumed after a period of inactivity.

[0007] The present invention is proposed on the basis of the above-mentioned novel finding and has the object of providing a stack of electrochemical cells which enables the prevention of damage to a solid electrolyte layer. Solution to the problem

[0008] The electrochemical cell stack according to the present invention includes a first separator, a second separator, and an electrochemical cell disposed between the first separator and the second separator. The electrochemical cell includes an anode, a cathode, and a solid electrolyte layer. The solid electrolyte layer is disposed between the anode and the cathode and contains a zirconium dioxide-based material as a main component. The solid electrolyte layer has an upstream part and a downstream part. The upstream part is located on the upstream side in the flow direction of the fuel gas flowing in the fuel flow channel between the anode and the first separator. The downstream part is located on the downstream side in the flow direction.The upstream portion includes a first region within 3 µm of the anode-side surface and a second region provided on the first region. An intensity ratio of tetragonal zirconia to cubic zirconia in a Raman spectrum of the first region is greater than an intensity ratio of tetragonal zirconia to cubic zirconia in a Raman spectrum of the second region. The intensity ratio of tetragonal zirconia to cubic zirconia in a Raman spectrum of the first region is greater than the intensity ratio of tetragonal zirconia to cubic zirconia in a Raman spectrum of the downstream portion. Advantageous effects of the invention

[0009] The present invention provides a stack of electrochemical cells that prevents damage to a solid electrolyte layer. Brief description of the drawings Fig. 1 is a perspective view of a fuel cell stack. Fig. 2 is a sectional view of a fuel cell stack. Fig. 3 is an enlarged view of Fig. 2. Fig. 4 is a perspective view of a cell. Description of embodimentsFuel cell stack 10

[0010] Fig. 1 is a perspective view of a fuel cell stack. Fig. 2 is a sectional view of a fuel cell stack. Fig. 3 is an enlarged view of Fig. 2.

[0011] The fuel cell stack 10 has a so-called "flat plate stack structure" in which a plurality of fuel cells 100 (hereinafter referred to as "cells") and a plurality of separators 200 are alternately stacked. 1. Cells 100

[0012] How Fig. As shown in Figure 3, the cells 100 include an anode 110, a solid electrolyte layer 120, and a cathode 130. Although the planar shape of the cells 100 is not limited, the length of an edge can, for example, form a square of 10 to 300 mm. Although the thickness of the cells 100 is not limited, it can, for example, be configured to be 110 to 2100 µm.

[0013] The anode 110 is composed of a material exhibiting oxide ion conductivity and a material exhibiting electron conductivity. The anode 110 may be made of, for example, NiO-8YSZ (yttrium-stabilized zirconia) or NiO-GDC (gadolinium-doped ceria), or the like. Although there are no restrictions on the thickness of the anode 110, it may be configured, for example, from 50 µm to 2000 µm. Although there are no restrictions on the porosity of the anode 110, it may be configured, for example, from 15 to 55%.

[0014] The solid electrolyte layer 120 is disposed between the anode 110 and the cathode 130. The solid electrolyte layer 120 acts as a sealing layer to prevent the mixing of fuel gas (e.g., hydrogen gas) and oxygen-containing gas (e.g., air). The solid electrolyte layer 120 includes a zirconia-based material as a main component. Including the main component means that the zirconia-based material is included in the solid electrolyte layer 120 in a proportion equal to or greater than 70 wt%. The zirconia-based material may include, for example, the use of 3YSZ, 8YSZ, or ScSZ (scandium oxide-stabilized zirconia), or the like. The detailed structure of the solid electrolyte layer 120 will be described below.

[0015] Although there are no particular restrictions on the thickness of the solid electrolyte layer 120, it can be configured, for example, to be 3 to 50 µm. Although there are no restrictions on the porosity of the solid electrolyte layer 120, it can be configured, for example, to be 0 to 10%.

[0016] The cathode 130 is formed, for example, by (LaSr)(CoFe)O3 (LSCF, lanthanum strontium cobalt ferrite), (La, Sr)FeO3 (LSF, lanthanum strontium ferrite), La(Ni, Fe)O3 (LNF, lanthanum nickel ferrite), (La, Sr)CoO3 (LSC, lanthanum strontium cobaltite), or the like. Although there are no particular restrictions on the thickness of the cathode 130, it can be configured with a thickness of 50 µm to 2000 µm. Although there are no restrictions on the porosity of the cathode 130, it can be configured with a porosity of 15% to 55%, for example. 2. Separator 200

[0017] The separator 200 may be configured with a Ni-based heat-resistant alloy (e.g., a ferrite-based SUS, Inconel 600, and Hastelloy, or the like). The planar shape of the separator 200 is identical to the planar shape of the cell 100.

