Aging of NTC Ceramics in the System Mn-Ni-Fe-O

The aging behaviour at 150°C of negative temperature coefficient ceramics in the system Mn-Ni-Fe-O has been investigated. Comparison is made between samples cooled in air and in a controlled atmosphere in which no oxidation can occur. The results show that the aging is dependent on composition and is strongly dependent on oxidation which occurs during cooling in air after sintering of the ceramics. From these results a model has been derived which describes the aging mechanism as a migration of cationic vacancies from the grain boundaries to the bulk. Additionally, cations and cationic vacancies migrate to their thermodynamically favourable site in the spinel structure.

[1]  A. Sinha,et al.  Vacancy Distribution and Bonding in Some Oxides of Spine1 Structure , 1957 .

[2]  R. Arnott,et al.  Preparation, semiconduction and low-temperature magnetization of the system Ni1−xMn12+xO4 , 1962 .

[3]  A. Rousset,et al.  Low-temperature oxidation of Mn3O4 hausmannite , 1998 .

[4]  H. Mizoguchi,et al.  New oxide phase with wide band gap and high electroconductivity, MgIn2O4 , 1992 .

[5]  M. Suzuki.  A.c. hopping conduction in Mn-Co-Ni-Cu complex oxide semiconductors with spinel structure , 1980 .

[6]  A. Rousset,et al.  Structural and electrical properties of iron manganite spinels in relation with cationic distribution , 1995 .

[7]  K. Becker,et al.  The Mechanism of Cation Equilibration in Nickel Aluminate Spinel, NiAl2O4 , 1998 .

[8]  L. S. Darken,et al.  The System Iron—Oxygen. II. Equilibrium and Thermodynamics of Liquid Oxide and Other Phases , 1946 .

[9]  N. Nachtrieb,et al.  The chemistry of imperfect crystals , 1973 .

[10]  C. Catlow,et al.  Defect structure and migration in Fe3O4 , 1985 .

[11]  I. Barin.  Thermochemical data of pure substances , 1989 .

[12]  V. Brabers.  Cation migration, cation valencies and the cubic-tetragonal transition in MnxFe3−xO4 , 1971 .

[13]  H. Verweij,et al.  Constant stoichiometry cooling of YBa2Cu3Ox , 1989 .

[14]  A. Rousset,et al.  Correlation Between Microstructure and Ageing of Iron Manganite Thermistors , 1997 .

[15]  V. Brabers,et al.  Electrical Conductivity and Cation Valencies in Nickel Manganite , 1982, January 16.

[16]  A. Rosencwaig,et al.  Mössbauer spectroscopy of stoichiometric and non-stoichiometric magnetite , 1969 .

[17]  Abel Rousset,et al.  Correlation between the structure, the microstructure and the electrical properties of nickel manganite negative temperature coefficient (NTC) thermistors , 1998 .

[18]  E. Otto.  Equilibrium Pressures of Oxygen over MnO2 ‐ Mn2 O 3 at Various Temperatures , 1965 .

[19]  R. Morineau,et al.  Chart of PO 2 versus temperature and oxidation degree for Mn-Zn ferrites in the composition range: 50 l Fe 2 O 3 l 54; 20 l MnO l 35; 11 l ZnO l 30 (mole %) , 1975 .