Synthesis by coprecipitation of india-stabilized zirconia and codoping with MoO

Coprecipitation synthesis of nanocrystalline india-stabilized zirconia with high surface area and codoping with MoO3, WO3, TaO2.5, or NbO2.5 is reported. The concentration of codopants was defined by the charge-compensating mechanism. Ethanol washing followed by azeotropic distillation and freeze drying were compared as dehydration techniques for the gels. As determined by XRD and Raman scattering, 9 mol% of InO1.5 plus charge-compensating dopants is sufficient to completely stabilize the high-temperature tetragonal phase of zirconia. The effect of alloying hexavalent and pentavalent oxides was secondary compared to the InO1.5 concentration in the retention of the tetragonal structure. Improved specific surface area of 106.1 m2 g‒1 and crystallite size between 8 and 9 nm were achieved through ethanol washing and subsequent azeotropic distillation even after calcination at 600 oC. This result is attributed to the effect of the incorporation of ethoxy and butoxy groups after the treatment of the gels in organic medium, as detected by FT-IR spectroscopy and DSC/TG.


Introduction
Thermal barrier coatings (TBC) have been widely used to protect metallic parts from high temperatures in gas turbines 1,2 , providing substantial benefits to the engine 3 .TBC acts as an insulating surface, reducing the underlying metal temperature by as much as 150 °C4,5 , therefore enabling the operation of metal-based components without melting or high-temperature corrosion.Also, the application of TBC allows higher inlet temperatures of the gas turbine, thereby reducing the amount of cooling air and increasing the volume of the working gas.These are key factors to improve the efficiency of land-based turbines for electrical power generation 4 .
Although some new systems are being studied 6,7 , yttria-stabilized zirconia (YSZ) is still the main industrial TBC material 8 .However, increasing the turbine temperature using YSZ coatings is limited.One restriction arises from the phase stability of tetragonal zirconia at high temperatures.Considerable improvements in this case have been found by alloying YNbO 4 and YTaO 4 to zirconia 9,10 .Another drawback is the low corrosion resistance of YSZ against molten salts 2 .As the variety of fuels that can be used in gas turbines is quite extensive, there is a growing interest in non-traditional fuels such as petroleum coke or residual and heavy oils 4,11,12 .In turbines burning these low-quality fuels, a high concentration of impurities, namely vanadium, phosphorus, and sulfur form salts responsible for the shortening of the YSZ-coating lifetime 11,13 .At high temperatures, these molten salts are responsible for the depletion of stabilizing Y 3+ ions from the YSZ lattice, resulting in coating cracks and detachment due to the 4-vol% expansion associated with the tetragonalto-monoclinic phase transformation 1,3 .
It is well known that the corrosion resistance of zirconia coatings can be improved by selecting and employing different stabilizers instead of yttria, since the nature of the stabilizer cation has been considered to be the most important factor affecting the corrosion resistance of zirconia-based TBCs 14,15 .Jones et al. 16,17 , who attempted to identify the most corrosion-resistant stabilizer, reported that the resistance of In 2 O 3 against molten salts is greater than that exhibited by Y 2 O 3 , suggesting that 7.8-12.2mol% InO 1.5 -stabilized zirconia (InSZ) may be an alternative coating to YSZ for gas turbines burning low-quality fuels.In the binary system ZrO 2 -InO 1.5 18 , stabilization of metastable tetragonal phase has been reported alloying up to 9 mol% de InO 1.5 , whereas the cubic phase stabilization starts at higher InO 1.5 additions.Despite its efficacy against corrosion by molten salts, a major problem associated with InSZ is the volatilization of In 2 O 3 19 , which results in poor-quality and inhomogeneous coatings.To date, few attempts have been made to lower the volatilization of In 2 O 3 in InSZ coatings.The most promising approach was reported by Hill et al. 20 , that the pre-reaction of certain oxides, such as Sm 2 O 3 and Gd 2 O 3 , with In 2 O 3 prevents loss of indium due the formation of a stable and less volatile perovskite compound.It appears that the alloying of a ternary oxide into InSZ can be the main route to improve its thermal stability.Considering this approach, in this paper we apply the concept of charge-compensation in the synthesis of InSZ codoped with MoO 3 , WO 3 , TaO 2.5 , or NbO 2.5, intended to establish a route for the preparation of codoped InSZ-based TBCs.The selection of pentavalent oxides is based on its good performance in YSZ 9,10 , in addition, it is explored further effects in the preparation of stabilized zirconia by charge-compensation using hexavalent ions.Moreover, we compared the effects of two different dehydration techniques applied to the coprecipitated gels to provide nanocrystalline powders with high specific surface area of these materials.

