| In this work, a novel method named as flame spray pyrolysis or FSP, was applied to the synthesis of yttria-stabilized zirconia (YSZ) ceramic materials for thermal barrier coatings (TBCs). After atomized precursor droplets were sprayed into the flame, tetragonal phase and micron-or nano-sized YSZ particles with a dense and spherical morphology were directly produced from droplets. Compared with conventional methods, such like co-precipitation, sol-gel and hydrothermal, the as-prepared YSZ particles synthesized by FSP have some superior advantages, such as easier size control, higher purity, narrower particle size distribution, higher production rate and easier to industrialization. So this novel method shows great potentials for industry application in producing micron-or nano-sized particles. Generally speaking, this research work can be divided into four parts:(1) investigation of the influences of precursor concentration and flame temperature on YSZ particles;(2) study of the influence of solvent composition on YSZ particles;(3) investigation of the formation mechanism of YSZ nanoparticles by using2-EHA/ethanol mixture as the solvent;(4) study of the formation pathway of YSZ nanoparticles by using SiC fiber collection method combined with SEM. Conclusions are summarized as follows:(1) Average particle size increased with increasing solution concentration. When concentration was lower than0.05M (M, solution concentration unit, mol), precursor concentration had a great influence on particle size-just a small rise of concentration led to a big particle size increase. However, when precursor concentration was higher than0.1M, concentration increase had a relatively small effect on increasing particle size. On the other hand, when fixing precursor concentration at0.1M and changing the flame temperature, there were no great differences between the four particle size distributions;(2) When using distilled water as the solvent, except submicron-sized, dense and spherical particles, there were micron-sized, thin film-like morphologies existed. When applying ethanol as the solvent, submicron-sized particles with dense and spherical morphologies were synthesized, and particles had a relatively narrower size distribution than those made by using distilled water as solvent. When using ethanol and2-Ethylhexanoic acid (2-EHA) mixture as solvent, the as-prepared YSZ particles exhibited a bimodel size distribution:both submicron and agglomerated nano-sized particles were formed. Furthermore, the effect of the volume ratio of the ethanol to2-EHA on the size distribution was investigated. It was found that more2-EHA was added, more homogenous nanoparticles were formed;(3) In order to investigate the formation mechanism of YSZ nanoparticles when using ethanol and2-EHA mixture as the solvent,2-EHA was replaced by butyric acid, octanoic acid and oleic acid respectively. The influences of solvent parameters such as boiling point, combustion enthalpy were discussed carefully. The results showed that neither the boiling point nor the combustion enthalpy was the mechanism that contributed to the synthesis of YSZ nanoparticles. What’s more,2-ethyl butyraldehyde was mixed with ethanol instead of2-EHA to study the influence of branched carboxylic acid structure or chemical reactivity with nitrates, and YSZ nanoparticles were synthesized, but not as much as those made from precursor using2-EHA and ethanol mixture as solvent. Compared with straight chain acid, such like butyric aicd, octanoic acid and oleic acid, branched chain acid like2-EHA is believed to have a higher chemical reactivity. When atomized droplets are sprayed into the flame, formation of volatile metal carboxylates when using ethanol/2-EHA or other branched chain acid (2-ethyl butyraldehyde) as solvent, is believed to be the real reason for YSZ nanoparticles formation. After these volatile metal carboxylates go through evaporation, pyrolysis, nucleation and coagulation, nanoparticles are synthesized from gas phase (gas phase pathway). On the other hand, some of the metal nitrates within the droplets decompose before they react with the acid, especially when the carboxylic acid is not enough. Thus submicron YSZ particles are formed through precipitation and pyrolysis of these nitrates within the droplet (liquid-phase pathway).And when ethanol/2-EHA ratio decrease, Zr/Y nitrates have more opportunities to react with2-EHA to form carboxylates within the droplets upon heat up in the flame. As a result, more homogenous YSZ nanoparticles are produced. On the other hand, when applying straight chain acid and ethanol mixture as solvent, there were no nanoparticles formed. It is probably because when using straight chain acid and ethanol mixture as solvent, there are no carboxylates formed, therefore only submicron particles are synthesized through liquid-phase pathway;(4) SiC fiber collection method was used to investigate the formation pathway of YSZ particles. For droplets at the bottom of the flame, since there is not enough time for nitrates to react with2-EHA to from carboxylates, most particles are synthesized from liquid-phase pathway, so few nanoparticles are formed. Closer to the flame front, higher the temperature is. For those droplets closer to the flame front, more nitrates within droplets react with2-EHA to form carboxylates and more nanoparticles are produced through gas-phase pathway. Moreover, submicron-sized particles synthesized from liquid-phase pathway are smaller than those formed far away from the flame front. At the center zone of the flame, temperature is relatively lower, so few nitrates within droplets are turned into carboxylates, thus there are few nanoparticles formed, and particles synthesized from liquid-phase pathway are relatively larger than those formed near the flame front. What’s more, because of the high droplets density at the center zone, large droplets are formed after small ones’ collision. Remaining droplets flow upward to the top of the flame, on the way upward, more and more nitrates within the droplets react with2-EHA to form carboxylates, and lots of nanoparticles are synthesized from gas-phase pathway. However, there are still some nitrates which go through liquid-phase pathway to form submicron-sized particles. |
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