Sealants For Solid Oxide Fuel Cells Research Articles

Performance evaluation for Ag and Au nanoparticle containing K 2 O‐MgO‐B 2 O 3 ‐Al 2 O 3 ‐SiO 2 ‐F glass sealants for SOFC application

Performance evaluation for Ag and Au nanoparticle containing K ₂ O‐MgO‐B ₂ O ₃ ‐Al ₂ O ₃ ‐SiO ₂ ‐F glass sealants for SOFC application

CaO-SiO2-B2O3 Glass as a Sealant for Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are promising devices for electrical power generation from hydrogen or hydrocarbon fuels. The paper reports our study of CaO-SiO2-B2O3 material with composition 36 mol.% SiO2, 26 mol.% B2O3, and 38 mol.% CaO as a high-temperature sealant for SOFCs with an operating temperature of 850 °C. The material was studied as an alternative to presently existing commercial glass and glass-ceramics sealants for SOFCs with operating temperature of 850 °C. Many of these sealants have limited adhesion to the surface of Crofer 22APU steel, commonly used in these SOFCs. The present study included X-ray diffraction, dilatometric, thermal, and microstructural analysis The study has shown that the softening point of the CaO-SiO2-B2O3 glass is around 900 °C, allowing sealing of the SOFCs with this glass at convenient temperature of 925 °C. The CaO-SiO2-B2O3 glass sealant has shown excellent adhesion to the surface of Crofer 22APU steel; SEM images demonstrated evidences of chemical reaction and formation of strong interface on sealant–steel contact surface. Furthermore, the glass has shown a coefficient of thermal expansion about 8.4 × 10−6 1/K after sealing, making it thermomechanically compatible with the existing SOFC materials.

The development of 3D technology for the creation of glass sealants for tubular oxide fuel cells

AbstractThe creation of gas‐tight connections in solid oxide fuel cells (SOFCs) requires finding a suitable glass sealant composition and developing high‐tech approaches to sealant molding. 3D printing has been used successfully to mold SOFC functional materials (anode, electrolyte), and this study tests its applicability to glass sealant printing. We have found the composition of the glass‐filled polymer compound to print glass sealants for tubular SOFCs: the polycaprolactone/filler ratio is 60/40 wt%. Silica has been shown to be a good model filler to polish the 3D printing technology. Polymer decomposition during sealing leads to the formation of voids and seal failure, which indicates that the filler content of 40 wt % is insufficient for such purposes.

Preparation and characterization of RHA based ceramic membrane for gas leak testing in SOFC seals

Solid Oxide Fuel Cell (SOFC) systems show great potential in future power generation applications. SOFC has many advantages, including high efficiency, low emission, and flexible modular structure. SOFC is an electrochemical device that converts fuel into electricity directly. If hydrogen gas is used, it will produce electricity and waste products in the form of heat and water vapor. The complete SOFC module system consists of a furnace, cell stack, fuel and oxygen. Several parameters that affect SOFC performance are fuel flow rate, furnace temperature, cell material, and collector current. SOFC consists of anode, electrolyte, cathode, and current collector. These parameters have a correlation with each other in building a SOFC system, resulting in a good cell and optimal output voltage. SOFC based on electrolyte material yttria stabilized zirconia (YSZ) operates at working temperatures between 600 to 1,000°C, so all materials used must be able to withstand these temperatures. In the empirical case to separate each function of the components in the fuel cell, a sealant is needed. The sealant function prevents fuel and oxidant leakage in the stack and electrically isolates the cells in the stack. So the other sealant material requirements are to have thermal and chemical compatibility with other cell components, chemically and physically stable at high temperatures and have good mechanical strength. The composition of the material (RHA, CaO, Al2O3, MgO and BaO) with six formulations (F1-F6) which was determined as a gas leak test specimen through a synthesis process using the ball mill method for 12 hours resulted in a grain variation between 3.808 um – 19.631 um. The production of gas leak test specimens was successfully carried out by molding which was designed to be effective using a material weight of 1.5 grams with a 96% PVA binder as much as 20% by weight. The size of the specimen obtained in the form of a membrane with a thickness of 1-1.1mm. The preparation of the test material can be continued in the next gas leak test process.

