**6. Conclusion**

The Na+-superionic conducting glass-ceramics with N5-type structure were successfully produced using the sodium rare earth silicophosphate composition of Na3+3*<sup>x</sup>*-*<sup>y</sup>*R1-*x*P*y*Si3-*y*O9, in which the rare earth elements of Sc to Sm were applicable to R. The possible combinations of *x* and *y* became more limited for the crystallization of the superionic conducting phase as the ionic radius of R increased, while the Na+ conduction properties were more enhanced in the glass-ceramics of larger R. The meaning of the composition formula can be signified in the thermodynamic and kinetic study of crystallization and phase transformation of metastable to stable phase in the production of N5-type glass-ceramics. It was demonstrated that the medium value of content product as [P]×[R] is important in the crystallization of N5 single phase. Conduction properties of these glass-ceramics were strongly dependent upon the crystallization conditions as well as compositions. Not only complex impedance analysis but also TEM observation confirmed that this dependence was attributed to the conduction properties of grain boundaries which were glasses condensed at triple points enclosed by grains.

Glass-ceramics of the N5-type superionic conductors in the system Na2O-Sm2O3-P2O5-SiO2 were prepared by crystallization of glasses with the composition Na3+3*x-y*Sm1-*x*P*y*Si3-*y*O9. The optimum conditions for crystallization were discussed with reference to the conduction properties and the preparation of crack free N5-type glass-ceramics. The crystallization of the N5 single-phase glass-ceramics was dependent strongly on the concentrations of both [R] and [P] (or *x* and *y* in the composition parameters) and the temperature for crystallization of glass specimens. The ionic conductivity of the glass-ceramic Na4.1Sm0.5P0.4Si2.6O9 was 4.78×10-2 S/cm at 300°C. The grain size of the specimen was about 3-5 μm. The state of grain growth is promoted with increase of heating temperature and heating time for crystallization. Although grain growth may cause high conductivity, it was difficult to prevent the sample heated for a long time from cracking during crystallization.

Glass-ceramics of the titanium-, germanium- or tellurium-containing N5-type superionic conductors were prepared by crystallization of glasses with the composition Na3+3*x*Y1−*<sup>x</sup>*X*y*Si3−*<sup>y</sup>*O9 (X=Ti, Ge, Te), and the effects of X elements on the separation of the phase and the microstructural effects on the conduction properties of glass-ceramics were discussed. The combination of x and y was most varied in N5YGeS and more limited in the order of N5YTeS>N5YTiS. Their conductivities and activation energies are of the order of 10−2 S/cm at 300°C and of 15 to 24 kJ/mol, respectively. The conductivity of the glassceramic N5YXS decreases giving the order N5YGeS>N5YTeS>N5YTiS. It is considered that this order corresponds to the N5 single phase region. Large enhancement of electrical conductivity was observed in the glass-ceramics as the grain growth was promoted with increase of heating temperature and heating time for crystallization.

Glass-ceramics of the vanadium- or molybdenum-containing N5-type superionic conductors were prepared by crystallization of glasses with the compositions Na3+3*<sup>x</sup>*−*<sup>y</sup>*Y1−*<sup>x</sup>*V*y*Si3−*<sup>y</sup>*O9 or Na3+3*<sup>x</sup>*−2*<sup>y</sup>*Y1−*<sup>x</sup>*Mo*y*Si3−*<sup>y</sup>*O9. The combination of *x* and *y* was most varied in N5-type NYPS and more limited in N5-type NYVS and NYMS. The conductivities of the glass-ceramic specimens with the Na3.9Y0.6V0.3Si2.7O9 (A) and Na3.7Y0.7Mo0.1Si2.9O9 (B) compositions were 0.87×10−2 and 3.58×10−2 S/cm at 300°C, respectively. The conductivity decreases giving the order NYPS>NYMS>NYVS. It is considered that this order corresponds to the N5 single phase region. We assume that the effect of the substitution of Si with V or Mo should be to bring about the difference of homogeneity in the N5 ring structure. The Na+ ionic transport numbers of these glass-ceramics determined by Wagner polarization method were nearly 0.9 for the specimen (A) and 1 for the specimen (B) at 300°C, respectively. It is considered that about 10% of total conduction is electronic conduction in the specimen (A). This result can explain following facts; the conductivity of the specimen (A) are lower than other N5 conductors, and it is seen in the temperature dependence Arrhenius plots for the specimen (A) that the lines drawn from the conductivity of grains are bending upwards.

We have successfully produced the N5-type glass-ceramic conductors by bias crystallization of the glasses with the composition Na4.05Y0.55P0.3Si2.7O9 in an electric field. The microstructure and the conduction properties were dependent on the current direction in the process of crystallization. The cross sections which are parallel and perpendicular to the electric field direction showed the ionic conductivities of 0.0923 and 0.132 mS/cm at 300°C, respectively. The microstructure and the electric conductivity of the glass-ceramics perpendicular to the electric field direction were significantly different from those in parallel.

#### **7. Acknowledgment**

I would like to thank Prof. Kimihiro Yamashita (Tokyo Medical and Dental University, Japan) and Professor emeritus Hideki Monma (Kogakuin University, Japan) for their support and warm encouragement.

