Structural Studies of WO₃‐Catalysed Mullite

Abstract

Mullite (3Al 2 O 3 ·2SiO 2 ) is well-known as superior engineering ceramic materials owing to its relatively high chemical stability, good refractory properties, high temperature mechanical strength and low thermal expansion coefficient. [1][2][3] This, together with excellent creep resistance and low thermal conductivity, make it useful for optical applications and high temperature structural components. [4,5] Mullite is also key material required for ceramic matrix composites for environmental barrier coatings. [6][7][8] Various techniques have been developed to fabricate a variety of one-dimensional nano-sized mullite materials such as nanowires, nanobelts, nanorods and more. [9][10][11][12][13] We also notice that the microstructures of mullite fabricated by any methods were not unique. Isotropic nano-or micro-sized particles always appeared and mixed in the one-dimensional mullite.In this study, ammonium tungstate (N 5 H 37 W 6 O 24 ·H 2 O) was employed as catalyst resource of WO 3 . Large quantities of nonstoichiometric mullite nanocolumns or isotropic particles could be synthesized by a simple sol-gel technique respectively through adjusting the concentration of catalyst. Anticipatorily, the micro-sized isotropic particles appeared among the nanocolumns. Detail structural characterizations suggested that the amount of isotropic particles and growth direction of mullite nanocolumns had great relation to the concentration of catalyst. Figure 1 shows the TG/DSC analysis of sample with WO 3 contented 3 mol % of Al element. It has been found that there is no evidence of crystallization of any phases prior to 800°C. The small endothermic occurred at 200°C is considered as the dehydration of aluminum hydrate and absorbed water. The AlWO 4 was formed before 800°C. [13] It was hard to observe the exothermic peak because of the lack of WO 3 . The sharp exothermic peak appeared around 800°C was related to the formation of c-Al 2 O 3 which was indicated by XRD later. TG analysis shows that the most mass loss of the precursor (∼ 23 %) happened before 900°C. There was a sudden weight loss (∼ 4 %) at 850°C followed the crystallization of c-Al 2 O 3 which was deduced to the volatilization of WO 3 . The molar ratio of Al and W was 100:3 and the weight percentage composition of WO 3 was counted to be 4 wt%, which matched the sudden weight loss perfectly. There was also a exothermic peak at 1000°C attributed to the crystallization of mullite. The broad exothermic peak in the temperature range of 1100°C to 1300°C was attributed to the absorption of SiO 2 and Al 2 O 3 which were formed before and the re-crystallization of mullite. Figure 2 shows the XRD patterns of the samples without catalyst sintered at different temperatures. The sample heated at 400°Cdoes not have any peaks of aluminum oxide or silicon oxides, indicating that both Al 2 O 3 and SiO 2 are amorphous at this temperature. This phase retained up to 800°C. Broad XRD peaks of c-Al 2 O 3 are obtained when the sample is calcined at 800°C. The sample heated at 1000°C...

Synthesis of nanocrystalline photoluminescent mullite using sacrificial cotton wool and filter paper templates

“…Doping with Mo and W introduced alumina and silica phases for cotton wool templated samples at both the sintering temperatures whereas in case of filter paper template only enhancement of mullitization occurred as a result of doping without any reflection of additional phases. Intermediate formation of aluminum molybdate (Al 2 (MoO 4 ) 3 ) and aluminum tungstate (AlWO 4 ) may be responsible for the segregation of alumina and silica phases and enhancement of mullitization . The influence of molybdenum on phase transformation of mullite is due to the formation and decomposition of aluminum molybdate resulting from the interaction of alumina and molybdena (MoO 3 ) phase which is governed from sodium molybdate doping.…”