Synthesis by coprecipitation
The initial raw materials were: ZrCl 4 (99.5%),InCl 3 (99.99%),NbCl 5 (99.9%),TaCl  1.The molar concentrations of the dopants in InSZ were adjusted respecting the charge-compensating mechanism 10,[21][22][23] .In this mechanism, we consider the partial substitution of Zr 4+ host by In 3+ acceptor ions along with the consequent compensation for Ta 5+ /Nb 5+ or W 6+ /Mo 6+ donor ions.For zirconia stabilized with InO 1.5 + TaO 2.5 /NbO 2.5 , the molar ratio of the cations was maintained at 1:1 (In 3+ :Ta 5+ /Nb 5+ ).This ratio can be understood through the Kröger-Vink notation 24 in Eqs. 1 and 2 wherein electroneutrality is achieved in case the doped acceptor and donor ions are in the same ratio.In the case of zirconia codoped with InO 1.5 + WO 3 /MoO 3 , the concentration of the doping ions was maintained at the ratio of 2:1 (In 3+ :W 6+ /Mo 6+ ).This ratio results from the fact that the incorporation of two In 3+ ions in the zirconia network requires one W 6+ /Mo 6+ to reach electroneutrality without creating oxygen vacancies, as can be seen in Eqs. 3 ( ) Codoped InSZ gels were synthesized by coprecipitation.For the synthesis of InTaSZ and InNbSZ-derived compositions, all precursors were mixed together in distilled and deionized water and stirred for 10 min at 25 ºC to obtain a 0.1M solution.The prepared precursor solution was added dropwise at rates of 2-8 mLmin -1 into a vigorously stirred solution of aqueous ammonia (28-30% NH 3 ) with pH value of 11.8, in which the coprecipitation took place.The gel was then left without stirring to settle for 48 h in the precipitating solution at room temperature.Following this process, the precipitate was filtered and washed eight times with water.In the first two water-washing steps, a solution of 1M NH 4 OH was used to prevent loss of cations.The following six washes were performed with distilled water until the absence of Cl -ions was detected (monitored by means of 1M AgNO 3 solution).
In the case of InMoSZ and InWSZ-derived compositions, since precipitation of MoO 3 and WO 3 does not occur in basic medium, their precipitation was performed separately in acid medium and then the precipitate was added in the appropriate amount to the water-washed InSZ gel.For the precipitation of MoO 3

25
, initially, 0.2M (NH 4 ) 6 Mo 7 O 24 •4H 2 O solution in distilled water was stirred for 20 min at 25 °C.Once homogenized, HNO 3 (64-66%) was used as precipitating agent and added in a dropwise manner to the ammonium-molybdate solution.This solution was magnetically stirred for 30 min at 80 °C, corresponding to the appropriate conditions at which precipitation of MoO 3 could be observed.The amount of HNO 3 was defined by using Reaction 5.For the precipitation of WO Once the codoped InSZ gels were synthesized, the effects of the dehydration techniques ethanol washing followed by azeotropic distillation (EW/AD) and freeze drying (FD) were compared.For EW/AD, the gels were first washed twice with ethanol for 30 min under stirring, then left to settle for 1 h, and filtered.The mass ratio of 1:2.5 (gel/ethanol) was used 27 .In the next step, the gels were mixed with iso-butanol in the mass ratio of 1:1 (gel/butanol), vigorously stirred for 1 h and submitted to azeotropic distillation.For FD dehydration, a Liotop freeze-dryer model L101 was used.Initially, the gels were cooled in liquid nitrogen (-196 ºC) for 5 min.The condenser temperature and storage camera were kept at -52 and 20 °C, respectively.Vacuum of 5 Pa was applied to the storage camera, and the entire freeze-drying process took 48 h.Finally, both EW/AD and FD-derived powders were calcined at 600 °C for 1 h with heating/cooling rates of 3 °C min -1 .

