MASTER'S THESIS. Synthesis of rare-earth oxide mesoporous structures by combustion synthesis

Transcription

1 MASTER'S THESIS 2009:114 Synthesis of rare-earth oxide mesoporous structures by combustion synthesis Etienne Letournel Luleå University of Technology Master Thesis, Continuation Courses Chemical and Biochemical Engineering Department of Chemical Engineering and Geosciences Division of Biochemical and Chemical Engineering 2009:114 - ISSN: ISRN: LTU-PB-EX--09/114--SE

2 ABSTRACT This work aims at characterizing the properties of rare earth oxides (ReO) produced by combustion synthesis. Three different studies are investigated. The first one consists in the study of ReO by the mean of thermogravimetry analysis. A mass spectrometer was used too, to get more information. Two different ReO are studied: yttrium oxide (Y 2 O 3 ) and gadolinium oxide (Gd 2 O 3 ). The ReO have been synthesized changing the most important initial parameter regarding the changes in the properties obtained. This parameter is the ratio glycine to rare earth nitrates (G/N), the two reactants used for the synthesis. The conclusions for this part are the following: for an excess of nitrates, the impurities in the oxides are mostly due to carbon residues, nitrates and carbonates. The amount of carbonates seems to scale with the specific surface area of the powder, while nitrates decrease when less in excess. The synthesis is carried out at 500 C in this study but the conclusion is that carrying out the synthesis at 600 C or increasing the temperature to 600 C after synthesis might allow for complete removal of nitrate without altering the structure too much. For the stoichiometric ratio, nitrates and carbonates can hardly be resolved by DTG. However, 46% of the total weight loss occurs above 460 C for Y 2 O 3. For an excess of glycine, unreacted glycine is found. A major difference in the chemical properties of the two oxides has been found as well. It is the heat of reaction released during the synthesis which does not show the same trend if we look at its evolution increasing the G/N ratio. The second part of this work consists in the study of the changes of some parameters in the synthesis on the ReO properties. Two parameters are tested: the mixing changing its speed and glycine from different brands (one aged of 20 years). It does not show major differences looking at adsorption and desorption of nitrogen measurements. Nevertheless, other ways have to be investigated like the concentration of rare earth nitrates solution for example. The last part is the study of the aging of ReO. Y 2 O 3 for a G/N ratio of 0.56 is studied. The conclusions are the following: X-ray analyses, adsorption and desorption of nitrogen measurements and thermogravimetry showed that the fresh rare-earth oxides have a metastable structure. Moreover, this fresh powder structure seems to change to a stable one with time. And we could have noticed that the moistening seems to increase a lot the speed of this structure evolution. 1

3 Table of contents Introduction...3 I. Scope of the past work...4 II. Thermogravimetric analysis Principle of the measurements Curves and results...6 III. Effects on product properties changing parameters in the synthesis Description of the synthesis Ways of investigation Results...20 IV. Study of the aging of the powders Introduction Plan of the study Results...26 a. Nitrogen desorption results...26 b. X-ray results...28 c. Thermogravimetric analysis Conclusions for the aging study...32 Conclusion

4 Introduction This study, entitled Synthesis of rare-earth oxide mesoporous structures by combustion synthesis, is the second part of a project which aims at characterizing the microstructure of mesoporous structures of rare-earth oxides produced by combustion synthesis. We will study here the oxides of two different elements: yttrium oxide (Y 2 O 3 ) and gadolinium oxide (Gd 2 O 3 ). These two elements have been considered together because of their close behaviors. It is interesting to do this work because when we produce rare-earth oxides by combustion synthesis, we can get at the same time the properties due to rare earth elements which are various, depending on the element we are working with, and the properties of nanocrystalline materials. We can mention for example among other properties of rare earth elements the high thermal stability or luminescence when doped with other rare earths. Then, we will see that thanks to the combustion synthesis, we can produce particles with a size below 10 nm. Nanocrystalline materials are also very interesting regarding to their specific properties such as, for example, the specific surface area or their catalytic properties. That is why it is interesting to synthesize rare earth oxides by combustion synthesis, to combine those properties. Then, we want to have a complete characterization of this product because the final goal of this study is the tailoring of the production. That is why we want to know all about these particles, to be able in the future to produce rare-earth oxides in a big quantity, with the properties we want (particle size, specific surface area ). The content of this work will be the following. First we will have a scope of the past work, in order to understand well what we are working on, and then, the actual work will begin. 3

