Morphology Evolution and Phase Interactions of Fe-containing Si₃N₄ in Vacuum High-temperature Environment

Abstract

To study the substitution of Fe₃Si–Si₃N₄ for refractories in the upper RH refiner, this paper simulated the service condition of RH refining and studied the change of the Fe₃Si–Si₃N₄ in the simulated condition. A Fe₃Si–Si₃N₄ specimen prepared by flash combustion was put in a vacuum sintering furnace with carbon lining, fired at 1450°C under 80 Pa of vacuum degree for 2 h, and then cooled. The morphological evolution before and after being treated and phase interactions of the Fe₃Si–Si₃N₄ specimen were studied and analyzed thermodynamically and dynamically. The results show that at high temperatures in vacuum, Fe volatilizes from the Fe₃Si melt in Fe₃Si–Si₃N₄ and reacts with Si₃N₄ on the Si₃N₄ crystal surface, forming new Fe_xSi melt there; then Fe continues to volatilize from the new Fe_xSi melt, causing Fe_xSi alloy particles finer and more uniform in Fe₃Si–Si₃N₄; the hexagonal columnar Si₃N₄ crystals begin to decompose partially, and become cylindrical with edges and corners disappearing; during prebaking or operation interval of RH refining, a SiO₂ film which has better stability than Si₃N₄ is developed on the surface of Si₃N₄ crystals or Fe₃Si–Si₃N₄ bricks, preventing the decomposition of Si₃N₄ and improving the application feasibility of Fe₃Si–Si₃N₄ in RH refining.

1. Introduction

RH refining, namely the vacuum circulation degassing refining method, is to refine the melting steel by decarbonizing and dehydrogenizing in the vacuum chamber circularly.^1,2) As the progress of refining technology, RH refining has developed from single vacuum degassing to multi-functional integrated refining including oxygen-blowing,³⁾ decarbonization,^4,5) powder injection,⁶⁾ desulphurization,⁷⁾ temperature compensation,⁸⁾ and component alloying,⁹⁾ which is featured with short processing period, large production capacity, good refining effect, and easy operation.¹⁰⁾ So it has been applied widely in the clean steel production.^11,12)

The low vessel and upper vessel of the inner lining in RH refining furnaces mainly adopt fused grain re-bonded magnesia-chromite bricks and direct-bonded magnesia-chromite bricks.^13,14,15,16) Although the application of magnesia-chromite bricks in RH refiners does not generate large quantities of Cr⁶⁺ as that in the burning zone of cement rotary kilns, the generation of Cr⁶⁺ is still inevitable during production.^15,17) So the chrome-free refractory is the developing trend of refractories for RH-vessel lining in consideration of environment protection.^18,19) So far, there have been some attempts of using chrome free MgO-spinel materials in the lower vessel of RH refining furnaces.²⁰⁾ Though the performance still lags, the application of MgO-spinel in refining some steels is pretty satisfied, approaching to that of the fused grain re-bonded magnesia-chromite bricks.^19,20) Compared with the lower vessel, the upper vessel is less vulnerable: it does not contact the melting steel and only suffers slag splashing and chemical corrosion, yet, good thermal shock resistance is required.¹⁶⁾ Silicon nitride (Si₃N₄) material, as a non-oxide material, has good thermal shock stability, and the components of CaO, SiO₂ and Al₂O₃ in splashing slag hardly wet or affect it, which is suitable for the RH upper vessel.²¹⁾ However, it is too expensive to adopt pure Si₃N₄ material in the RH upper vessel.

Therefore, this paper proposed a new idea applying the low-cost Fe₃Si–Si₃N₄ material in the RH upper vessel. The Fe₃Si–Si₃N₄ used in this work was a mixture prepared by flash combustion technology using FeSi75 with the particle size under 0.074 mm as the raw material.²²⁾ The main phases of the Fe₃Si–Si₃N₄ are Fe₃Si (15 wt%), α-Si₃N₄ and β-Si₃N₄ (75 wt%).^22,23) Fe₃Si is a coexisting phase with Si₃N₄ during the high temperature synthesis. The density of the Fe₃Si–Si₃N₄ is 2.0 g/cm³ and the porosity is 42%.²³⁾