#### **8. References**

104 Advances in Crystallization Processes

The Na+-superionic conducting glass-ceramics with N5-type structure were successfully produced using the sodium rare earth silicophosphate composition of Na3+3*<sup>x</sup>*-*<sup>y</sup>*R1-*x*P*y*Si3-*y*O9, in which the rare earth elements of Sc to Sm were applicable to R. The possible combinations of *x* and *y* became more limited for the crystallization of the superionic conducting phase as the ionic radius of R increased, while the Na+ conduction properties were more enhanced in the glass-ceramics of larger R. The meaning of the composition formula can be signified in the thermodynamic and kinetic study of crystallization and phase transformation of metastable to stable phase in the production of N5-type glass-ceramics. It was demonstrated that the medium value of content product as [P]×[R] is important in the crystallization of N5 single phase. Conduction properties of these glass-ceramics were strongly dependent upon the crystallization conditions as well as compositions. Not only complex impedance analysis but also TEM observation confirmed that this dependence was attributed to the conduction properties of grain boundaries which were glasses condensed at triple points enclosed by

Glass-ceramics of the N5-type superionic conductors in the system Na2O-Sm2O3-P2O5-SiO2 were prepared by crystallization of glasses with the composition Na3+3*x-y*Sm1-*x*P*y*Si3-*y*O9. The optimum conditions for crystallization were discussed with reference to the conduction properties and the preparation of crack free N5-type glass-ceramics. The crystallization of the N5 single-phase glass-ceramics was dependent strongly on the concentrations of both [R] and [P] (or *x* and *y* in the composition parameters) and the temperature for crystallization of glass specimens. The ionic conductivity of the glass-ceramic Na4.1Sm0.5P0.4Si2.6O9 was 4.78×10-2 S/cm at 300°C. The grain size of the specimen was about 3-5 μm. The state of grain growth is promoted with increase of heating temperature and heating time for crystallization. Although grain growth may cause high conductivity, it was difficult to prevent the sample heated for a long time from cracking during crystallization. Glass-ceramics of the titanium-, germanium- or tellurium-containing N5-type superionic conductors were prepared by crystallization of glasses with the composition Na3+3*x*Y1−*<sup>x</sup>*X*y*Si3−*<sup>y</sup>*O9 (X=Ti, Ge, Te), and the effects of X elements on the separation of the phase and the microstructural effects on the conduction properties of glass-ceramics were discussed. The combination of x and y was most varied in N5YGeS and more limited in the order of N5YTeS>N5YTiS. Their conductivities and activation energies are of the order of 10−2 S/cm at 300°C and of 15 to 24 kJ/mol, respectively. The conductivity of the glassceramic N5YXS decreases giving the order N5YGeS>N5YTeS>N5YTiS. It is considered that this order corresponds to the N5 single phase region. Large enhancement of electrical conductivity was observed in the glass-ceramics as the grain growth was promoted with

Glass-ceramics of the vanadium- or molybdenum-containing N5-type superionic conductors were prepared by crystallization of glasses with the compositions Na3+3*<sup>x</sup>*−*<sup>y</sup>*Y1−*<sup>x</sup>*V*y*Si3−*<sup>y</sup>*O9 or Na3+3*<sup>x</sup>*−2*<sup>y</sup>*Y1−*<sup>x</sup>*Mo*y*Si3−*<sup>y</sup>*O9. The combination of *x* and *y* was most varied in N5-type NYPS and more limited in N5-type NYVS and NYMS. The conductivities of the glass-ceramic specimens with the Na3.9Y0.6V0.3Si2.7O9 (A) and Na3.7Y0.7Mo0.1Si2.9O9 (B) compositions were 0.87×10−2 and 3.58×10−2 S/cm at 300°C, respectively. The conductivity decreases giving the order NYPS>NYMS>NYVS. It is considered that this order corresponds to the N5 single phase region. We assume that the effect of the substitution of Si with V or Mo should be to bring about the difference of homogeneity in the N5 ring structure. The Na+ ionic transport

increase of heating temperature and heating time for crystallization.

**6. Conclusion** 

grains.


**5** 

*Vadodara, India* 

Arun Pratap and Ashmi T. Patel

*–David Oxtoby, Nature, August 3, 2000.* 

**Crystallization Kinetics of Metallic Glasses** 

*Faculty of Technology & Engineering, The Maharaja Sayajirao University of Baroda,* 

Metallic glasses are kinetically metastable materials. Metallic glass is defined as "A liquid, which has been cooled into a state of rigidity without crystallizing". Properties of metallic glasses differ form non metallic glasses. Ordinary glasses are made up of silica while metallic glasses are made of alloy metals. Ordinary glasses are transparent whereas metallic glasses are opaque. In ordinary glasses, covalent bond is observed while in metallic glasses metallic bond is observed. On the basis of internal arrangement of atoms or molecules and type of force acting between them, the material can be classified into the following two

i. Crystalline solid: Those materials in which the constituent ions or atoms and molecules are arranged in regular pattern are called crystalline solids. Besides, crystalline solids

ii. Amorphous or glassy solid: Those materials do not have definite geometric pattern are called amorphous solids. In amorphous solid atoms, ions or molecules are not arranged

Also, an amorphous solid is a solid in which there is no long range order of the positions of the atoms. Solids in which there is long-range atomic order are called crystalline solids.

At high cooling rate, any liquid can be made into an amorphous solid. Cooling reduces molecular mobility. If the cooling rate is faster, then molecules can not organize into a more thermodynamically favourable crystalline state and an amorphous solid will be formed. Materials in which such a disordered structure is produced directly from the liquid state during cooling are called "Glasses" and such amorphous metals are commonly referred to as "Metallic Glasses" or "Glassy Metals". The metallic glasses have a combination of amorphous structure and metallic bond. This combination provides a metallic glass a new

and unique quality, which cannot be found in either pure metals or regular glass.

**1. Introduction** 

categories:

in definite pattern.

have a definite external geometrical form. e.g. Quartz, Calcite, Diamond, Sugar, and Mica

e.g. Rubber, Glass, Plastic and Cement

*Condensed Matter Physics Laboratory, Applied Physics Department,* 

*"Crystallization is still in many ways, more an art than a science."* 