5 I. Scope of the past work The past work consisted in the study of the rare earth oxides from the very beginning. What has been done first is a bibliographical research on the rare earth elements and rare earth oxides. Important information could have been got thanks to this study such as: - the location of rare earth elements in the periodic table - the valence of ions formed from those elements - the kind of oxides we can form with rare earth elements - the crystal structure of the oxides, depending on the ionic radius and the synthesis temperature Then, the second part of the bibliographical research was to investigate the several ways of production of these rare earth oxide particles, being given that the next step is to synthesize those particles under different conditions and to analyze them. Among a lot of different methods to synthesize the particles, the one which has been chosen for this study is the combustion synthesis and this for two main reasons. First, this method allows us to get a particle size below 10 nm which is an important parameter for this study. Then, the second reason why we chose this method is that it is very time efficient. Some methods can take a lot of time to process whereas this one is very quick. In the publications, the most used method to get the product is the synthesis by precipitation. Its drawback compared to the combustion synthesis is that when the precipitation is finished it takes a lot of time to get a pure product. There are long steps of separation (filtration for example) and then steps like washing and drying to get rid of the liquid phase. With the combustion method, you can get a product almost pure just after the synthesis. That is why it is interesting to work with this method. We will not describe this method in details in this part because it is not the main purpose here and moreover, it is going to be done in a next part. We will just give here the chemical reaction of this combustion synthesis: Where we have : - Re(NO 3 ) 3 : rare earth nitrates, the oxidizer (Re is representing the rare earth element, in our case yttrium or gadolinium) - NH 2 CH 2 COOH : glycine, the fuel - Re 2 O 3 : rare earth oxide Among several kinds of fuels, the one chosen has been the glycine owing to its popularity in many publications. 4

6 Then several samples of rare earth oxides were synthesized changing the most important initial parameter which is the fuel to oxidizer ratio, because of its big impact on the morphology and the properties of the powders produced. That is why series of 8 samples had been synthesized for each rare earth element (yttrium and gadolinium), changing the fuel to oxidizer ratio from 0.25 to 0.80 (with the stoichiometric ratio 0.56 in between). The following work was to analyze these series of samples using several methods, in order to characterize the different properties of these powders for each ratio. Three different analysis methods had been used for the moment: - X-ray analysis: to establish the crystal structure and to determine the crystallite size of the fine powders - SEM characterization: to study the powder morphology - Adsorption and desorption of nitrogen measurements: to find the surface area and the pore size distribution of the different powders But these three methods of analysis are not enough to get a perfect characterization of the powders. Thermogravimetric analysis and TEM observations are required to learn more about rare earth oxides. In this actual work, a complete study of the powders by thermogravimetric analysis will be done. II. Thermogravimetric analysis 1. Principle of the measurements The thermogravimetric analysis consists in the study of the weight loss of samples while the temperature is increasing. In our case, will we study the weight losses of our samples for a temperature increasing from 50 to 1300 C. If we just look to the weight loss curves, we can get only low information. It is important to look at the derivatives of these curves as well, to be able to distinguish events, i.e. weight loss steps. On top of that, we had at the outlet of the thermogravimetric analyzer a mass spectrometer. This is a particularly interesting combination which will allow us to see which elements or fragments of molecules are responsible for a variation of weight of one sample. The principle is the following: the mass spectrometer will first ionize the different molecules we can get at the outlet of the thermogravimetric analyzer. Then it will separate the different ions thanks to their mass-to-charge ratio. Finally, the analyzing part consists in counting the ions with a given mass-to-charge ratio. Thanks to this analysis, we can get the quantity of some elements or fragments of molecules released by our sample, while the temperature is increasing. 5

7 2. Curves and results As we can see on figure 1, the minimum weight loss for Y 2 O 3 is observed for the stoichiometric G/N ratio (0.56), which is consistent with the maximum release of heat associated with it. However, the minimum weight loss of Gd 2 O 3 is observed for a fuel-rich ratio of Figure 1: weight loss vs F/O ratio for gadolinium and yttrium oxide 6