[0018] As in Fig. 2 and Fig. As shown in Figure 3, the separator 200 includes a flat plate portion 210 and a frame body 220. The peripheral edge portion of the flat plate portion 210 is enclosed by the frame body 220 over the entire circumference. The thickness of the frame body 220 is greater than the thickness of the flat plate portion 210. The frame body 220 protrudes on both the top and bottom surfaces relative to the flat plate portion 210.

[0019] How Fig. As shown in Figure 3, the cell 100 is arranged between the first separator 200a and the second separator 200b. The cell 100 is held by the first separator 200a and the second separator 200b. The cell 100 is connected to the respective frame bodies 220 of the first separator 200a and the second separator 200b. The peripheral edge portion of the cell 100 may be connected to the respective frame body 220 by a connecting material (glass material or the like).

[0020] The space between the first separator 200a and the anode 110 of the cell 100 forms a fuel flow channel 110S through which fuel gas flows. The space between the second separator 200b and the cathode 130 of the cell 100 forms an air flow channel 130S through which an oxygen-containing gas flows.

[0021] Power generation is performed based on the following chemical reaction equations (1) and (2) as a result of the flow of a fuel gas in the fuel flow channel 110S together with the air flow in the air flow channel 130S and the electrical connection of the cell 100 to a load of an external unit. (1 / 2) · O2 + 2e - → O 2 (at cathode 130) (1) H2 + O 2- → H2O + 2e - (at anode 110) ... (2) Configuration of the solid electrolyte layer 120

[0022] The configuration of the solid electrolyte layer 120 of each cell 100 is described below.

[0023] The solid electrolyte layer 120 has an upstream part 120a and a downstream part 120b.

[0024] The upstream part 120a is formed integrally with the downstream part 120b. The upstream part 120a is arranged on an upstream side of the downstream part 120b with respect to the flow direction FG of the fuel gas. That is, the upstream part 120a is located in an area near the flow inlet (not shown) for the fuel gas in the solid electrolyte layer 120. More specifically, the upstream part 120a may be arranged with respect to a 1 / 4 area (L / 4) of the total length L of the solid electrolyte layer 120 in the flow direction FG.

[0025] The downstream part 120a is arranged on a downstream side of the upstream part 120a with respect to the flow direction FG of the fuel gas. That is, the downstream part 120b is located in an area near the flow outlet (not shown) for the fuel gas in the solid electrolyte layer 120. More specifically, the downstream part 120b may be arranged with respect to a 3 / 4 area (3L / 4) of the total length L of the solid electrolyte layer 120 in the flow direction FG.

[0026] In this context, as in Fig. 4, the upstream part 120a has a first area a1 and a second area a2.

[0027] The first region a1 is a region within 3 µm of the anode-side surface 120S of the upstream part 120a. The anode-side surface 120S is the interface between the anode 110 and the solid electrolyte layer 120. The anode-side surface 120S is a straight line approximated, using a least-squares method, to a line of rapid change in the element densities contained in the solid electrolyte layer 120 when the component densities are mapped in a cross-section of the cell 100.

[0028] contains a zirconia-based material as a main component. The first region a1 contains tetragonal zirconia and cubic zirconia as the zirconia-based material.

[0029] Cubic zirconia is zirconia formed from a crystalline phase, with the phase essentially consisting of cubic crystals. Cubic zirconia contains, for example, 8YSZ (8 mol% yttrium-stabilized zirconia) or 10YSZ (10 mol% yttrium-stabilized zirconia).

[0030] Tetragonal zirconia is zirconia formed from a crystalline phase, with the phase comprising essentially tetragonal crystals. Tetragonal zirconia contains, for example, yttrium-stabilized zirconia containing equal to or less than 3 mol%, such as 2.5YSZ (2.5 mol% yttrium-stabilized zirconia) or 3YSZ (3 mol% yttrium-stabilized zirconia).

[0031] The second region a2 is a region located between the cathode 130 and the first region a1. The second region a2 is a region in the solid electrolyte layer 120 that is different from the first region a1. Although the thickness of the second region a2 is not particularly limited, it can be set to be greater than or equal to 1 μm and less than or equal to 30 μm. The thickness of the second region a2 is preferably less than or equal to 80% of the total thickness of the solid electrolyte layer 120 when considering preventing damage to the solid electrolyte layer 120. The thickness of the second region a2 is preferably greater than or equal to 20% of the total thickness of the solid electrolyte layer 120 when considering preventing a reduction in the oxide ion conductivity of the solid electrolyte layer 120.