Structural and thermophysical behaviour of barium zinc aluminoborosilicate glasses for potential application in SOFCs

Structure property relationships in two series of BaO-ZnO-La2O3-Al2O3-B2O3-SiO2 glasses have been investigated to assess their suitability for use as sealants in solid oxide fuel cell (SOFC) applications. Density, mechanical properties, thermal and electrical properties have been investigated as a function of composition. Raman and Fourier transfer infra-red spectroscopies were undertaken to obtain insights into the glass structure. The mechanical properties are generally relatively high but the thermal expansion coefficients are lower than ideal for SOFC sealants. Of the glasses studied 9.74BaO-6.39ZnO-15.84La2O3-5.04Al2O3-15.63B2O3-47.37SiO2 (mol%) has the most promising properties for SOFC sealing applications.

Progress in Glass-Ceramic Seal for Solid Oxide Fuel Cell Technology

Solid oxide fuel cells (SOFCs) have emerged as promising energy conversion devices nowadays. SOFC consists of several components such as cathode, anode, electrolyte, interconnects, and sealing materials. In planar SOFC stack construction, the sealant and interconnection functions play an important role. Glass and ceramics are quite popularly used as SOFC sealing materials to achieve several functions including preventing leakage of fuel and oxidants in the stack and electrically isolating cells in the stack. In this review, material preparation, material composition, ceramic properties especially thermal properties are compared from various systems that have been developed previously. The main challenges and complexities in the functional part of SOFC sealants include: (i) chemical incompatibility and instability in the oxidizing and reducing environment by adjusting the value of the thermal expansion coefficient (CTE) with the interconnecting material during SOFC operation, and (ii) insulation of oxidizing fuels and gases by matching CTE anode and cathode. Also, the sealant glass transition determines the maximum permissible working temperature of the SOFC. The choice of method and analysis will provide data on various ceramic attributes. The search for thermal attributes consisting of Glass transition (Tg), Deformation temp (Td), Crystallization temp (Tx), Melting pt (Tm) became a focus on SOFC sealant development.

Feldspar-based CaO–K2O–Na2O–BaO silicate glass sealant for solid oxide fuel cells

Sealants have been widely used in solid oxide fuel cells (SOFCs), and glass sealants have shown considerable potentials due to their low leakage rate. We report a type of feldspar-based glass sealant, mainly composed of CaO–K2O–Na2O–BaO, with a coefficient of thermal expansion (CTE) of 10.11 × 10−6 k−1, a glass transition temperature at 528 °C, and a softening temperature at 573 °C. Using this sealant to seal anode-supported SOFCs, the open-circuit voltage (OCV) remains at 1.08 V after five thermal cycles and 50-h aging at 750 °C, close to the theoretical OCV value. The SEM and EDS reveal stable interface between the glass sealant and the anode, indicating excellent performance of the CaO–K2O–Na2O–BaO glass sealant for SOFC sealing applications.

Mica (KMg3AlSi3O10F2) based glass-ceramic composite sealant with thermal stability for SOFC application

In this work, pre-synthesized fluorophlogopite mica (KMg3AlSi3O10F2) crystalline powder was added into potassium-magnesium boro-alumino-silicate (K2O–MgO–B2O3–Al2O3–SiO2) glass powder to synthesize glass-ceramic composite (GCC) sealant for solid oxide fuel cell (SOFC) application. The basic purpose was to report the influence of KMg3AlSi3O10F2 on microstructure, thermal expansion and volume shrinkage behavior of GCCs in terms of their SOFC sealing ability. The predominant crystalline phase was also identified as fluorophlogopite mica (KMg3AlSi3O10F2) by X-ray diffraction in the GCCs heat-treated at 900 °C (for 2 h) followed by 800 °C (for 10 h). Higher thermal expansion (10.65–10.81 × 10−6/K at 50–800 °C) compatible with SOFC components (metallic electrode/interconnect, ceramic electrolyte) was realized for 5–10 wt % mica containing GCC. However, it was found decreased linearly with adding further KMg3AlSi3O10F2 content (20–40 wt%) in the composites. GCC with 5 wt % of mica was stable under thermal cycling testing and no significant degradation was observed in thermal expansion till 10 cycles. Field emission scanning electron microscopy revealed the spherical shaped nanocrystallites morphology of those GCCs and that was ascribed for their compact bonding with SOFC components. Heating stage microscopy (HSM) ensured the lower softening temperature (752 ± 7 °C) for 5 wt % mica containing GCC which furthermore showed 11–13% volume shrinkage (to ensure the self-healing) in SOFC operating temperature. In a summary, the above factors confirmed the suitability of 5 wt % mica (KMg3AlSi3O10F2) containing K2O–MgO–B2O3–Al2O3–SiO2 system (GCC) for SOFC sealant application.