However, what happens in Fe₃Si–Si₃N₄ material at high temperatures in vacuum? What is the stability? Do its phases interact? How does the morphology change? These are all important factors influencing the application of Fe₃Si–Si₃N₄ material in the upper vessel of RH refiners. BATHA and WHITNEY researched the stability of Si₃N₄ at high temperatures in vacuum, Ar atmosphere, and N₂ atmosphere, and analyzed its decomposition dynamically.²⁴⁾ Their study showed that Si₃N₄ decomposed obviously at 1400–1500°C in vacuum and the main mechanism was that the N atoms escaped from the surface covalent bonds to the atmosphere. The Fe₃Si in the Fe₃Si–Si₃N₄ material coexists with Si₃N₄ in the synthesis condition of Fe₃Si–Si₃N₄, but what are the stability and volatility of Fe₃Si at high temperatures in vacuum? There is few report on that.

Therefore, this paper studied the stability, phase interaction and microstructure change of Fe₃Si–Si₃N₄ material in high temperature vacuum condition to seek the application of Fe₃Si–Si₃N₄ feasibility in the upper vessel of RH refiners and to investigate the Si₃N₄ crystallization and the morphology control at high temperatures in vacuum.

2. Experimental Procedure

2.1. Experimental Procedure

A Fe₃Si–Si₃N₄ specimen (10×10×10 mm) prepared by flash combustion was put in a vacuum sintering furnace (graphite heaters and carbon lining) filled with high purity Ar with a vacuum degree of about 80 Pa. Then the furnace temperature was raised to 1450°C and hold for 2 h. After cooling, the specimen was investigate by SEM and EDS.

2.2. Characterization

The microstructure and element distribution of the Fe₃Si–Si₃N₄ specimen before and after the treatment were obtained using a scanning electron microscope (nova^TM nano SEM 230, FEI Company, USA) equipped with an energy dispersive spectrometer (Inca Energy 250XM 50mm/IW500, Oxford, UK).

3. Results

Figure 1 gives the SEM images of the Fe₃Si–Si₃N₄ before treatment. There are typically two morphologies in the Fe₃Si–Si₃N₄ material, namely the grey columnar or needle-like crystals and the bright white particles. The Fe₃Si–Si₃N₄ is mainly composed of the micron grade grey radial cross columnar crystals with aspect ratio more than 20. The white particles with the diameter of about several microns to tens of microns, are wrapped in the centers of the grey crystals and dispersively distribute in the Si₃N₄ matrix. The surface of the white particles is closely connected with the grey crystals, as shown in Fig. 1(a). It is analyzed that the grey columnar or needle-like crystals are Si₃N₄ single crystals, as shown in Fig. 1(b), while the white particles are Fe-rich phases with the main phase of Fe₃Si, as shown in Fig. 1(c). The Si₃N₄ crystals develop well with clear edges and corners as well as smooth surfaces, and there is no impurity observed around the crystals. The surface of Fe₃Si connects with the Si₃N₄ crystals closely, and partial Fe₃Si is directly exposed in the air.

Fig. 1.

SEM images of Fe₃Si–Si₃N₄ before treatment: (a) Image of Fe₃Si–Si₃N₄ morphology; (b) Image of β-Si₃N₄ crystal in Fe₃Si–Si₃N₄; (c) Image of Fe₃Si in Fe₃Si–Si₃N₄.

Figure 2 shows the SEM images and the EDS results of the Fe₃Si–Si₃N₄ specimen treated at high temperatures in vacuum. Compared with the untreated material, the distribution of the white Fe-rich phases changes greatly. Before treatment, the Fe-rich phases (Fe₃Si) are wrapped in the centers of columnar Si₃N₄ crystals; while after treating, the Fe-rich phases change into widely spread Fe-rich particles with diameter under 1 μm, and most of the particles adhere to the surface of Si₃N₄ crystals, as shown in Fig. 2(a).

Fig. 2.

SEM images and EDS result of Fe₃Si–Si₃N₄ after treatment: (a) SEM image of Fe₃Si–Si₃N₄ after treatment; (b) Image of columnar Si₃N₄ crystals in Fe₃Si–Si₃N₄; (c) Image of Si₃N₄ surface; (d) SEM image of white spots in Si₃N₄ surface; (e) EDS result of point A.

The morphology of the columnar Si₃N₄ crystals changes a lot as well. The smooth and clear surfaces of the hexagonal columns become rough and uneven. Some holes and strips are observed in the Fe-rich particle-adhering areas, and their diameters are similar to the diameter of the Fe-rich particles. So it is forecasted that the holes and strips are formed by the reaction between the Fe-rich particles and the surface of the Si₃N₄ crystals at high temperatures, as shown in Fig. 2(b). In addition, the sharp edges and corners of the hexagonal columnar Si₃N₄ crystals disappear and tend to be planar, changing the hexagonal columnar Si₃N₄ crystals into cylindrical.