Powder characterization
The crystal structures of the samples were identified by X-ray diffraction (XRD) using a diffractometer model Siemens D5005 in Bragg-Brentano geometry with CuKα radiation in the 2θ range from 5 to 85º and at the scan rate of 2º min -1 .The fraction of tetragonal zirconia (v t = 1 -v m ) existent in the calcined powders was determined following the Eqs.7 and 8 described by Toraya et al. 28 , where X m is the integration of the intensities ratio and I represent the diffraction intensity of the respective lattice plane for I m (monoclinic) and I t (tetragonal) phases.For the samples composed of the t-phase, the crystallite size was measured by the Scherrer equation 29 in the (101) t reflection of the tetragonal phase.Raman spectra of the calcined samples were obtained with a Renishaw inVia micro-spectrometer at 25 °C.Excitation was performed using the 532 nm line and spectra were recorded in the spectral range between 1 and 1000 cm -1 .
Fourier-transform infrared (FT-IR) spectra were recorded with a Thermo Scientific Nicolet TM 6700 spectrometer with 2 cm -1 resolution and 128 scans in the spectral range between 400 and 4000 cm -1 .Powder samples were mixed with KBr in a mass weight of 1:1 for FT-IR recording.Differential scanning calorimetry and thermogravimetric analysis (DSC/TG) of the gels were recorded with the heating rate of 10 °C min -1 up to 700 °C in an Ar atmosphere.
The specific surface areas S BET of the calcined powders were measured by the Brunauer-Emmett-Teller (BET) method using N 2 adsorption-desorption (Micrometrics Gemini 2370 V1.02).The particle size and morphology of the samples were observed by transmission electron microscopy (TEM) with the instrument FEI Tecnai G 2 F20 at the acceleration voltage of 200 kV.For TEM observations, the powders were ultrasonically dispersed in acetone for 10 min and then dripped onto carbon films on top of copper grids (200 mesh) and dried at room temperature.