Structural and electrical behaviour of glass ceramic 00B under sandwiched condition between two Crofer22APU substrates

Obtaining high temperature glass and glass ceramic based seals is one of the biggest challenges in the present day fabrication of Solid Oxide Fuel Cell (SOFC) stacks. Formation of BaCrO 4 phase in BCAS glasses leads to degradation of cell performance. This work focusses on one glass composition derived from a BCAS species named 00B which is free of BaO and Al 2 O 3 contents. 00B was sandwiched between two Crofer22APU plates forming CGC sandwiches and heated for repeated thermal cycles from RT to 800 °C to check their compatibility with the metallic substrate. Microscopic images of the CGC interface and XRD data of the internal glass surface of the broken sandwich were recorded to observe the structural properties of the prepared sandwich. DC resistivity measurements of the glass under sandwiched conditions between two Crofer22APU plates, at RT and at elevated temperatures were carried out, to determine its resistivity at high temperatures, under SOFC operating conditions. The CGC sandwiches prepared using sample 00B showed good structural properties after repeated thermal cycles and sufficiently high specific resistance at room and elevated temperatures. The 00B glass without BaO and Al 2 O 3 content, but having structural and electrical behaviour suitable for coating on Crofer22APU, is suggested to be a potentially promising candidate for SOFC sealant applications.

Non-crystallising glass sealants for SOFC: Effect of Y2O3 addition

The joining of ceramic and metal (interconnect) parts is one of the main challenges in the development of solid oxide fuel cells (SOFC). A promising approach to solving this problem is the use of glassy sealants. In this work, we investigated the effect of yttria additions on the properties of SiO2–Al2O3–CaO–Na2O–ZrO2–Y2O3 glass sealants. An increase in the concentration of yttria is shown to reduce the tendency of the glasses under study to crystallisation. A glass containing 4 wt% of Y2O3 is found to be amorphous, even after exposure at 850 °C for 100 h. Moreover, the defectiveness of the glass microstructure, after sealing, is found to decrease along with a growth in the Y2O3 concentration. The developed non-crystallising sealant was successfully applied for joining a YSZ ceramic and an Fe–Ni–Co alloy having the phase transition of around 500 °C. The use of the non-crystallising sealant allows us to join materials with very different thermal expansion coefficients and to avoid cracking under cooling, which might occur due to a large difference in thermal expansion coefficients.

Crystallization kinetics of basalt-based glass-ceramics for solid oxide fuel cell application

Solid oxide fuel cell (SOFC) is a very efficient and clean source of energy. The glass and glass-ceramics are the most suitable and compatible sealing materials for SOFC. The crystallization kinetics of glass sealants is an essential parameter to check the suitability of glass as a sealant. In this study, the crystallization behavior of a novel sealing glass developed from the volcanic rock basalt suitable as an SOFC sealant is investigated via differential thermal analysis using the Kissinger kinetic model with several different heating rates. Development of crystalline phases on thermal treatments of the glass at various temperatures has been followed by X-ray diffraction. Augite and diopside are the primary crystalline phases in crystallized glass-ceramic seals. The results show that the basalt base glass-ceramic sealant material exhibited promising properties to use for SOFC.

Novel glass-ceramic SOFC sealants from glass powders and a reactive silicone binder

The processing of sintered ceramics is often conditioned by the debinding step. The binders may determine some defects in the final product directly, by causing some gas evolution even at an advanced state of densification, due to incomplete decomposition at low temperature, or indirectly, by offering poor adhesion between particles, so that ‘green’ compacts may be easily damaged. The present investigation is aimed at exploring a novel concept for sintered glass-ceramics, based on the adoption of a silicone polymer as reacting binder, providing an abundant ceramic residue after firing. A glass belonging to the CaO-MgO-Al2O3-SiO2 system, already studied as a sealant in solid oxide fuel cell (SOFC) planar stack design, was reproduced in form of ‘silica-defective’ variants, featuring a SiO2 content, in the overall formulation, reduced up to 15 wt%. The overall silica content was recovered by mixing powders of the new glasses with the silicone: upon firing in air, the interaction between glass powders and polymer-derived silica led to glass-ceramics with the same phase assemblage than that formed by the reference glass and with a CTE of 9.5 × 10−6 K-1. The new approach has been successfully applied to the manufacturing of glass-ceramic seals as joining materials for solid oxide cells.