Fe-rich particles bond with the Si₃N₄ crystal surface to some extents. Besides, the Fe-rich particles consist of bright parts and grey parts, which should be caused by the different Si contents in the Fe–Si alloy, as shown in Fig. 2(c). The EDS result of point A in Fig. 2(d) is shown in Fig. 2(e). The spherical Fe-rich particle is mainly composed of Fe and Si, so the spherical Fe-rich particle should be Fe–Si alloy. The presence of N could be the influence of the Si₃N₄ around the Fe–Si alloy particle and some trace impurities such as Al and Ca which come from the raw FeSi75 exist as well.

4. Discussion

4.1. Volatilization of Fe₃Si and Reaction between Fe₃Si and Si₃N₄

The dispersive arrangement of Fe-rich particles is caused by the volatilization of Fe–Si alloy in vacuum at high temperatures. At 1450°C, the Fe–Si alloy in Fe₃Si–Si₃N₄ material is in melt state, Fe vapor and Si vapor release from the melt continuously. The saturation vapor pressure of Fe and Si is shown as follows according to references:^25,26)   

$$P_{\text{Fe}}^{\text{o}}=9.12\times {10}^{-1}\text{Pa}\hspace{1em}{P}_{\text{Si}}^{\text{o}}=9.77\times {10}^{-2}\text{Pa}$$(1)

The activity coefficients γ_Fe and γ_Si in Fe–Si melt at 1450°C vs the mole fraction of Fe x_Fe is drawn in Fig. 3(a).²⁷⁾ The calculated activities a_Fe and a_Si vs x_Fe by formula (2) is shown in Fig. 3(b).

Fig. 3.

Activity coefficients γ_Fe and γ_Si and activities a_Fe and a_Si in Fe–Si melt vs mole fraction of Fe x_Fe: (a) Activity coefficients γ_Fe and γ_Si in Fe–Si melt vs x_Fe; (b) Activities a_Fe and a_Si in Fe–Si melt vs x_Fe.

Before treatment, the main composition of Fe-rich phases in the Fe₃Si–Si₃N₄ material is Fe₃Si, so at high temperatures, x_Fe is about 0.75 and x_Si is about 0.25.²²⁾   

$$a_{\text{Fe}}=x_{\text{Fe}}\cdot {\gamma }_{\text{Fe}}\hspace{1em}{a}_{\text{Si}}=x_{\text{Si}}\cdot {\gamma }_{\text{Si}}$$(2)

  

$$P_{\text{Fe}}=a_{\text{Fe}}\cdot P_{\text{Fe}}^{\text{o}}\hspace{1em}{P}_{\text{Si}}=a_{\text{Si}}\cdot P_{\text{Si}}^{\text{o}}$$(3)

According to Fig. 3 and formula (2), the corresponding γ_Fe=0.63, γ_Si=0.025, and a_Fe=0.47, a_Si=0.0063. As calculated by formula (3), at 1450°C, the Fe vapor pressure around Fe₃Si melt P_Fe=4.29×10⁻¹Pa while the Si vapor pressure P_Si=6.16×10⁻⁴Pa.

The above calculation shows that the vapor pressures of Fe and Si around Fe₃Si melt are different; Fe is the majority in the vapor of Fe₃Si melt while Si accounts little in vacuum high-temperature environment: P_Si is only millesimal of P_Fe.

In the high temperature melt, Fe is hard to volatilize and its volatilization rate is controlled by interface volatilization reaction. So the volatilization rate of Fe can be calculated by formula (4) as follows:²⁸⁾   

$$w_{\text{Fe}}=P_{\text{Fe}}\sqrt{\frac{M_{\text{Fe}}}{2\pi RT}}=a_{\text{Fe}}{P}_{\text{Fe}}^{\text{o}}\sqrt{\frac{M_{\text{Fe}}}{2\pi RT}}={\gamma }_{\text{Fe}}{x}_{\text{Fe}}{P}_{\text{Fe}}^{\text{o}}\sqrt{\frac{M_{\text{Fe}}}{2\pi RT}}$$(4)

