Shou-Ren Gu

Oxidation resistance of mild steel by zirconia sol–gel coatings

Abstract The effect of zirconia coatings deposited by the Sol–gel technique on the oxidation of mild steel was studied by scanning electron microscopy (SEM), X-ray diffraction (XRD) and measuring the weight gain. It was found that the thickness of zirconia coatings is a crucial factor affecting the oxidation kinetics and that a multi-layer coating is much more effective than the single-layer one. Auger electron spectroscopy (AES) was used to evaluate the depth composition of the coatings. The results suggest that the zirconia coatings deposited on mild steel just retard the oxidation, but do not suppress the oxidation.

Effect of changing TiO2 content on structure and crystallization of CaO-Al2O3-SiO3 system glasses

Abstract This paper studied that the influence of changes in the TiO 2 content on the glass structure of CaO-Al 2 O 3 -SiO 3 system and the formation of crystalline phases on reheating. The activation energy (E) for crystal growth and the Avrami parameter (n) have also been evaluated by means of differential thermal analysis (DTA) techniques. The value of the Avrami parameter (n) agrees well with scanning electron microscopy (SEM) observations of dendritic crystal growth from surface nuclei. In all the CaO-Al 2 O 3 -SiO 3 system glasses studied, dendritic crystals were observed. The greater the TiO 2 content, the lower were the glass transition temperature, T g , and crystallization peak temperature, T p . The high value of T p did not mean that value of E was large. The most effective addition of TiO 2 was about 10.8% (by mass) in the CaO-Al 2 O 3 -SiO 2 system glasses. The difference of E between the base glass (no TiO 2 added) and the glass containing most effective addition of TiO 2 was very little. The experimental results suggest that in the studied samples, TiO 2 cannot promote the crystallization very effectively.

A new criterion for the stability of glasses

Abstract A new criterion for the stability of glasses, kD(T), kD(T) = ν · exp ( − E R T ⋅ D ) , where D = T c ( T p − T c ) T l ( T l − T g ) , was proposed, which included both the kinetic factor and the thermodynamic factor by means of bringing the correction factor, D, into the crystallization kinetic parameter, k(T), k(T) = ν · exp ( − E R T ) . The lower the value of kD(T), the more stable glass. The new criterion, kD(T), was not influenced by the heating rate of differential thermal analysis (DTA). Especially when the values of frequency factor ν of glasses were near, the new criterion, kD(T), could accurately determine the stability of the glasses. The correctness of the new criterion, kD(T), had been verified through being used in the CaO-Al2O3-SiO2 system glasses and the Nd doped fluorozirconate glasses.

The effect of additives on the crystallization of Na2O–CaO–MgO–Al2O3–SiO2–TiO2 system glasses

The differential thermal analysis (DTA) curve of Na2O–CaO–MgO–Al2O3–SiO2–TiOs system glasses has two exothermic peaks, which means that at least two kinds of main crystal are produced in the process of heat treatment. Some additives such as typical network modifier (Na2O, CaF2), and network intermediate (Al2O3) are added in the above glasses. After the glasses with additives are heat treated in the different processes, they are investigated by the method of scanning electron microscopy (SEM) and X-ray diffraction (XRD). Some important results are obtained. The main crystal corresponding to the first exothermic peak temperature is fassaite (Ca(Ti,Mg,Al)(Si,Al)2O6), and another main crystal corresponding to the second exothermic peak temperature is anorthite (CaAl2Si2O8). The additives of Na2O and CaF2 play a role in decreasing the nucleation and growth of fassaite. Na2O has no obvious effect on the nucleation and growth of anorthite, but CaF2 suppresses the nucleation and growth of anorthite and itself comes out of the base glasses. B2O3 hinders fluorite’s coming out of the base glasses and improves the nucleation and growth of fassaite and anorthite.

A study on the mechanism of crystal growth in the process of crystallization of glasses

Abstract This paper investigates the CaO–Al 2 O 3 –SiO 2 system glass. When the glasses were heat treated at different temperatures and times, they crystallized from the surface. The main crystals were dendrites of anorthite (CaAl 2 Si 2 O 8 ). The morphology of crystallization was observed using scanning electron microscopy, and the crystal size was measured by optical microscopy. It was found that the growth of anorthite (CaAl 2 Si 2 O 8 ) crystals was diffusion controlled. The apparent activation energy for crystal growth was about 581 kJ/mol. The rate of crystal growth was not constant during the growth process; the rate was highest at the beginning of crystallization and gradually decreased with increasing time. The higher the temperature, the less time required to complete crystal growth. The total process of diffusion may be determined by the diffusion rate of Ca 2+ .

Effect of Na+ ion on the structure of 27CaO.12Al2O3.61SiO2 glass

Abstract Raman spectra and differential thermal analysis (DTA) were used to investigate structure of 27CaO·12Al2O3·61SiO2 glass with and without Na2O. A new phenomenon is found: that is, the addition of a low content of Na2O to 27CaO·12Al2O3·61SiO2 glass resulting in an increase of the aggregation extent of the glass network. Addition of the Na+ ions to glass leads to the Al3+ ions, existing as network modifiers, further entering as network formers. This results in the decrease of the amount of nonbridging oxygen ions. Al3+ ions located as network modifiers enter as network formers, which corresponds to the highest value of the aggregation extent of the glass network. The nonbridging oxygen ions will re-increase continuously with the Na2O content, which corresponds to the continuous decrease of the aggregation extent of glass network. The most effective content of the Na2O added is about 4.3 mol%.

Controlled phase separation by an electric field in glasses

Abstract Externally applied electric field has a significant effect on phase separation of glasses. At low BaO contents, spherical barium-rich phase separation is promoted due to the greater ( e 2 / e 1 >1) ratio of the dielectric constants of the new phases than the old. Increasing BaO contents to 0.1 mol%, owing to the interconnected phase separation produced, the electric field has no effect on the phase separation. At higher BaO contents, spherical silica-rich phase separation is retarded. This means that the electric field can either stimulate or inhibit the phase separation process. Furthermore, we may control phase separation of glasses. These results suggest a new method for the preparation of nanocrystalline glass ceramics.

A new kinetic description for phase separation of materials

Abstract This Letter presents a new kinetic description for phase separation of materials. The unstable non-uniform regions fluctuate to form the steadily critical non-uniform regions, which diffuse to grow by either nucleation growth or Spinodal decomposition mechanism. The equation describing fluctuation stage is c( r ,t)−c 0 =P cos (βWt) cos ( β · r ) , and the equations describing diffusion stage are λ2=Ht (nucleation growth) and c( r ,t)−c 0 =PQ 2 exp [R 2 (β)t] cos ( β · r ) (early stage of Spinodal decomposition). The process of forming a steadily critical non-uniform region is that the region size fluctuates near the critical one, the amplitude of fluctuation decreases gradually, and the composition of region tends to stable.