Torsional shear strength of novel glass‐ceramic composite sealants for solid oxide fuel cell stacks

AbstractTo improve the mechanical strength of sealants for solid oxide fuel cell (SOFC) stacks, several fillers were proposed in this work as reinforcement for a glass‐ceramic sealant based on the system BaO‐CaO‐SiO2. The chosen reinforcement additives were metallic particles including nickel, a nickel‐chromium alloy and silver, as well as ceramics such as gadolinium‐doped ceria particles and yttrium‐stabilized zirconia particles or fibers. The glass‐ceramic sealant, with and without reinforcements, was used to join two parts made of stainless steel (Crofer22APU). A torsion test was used to measure the shear strength of the joined samples. The shear strength measurement by torsion gave reliable and reproducible results for all the joined samples. The lowest shear strength was found for the glass‐ceramic sealant without any reinforcement. The composite prepared of a combination of glass matrix and 20 wt.‐% Ni powder seems to be the best candidate sealant, able to fulfill all the requirements of a SOFC sealant. The samples made with this composite sealant presented the highest shear strength values as prepared and also after aging the sample in air atmosphere for 500 hours at 800°C.

The analysis of torsional shear strength test of sealants for solid oxide fuel cells

Abstract A torsion test recently implemented for solid oxide fuel cell sealant materials is analyzed as a method for measuring the shear strength of sealant for solid oxide fuel cells. The finite element method is used to simulate the stress distribution in the hourglass-shaped steel specimens with intermediate sealant layer with different specimen's dimensions and configurations. Also, it is analyzed how stress concentration changes if the sealant does not completely fill the gap or is squeezed out of gap. The reduction of seal thickness to outer radius ratio results in an increase in stress concentration at the outer edge of sealant. The developed specimens with a hollow halve steel plate as well as the ones with two hollow halve steel plates appear to be suitable choices for torsional shear strength test, reducing the torque for fracture and stress concentrations. Effects of lack of filling and squeezing out of gap onto the stress distribution are negligible compared to the effect of pre-existing discontinuities.

Characterization of Reliability of Solid Oxide Fuel Cell Sealants and Anodes

Typically brittle sealant and anode materials are critical for the reliability of solid oxide fuel cells (SOFCs). Hence, characterization of these failure-relevant materials is crucial for the long term operation. In particular, the mechanical behaviour and elevated temperature stability which is strongly affected by creep has to be thoroughly understood and in terms of property – microstructure correlation, related to the materials’ compositions, crystallization, porosity and phase stability. Since sealant leakage can lead to malfunction of the whole system, the structural integrity of the sealant is a key issue for the operation of SOFC stacks and systems. Fracture properties and elevated temperature deformation need to be assessed, particularly for partially crystallized glass-ceramic sealants that might suffer from instability issues at operation relevant temperatures due to viscoelastic deformation of the residual glass phase. In this work, a modified sealant, which consists of a composition from the BaO-CaO-SiO2 ternary system with Ag particles reinforcement, was characterized. Bending tests were carried out at room temperature and typical stack operation temperatures on a head-to-head specimen geometry in an as-sintered and annealed state, yielding average fracture stresses and apparent creep data. Since the sealants are exposed to a complex loading state in the stack environment, complementary torsion tests were used to investigate the shear strength. Advanced microstructural characterization was carried out to gain insight into the annealing and reinforcement filler effects. The results are compared and discussed with respect to literature data and their relevance for the application in SOFCs operating at different temperatures in the range of 600°C to 800°C. Typical anode substrate materials (NiO-3YSZ and NiO-8YSZ) along with their reduced and re-oxidized state were analysed by means of fracture toughness at room temperature and 800 °C. Effects related to porosity and transformation toughening could be verified. Furthermore, creep of porous Ni-YSZ composite has been investigated under H2/Ar atmosphere at typical operating temperatures. Ni-YSZ materials with different porosities and Ni/YSZ ratios were tested in order to investigate material´s composition and porosity effects. Different loading configurations such as compression, four-point bending and ring-on-ring bending have been used to assess the differences in creep response of these materials. Interpretation and analysis of the data were based on analytical and finite element simulation. The systematic comparison and discussion of the results was aided by microstructural investigations.