Where, M_Fe is the gram-molecular weight of Fe 55.85 g; R is the gas constant; T is the absolute temperature; so the volatilization rate of Fe element at 1450°C is calculated as follows:   

$$w_{\text{Fe}}=P_{\text{Fe}}\sqrt{\frac{M_{\text{Fe}}}{2\pi RT}}=1.07\times {10}^{-1}\text{g}\cdot {\text{cm}}^{-2}\cdot {\text{s}}^{-1}$$(5)

The volatilized Fe vapor condenses into solid Fe on the surface of Si₃N₄ because the system temperature is under the melting point of Fe. The solid Fe reacts with columnar Si₃N₄ crystals as formulas (6), (7), (8).^25,26)   

$$\begin{array}{l}\text{5Fe}+{\text{Si}}_{\text{3}}{\text{N}}_{\text{4}}={\text{Fe}}_{\text{5}}{\text{Si}}_{\text{3}}\text{(l)}+{\text{2N}}_{\text{2}}\text{(g)}\\ {\mathrm{\Delta }}_{\text{r}}{G}^{\mathrm{\theta }}=\text{(496}\hspace{0.17em}\hspace{0.17em}\text{704}-\text{340}\text{.961}T)\hspace{0.17em}\hspace{0.17em}\text{J}\cdot {\text{mol}}^{-1}\\ {\mathrm{\Delta }}_{\text{r}}G={\mathrm{\Delta }}_{\text{r}}{G}^{\mathrm{\theta }}+RTln(P_{{\text{N}}_{\text{2}}}/P^{\mathrm{\theta }}{)}^2<0\end{array}$$(6)

  

$$\begin{array}{l}\text{3Fe}+{\text{Si}}_{\text{3}}{\text{N}}_{\text{4}}=\text{3FeSi(l)}+{\text{2N}}_{\text{2}}\text{(g)}\\ {\mathrm{\Delta }}_{\text{r}}{G}^{\mathrm{\theta }}=\text{(5}07\hspace{0.17em}\hspace{0.17em}628-377.967T)\hspace{0.17em}\hspace{0.17em}\text{J}\cdot {\text{mol}}^{-1}\\ {\mathrm{\Delta }}_{\text{r}}G={\mathrm{\Delta }}_{\text{r}}{G}^{\mathrm{\theta }}+RTln(P_{{\text{N}}_{\text{2}}}/P^{\mathrm{\theta }}{)}^2<0\end{array}$$(7)

  

$$\begin{array}{l}\text{9Fe}+{\text{Si}}_{\text{3}}{\text{N}}_{\text{4}}={\text{3Fe}}_{\text{3}}\text{Si(l)}+{\text{2N}}_{\text{2}}\text{(g)}\\ {\mathrm{\Delta }}_{\text{r}}{G}^{\mathrm{\theta }}=\text{(204}\hspace{0.17em}\hspace{0.17em}\text{296}\text{.8}-\text{656}\text{.83}T)\hspace{0.17em}\hspace{0.17em}\text{J}\cdot {\text{mol}}^{-1}\\ {\mathrm{\Delta }}_{\text{r}}G={\mathrm{\Delta }}_{\text{r}}{G}^{\mathrm{\theta }}+RTln(P_{{\text{N}}_{\text{2}}}/P^{\mathrm{\theta }}{)}^2<0\end{array}$$(8)

The volatilized Fe vapor condenses into solid Fe and reacts on the surface of Si₃N₄ crystals as formulas (6), (7), (8), forming Fe_xSi melt and leading to the holes or strips. The subsequent Fe vapor is easy to react in the holes or strips, expanding the holes or strips further. As the temperature decreases, the Fe_xSi melt solidifies, leaving the Fe_xSi alloy particles on the surface of Si₃N₄ crystals. The main elements of Fe_xSi alloy particles are Fe and Si, which is consistent with the EDS result. And yet, the formed Fe_xSi at high temperatures in the system keeps releasing Fe vapor and Si vapor.