8 - Ratio 0.25: Y 2 O 3 (c/x) Gd 2 O 3 (x) Figure 2: weight loss and derivative of weight loss vs temperature for gadolinium and yttrium oxide, ratio 0.25 Figure 2 shows that both compounds show one main weight loss above 332 C and which accounts for 82 and 80% of the total weight loss for Y 2 O 3 and Gd 2 O 3, respectively. This weight loss for Gd 2 O 3 (peak maximum at 575 C) is almost exclusively associated with the masses 44, 12 and 16 (figure 3 below). The two former indicates the emission of CO 2, while the latter might correspond to either CH 4 or a single oxygen atom, although no corresponding variation of O 2 was detected. We strongly believe that this combination of mass 44 and 16 correspond to the decomposition of carbonate groups. Slight release of water simultaneously suggests also the decomposition of hydroxyl groups. Peaks of 30, 32 and 14 at 519 C indicate the decomposition of nitrates due to their excess during synthesis. The aspects of the masses 16, 28, 30, 32, 44 are shown on figure 3: 7

9 Figure 3: Mass spectrums, ratio 0.25, gadolinium oxide The same observations were made for Y 2 O 3, except that the main weight loss was found to correspond to a double peak when looking at its derivative as we can see on figure 3. Both peaks (512 and 582 C) overlap and are due mostly to evolution of CO 2 accompanied with mass 16 as shown on figure 4 just below. In a less extent, decomposition of nitrates also contributes to this double weight loss. The latter is thought as to stem from the presence of two phases in the case of Y 2 O 3 as shown by x-ray diffraction: an unknown phase similar to that of Gd 2 O 3 for the same ratio and low amount of the cubic phase. In both cases, the two series of mass spectrums (figures 3 and 4) show that O 2 is consumed while CO 2, H 2 O and multiple organic fragments are detected between 330 and 450 C. This suggests the decomposition of organic residues at these temperatures. Below 320 C, adsorbed H 2 O and CO 2 are released and account for 16 and 19% of the total weight loss of Y 2 O 3 and Gd 2 O 3, respectively. Adsorbed H 2 O evolves mainly below 200 C and CO 2 between 200 C and 320 C. 8

10 Figure 4: Mass spectrums, ratio 0.25, yttrium oxide - Ratio 0.29: Y 2 O 3 (c) Gd 2 O 3 (c/x) Figure 5: weight loss and derivative of weight loss vs temperature for gadolinium and yttrium oxide, ratio

11 As we can see on figure 5, both compounds show one main weight loss above 400 C and which accounts for 58 and 66% of the total weight loss for Y 2 O 3 and Gd 2 O 3, respectively. This weight loss for Gd 2 O 3 (peak maximum at 559 C, figure 5) is almost exclusively associated with the masses 44, 12 and 16 indicating decomposition of carbonate groups. Slight release of water simultaneously suggests also the decomposition of hydroxyl groups. Peaks of 30, 32 and 14 at 526 C indicate the decomposition of nitrates due to their excess during synthesis. The behaviour of Y 2 O 3 is somewhat different. The main weight loss was again found to correspond to a double peak when looking at its derivative on figure 5. Both peaks take the same positions and overlap (512 and 587 C), but this time they can be attributed to two different events. The peak at 512 C is due to the decomposition of nitrates, that lying at 587 C corresponds to the evolution of CO 2 accompanied with mass 16. The decomposition of nitrates is stronger in quantity than that of carbonates. The fact that there is no double peak corresponding to the same event is consistent with the presence of only one phase. In both cases, O 2 is consumed while CO 2, H 2 O and multiple organic fragments are detected between 330 and 450 C. This suggests the decomposition of organic residues at these temperatures. Below 320 C, adsorbed H 2 O and CO 2 are released and account for 42 and 34% of the total weight loss of Y 2 O 3 and Gd 2 O 3, respectively. Adsorbed H 2 O evolves mainly below 200 C and CO 2 between 200 C and 320 C. - Ratio 0.32: Y 2 O 3 (c) Gd 2 O 3 (c) Figure 6: weight loss and derivative of weight loss vs temperature for gadolinium and yttrium oxide, ratio

