The Solid State Reactions Under Normal Pressure And High-pressure And High-temperature In SiO₂-ZrO₂ System

When the crystallite size of articles reduced to nano level, the number atoms on the surface and the surface energy would be increased rapidly because the atoms on the surface are a large number and the crystal field and the bonding energy for them are different from the inner atoms. The atoms on the surface lack surrounding atoms and there is lot of unsaturated bonds. It is easy to combine with other atoms and be stabilization. So the atoms have large chemistry active and the surface has large energy when the atoms pressing to block. During the sinter process, the high surface energy act as driver for the movement of the atoms and it is accelerate for the shrink of inanition on surface. So sinter at lower temperature can achieve the compactness of samples. There are many means of preparation of nanoparticles. mechanical ball milling(MBM) and sol-gel method are common in laboratory.High-pressure method, being a typical extreme method of physics, have important effect on solid state reaction compare with the rection under ambient. It can increase the speed of reaction, transformation rate, reduce the temperature of reartion and reduce the time of synthesis. At the same time, high-pressure can increase the dense, symmetry, coordination number and can reduce the bonding length. The development of high-pressure technology arouse the extensity application and obtain many important results. It is important to simulate the high-pressure and high-temperature conditions in the Earth and have important meaning for the geoscience.In this paper, SiO₂-ZrO₂ system has been studied. first, prepared nano precursor by mechanical ball milling and then synthesized by solid state rection under ambient and high-pressure and high-temperature. Using XRD and Raman measurements to study the synthesis production and the phase transformation.ZrSiO₄ was synthesized by solid state reartion under ambient conditions. The start temperature of synthesis was 1000℃(SiO₂:ZrO₂=1:3); at 1500℃the reaction completede. The synthesis temperature depressed evidently compared with the synthesis using non-MBM raw materials. Under high-pressure, the synthesis conditions of ZrSiO₄ were 920℃(3.6GPa) and 815℃(3.9GPa), and the synthesis temperature depressed evidently when the pressure increased.The processes of the formation of ZrSiO₄ were different according to the pressure and temperature. Under ambient conditions, the process at 1000℃was:α-SiO₂+t-ZrO₂→ZrSiO₄; above 1300℃, the superfluous SiO₂ has transformated toα-cristobalite, so the process was: SiO₂ (α-cristobalite) + t-ZrO₂→ZrSiO₄. Under high-pressure, the raw materialα-SiO₂ didn't transformate under 3.6GPa, so the transformation procession of ZrSiO₄ was α-SiO₂+t-ZrO₂→ZrSiO₄; under 3.9GPa, when the temperature below 815℃,α-SiO₂ didn't transformate too, the transformation procession of ZrSiO₄ wasα-SiO₂+t-ZrO₂→ZrSiO₄; above 990℃,theα-SiO₂ transformated to coesite first, then the coesite reaction with t-ZrO₂. so the the transformation procession of ZrSiO₄ was SiO₂(coesite)+t-ZrO₂→ZrSiO₄.When SiO₂:ZrO₂=3:1, the synthesis condition of coesite was lower that of zircon (ZrSiO₄), 990℃, at 3.9GPa. At 3.9GPa, 1200℃, the sample only contain the pure phase of coesite and zircon. The result can explain the fact that coesite on surface of the Earht often occurs as inclusions in zircon. In the shear zones of Earth's plates, if the local temperature and pressure (produced by the seismic wave and/or stress or collision) achieve the lowest conditions of the formation of coesite, coesite would be as inclusions in zircon. When the local temperature and pressure achieve the lowest conditions of the formation of zircon, coesite can not form andα-SiO₂ would be as inclusions in zircon. Therefore, ZrSiO₄ can work as a"pressure vessel"for the coesite and the natural coesite was always found along with ZrSiO₄.The pure ZrO₂ became amorphous after 40 hours milled. Mostly m-ZrO₂ transformd to t-ZrO₂ and the t-ZrO₂ was stabl under ambient conditions. In SiO₂-ZrO₂ system, the increase of content SiO₂ baffled the transformation from m→t phase of ZrO₂. We have the same conclusion when replace the amorphous by crystalloid SiO₂.In MBM treated SiO₂-SiO₂, treated by sinter and high-pressure and high-temperature, the stabilization of t-ZrO₂ was due to the formation of Si-O-Zr bond. When the content of SiO₂ below 50%, the Si-O-Zr bond can formation at 800℃under ambient condition. Sintered at 1000℃,the Si-O-Zr bond can formation in all samples with different mol ratio of SiO₂ and ZrO₂. Under high-pressure, the Si-O-Zr bond can formation in the samples with SiO₂:ZrO₂=3:1 at 920℃; when the pressure increased to 3.9GPa, the formation temperature decreased to 815℃. Compared with the formation temperation of Si-O-Zr bond at ambient (1000℃), it can be seen that high-pressure can accelerate the formation of Si-O-Zr bond.We also synthesized BaZr1-xYxO3 series samples by solid state reaction under ambient conditions, by high-pressure and high-temperature method and by sol-gel method. No pure phase of cubic perovskites phase BaZr1-xYxO3 obtained at 1300℃under ambient conditions synthesis by solid state reaction. When x≥0.1, the cell patameters and cell volumes show a linearity change with the substitute amount"x"of Y3+. In is means that the Y3+ have substituted the Zr4+.Treated by high-pressure (3.6GPa) and high-temperature( 500℃) for the samples prepared by sinter under ambient conditions, a pure phase of cubic perovskite phase was obtained in lower substituted samples(x=0, 0.5). High-pressure can accelerate the reactions and can increased the single-phase of BaZr1-xYxO3 series samples under relatively lower temperature. For the high substitute samples(x≥0.1), little BaCO3 and m-ZrO₂ appeated and high-temperature can resist the reactions. Only below 500℃a pure phase can obtained under high-pressure.Pure phase of BaZrO₃ occurred at 300℃in samples with x=0 and x=0.05 synthesized by sol-gel method. The temperature of samples with x=0.1 and x=0.2 were 700℃and 500℃. At 300℃, the superfluous Ba2+ existed as Ba(NO₃)₂ and decomposed at 500℃, then the BaCO3 formed. At 700℃the Ba(NO₃)₂ decomposed completed. Sintered for 5 hours at 900℃, the main phase in all samples were BaZrO₃ phase, but little BaCO3 appeared at this temperature. The contain of BaCO3 increared with the increased of Y3+ and the diffraction peaks of BaZrO₃ broadered clearly, and the trend of asymmetry clearly too. It meant the distortion of crystal lattice increased. The Raman spectra indicated that the distortion of crystal lattice of samples sintered at 900℃clearly.