Study of sealants for SOFC

The development of suitable sealants for high-temperature solid oxide fuel cells (SOFC) is a major challenge, as such sealants must withstand harsh conditions. They must be inert at the high temperatures used in SOFC operation (i.e., resistant to environments composed of oxidative and reducing gases), as well as thermo-chemically and thermo-mechanically compatible with the materials with which they are in contact. This paper presents a post-experimental analysis of variously sealants—mica paper, flexible mica paper, and thermiculite 866—used in high-temperature SOFC operation. The sealants are exposed to air or hydrogen at 600 °C–1000 °C for 100 h. The goal of this work is to investigate the thermal expansion properties (thickness expansion, coefficients of thermal expansion CTE and porosity), mechanical stability, and leakage during midterm operation. After the sealants fired at 1000 °C, their relative thicknesses increased to around 0.98, 1.01, and 0.568 mm, respectively. The coefficients of thermal expansion CTEs (600–1000 °C) for mica paper and flexible mica paper were calculated to be from 8.0 × 10−4 K−1 to 10.0 × 10−4 K−1, the CTEs of thermiculite 866 were around 1.0 × 10−4 K−1. The relative porosity of thermiculite 866, as determined through the density method, changed from 15.4% to 28.7% for temperatures from 600 °C to 1000 °C, respectively. Scanning electron microscopy is used to investigate the structure of thermiculite 866. It is tested for leakage using hydrogen from 500 to 3000 cc min−1 at 25 °C and 800 °C. The leakage rates at 3000 cc min−1 are 4.06% and 8.4% at 25 °C and 800 °C, respectively, showing that thermiculite 866 is a suitable sealant for SOFC applications.

Extreme nanomechanics: vacuum nanoindentation and nanotribology to 950 °C

Elevated temperature mechanical and tribological properties can be more relevant for practical wear situations than corresponding measurements at room temperature. However, high temperature nanomechanics and nanotribology is highly challenging experimentally. To overcome these challenges the NanoTest*** has been developed with active heating of the indenter and sample with resistive heaters, horizontal loading, patented thermal control method and stage design. By separately actively heating*** and controlling the temperatures of indenter and sample their temperatures can be precisely matched so that there is no heat flow and minimal/no thermal drift during the high temperature indentation,*** and measurements can be performed as reliably as at room temperature. Above 500 °C it is beneficial to use a cubic Boron Nitride indenter with gas purging to limit oxidation of samples. To achieve higher temperatures without indenter or sample oxidation an ultra-low drift high temperature vacuum nanomechanics/tribology system capable of testing to*** much higher temperatures has been recently developed (NanoTest Xtreme). The influence of time-dependent deformation on elevated temperature nanomechanical behaviour is discussed, using published results in Argon on glass-ceramic solid oxide fuel cell seal materials and previously unpublished nanoindentation measurements on single crystal silicon and polycrystalline tungsten using the NanoTest Xtreme in vacuum at temperatures up to 950 °C. Studies of the elevated temperature nano-/micro-tribological*** behaviour of wear-resistant*** nitride-based and MAX-phase coatings are also briefly reviewed.

Effects of Al2O3 on (1−x)[SrO–SiO2–B2O3]–xAl2O3 glass sealant for intermediate temperature solid oxide fuel cell

The effects of Al2O3 on (1−x)[SrO–SiO2–B2O3]–xAl2O3 (SASB, x=0, 0.02, 0.04, 0.06, 0.08, and 0.1) sealing glasses for intermediate temperature solid oxide fuel cell (SOFC) have been investigated to determine the proper amount of Al2O3 for the application of the SASB glasses to a SOFC sealant. The structural, thermal, mechanical and adhesion properties were determined by using DTA, TMA, XRD, SEM, and Raman spectroscopy. It is found that the thermal expansion coefficients of the glasses and crystallized samples are 8.0–11.0×10−6°C−1, and ~10.0×10−6°C−1, respectively, which are within the required values for the SOFC sealant. The change of mode population in Raman scattering analysis shows that Al2O3 acts as a network modifier. When annealed at 800°C, the glasses with x=0 and 0.1 start to crystallize in 5h, but the sample with x=0.6 mostly remains as a glass phase even after 50h, and this crystallization behavior has a close relationship with adhesion properties. Comparing the cross-sectional views of SEM images between x=0 and x=0.06, the sample with x=0.06 shows homogeneous morphologies with good adhesion and clean surface boundaries between the sample and substrate. But for the x=0 case, the boundaries are not clearly defined and devitrifications with heterogeneous morphologies exist.