Crystallization and phase formation
As shown in Fig. 1, the uncalcined gels dehydrated by EW/AD and FD are amorphous, regardless of the concentration of the stabilizer alloyed.For compositions containing 3 mol% of InO 1.5 , it is possible to observe some retention of the tetragonal symmetry (PDF No. 79-1771) after calcination at 600 ºC.However, in these compositions, the concentration of the stabilizer is insufficient for the total stabilization of the t-phase and, therefore, the monoclinic phase (PDF No. 37-1484) of zirconia (m-) was detected (Fig. 1a and b).For the FD-dehydrated samples, increasing the concentration of InO 1.5 to 6 mol% generates exclusively the t-phase without small background peaks (-111) m and (111) m of the monoclinic phase.With respect to the samples dehydrated by EW/AD, the complete stabilization of the t-phase was only achieved for samples with 9 mol% of InO 1.5 plus additional charge-compensating dopants: 9 mol% of NbO 2.5 /TaO 2.5 or 4.5 mol% of MoO 3 /WO 3 .DSC analysis for the InNbSZ compositions (Fig. 2) shows that the crystallization temperature rises with increasing InO 1.5 and NbO 2.5 concentration.The crystallization at 491 °C for the 9 mol% InNbSZ sample is higher than 480 °C30 and 486 ºC 27 recently reported for the crystallization of tetragonal YSZ.Comparing these DSC results with the XRD patterns shown in Fig. 1b, it can be seen that increased crystallization temperatures correspond to the stabilization of the t-phase, in agreement with the results obtained for the Y 2 O 3 -ZrO 2 system, as reported by Haberko et al. 31 .
From the XRD patterns in Fig. 1, the distinction between tetragonal and cubic phase (c-) is not apparent, since the distinction between the peaks (002) t /(110) t at 2θ ≈ 35º cannot be observed unambiguously because of peak-broadening effects 32,33 .For this reason, visible Raman spectra were used to observe the existence of the single t-phase 2,34 .Raman spectra for InTaSZ compositions are shown in Fig. 3.It can be seen in Fig. 3a that the 3InTaSZ sample gives major bands at 176, 188, 219, 330, 343, 381, 460, 501, 538, 558, and 618 cm -1 , which are characteristic bands of the m-phase.These are 11 of the 16 Raman modes predicted by group theory for monoclinic zirconia 2,[35][36][37] .The detection of the m-phase in the Raman spectra of the 3InTaSZ sample is in agreement with the presence of 58 vol% of monoclinic zirconia calculated on the basis of XRD.In the case of the 9InTaSZ sample (Fig. 3c), bands at 147, 276, 310, 470, and 637 cm -1 represent 5 of the 6 Raman-active modes predicted by group theory for tetragonal symmetry 36,38,39 .Similar Raman spectra for YSZ were recorded by Ghosh et al. 33 .Therefore, the 9InTaSZ sample is composed exclusively by the t-phase and we can consider that this holds true also for the others EW/AD dehydrated samples: 9InNbSZ, 9In4.5MoSZ, and 9In4.5WSZdue the similarity of their XRD patterns (Fig. 1) and the same InO 1.5 concentration.Moreover, the Raman spectrum of the 9InTaSZ sample does not provide evidence for the existence of the c-phase 32,35,36,39 , since cubic symmetry must theoretically be reflected by a single broad band between 400 and 670 cm -1 , which is not observed in Fig. 3c.
A comparison of the XRD and Raman results reveals that alloying 9 mol% of InO 1.5 into zirconia and dehydrated by EW/AD provides the complete stabilization of the t-phase.On the other hand, the charge-compensating dopants on definition of crystal structure could not be observed in the present work due the lower calcination temperature.This condition does not allow the codopants diffusion, especially for molybdenum and tungsten oxides, that were not coprecipitated with indium oxide.Li et al. 21found that the substitution of Zr 4+ host ions by Nb 5+ (and also Ta 5+ ) in YSZ increases the temperature of the t→m transformation.However, these changes only occur after thermal treatments at 1500 ºC, a condition at which Nb 5+ /Ta 5+ ions are in substitutional positions with respect to the cation network 22,40,41 .We observed through TEM images (not shown) the existence of isolated Nb 2 O 5 and Ta 2 O 5 particles in the calcined 9InTaSZ and 9InNbSZ samples.The presence of these particles represents an evidence that at low calcination temperatures, the Nb 5+ /Ta 5+ ions are not completely alloyed into the InSZ network.In the case of hexavalent oxides, the effect of MoO 3 and WO 3 in the retention of the t-phase has been reported by Reddy and Reddy 42 .Comparing MoO 3 /ZrO 2 and WO 3 /ZrO 2 catalysts, the authors observed that MoO 3 seems to be more effective in the retention of the tetragonal symmetry of zirconia.However, no apparent difference between MoO 3 and WO 3 regarding  Synthesis by coprecipitation of india-stabilized zirconia and codoping with MoO 3 , WO 3 , TaO 2.5 , or NbO 2.5 for application as thermal barrier coatings the stabilization of the t-phase in InSZ was observed here (see Fig. 1c and d).Therefore, as in the case of pentavalent oxides, the effects of the charge-compensating dopants appear to be secondary in relation to the InO 1.5 concentration at low calcination temperature.

