3.1. Analysis of Experimental Raw Materials
Pulverized coal furnace fly ash is the discharged solid waste of coal combustion, which has a large amount of amorphous phase composed of silica and alumina. Natural diatomite contains the highest content of amorphous SiO2 and the lowest Al:Si ratio, while raw illite is mainly composed of crystalline aluminosilicate phase, which has the highest alkali metal content. In terms of loss on ignition (LOI) caused by combustible organic matter and absorbed water, pulverized coal furnace fly ash is relatively low and close to raw illite.
The chemical compositions of these starting materials are listed in Table 1
, and were detected by methods of chemical analysis for silicate rocks (Chinese GB/T 14506−2010). In terms of chemical composition, fly ash and raw illite are mainly composed of silica and alumina, while diatomite is mainly composed of silica. Variations of Al:Si ratio in the starting materials will affect the forming process of aluminum-containing products such as corundum and SiAlON during CRN synthesis. From the perspective of alkali metal oxides which have greatest influence on thermal reactivity, raw illite displays the highest content of potassium oxide, indicating the lowest melting point of the resultant mixture. This means that differences in the chemical composition of the starting materials will greatly affect the thermal reactivity, morphology, phase forming and transition process during CRN.
The crystalline structure analyzed by the XRD method for different starting materials is shown in Figure 1
. The major phases of the pulverized coal furnace fly ash are mullite (Al6
), corundum (Al2
) and amorphous silica phase. These correspond to the high content of alumina in the chemical composition (Table 1
). Diatomite is mainly composed of an amorphous phase, with no other crystalline phase except for quartz. The crystalline phases of raw illite are mainly composed of quartz and illite. The source of silicon in synthesizing SiAlONs is mainly the amorphous phase and mullite in fly ash, the amorphous phase and quartz in diatomite, and the quartz and illite in raw illite. This means that differences in the crystalline structure of starting materials will also greatly affect the phase forming of SiAlONs.
3.2. The Effect of the Si Sources on the Phase Composition of Products
During CRN calcination, these raw materials are partly reduced and nitrified. The XRD patterns of the products obtained from different starting materials are shown in Figure 2
. The phase compositions of the products are identified by their XRD patterns [6
]. At 1350 °C, the intermediate phase Si2
O exists in the products of the pulverized coal furnace fly ash, diatomite and raw illite. This means that Si2
O exists at all of the initial stages of carbothermal reduction and nitridation reaction, regardless of the various initial chemical compositions and crystal structures of silicon sources. Although the nitridation process has started at 1350 °C, the β-SiAlON phase only exists in the products of the pulverized coal furnace fly ash and raw illite, both of which are rich in alumina. There is no Si3
phase in the product of the diatomite. Therefore, it implies that, in our CRN system, the forming temperature of β-SiAlON is lower than that of the Si3
phase, and rich alumina species in fly ash and raw illite improve the nitridation process to form SiAlONs. Considering that a large amount of silicon carbide is produced in diatomite instead of Si3
, the carbonization process must take precedence over the nitridation process in a nitrogen atmosphere for diatomite, even when part of the silicon carbide acts as the intermediate product of the nitridation process. Mullite and corundum from the starting materials remain in the 1350 °C products of fly ash. Meanwhile, quartz from diatomite and raw illite is completely converted into cristobalite and other phases below 1350 °C.
At 1400 °C, the amount of Si2N2O decreases sharply and only remains in the products of pulverized coal furnace fly ash due to its high Al:Si ratio. As an intermediate nitride product at low temperatures, the vanishing of Si2N2O indicates that the nitridation process is completed. Thus, more β-SiAlON and SiC phase exist in the products. The phases in the products from illite mineral are basically pure β-SiAlON and a small amount of SiC, which can be regarded as an almost complete nitriding process. As for diatomite, SiC phase in the products is greatly reduced, and replaced by Si3N4 with two structures: α-Si3N4 and β-Si3N4. These two Si3N4 phases with different structures appear almost at the same time.
At 1450 °C, in addition to the phases in the products at 1400 °C, pulverized coal furnace fly ash also produces aluminum-rich nitride phase 15R-SiAlON (SiAl4
). In the products of pulverized coal furnace fly ash, corundum is always accompanied. That means aluminum is excessive in the chemical composition of pulverized coal furnace fly ash corresponding to β-SiAlON. There is no obvious evidence of AlN phase in XRD patterns of products, and, even if formed, they are below detectable limits, similar to the result in [31
]. For diatomite at 1450 °C, the resultant phases are the same as that of 1400 °C. Illite mineral produced a small amount of corundum phase on the basis of β-SiAlON and a small amount of SiC.
To sum up, the aluminum species in the starting materials promote the nitriding reaction. Thus, the initial forming temperature of β-SiAlON is lower than silicon nitride, and aluminum-rich nitride phase will be formed at higher temperature, 1450 °C. The products of different raw materials are all β-SiAlON phase, while α-Si3N4 and β-Si3N4 are produced at almost the same time. The higher alkali metal content in the raw materials, such as K2O, will promote the production of SiAlON at lower temperatures, although no independent phase or new phase different from other aluminosilicate starting materials is observed.