Development of bilayer glass-ceramic SOFC sealants via optimizing the chemical composition of glasses—a review

Glasses and glass-ceramics (GCs), in particular alkaline-earth alumino silicate-based compositions, are becoming the most common sealing materials for gas-tight sealing applications in solid oxide fuel cells (SOFCs). The present review aims at reporting the systematic procedures put forward developing a novel concept of diopside-based bilayer GC seal, which contains a rigid GC and a self-healing (SH) GC. The concept behind the bilayer GCs is (i) a small gradient in the coefficient of thermal expansion (CTE) will lead to lower thermal expansion mismatch between the sealing layers and other SOFC components, thus providing enhanced mechanical reliability for the stack; and (ii) cracks produced due to minor thermal stresses in the rigid GC layer can be healed by the SH GC layer due to sufficient amorphous content. In general, at high temperature, highly crystallized glass behaves as a rigid glass. On the other hand, due to low viscosity behavior, partially crystallized glass provides a SH behavior. Various glasses in the field of diopside crystalline materials have been systematically designed by varying the chemical composition of glass to achieve desired combination of functional properties for the rigid and SH GC layers. The glass Sr-0.3 where Sr replaced 30 % of Ca was revealed as a highly reliable rigid GC seal for high-temperature electrochemical applications. On the other hand, SH features have been achieved in 30 % Sr-containing diopside-based glass with Gd2O3 for MgO + SiO2 substitution (denoted as Gd-0.3). These GCs exhibit similar thermal properties and excellent thermal stability along a period of 1000 h, while differing in their amorphous fractions, and revealed excellent thermal stability along a period of 1000 h. The bilayered GC synthesized from Sr-0.3 and Gd-0.3 showed good wetting and bonding ability to the SOFC metallic Crofer22APU components. The results revealed superior performance for the newly proposed bilayer GCs in comparison to single-layer sealants.

THE INVESTIGATION ON INTERFACE CHARACTERISTIC OF CERAMIC SEALANTS PRODUCED FROM NATURAL ROCKS FOR SOLID OXIDE FUEL CELLS

Solid oxide fuel cells (SOFC), which are green electrochemical devices, transform directly chemical energy of fuel to electricity, and striking heat energy by using solid fuels as electrolyte [1-3]. One of the essential problems for SOFC is to mix the gases, which used in anode and cathode, reacting electrochemically at high temperature before the reaction and/or the gas infiltration to outside of SOFC. It makes security problem and low efficiency. To use safe sealing material for SOFC is very important due to high operating temperature such as 600-1000°C. The actual studies have shown that the sealing materials produced from glass-ceramics have the greatest potential about sealing. Glass-ceramics are polycrystalline materials produced by controlled crystallization of glass that is suitable for crystallization. Glass-ceramics that can be produced by using various compositions are ideal sealing material in terms of accordance between thermal expansion coefficients of SOFC components and glass-ceramic, high temperature strength and corrosion resistance. However, there are also problems resulting from glass-ceramic sealants in SOFC. Nowadays, the glass-ceramics including BaO and SrO are used in commercial glassceramic sealants for SOFC. In the operation condition of SOFC, BaCrO4 and/or SrCrO4 phases can form because of different thermal expansion coefficient, when the glassceramics including BaO and SrO are combined with Crofer Apu 22 as interconnect material. This situation may cause decrease in bonding strength, sealing failure and crack formation. In the current study, the glass-ceramic compositions not including BaO and SrO were prepared for sealant material of SOFC and interface characteristics with CroferAPU22 were investigated. The results indicated that the glass-ceramic materials exhibiting good bonding with CroferAPU22 and not containing chromate phase can be produced by natural rocks.