12 Figure 6 enlighten on the facts that both compounds do not show one main weight loss, but several of relatively similar importance. Nevertheless both cubic phases here show a similar behaviour to cubic Y 2 O 3 for a ratio of The weight losses above 430 C for Y 2 O 3 and Gd 2 O 3 represent only 52 and 42% of the total weight loss, respectively. They were found to correspond to double peak when looking at their derivative. Both peaks overlap (510 and 607 for Y 2 O 3, 519 and 540 C for Gd 2 O 3 ) and can be attributed to two different events. The peaks at 510 and 519 C are due to the decomposition of nitrates, while those centred on 607 and 540 C correspond to the evolution of CO 2 accompanied with mass 16. The decompositions of nitrates are stronger in quantity than those of carbonates. In both cases, O 2 is consumed while CO 2, H 2 O and multiple organic fragments are detected between 330 and 430 C (413 C for Y 2 O 3 and 401 C for Gd 2 O 3 ). This suggests the decomposition of organic residues at these temperatures. In the case of Y 2 O 3, this event can be well distinguished on the first derivative of the thermogravimetry curve shown on figure 6and represents 13% of the total weight loss. Below 330 C, adsorbed H 2 O and CO 2 are released. Adsorbed H 2 O evolves mainly below 200 C and CO 2 between 200 C and 320 C. - Ratio 0.38: Y 2 O 3 (c) Gd 2 O 3 (c) Figure 7: weight loss and derivative of weight loss vs temperature for gadolinium and yttrium oxide, ratio

13 Again both cubic phases show a similar behaviour to cubic Y 2 O 3 for a ratio of The weight losses above 460 and 430 C for Y 2 O 3 and Gd 2 O 3 represent only 48 and 43% of the total weight loss, respectively. They were found to correspond to double peak when looking at their derivative as we can see on figure 7. Both peaks overlap (510 and 642 for Y 2 O 3, 510 and 573 C for Gd 2 O 3 ) and can be attributed to two different events. The peaks at 510 in both cases are due to the decomposition of nitrates, while those centred on 642 and 573 C correspond to the evolution of CO 2 accompanied with mass 16. The decompositions of nitrates and carbonates are of equal intensity for Gd 2 O 3, while the weight loss of nitrates can hardly be resolved stronger for Y 2 O 3. In both cases, O 2 is consumed while CO 2, H 2 O and multiple organic fragments are detected between 330 and 430 C (422 C for Y 2 O 3 and 404 C for Gd 2 O 3 ). This suggests the decomposition of organic residues at these temperatures. In the case of Y 2 O 3, this event can be well distinguished on the first derivative of the thermogravimetry curve (figure 7) and represents 13% of the total weight loss. Below 330 C, adsorbed H 2 O and CO 2 are released. Adsorbed H 2 O evolves mainly below 200 C and CO 2 between 200 C and 320 C (237 C for Y 2 O 3 and 285 C for Gd 2 O 3 ). - Ratio 0.44: Y 2 O 3 (c) Gd 2 O 3 (50%c/50%m) Figure 8: weight loss and derivative of weight loss vs temperature for gadolinium and yttrium oxide, ratio 0.44 The figure 8 gives us the information that the weight losses below 422 and 457 C for Y 2 O 3 and Gd 2 O 3 represent 56 and 28% of the total weight loss, respectively. In both cases, O 2 is consumed while CO 2, H 2 O and multiple organic fragments are detected between 320 and 450 C (417 C for Y 2 O 3 and 387 C 12

14 for Gd 2 O 3 = large discrepancy compared with before due to the change of phase). This suggests the decomposition of organic residues at these temperatures. In the case of Y 2 O 3, this event can be well distinguished on the first derivative of the thermogravimetry curve and represents 17% of the total weight loss. Below 320 C, adsorbed H 2 O and CO 2 are released. Adsorbed H 2 O evolves mainly below 200 C and CO 2 between 200 C and 320 C (237 C for Y 2 O 3 and 269 C for Gd 2 O 3 ). Above 430 C, weight loss can still be related to the decompositions of nitrates and carbonates. Decomposition of nitrates is centred on 500 C for Y 2 O 3 and gives rise to two peaks centered on 343 and 500 C for Gd 2 O 3 we can see on figure 8, which is consistent with the copresence of the cubic and monoclinic phases. Similarly, decomposition of carbonates is centred on 605 C for Y 2 O 3 and gives rise to two peaks centered on 468 and 563 C for Gd 2 O 3. The decompositions of nitrates and carbonates are of equal intensity for Gd 2 O 3, while the weight loss of nitrates can hardly be resolved stronger for Y 2 O 3. - Ratio 0.80: Y 2 O 3 (c) Gd 2 O 3 (66%c/33%m) Figure 9: weight loss and derivative of weight loss vs temperature for gadolinium and yttrium oxide, ratio 0.80 The decomposition of nitrates and carbonates above 460 C cannot be resolved. In both cases, O 2 is consumed while CO 2, H 2 O and multiple organic fragments are detected below 460 C (418 C for Y 2 O 3 and 392 C for Gd 2 O 3 ). This suggests the decomposition of organic residues at these temperatures. For Y 2 O 3, this event can be well distinguished on the first derivative of the thermogravimetry curve as we can see on figure 9 and represents 10% of the total weight loss. Below 330 C, adsorbed H 2 O and 13