Powder characteristics after dehydration
The powder characteristics after EW/AD and FD dehydration are shown in Table 2.The specific surface-area values of the samples dehydrated by EW/AD were considerably higher than those obtained after FD in agreement with a previous work 43 .Practically the same S BET values were achieved for 9InTaSZ and 9InNbSZ, whereas the Nb 2 O 5 and Ta 2 O 5 oxides can be considered as having similar effects in the gel formation during coprecipitation.On the other hand, InSZ codoped with MoO 3 and WO 3 presented higher S BET values than those observed for pentavalent oxides.The highest S BET of 106.1 m 2 g -1 was found in the 9In4.5MoSZsample after EW/AD dehydration.The fact that the addition of hexavalent oxides increases S BET was also observed in zirconia-supported catalysts by Scheithauer et al. 44 .The authors suggested that the formation of WO x clusters anchored on the zirconia surface provides W-O-W and W=O bonds responsible for the lowering of the diffusion, thus inhibiting coalescence of zirconia crystallites and maintaining high specific surface area.Similar S BET values found for the 9In4.5WSZand 9In4.5MoSZsamples show an equivalent behavior when WO 3 or MoO 3 are alloyed into InSZ.
Fig. 4 shows TEM images of the calcined 9InTaSZ sample after different dehydration techniques have been applied.The images obtained after EW/AD (Fig. 4a) indicate a low level of agglomeration.This fact is consistent with the high S BET values presented in Table 2.In contrast, the 9InTaSZ sample dehydrated by FD exhibited a higher level of agglomeration, as shown in Fig. 4b, whereas the lower S BET values after FD is a direct effect of the formation of hard agglomerates.For both EW/AD and FD powders, the electron diffraction patterns show tetragonal zirconia 45,46 , along with the interplanar spacing of 0.29 nm for the (101) t plane, once again consistent with the values reported for tetragonal symmetry 47,48 .
For all the samples, the crystallite size was smaller after EW/AD dehydration (see Table 2).In the 9InTaSZ powders dehydrated by EW/AD, the small crystallite size of 8.4 nm calculated by the Scherrer formula is in agreement with the TEM observation (Fig. 4a).Furthermore, the high degree of agglomeration after FD dehydration is probably the main cause for complete stabilization of the t-phase at concentrations of InO 1.5 that are lower than those required after EW/AD.Apparently, it is a result of the constriction exerted on the dense agglomerates of zirconia crystallites in case of FD.More details of this stabilization mechanism can be found in the work of Shukla et al. 49 .

Modifications in the gels after EW/AD and FD
The DSC/TG curve of the 6InNbSZ sample dehydrated by FD (Fig. 5a) shows two peaks.First, an endothermic peak at ~130 °C originating from the elimination of physically adsorbed water [50][51][52] .Water removal progresses continuously up to 495 °C and, at this temperature, an exothermic peak due the crystallization of zirconia can be observed.For this FD dehydrated sample, the mass loss of 17.2 wt% between 25 and 495 °C occurs because of desorption of water and removal of hydroxyl groups (OH-) 30 .In contrast to FD, the dehydration of the 6InNbSZ sample by EW/AD leads to significant changes in the DSC/TG curve (Fig. 5b).The broad endothermic peak at ~130 °C after FD dehydration is replaced by a narrow peak at 100 °C, which is characteristic of alcohol dehydration 53 .Ethanol washing prior to azeotropic distillation led to chemical changes in the gel, as reflected by the endothermic peak at 290 °C.This endothermic reaction is due to the replacement of original hydroxyl by ethoxy groups (CH 3 CH 2 O-) during prior washing of the gels with ethanol 27 .Further effects of the azeotropic distillation with butanol can be observed in the DSC/TG curve in form of one endothermic peaks at 250 °C and another less visible one around 300 °C, all related to the removal of butoxy groups (C 4 H 9 O-) 54 .Thus, the interaction between codoped hydroxide and organic solvents incorporated by EW/AD seems to be responsible for the high specific surface area, low crystallite size, and low agglomeration level, as presented in Table 2.
These chemical interactions between gel and solvent can also be observed in the FT-IR spectra.As shown in Fig. 6a, the FT-IR spectrum of the 6InNbSZ sample dehydrated by FD presents a band at 3400 cm -1 due to the stretching vibration of structural -OH groups and adsorbed water molecules 55 .The band at 1630 cm -1 accounts for the vibration of Zr-OH in the codoped InSZ hydroxide 51,56 .Furthermore, we can observe a band at 1360 cm -1 which indicates the presence of carbonate species resulting from the incorporation of atmospheric CO 2 into the gels 48 .With respect to the gel after EW/AD dehydration (Fig. 6b), a band at 2971 cm -1 characteristic of C-H groups after ethanol washing, and a small band at 1159 cm -1 probably originating from adsorbed ethanol 55 are present.Azeotropic distillation with butanol provided an additional small band at 2861 cm -1 , corresponding also to C-H groups, although the characteristic band at 1446 cm -1 reflecting C-C units from adsorbed butanol cannot be identified because of the overlap with the zirconium-hydroxide region 57 .These bands provide further evidence regarding the adsorption of ethoxy and butoxy organic complexes after EW/AD dehydration, in accordance with the DSC/TG curves (Fig. 5).Indeed, the better powder characteristics provided by this technique is a result of the chemical modification of the InSZ codoped gels.Synthesis by coprecipitation of india-stabilized zirconia and codoping with MoO 3 , WO 3 , TaO 2.5 , or NbO 2.5 for application as thermal barrier coatings