For Fe₅Si₃ melt, the molar fraction of Fe is 0.625 and that of Si is 0.375. As calculated by formulas (1), (2), (3), the vapor pressure of Fe is P_Fe=1.64×10⁻¹ Pa while that of Si is P_Si=5.18×10⁻³ Pa. For FeSi melt, the molar fractions of Fe and Si are both 0.5, P_Fe=4.65×10⁻² Pa and P_Si=2.15×10⁻² Pa. The volatilized Fe vapor will condense and react on the surface of new Si₃N₄ crystals. The volatilized Si vapor, Fe₅Si₃ melt and FeSi melt may react with N₂ in the atmosphere as formulas (9), (10), (11):^25,26)   

$$\begin{array}{l}\text{3Si(g)}+{\text{2N}}_{\text{2}}\text{(g)}={\text{Si}}_{\text{3}}{\text{N}}_{\text{4}}\\ {\mathrm{\Delta }}_{\text{r}}{G}^{\mathrm{\theta }}=\text{(}-2\hspace{0.17em}\hspace{0.17em}060\hspace{0.17em}\hspace{0.17em}656+\text{739}\text{.15}T)\hspace{0.17em}\hspace{0.17em}\text{J}\cdot {\text{mol}}^{-1}<0\end{array}$$(9)

  

$$\begin{array}{l}{\text{9Fe}}_{\text{5}}{\text{Si}}_{\text{3}}\text{(l)}+{\text{8N}}_{\text{2}}\text{(g)}={\text{4Si}}_{\text{3}}{\text{N}}_{\text{4}}+{\text{15Fe}}_{\text{3}}\text{Si(l)}\\ {\mathrm{\Delta }}_{\text{r}}{G}^{\mathrm{\theta }}=\text{(}-3\hspace{0.17em}\hspace{0.17em}449\hspace{0.17em}\hspace{0.17em}298-218.333T)\hspace{0.17em}\hspace{0.17em}\text{J}\cdot {\text{mol}}^{-1}<0\end{array}$$(10)

  

$$\begin{array}{l}\text{9FeSi(l)}+{\text{4N}}_{\text{2}}\text{(g)}={\text{2Si}}_{\text{3}}{\text{N}}_{\text{4}}+{\text{3Fe}}_{\text{3}}\text{Si(l)}\\ {\mathrm{\Delta }}_{\text{r}}{G}^{\mathrm{\theta }}=\text{(}-1\hspace{0.17em}\hspace{0.17em}318\hspace{0.17em}\hspace{0.17em}676+476.507T)\hspace{0.17em}\hspace{0.17em}\text{J}\cdot {\text{mol}}^{-1}<0\end{array}$$(11)

The volatilized Si vapor, Fe₅Si₃ melt and FeSi melt may continue to capture and to react with N₂ in the atmosphere. The reaction between Fe and Si₃N₄ proceeds continually on the surface of Si₃N₄ crystals and the Fe_xSi melt keeps releasing Fe vapor.

So, after treated at high temperatures in vacuum, the Fe_xSi alloy particles widely spread in Fe₃Si–Si₃N₄ with more uniform distribution and smaller particles size because of the Fe volatilization and reaction between Fe and Si₃N₄.

4.2. The Stability of Silicides and Si₃N₄ with O₂ in the Reaction Atmosphere

The pressure of Ar is 80 Pa with the purity of 99.999%, so the initial O₂ partial pressure is 8×10⁻¹⁰ MPa in the vacuum sintering furnace. The graphite heaters and carbon lining of the vacuum sintering furnace bring about reducing atmosphere with excess carbon content in the experimental process. The main reactions between carbon and oxygen in reducing atmosphere at high temperature are as formulas (12), (13), (14):³²⁾   

$$\begin{array}{l}\text{C}+{\text{O}}_{\text{2}}\text{(g)}={\text{CO}}_{\text{2}}\text{(g)}\\ {\mathrm{\Delta }}_{\text{r}}{G}^{\mathrm{\theta }}=\text{(}-\text{395}\hspace{0.17em}\hspace{0.17em}\text{350}-\text{0}\text{.54}T)\hspace{0.17em}\hspace{0.17em}\text{J}\cdot {\text{mol}}^{-1}\\ {\mathrm{\Delta }}_{\text{r}}G={\mathrm{\Delta }}_{\text{r}}{G}^{\mathrm{\theta }}+RTln(P_{{\text{CO}}_{\text{2}}}/P^{\mathrm{\theta }})/(P_{{\text{O}}_{\text{2}}}/P^{\mathrm{\theta }})\end{array}$$(12)