15 CO 2 are released. Adsorbed H 2 O evolves mainly below 200 C and CO 2 between 200 C and 330 C (244 C for Y 2 O 3 and 297 C for Gd 2 O 3 ). Interestingly, Y 2 O 3 shows a main weight loss above 460 C (figure 10 just below), which matches a consumption of O 2 and release of masses 44, 30, 16, and 14, which might correspond to the decomposition of unreacted glycine (NH 2 -CH 2 -COOH) with the groups COO-, NH 2 -CH 2, NH 2, and CH 2, respectively. It accounts for 58% of the total weight loss. Possible decomposition of unreacted nitrates around 440 C. Figure 10: Mass spectrums, ratio 0.80, yttrium oxide 14

16 - Ratio 0.68: Y 2 O 3 (c) Gd 2 O 3 (20%c/80%m) Figure 11: weight loss and derivative of weight loss vs temperature for gadolinium and yttrium oxide, ratio 0.68 The decomposition of nitrates and carbonates above 500 C cannot be resolved, except for Gd 2 O 3 with a small centred on 311 C, which indicates that the latter is related to the monoclinic phase and that this powder might not have been synthesized with a slightly fuel-lean ratio. In both cases, O2 is consumed while CO 2, H 2 O and multiple organic fragments are detected below 500 C (400 C for Y 2 O 3 and 385 /435 C for Gd 2 O 3 ). This suggests the decomposition of organic residues at these temperatures. For Y 2 O 3, this event can be well distinguished on the first derivative of the thermogravimetry curve shown on figure 11 and represents 16% of the total weight loss. In the case of Gd 2 O 3, a double peak seems to be present for CO 2 (385 /435 C), which is consistent with the presence of both the monoclinic and cubic phases. Below 350 C, adsorbed H 2 O and CO 2 are released. Adsorbed H 2 O evolves mainly below 200 C and CO 2 between 200 C and 350 C (267 C for Y 2 O 3 and 280 C for Gd 2 O 3 ). Interestingly, Y 2 O 3 shows a weight loss above 600 C, which matches a consumption of O 2 and release of masses 44, 30, 16, and 14, which might correspond to the decomposition of unreacted glycine (NH 2 -CH 2 -COOH) with the groups COO-, NH 2 -CH 2, NH 2, and CH 2, respectively. Possible decomposition of unreacted nitrates around 440 C. 15

17 - Ratio 0.56: Y 2 O 3 (c) Gd 2 O 3 (20%c/80%m) Figure 12: weight loss and derivative of weight loss vs temperature for gadolinium and yttrium oxide, ratio 0.56 The decomposition of nitrates and carbonates above 460 C cannot be resolved except for Gd 2 O 3 with a small centred on 311 C as we can see on figure 13 just below, which indicates that the latter is related to the monoclinic phase and that this powder might not have been synthesized with a slightly fuel-lean ratio. The weight losses above 460 C for Y 2 O 3 and Gd 2 O 3 represent 46 and 13% of the total weight loss, respectively, as we can see on figure