[0032] The second region a2 contains a zirconia-based material as a main component. The second region a2 contains cubic zirconia as the zirconia-based material. The second region a2 may contain tetragonal zirconia.

[0033] In the present context, a ratio R1 of the peak intensity of the tetragonal zirconia to the peak intensity of the cubic zirconia in a Raman spectrum of the first region a1 (hereinafter appropriately abbreviated as "intensity ratio R1 of the first region a1") is greater than a ratio R2 of the peak intensity of the tetragonal zirconia to the peak intensity of the cubic zirconia in a Raman spectrum of the second region a2 (hereinafter appropriately abbreviated as "intensity ratio R1 of the second region a2").

[0034] To this end, it is possible to strengthen the framework structure of the porous first region a1 with a configuration of robust connections between the cubic zirconia particles by using tetragonal zirconia particles, which have a relatively smaller particle diameter compared to cubic zirconia. Therefore, damage to the upstream portion 120a resulting from thermal stress can be suppressed even if the temperature of the upstream portion 120a of the solid electrolyte layer 120 is reduced as a result of insufficient preheating of the fuel gas on the upstream side of the fuel flow channel 110S.

[0035] Furthermore, the downstream portion 120b has a main component made of a zirconia-based material. The downstream portion 120b contains cubic zirconia as the zirconia-based material. The downstream portion 120b may contain tetragonal zirconia. The material composition of the downstream portion 120b may be the same as the material composition of the second region a2 of the upstream portion 120a.

[0036] The intensity ratio R1 of the first region a1 is preferably greater than an intensity ratio R3 of the tetragonal zirconia to the cubic zirconia in a Raman spectrum of the downstream portion 120b (hereinafter appropriately abbreviated as "intensity ratio R3 of the downstream portion 120b"). Since it is possible to increase the oxide ion conductivity in the downstream portion 120b and the second region a2 of the upstream portion 120a to be greater than an oxide ion conductivity in the first region a1 of the upstream portion 120a, the overall oxide ion conductivity of the solid electrolyte layer 120 can be maintained.

[0037] The intensity ratio R1 of the first area a1 is obtained as described below.

[0038] First, in a cross-section parallel to the thickness direction of the first region a1, a Raman spectrum is obtained at five positions that evenly divide the first region a1 in a planar direction orthogonal to the thickness direction. It is preferable that the locations of the five positions for obtaining the Raman spectrum be approximately at the same position in the thickness direction.

[0039] Next, the ratio of the spectral intensity of tetragonal zirconia to the spectral intensity of cubic zirconia is calculated by analyzing the Raman spectrum at the 5 positions using the respective unique Raman spectra (known spectral data) for tetragonal zirconia and cubic zirconia, respectively. The means for analyzing the known spectral data for the Raman spectrum using a known spectrum is a CLS method, which is a known method for inferring a chemical species based on a Raman spectrum.

[0040] Subsequently, the intensity ratio R1 for the first region a1 is calculated as the arithmetic average of the intensity ratios calculated with respect to the Raman spectra at five positions. The intensity ratio R1 is an index indicating the concentration ratio (excess ratio) of tetragonal zirconia to cubic zirconia in the first region a1. The unit used for the intensity ratio R1 of the first region a1 is "%".

[0041] Although the intensity ratio R1 of the first region a1 is not particularly limited, it can be configured to be greater than or equal to 0.5% and less than or equal to 10%. The intensity ratio R1 of the first region a1 is preferably greater than or equal to 1%, and more preferably less than or equal to 8%.

[0042] The intensity ratio R2 of the second area a2 is obtained in a similar manner to the intensity ratio R1 of the first area a1 as described below.

[0043] First, in a cross-section parallel to the thickness direction of the second region a2, a Raman spectrum is obtained at five positions that evenly divide the second region a2 in a planar direction. It is preferable that the locations of the five positions for obtaining the Raman spectrum be approximately at the same position in the thickness direction.

[0044] Next, the ratio of the spectral intensity of the tetragonal zirconia to the spectral intensity of the cubic zirconia is calculated by analyzing the Raman spectrum at the 5 position respectively by using the respective unique Raman spectra (known spectral data) for tetragonal zirconia and cubic zirconia, respectively.

[0045] Subsequently, an intensity ratio R2 for the second region a2 is calculated as the arithmetic average of the intensity ratios calculated with respect to the Raman spectra at five positions. The intensity ratio R2 is an index indicating the concentration ratio (excess ratio) of tetragonal zirconia to cubic zirconia in the second region a2. The unit used for the intensity ratio R2 of the second region a2 is "%".