Conclusions
A route for producing tetragonal india-stabilized zirconia by coprecipitation and codoping with hexavalent (WO 3 , MoO 3 ) or pentavalent (NbO 2.5 , TaO 2.5 ) oxides was presented.The effect of charge-compensating oxides in stabilizing the tetragonal zirconia phase seems to be secondary in relation to the amount of InO 1.5 , which probably occurs due to the low calcination temperature used, thus not allowing direct observation of the effect of codopants.However, WO 3 and MoO 3 oxides are more effective in promoting modifications in the gels, leading to specific surface areas of up to 106.1 m 2 .g - .Ethanol washing followed by azeotropic distillation is the preferable dehydration technique, rather than freeze-drying, to produce highly non-agglomerated powders of these systems.The effectiveness of this technique is derived by the substitution of surface -OH group in the InSZ codoped hydroxide for CH 3 CH 2 O-after ethanol washing and further C 4 H 9 O-after azeotropic distillation.It is important to note that further exploration regarding the high-temperature stability, long time phase stability, ability to be deposited as a homogeneous coating, and others properties, are required to evaluate the adequacy of 9In4.5MoSZ,9In4.5WSZ,9InTaSZ, and 9InNbSZ as TBC materials.

Fig. 1
Fig. 1 XRD patterns after EW/AD and FD dehydration for the systems: (a) InTaSZ, (b) InNbSZ, (c) InMoSZ, and (d) InWSZ.The subscripts t and m represent the tetragonal and monoclinic phases, respectively.Each composition is represented by numbers and labeled by either UC -uncalcined or C -calcined at 600 ºC for 1 h.

Fig. 4
Fig. 4 TEM images of 9InTaSZ dehydrated by (a) EW/AD and (b) FD.Electron diffraction patterns and high-resolution images are shown for the two dehydration techniques.

Fig. 6 FT
Fig. 6 FT-IR spectra of the 6InNbSZ sample dehydrated by (a) FD and (b) EW/AD.

Fig. 5
Fig. 5 DSC/TG curves of the 6InNbSZ sample after the application of different dehydration techniques: (a) FD and (b) EW/AD.

Table 1 .
Compositions of codoped InSZ samples and corresponding codes used in this work.The numbers in front of the chemical elements indicate their molar percentage.Due to the same molar ratio, only a single number is shown in the codes of the pentavalently doped compositions.by coprecipitation of india-stabilized zirconia and codoping with MoO 3 , WO 3 , TaO 2.5 , or NbO 2.5 for application as thermal barrier coatings Synthesis

Table 2 .
Characteristics of InSZ codoped samples dehydrated by EW/DA and FD after calcination.