18 Figure 13: Mass spectrums, ratio 0.56, gadolinium oxide Figures 13 and 14 shows that In both cases, O 2 is consumed while CO 2, H 2 O and multiple organic fragments are detected between 330 and 500 C (394 C for Y 2 O 3 and 422 C for Gd 2 O 3 ). This suggests the decomposition of organic residues at these temperatures. In both cases, this event can be well distinguished on the first derivative of the thermogravimetry curve (figure 12) and represents 17% and 13% of the total weight loss for Y 2 O 3 and Gd 2 O 3, respectively. In the case of Gd 2 O 3, a double peak seems to be present for CO 2 (390 and 413 C), which is consistent with the presence of both the monoclinic and cubic phases. Below 320 C, adsorbed H 2 O and CO 2 are released. Very little H 2 O desorbs and CO 2 evolves between 200 C and 320 C (242 C for Y 2 O 3 and 290 C for Gd 2 O 3 ). 17

19 Figure 14: Mass spectrums, ratio 0.56, yttrium oxide Conclusions: G/N<0.56: The impurity is mostly due to carbon residues, nitrates and carbonates. The amount of carbonates seems to scale with the specific surface area of the powder, while nitrates decrease when less in excess. The amount of carbon residues seems fairly constant for all runs. Carrying out the synthesis at 600 C or increasing the temperature to 600 C after synthesis might allow for complete removal of nitrate without altering the structure too much. G/N=0.56: Nitrates and carbonates can hardly be resolved by DTG. However, 46% of the total weight loss occurs above 460 C for Y0.56. G/N>0.56: unreacted glycine. 18

20 III. Effects on product properties changing parameters in the synthesis 1. Description of the synthesis First, we said that we wanted to synthesize the powder for different fuel-to-oxidizer ratios because it was the most important initial parameter. The determination of the quantities required of fuel and oxidizer to obtain powders for the different ratios had been made like this (example for the synthesis of the yttrium oxide): In order to obtain 0.3 g of product, the solution was prepared mixing of Yttrium nitrate and the corresponding quantity of glycine depending on the different ratios: (0.25 ; 0.29 ; 0.32 ; 0.38 ; 0.44 ; 0.56 ; 0.68 ; 0.80). All these numbers are summarized in this following table: F/O m glycine (g) The procedure to obtain the rare earth oxides by the combustion method is the following: - First weigh the glycine which is a powder with a fold paper - Then weigh the yttrium nitrate into the platinum crucible where the reaction will take place - Add the glycine into the crucible - Very quickly, add the magnetic agitator and seal the crucible to avoid any evaporation - Then, stir the solution during 45 min in order to get a solution absolutely transparent and without any impurities - When it is ready, put the crucible (removing sealing film and agitator) in a furnace at 500 C - After a few minutes, red fumes are going out from the furnace: the reaction is done. Wait 10 min before removing the powder from the furnace. Then, the powder synthesized can either be analyzed or stored under given conditions. 19

21 2. Ways of investigation In this actual study, we changed two major parameters: - Mixing time - Glycine Of course for each study, all the other parameters were unchanged (ratio, temperature ). All the samples we are going to study had been synthesized for a fuel-to-oxidizer ratio of 0, Results Mixing time We saw in the presentation of the synthesis that the fuel and the oxidizer were mixed during 45 minutes, to obtain a solution absolutely transparent and without any impurities. We decided to change this mixing time to 10 minutes. Then several speeds of mixing had been tried because of the presence of air bubbles in the solution for a fast mixing. We wanted to know if the air in the solution had an influence or not on the product. Before to show the patterns of adsorption and desorption of nitrogen for the different synthesizes, it is important to notice that with a mixing time of 10 minutes, the solution is already clear and without any impurities. The graphs which are going to be shown will give the pore volume as a function of the logarithm of the pore diameter measured for the BJH desorption. During the measurements, we could get the specific surface area for each sample as well. 20

22 Figures 15 to 17 are showing patterns of adsorption and desorption of nitrogen for different powders, changing the mixing during the synthesis. S.A. = 93 m²/g S.A. = 85 m²/g Figure 15: mixing 10 min (fast) Figure 16: mixing 45 min S.A. = 84 m²/g Figure 17: mixing 10 min (slow) As we can see on figures 15 to 17, the mixing time does not affect at all the pore size distribution. We can notice that we got almost exactly the same aspects for the three curves. Moreover, we can see that the specific area, shown on the top right corner of each figure, does not change in a significant way neither. We can conclude here that the mixing time does not affect the properties of the rare earth oxides. 21