The FT-IR spectra of the products obtained from different starting materials are shown in Figure 3
. At 1350 °C, the spectrum of the products of pulverized coal furnace fly ash does not exhibit the characteristic absorption at 1100 cm−1
of the Si-O bond [32
]. The spectrum of the products of raw illite does not obviously exhibit characteristic absorption at 950 cm−1
of the N-Si-O bond [32
], which corresponds to the complete nitridation reaction. The peaks at 1100 cm−1
are generally assigned to the characteristic absorption of the Si-O bond [32
]. The peaks at 1000, 950, 900 and 540 cm−1
are generally assigned to the characteristic absorption of N-Si-O bond. The peaks at 490 cm−1
are generally assigned to the characteristic absorption of the Si-N bond [33
]. The peaks at 460 cm−1
are generally assigned to the characteristic absorption of the Al-O bond [32
]. The peaks at 430 cm−1
are generally assigned to the characteristic absorption of the Si-O bond [32
At 1400 °C, the FT-IR spectrum of the products of raw illite does not exhibit characteristic absorption at 1100 cm−1
of the Si-O bond [32
]. The characteristic absorption of the N-Si-O bond at 950 cm−1
only exists in the products of diatomite [32
]. It corresponds to the fact that the nitridation reaction of the diatomite with less impurity is not easy. The peaks at 490 cm−1
, assigned to α-Si3
do not exist in the products of pulverized coal furnace fly ash and raw illite, consistent with the XRD results. For diatomite, the characteristic absorption of the Si-N bond at 490 cm−1
indicates the forming of α-Si3
]. The existence of the characteristic absorption at 460 cm−1
and the disappearance of the characteristic absorption at 430 cm−1
is caused by the larger amount of aluminum and the lower amount of silicon in the products of pulverized coal furnace fly ash [33
At 1450 °C, the characteristic absorption at 1100 cm−1 in the products of pulverized coal furnace fly ash appears again, which is assigned to the 15R SiAlON phase considering the XRD results. The peaks of the N-Si-O bond at 950 cm−1 in all products disappears, indicating the completed nitridation reaction at 1450 °C. The Si-N peaks at 490 cm−1 do not exist in the products of pulverized coal furnace fly ash and raw illite, and only appear in diatomite samples. The disappearance of the 900 cm−1 peak in the pulverized coal furnace fly ash sample may also be related to the forming competition between the 15R SiAlON phase and β-SiAlON phase during CRN.
3.4. The Effect of the Si Sources on the Photoluminescence Properties of Products
The photoluminescence spectra of the products obtained from different starting materials are shown in Figure 5
. The photoluminescence excitation spectra of the products obtained from different starting materials at 1350 °C, 1400 °C and 1450 °C are shown in Figure 5
a–c, respectively. The photoluminescence emission spectra of the products obtained from different starting materials at 1350 °C, 1400 °C and 1450 °C are shown in Figure 5
d–f, respectively. The full width of half maximum (FWHM) values of the excitation spectra of the products obtained from fly ash, diatomite and raw illite at 1350 °C are approximately 97, 80 and 99 nm respectively. The FWHM values of the excitation spectra of the products obtained from fly ash, diatomite and raw illite at 1400 °C are approximately 124 nm, 102 nm and 114 nm, respectively. The FWHM values of the excitation spectra of the products obtained from fly ash, diatomite and raw illite at 1450 °C are approximately 121 nm, 120 nm and 119 nm, respectively. The FWHM values of the emission spectra of the products obtained from fly ash, diatomite and raw illite at 1350 °C are approximately 92 nm, 93 nm and 111 nm, respectively. The FWHM values of the emission spectra of the products obtained from fly ash, diatomite and raw illite at 1400 °C are approximately 85 nm, 107 nm and 127 nm, respectively.
The FWHM values of the emission spectra of the products obtained from fly ash, diatomite and raw illite at 1450 °C are approximately 140, 107 and 146 nm, respectively. At 1350 °C, the sample of raw illite displays the strongest luminescence. This may be related to the priority of β-SiAlON in the products of raw illite. In all temperature ranges, the photoluminescence intensities of the products from pulverized coal furnace fly ash are the weakest, which is probably related to the existence of the corundum phase. As revealed by XRD analysis, in the product system from pulverized coal furnace fly ash, corundum is the exact phase which always exists in all temperature ranges and differs from the phases of other starting materials. At 1400 °C and 1450 °C, the photoluminescence intensities of the products from diatomite rise sharply, and are much higher than those of the other two systems. This indicates that the photoluminescence intensity of Si3N4 is higher than that of the other two systems. From the above two aspects, although aluminum promotes the nitridation process from the point of phase evolution, it can be considered that the existence of aluminum has a negative impact on the photoluminescence performance.