  

$$\begin{array}{l}\text{2C}+{\text{O}}_{\text{2}}\text{(g)}=\text{2CO(g)}\\ {\mathrm{\Delta }}_{\text{r}}{G}^{\mathrm{\theta }}=\text{(}-\text{228}\hspace{0.17em}\hspace{0.17em}\text{800}-\text{171}\text{.54}T)\hspace{0.17em}\hspace{0.17em}\text{J}\cdot {\text{mol}}^{-1}\\ {\mathrm{\Delta }}_{\text{r}}G={\mathrm{\Delta }}_{\text{r}}{G}^{\mathrm{\theta }}+RTln({P_{\text{CO}}/P^{\mathrm{\theta }})}^2/(P_{{\text{O}}_{\text{2}}}/P^{\mathrm{\theta }})\end{array}$$(13)

  

$$\begin{array}{l}\text{C}+{\text{CO}}_{\text{2}}\text{(g)}=\text{2CO(g)}\\ {\mathrm{\Delta }}_{\text{r}}{G}^{\mathrm{\theta }}=\text{(166}\hspace{0.17em}\hspace{0.17em}\text{550}-\text{171}\text{.00}T)\hspace{0.17em}\hspace{0.17em}\text{J}\cdot {\text{mol}}^{-1}\\ {\mathrm{\Delta }}_{\text{r}}G={\mathrm{\Delta }}_{\text{r}}{G}^{\mathrm{\theta }}+RTln({P_{\text{CO}}/P^{\mathrm{\theta }})}^2/(P_{{\text{CO}}_{\text{2}}}/P^{\mathrm{\theta }})\end{array}$$(14)

Combined with the formulas (12), (13), (14), the O₂ partial pressure at 1450°C in reaction process can be calculated as follow formula (15):   

$$P_{{\text{O}}_{\text{2}}}=3.3\times {10}^{-36}\text{MPa}$$(15)

Under this O₂ partial pressure, the Si₃N₄ cannot be oxidized by O₂ according to the stable area diagram of condensed phase of Si–N–O at 1450°C (Fig. 4).³²⁾

Fig. 4.

Stable area of condensed phase of Si–N-O at 1450°C.³²⁾

At 1450°C, the Fe–Si alloy including FeSi, Fe₅Si₃ and Fe₃Si in Fe₃Si–Si₃N₄ material is in melt state. The activity of Fe and Si in alloy melt are less than in pure Fe and Si (Fig. 3), the possible reactions between the pure Fe and Si, and O₂ are as follows formulas (16), (17):³²⁾   

$$\begin{array}{l}\text{Si(l)}+{\text{O}}_{\text{2}}\text{(g)}={\text{SiO}}_{\text{2}}\text{(s)}\\ {\mathrm{\Delta }}_{\text{r}}{G}^{\mathrm{\theta }}=\text{(}-\text{946}\hspace{0.17em}\hspace{0.17em}\text{350}+197.64T)\hspace{0.17em}\hspace{0.17em}\text{J}\cdot {\text{mol}}^{-1}\\ {\mathrm{\Delta }}_{\text{r}}G={\mathrm{\Delta }}_{\text{r}}{G}^{\mathrm{\theta }}+RTln1/(P_{{\text{O}}_{\text{2}}}/P^{\mathrm{\theta }})>0\end{array}$$(16)

  

$$\begin{array}{l}\text{2Fe(s)}+{\text{O}}_{\text{2}}\text{(g)}=\text{2FeO(l)}\\ {\mathrm{\Delta }}_{\text{r}}{G}^{\mathrm{\theta }}=\text{(}-\text{441}\hspace{0.17em}\hspace{0.17em}\text{410}+77.82T)\hspace{0.17em}\hspace{0.17em}\text{J}\cdot {\text{mol}}^{-1}\\ {\mathrm{\Delta }}_{\text{r}}G={\mathrm{\Delta }}_{\text{r}}{G}^{\mathrm{\theta }}+RTln1/(P_{{\text{O}}_{\text{2}}}/P^{\mathrm{\theta }})>0\end{array}$$(17)

The pure Fe and Si are not react with O₂ according to the formulas (16), (17). The activity of Fe and Si in such melt as FeSi, Fe₅Si₃ and Fe₃Si are less than that in pure Fe and Si, so they can not react with O₂, leading to the Si₃N₄, FeSi, Fe₅Si₃ and Fe₃Si can stably exist in the reaction atmosphere.

4.3. Morphological Evolution of Columnar Si₃N₄ Crystals

The morphological evolution of Si₃N₄ crystals relates with not only the Fe vapor and its reactions, but also the vacuum condition. At 1450°C in vacuum, the sharp edges and corners of the Si₃N₄ crystals become smooth. Edges disappear and corners and edges of the hexagonal columnar crystals get planarized, as shown in Fig. 4. This is because that at this temperature, the sharp edges and corners of the Si₃N₄ crystals decompose.