23 Glycine We decided to synthesize the powder with different glycines to see the effects produced on the properties on of the powders. Different glycines had been used with one aged of more than 20 years. With that study, we will clearly see if the aging of the glycine has an influence or not. Figures 18 to 20 are showing patterns of adsorption and desorption of nitrogen for different powders, using a different glycine in each synthesis. S.A. = 88 m²/g S.A. = 85 m²/g Figure 18: Glycine Sigma Figure 19: Glycine Riedel-de Haën S.A. = 89 m²/g Figure 20: Glycine Merck (20 year-old) 22

24 We can see, thanks to the graphs above, that the aging of the glycine has no effect on the properties of our powders. In fact, we can notice that both the pore size distribution and the specific area are very close for the different glycines that have been tested. Even if the glycine is aged of more than 20 years, the properties of the products to remain the same. The conclusion of this part is that the parameters we studied do not have any effects on the properties of the oxides synthesized. However, to know all about the combustion synthesis for rareearth oxides, some more parameters have to be investigated like the time passed in the furnace, the temperature or the rare-earth solutions changing the ph or the nitrates concentration. However, we noticed that another parameter seems to affect a lot the properties of rare-earth oxides: the aging. IV. Study of the aging of the powders 1. Introduction To understand why it is interesting to study the aging of rare-earth oxides, we will study the BJH desorption patterns for two different powders: - One which has been analyzed just after synthesis - Another one which is a 3-year old powder These two powders are yttrium oxides, synthesized for a fuel-to-oxidizer ratio of 0,32. We can see their BJH desorption patterns on the figures 21 and 22 just below. S.A. = 85 m²/g S.A. = 50 m²/g Figure 21: fresh powder Figure 22: 3-year old powder 23

25 As we can notice comparing the figures 21 and 22, the properties of the powders are changing a lot with the time passing. Both the pore size distribution and the specific surface area are completely different from one pattern to another. Figure 23 shows the X-ray patterns of these two powders. Figure 23: X-ray patterns for the fresh and the old powders With the X-ray patterns shown on figure 23, we can see that we have the same observations than for the nitrogen desorption patterns. For the old powder, we notice that the structure is a lot more amorphous compared to the fresh powder which has a more crystalline structure. These two methods of analysis showed us that the properties of rare-earth oxides synthesized by combustion synthesis are changing a lot while the time is passing. That is why we decided to study the aging, to understand how it was changing. 24

26 2. Plan of the study The plan was to synthesize first a big quantity of powder (Y 2 O 3, x 10, ratio: 0,32), to store it under two different conditions, and then to analyze it week after week with different analysis methods (TA, X-ray, nitrogen desorption). Here are the two different storage conditions: - In a desiccator with silica gel as a desiccant. - In a desiccator with at the bottom a saturated KCl solution giving a controlled humidity rate of 92 % 25

27 3. Results a. Nitrogen desorption results This study has been done over three weeks. Results for the nitrogen desorption analysis Figure 24 is showing the specific surface area plotted as a function of the time 3-year old powder Figure 24: S.A. vs time (Aging study) We can observe on figure 24 that the specific surface is decreasing with time and that this phenomenon is much faster when the humidity rate is high. In both cases, it seems that we are reaching an equilibrium state (the 50 m²/g of the 3-year powder). We will try to confirm that later with the next studies. 26

28 Evolution of the pore size distribution week after week Fresh powder With KCl 1 week after With KCl 3-year old powder 2 weeks after Figure 25: Evolution of the pore size distribution week after week (absorption and desorption of nitrogen patterns Just above we have the evolution of the pore size distribution week after week. The powder in the desiccator with the saturated KCl solution is shown on the figure 25 because it is the one with the quickest evolution. Once again, it is clear that the powder seems to reach the properties of the 3- year old powder. 27

29 b. X-ray results Figure 26: X-ray patterns (Aging study) Another time, thanks to the X-ray patterns shown on figure 26, we can see that week after week, we are reaching the characteristics of the 3-year old powder. Week after week, the powder is losing its crystalline properties to become more amorphous. 28

30 Another interesting thing for the conclusions of this study is to look at the evolution of the crystallites size. Figure 26 shows the evolution of it during the two weeks of investigations: Figure 27: crystallites size vs time We saw before that the aging was affecting a lot the properties of the powders we are studying. Here, it is interesting to notice that it is not the same thing with the crystallites size calculated with the Debye-Scherrer method. The crystallites size is not changing a lot with the weeks passing, as seen on figure 27. It will give us interesting conclusions for the end of this study. 29