According to peak position analysis of the photoluminescence excitation spectra, the samples were excited at 325nm. According to the peak position analysis of the photoluminescence emission spectra, the peak positions of the product of all of the samples calcined at 1350 °C are close to 435 nm, approximately, attributing to blue spectral range. When the CRN temperature increases to 1400 °C, the peak positions of the products from diatomite change significantly to the green spectral range, centered at approximately 530 nm. The result is similar to related studies [6
]. Ryu et al. [6
] reported β-SiAlON:Eu2+
showed a single intense broadband emission in the range from 525 to 540 nm, and exhibited broad bands of the spectra of the Eu2+
ions in the present β-SiAlON lattice. Zhu et al. [9
] reported Eu2+
-doped β-SiAlON phosphor emission spectra exhibited two broad bands with maxima at about 415 nm (violet) and 540 nm (green), and the broad band excitation is attributed to the 4f7
5d transition of Eu2+
. Yu et al. [35
] reported Eu-doped Si3
emission spectra exhibited a broad band ranging from 500 to 700 nm, and the emission band was attributed to the allowed 4f6
transition of Eu2+
ion. In summary, the emission bands in our study are attributed to the allowed 4f6
transition of Eu2+
ion. The emission spectra of the products calcined at 1400 °C and 1450 °C from diatomite are redshifted. This is related to the change of phase composition, especially the production of two different structures of Si3
. The above results imply that the photoluminescence performance of the Si3
system differs from that of the SiAlON system. It can also be considered that the presence of aluminum will lead to the blue shift in the photoluminescence spectrum. The blue shift of the peaks of pulverized coal furnace fly ash and raw illite at 1400 °C and 1450 °C can be regarded as the change in the photoluminescence properties of the two systems caused by nitridation.
3.5. Product Morphology
SEM micrographs of the different starting materials and their typical nitridation products are shown in Figure 6
. In pulverized coal furnace fly ash, there are spherical particles and irregular particles, which may affect the morphology of later products. Diatomite displays relatively complete or broken porous algal disc morphology. Raw illite has a multi-layer lamellar morphology, which is typical morphology of clay minerals.
In the products of pulverized coal furnace fly ash, many particles maintain the spherical shape at 1350 °C, and a small amount of fibers also exists, which is related to the nitridation process. But the EDS results show that these fibers are not nitrides but carbides. It is likely that the nitridation products from the silicon carbide transformation maintain the morphology of silicon carbide. At 1400 °C, the aggregations of prismatic particles increased, and the spherical products disappeared, indicating that most of the silica species from the spheres reacted in the CRN condition. Although the change in the phase composition in XRD patterns was not obvious, the nitridation process and the production of β-SiAlON led to the replacement of spherical particles by prismatic particles directly. At 1450 °C, with the deepening of nitridation process, the products maintain the aggregation morphology of prismatic particles and the particle sizes obviously increase. With the increasing temperature, more β-SiAlON phase is formed, and the morphology of fly ash changes from a spherical to a prismatic shape.
In products from diatomite, the particles are mostly tiny, agglomerated vermicular fibers and large plates at 1350 °C. At 1400 °C, the fibers grow longer and straighter than at 1350 °C. The morphologies of the products are a mixture of fibers, small spherical particles and large plates. At 1450 °C, large plates disappear in the products, and long straight fibers and their aggregations fill the field of vision. Although the phase composition of the XRD patterns remained basically unchanged, the morphology changed greatly due to the growth of fibers. With an increase in reaction temperature, diatomite particles tend to be smaller with more fibrous products from large plates.
In the products from raw illite, the morphologies are mostly fine long straight fibers and spherical particles at 1350 °C. At 1400 and 1450 °C, the morphologies of the products change a little, which are composed of larger long straight fibers and irregular particle aggregations. Additionally, the corresponding phase composition, analyzed by XRD patterns, remains basically unchanged. With an increase in reaction temperature, the product particles from natural illite become irregular aggregations from spherical particles, and the fibers become longer and straighter.
At 1350 °C, the products of pulverized coal furnace fly ash are mostly spherical in shape, and came from the starting materials, unlike the products of diatomite and raw illite. There are a few spherical particles in the products of raw illite, but the origin of these spheres differs from that of fly ash spheres. They are newly formed spheres, indicating that part of raw the illite is melted in the samples at 1350 °C due to the higher potassium content and lower melting point of the illite system. At 1400 °C, the products of pulverized coal furnace fly ash transformed into prismatic particles caused by the formation of β-SiAlON phase (revealed by XRD analysis), while the products of diatomite and raw illite transformed into more fibers. Revealed by XRD analysis, the fibers are α-Si3N4 and β-Si3N4 phase in the system of diatomite and β-SiAlON phase in the system of raw illite. There are still large plate particles in the products of diatomite, and some of them are transformed into smaller spherical particles. At 1450 °C, the prismatic particles in the products of fly ash and the fibers in the products of illite grow and the intensity of PL spectra becomes stronger. The spherical particles and plate particles in the products of diatomite disappear. With the progress of the nitridation process, the morphologies of products from different silicon sources show that the fibers are increased, becoming longer and straighter, and the prismatic particles are more obvious. The growth of particles could enhance the PL properties of the products.