[0046] The intensity ratio R2 of the second region a2 is not subject to any particular restrictions as long as it is smaller than the intensity ratio R1 of the first region a1, and it can be configured to be less than or equal to 0.1%. The intensity ratio R2 of the second region a2 is preferably less than or equal to 0.05%.

[0047] The intensity ratio R3 of the downstream part 120b is obtained in the same manner as the intensity ratio R1 of the first area a1 of the upstream part 120a as described below.

[0048] First, in a cross-section parallel to the thickness direction of the downstream portion 120b, a Raman spectrum is obtained at five positions that evenly divide the downstream portion 120b in a planar direction. It is preferable that the locations of the five positions for obtaining the Raman spectrum be approximately at the same position in the thickness direction.

[0049] Next, the ratio of the spectral intensity of the tetragonal zirconia to the spectral intensity of the cubic zirconia is calculated by analyzing the Raman spectrum at the 5 position respectively by using the respective unique Raman spectra (known spectral data) for tetragonal zirconia and cubic zirconia, respectively.

[0050] Subsequently, the intensity ratio R3 for the downstream portion 120b is calculated as the arithmetic average of the intensity ratios calculated with respect to the Raman spectra at five positions, respectively. The intensity ratio R3 is an index indicating the concentration ratio (excess ratio) of tetragonal zirconia to cubic zirconia in the downstream portion 120b. The unit used for the intensity ratio R3 of the downstream portion 120b is "%."

[0051] The intensity ratio R3 of the downstream part 120b is not subject to any particular restrictions as long as it is smaller than the intensity ratio R1 of the first prayer a1, and it can be configured to be less than or equal to 0.1%. The intensity ratio R3 of the downstream part 120b is preferably less than or equal to 0.05%. Method for producing the fuel cell stack 10

[0052] The method for manufacturing the fuel cell stack 10 is described below.

[0053] First, a slurry for forming the anode 110 is prepared by mixing an organic binder and a solvent into a powder mixture (for example, a powder mixture of NiO powder and YSZ powder). The slurry is used to manufacture an anode plate (green body for the anode 110).

[0054] Subsequently, after dip-forming the zirconia-based material for the first region a1 on 1 / 4 of the region in the flow direction FG from the upstream end in the fuel gas flow direction FG of the green body for the anode 110, a green body for the upstream part 120a of the solid electrolyte layer 120 is formed by dip-forming the zirconia-based material for the second region a2 on this region.

[0055] Here, the mixing ratio of the tetragonal zirconia contained in the zirconia-based material for the first region a1 is higher than the mixing ratio of the tetragonal zirconia contained in the zirconia-based material for the second region a2. The intensity ratio R1 of the first region a1 can be adjusted with respect to the mixing ratio of the tetragonal zirconia relative to the cubic zirconia contained in the zirconia-based material for the first region a1. In the same way, the intensity ratio R2 of the second region a2 can be adjusted with respect to the mixing ratio of the tetragonal zirconia relative to the cubic zirconia contained in the zirconia-based material for the second region a2. However, the zirconia-based material for the second region a2 can be formed only by cubic zirconia.

[0056] Subsequently, a green body for the downstream part 120b of the solid electrolyte layer 120 is formed by dip-molding the zirconia-based material in a region that exceeds 1 / 4 of the green body for the anode 110 in the flow direction FG from the upstream end in the fuel gas flow direction Fg. Here, the mixing ratio of the tetragonal zirconia contained in the zirconia-based material for the downstream part 120b is lower than the mixing ratio of the tetragonal zirconia contained in the zirconia-based material for the first region a1 of the downstream part 120b. The zirconia-based material used in the downstream part 120b does not need to contain tetragonal zirconia and may only contain cubic zirconia.For example, the downstream part 120b may use the same zirconia-based material as the second region a2 of the upstream part 120a.

[0057] Then, after performing a heat treatment as a binder removal treatment on the stacked body of the green body for the solid electrolyte layer 120 and the green body for the anode 110, a co-fired body for the solid electrolyte layer 120 and the anode 110 is obtained by co-firing in an oxygen-containing atmosphere at 1300 to 1600 °C.

[0058] Then, a green body for the cathode 130 is formed by dip-molding a coating liquid containing a dispersion of a powder for forming the cathode 130 (for example, a LaFeO3 oxide powder) in a solvent on the surface of the solid electrolyte layer 120.

[0059] Then, the cathode 130 is formed by firing the green body for the cathode 130 at 1000 to 1300 °C. Further embodiments

[0060] The present invention is not limited to the above embodiment, and various changes or modifications may be added within a scope that does not deviate from the scope of the invention.