The decomposition of the Si₃N₄ crystals sharp edges and corners can be explained by weighted mean curvature and surface free energy. As a polyhedron crystal, the sharp edges and corners of Si₃N₄ crystals show larger weighted mean curvature with higher surface free energy compared with the flat segments. In order for a polyhedral surface S to avoid having a singular part to its weighted mean curvature, individual corners and edges of S must be surface energy minimizing.³⁵⁾ The volume sweeping out under a deformation and the motion of the surface at corners and edges result in the decrease of weighted mean curvature and surface energy, so the sharp edges and corners of the Si₃N₄ crystals decompose.

From the aspect of atom arrangement, Si₃N₄ is a covalent compound, each Si atom is connected with four N atoms and each N atom is connected with three Si atoms, as shown in Fig. 5(a).^30,31) In the corners and edges of the crystal, the outer atoms have less atoms connected and less bonding. So they are easy to escape from the covalent bonds, thus the corners and edges of the Si₃N₄ crystal decompose firstly.²⁴⁾

Fig. 5.

Decomposition of edges and corners of Si₃N₄ crystals: (a) Edges and corners of Si₃N₄ crystals before treatment; (b) Edges and corners of Si₃N₄ crystals after treatment.

Fig. 6.

Decomposition schematic diagram of Si₃N₄ crystal.

The analysis of the vapor pressure and atom arrangement shows that the corners and edges of the crystal get decomposed firstly, so the corners and edges disappear, namely at high temperatures in vacuum, Si₃N₄ crystals get decomposed partially.

4.4. Application in RH Refining

During the actual RH refining, the atmosphere of the refining furnace is not always in vacuum. During the prebaking before refining or the operation interval of the refining, the atmosphere is hot air in normal pressure. In these situations, the hot air with temperature higher than 1000°C can oxidize the Si₃N₄, forming a SiO₂ film on the surface. In other words, in the actual RH refining, a SiO₂ film forms on the surface of the Si₃N₄ crystals or the Fe₃Si–Si₃N₄ bricks.³²⁾

The SiO₂ film has very low vapor pressure at high temperatures in vacuum, so, in the actual RH condition, the SiO₂ film has poor volatility and little volatilization.³³⁾ Besides, the gas penetration rate of the film is low, isolating the vacuum atmosphere to some extent and restraining the decomposition of Si₃N₄.^32,34) Therefore, in the on-site RH refining, the Si₃N₄ inside of Fe₃Si–Si₃N₄ hardly decomposes. From this point, it is deduced that the Si₃N₄ in the Fe₃Si–Si₃N₄ material is stable and the Fe₃Si–Si₃N₄ material can be applied in the upper vessel of the RH refiner.

5. Conclusions

At high temperatures in vacuum, large quantities of Fe volatilizes from the Fe₃Si melt in Fe₃Si–Si₃N₄ and reacts with Si₃N₄ on the Si₃N₄ crystal surface, forming new Fe_xSi melt there. Fe continues to volatilize from the new Fe_xS melt, causing Fe_xSi alloy particles finer and more uniform in Fe₃Si–Si₃N₄.

Under the low O₂ partial pressure, the silicides and Si₃N₄ cannot react with O₂ at 1450°C in reaction process.

Meanwhile, in this condition, the hexagonal columnar Si₃N₄ get decomposed partially and the appearance of the crystal tends to be planar; the corners and edges gradually disappear, the crystal becomes cylindrical and the crystal top tends to be spherical.

In the actual RH refining, a SiO₂ film always exists on the surface of the Si₃N₄ crystals or the Fe₃Si–Si₃N₄ bricks. During RH refining, the film exists stably on the surface of the Si₃N₄ or the Fe₃Si–Si₃N₄ bricks with very little volatilization, insulating the vacuum atmosphere and thus restraining the decomposition of Si₃N_4. From this point, it is deduced that the Si₃N₄ in the Fe₃Si–Si₃N₄ material is stable and the Fe₃Si–Si₃N₄ material can be applied in the upper vessel of the RH refiner.

Acknowledgement

This research was supported by the National Nature Science Foundation of China (No. 51572019) and National Science-technology Support Plan Projects (Grant No.2013BAF09B01).

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