31 c. Thermogravimetric analysis Figure 28: weight loss vs Temperature (Aging) Figure 29: DTG vs temperature (Aging study) 30

32 - Y 2 O , as prepared: In contrast to Y0.32, the week 0 sample shows a larger total weight loss (11.06% against 9.13%), although the former had been stored for 6 months before measurement as we can see on figure 28. The main discrepancies lie in the last weight loss related to the decomposition of nitrates and carbonates. It accounts only for 4.51% in the Y0.32, while it is up to 6.08% in week0. Another difference is that the weight loss related to the decomposition of organic residues around 400 C cannot be resolved on the first derivative of the thermogravimetry curve of week0, whereas it was for Y0.32 (Was there any corresponding consumption of O 2 for week0?). - Y 2 O , after two weeks in a dry atmosphere: In comparison to week 0, this sample shows more carbonates decomposing at 600 C and an associated stronger desorption of CO 2 at 250 C. The latter is associated with higher evolution of water at the same temperature. It could be due to adsorbed water or decomposition of hydroxyl groups. Interestingly, the decomposition of organic residues seems to be shifted to 500 C as indicated by a small peak of CO 2 and the absence of a negative peak of O 2 might be offset by the evolution of the latter during decomposition of the nitrates at this temperature. It was well present at 400 C after one week. The absence of organic groups at 500 C suggests the absence of organic residues and that their presence in the powder might not be homogeneous. - Y 2 O , after two weeks at 96% humidity: In comparison to week0, this sample also shows more carbonates decomposing at 600 C, but also a strong evolution of CO 2 between 200 and 500 C as we can see on figure 30 just below. This suggests the formation of a second type of carbonates or a strong adsorption of CO 2 in addition to the CO 2 emitted due to the decomposition of organic residues. However, the larger total weight loss of this sample stems from the evolution of water at 100 and 230 C (8.50% against 4.65% for week0 between 50 and 385 C). The latter might correspond to hydroxyl groups. Some hydroxyl groups are also thought to decompose during the decomposition of the carbonates. The remaining nitrates in the powder remain unchanged during aging. 31

33 Figure 30: Mass spectrums 2nd week, KCl 4. Conclusions for the aging study With all these analyses before, we could have find out the fresh rare-earth oxides have a metastable structure. Moreover, this fresh powder structure seems to change to a stable one with time. And we could have noticed that the moistening seems to increase a lot the speed of this structure evolution. Here is a final analysis. We will see just after on the figures 31 and 32 that the nitrogen desorption graphs and the X-ray patterns for the 3-year old sample before and after calcinations. The calcinations have been processed during one hour at 500 C with an oxygen stream. Here are the results. 32

34 Figure 31: 3-year old sample before calcination Figure 32: 3-year old sample after calcination For the nitrogen desorption graphs, we cannot see any significant changes between them. We just have an opening of small pores, but the general pore size distribution remains the same. Let us check the X-ray patterns, to see if we can find any differences. Figure 33: X-ray patterns for the two samples One more time, we cannot see here any significant differences between the two analyses if we compare the X-ray patterns shown on the figure 33. This brings us to the conclusion that the metastable structure we get just after the synthesis of the product is changing to a very stable one. In fact, we could have seen the properties remained unchanged after the calcinations. 33

35 Conclusion The conclusions of this study are that we could have observed that rare-earth oxides freshly synthesized are showing a metastable structure. This metastable structure changes with the time passing to a very stable one. The moistening of the powder increases a lot the speed of those properties changes. The properties that are affected a lot during the change of structure are the specific surface area, the pore size distribution and the powder morphology. But we saw that the crystallites size was not changing that much compared to the other properties. This is an important point that has to be investigated: how can the structure change so much, keeping the same crystallites size? As we said just before, this aspect needs more investigations and to learn more about that, it could be very interesting to analyze the different samples with transmission electron microscopy. Another important conclusion is that this work has enlightened on the fact that Y2O3 and Gd2O3 have a close behavior but have different chemical properties. They most exothermic reaction does not come from the G/N ratio for the two oxides for example. This is another important point for what could allow this study in the future: tailoring of the production of rare earth oxides. 34