[0061] Although in the above-described embodiment, the solid electrolyte layer 120 according to the present invention is applied to all of the cells 100 provided in the fuel cell stack 10, it is sufficient if the solid electrolyte layer 120 according to the present invention is applied to at least one of the cells 100.

[0062] Although in the above-described embodiment, the fuel cell stack 10 is provided with a plurality of cells 100 and a plurality of separators 200, it is sufficient if at least one cell 100 and two separators (first separator 200a and second separator 200b) for holding the cell 100 are provided.

[0063] Although the cells are formed as square plates in the above-described embodiment, there is no limitation in this regard. For example, the cells 100 can be formed as circular plates, rectangular plates, triangular plates, or a polygonal-shaped plate with five or more corners. Even in such a configuration, the upstream portion 120a of the solid electrolyte layer 120 is 1 / 4 of the area in the flow direction FG from the downstream end of the solid electrolyte layer 120 in the fuel gas flow direction FG.

[0064] Although in the above-described embodiment, as shown in Fig. 4, the fuel gas flow direction FG is configured as a direction parallel to both sides of the outer edges of the cells 100, but there is no limitation in this regard. The fuel gas flow direction FG is not limited with respect to the shape of the cells 100 and may be oblique with respect to the outer edge of the cell 100. Even in such a configuration, the upstream part 120a of the solid electrolyte layer 120 is 1 / 4 of the area in the flow direction FG from the downstream end of the solid electrolyte layer 120 in the fuel gas flow direction FG.

[0065] Although in the present embodiment, the cells 100 are configured with the anode 110, the solid electrolyte layer 120, and the cathode 130, there is no need for a direct connection between the anode 110 and the solid electrolyte layer 120 or for a direct connection between the solid electrolyte layer 120 and the cathode 130. For example, a barrier layer for inhibiting the formation of a high-resistivity layer may be disposed between the solid electrolyte layer 120 and the cathode 130. The barrier layer may be formed, for example, using a cerium oxide-based material containing cerium oxide and a rare earth metal oxide in a solid solution in the cerium oxide. This type of cerium oxide-based material includes the use of GDC (gadolinium-doped cerium oxide), SDC (samarium-doped cerium oxide), or the like.

[0066] Although in the present embodiment, a configuration in which the solid electrolyte layer according to the present invention was applied to a solid oxide fuel cell was described, the solid electrolyte layer according to the present invention can also be applied to a solid oxide type electrochemical cell such as a solid oxide type electrolytic cell in addition to the solid oxide fuel cell. List of reference symbols 10 fuel cell stack 100 cells 110 Anode 110S fuel flow channel 120 solid electrolyte layer 120a upstream part 120S fuel cell side surface a 1 first area a2 second area 120b downstream part 130 Cathode 130S air flow duct 200 separators FG fuel gas flow direction

Claims

[1] Stack (10) of electrochemical cells (100) containing: a first separator (200); a second separator (200); and an electrochemical cell (100) arranged between the first separator (200) and the second separator (200), which electrochemical cell (100) contains an anode (110), a cathode (130) and a solid electrolyte layer (120), which solid electrolyte layer (120) is arranged between the anode (110) and the cathode (130) and contains a zirconium dioxide-based material as a main component, wherein the solid electrolyte layer (120) has an upstream part (120a) and a downstream part (120b), wherein the upstream part (120a) is positioned on an upstream side in a flow direction (FG) of the fuel gas flowing in a fuel flow channel (110S) between the anode (110) and the first separator (200), wherein the downstream part (120b) is positioned on a downstream side in the flow direction (FG), wherein the upstream part (120a) includes a first region (a1) within 3 µm of an anode-side surface and a second region (a2) provided between the first region (a1) and the cathode (130), wherein an intensity ratio (R1) of tetragonal zirconia to cubic zirconia in a Raman spectrum of the first region (a1) is greater than an intensity ratio (R2) of tetragonal zirconia to cubic zirconia in a Raman spectrum of the second region (a2), and wherein the intensity ratio (R1) of tetragonal zirconia to cubic zirconia in a Raman spectrum of the first region (a1) is greater than the intensity ratio of tetragonal zirconia to cubic zirconia in a Raman spectrum of the downstream part (120b). [2] Stack (10) of electrochemical cells (100) according to claim 1, wherein the intensity ratio (R1) of tetragonal zirconia to cubic zirconia in a Raman spectrum of the first region (a1) is greater than or equal to 1%. [3] Stack (10) of electrochemical cells (100) according to claim 1 or 2, wherein the intensity ratio (R2) of tetragonal zirconia to cubic zirconia in a Raman spectrum of the second region (a2) is less than or equal to 0.1%.

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