DD295941A5 - METHOD FOR PRODUCING SINTER CERAMICS FOR THERMISTORS INCREASED SENSITIVITY AND STABILITY - Google Patents

Abstract

A process for the production of high sensitivity thermistors is described. Previously known technical solutions are based on semiconducting oxides of the transition elements and their combinations in spinels and perovskites. Such multiphase systems require a differentiated adapted reaction regime, so that a sufficiently narrowly tolerated assortment of long-term stable thermistors can only be obtained by various forms of thermal and electrical aftertreatment as well as by classifying and separating. The aim of the invention is to counteract the decomposition into different semiconducting oxide phases when the temperature falls below a certain temperature in the air by incorporation of magnesium oxide instead of manganese oxide in the nickel-manganese oxide spinel system and in the composition Mg5 / 6NiMn7 / 6O4 a complete thermal stability to reach. The decomposition of oxygen formed in the process of sintering at 1 000 to 1 050C and the associated decomposition into different phases proves to be reversible during annealing in the range 600 to 700 ° C, assuming sufficient porosity of the ceramic. At the same time, the B constant is increased to increase the sensitivity, without the resistivity simultaneously increasing unfavorably. uniform structure; Magnesium; Nickel-manganese-spinell; high B-constant; thermodynamically stable phase}

Description

Explanation of the essence of the invention

The invention is based on the object, for the Anwendungszwock as thermistors suitable semiconductor ceramics, consisting of a uniform and thermodynamically stable as possible phase, based on the nickel-manganese oxide Sp! ' to produce -jllsystoms.

Erfindungsgomäß the object is achieved in that by dissolving magnesium carbonate, nickel carbonate and manganese carbonate in acetic acid, adding oxalic acid in a slight excess and evaporation oxalate mixed crystals

Mg _x NIyMn ₃ - "- _Y (C ₂ O ₄ ) ₃ .6H ₂ O

1,27b; O S χ S 1)

produced whose precipitation by stepwise heating to 65O ⁰ C under oxygen leads to uniform spinel phases, which occur as sinteraktivaktivn powder with a specific surface area of about 10mVg.

Mg _x Ni _y Mn ₃ - "- _y (C ₂ O ₄ ) ₃ · 6H ₂ O + γ O ₂

Mg _K Ni _y M n ₃ _ χ _ _Y O ₄ + 3 CO ₂ + 3 CO + 6 H ₂ O

The nickel cations are gradually replaced by magnesium cations at the tetrahedral sites, allowing them to escape to the more energy-efficient octahedral sites, which, in conjunction with the formation of manganese (IV) cations, stabilizes the spinel phase. At the same time, a trigonal deformation of the manganese oxygen octahedra is induced on the tetrahedral sites due to the lower radius of action of the magnesium ions compared with nickel (II), which is associated with an increase in the B constant. For example, the spinels can be represented by y = 1 and χ = 1 / 3.2 / 3.5 / 6.1 in this way. The resulting powder with increased sintering activity after mixing the sintering aid, compression molding of tablets and sintering at 1000 to 1050 ⁰ C in an oxygen atmosphere initially an inconsistent structure intimately mixed phases, since at the sintering temperature with elimination of oxygen, a decomposition of the spinel takes place. The microstructure still has sufficient porosity to take up oxygen in a subsequent annealing step at 65O ⁰ C and to rebuild the uniform spinel phase.

NiMn ₂ O ₄ (y = 1, χ = 0) is subject below 72O ⁰ C in air of decomposition in NiMnO ₃ and 0-Mn ₂ O ₃ . The spinel Mgi / ₃ NiMn _6/3 Oi (y = 1, x = 1/3) turns out to 65O ⁰ C under an oxygen atmosphere during prolonged tempering to be unstable, although the connection initially formed at this temperature as a result of decomposition of the Oxalatmischkristalle becomes. The decomposition products were found to be NiMnO ₃ , a tetragonal MgMn ₂ O ₄ and an apparently Mg-richer cubic spinel phase. The spinel Mg _2/3 NiMn _4/3 0 ₄ (x = 2/3, y = 1), on the other hand, is stable for a long time at 65O ⁰ C, but oxidative decomposition also occurs slowly at 450 "C. For the spinel Mg ₆ Z ₆ NiMn ₇ Z ₆ O ₄ (x = 5/6, y = 1) could not be observed to decompose into several phases at long-term temperatures at 45O ⁰ C and lower temperatures, and similarly MgNiMnO ₄ turns out to be a thermodynamically stable spinel phase.

The advantage achieved by increasing the incorporation of magnesium to form a spinel phase which is stable with respect to thermal stress is counteracted by the elimination of oxygen which, as a result of the magnesium content of the manganese (IV) in the spinel lattice, shifts to lower temperatures the higher it is the magnesium or manganese (IV) content is. As a result, an increasing phase inhomogeneity occurs at the sintering temperature, which can only be reversed in the case of a sufficient porosity of the ceramic by re-forming the spinel phase with oxygen uptake.

Ausführungsbelsplele

The table shows examples of sintered ceramics according to the invention for thermistors of increased sensitivity.

No. composition D, egg decomposition temperature 750 lower, lower σ (20) B % upper, lower 740 O ₂ ads. [0 " ¹ CrTT ¹ I [K] O ₂ -AbSp. 700 720 1 NiMn ₂ O ₄ 95 1000 powder 650 (4.97 ± 0.08) 10 " ⁴ 3774 ± 3 2 Mgiz ₃ NiMn _6/3 0 ₄ - - 450 - - 3 Mg ₂ Z ₃ NiMn ₄ Z ₃ O ₄ - (6.7 ± 0.2) 10 " ⁶ 4099 ± 5 4 Mg ₆ / _e NiMn _7/6 0 ₄ 80 - (6 ± 2) 10 " ^β 4120 ± 3 5 MgNiMnO ₃₆₄ 97 (2.8 ± 0.3) 10 " ^Β 3800 6 MgNiMnO ₄ (powder) -

Claims (2)

1. A process for producing a sintered ceramic for thermistors increased sensitivity and stability, characterized in that by dissolving magnesium carbonate, nickel carbonate and manganese carbonate in acetic acid, adding oxalic acid in a slight excess and evaporation oxalate mixed crystals Mg _x Ni _y Mn ₃ -xy ( ₂ = 4) 3 x 6H ₂ O (0 <y <1.275; 0 <χ <1) are prepared, the decomposition by stepwise heating to 65O ⁰ C under oxygen leads to uniform spinel phases, which are obtained as sintering active powder. 2. The method according to claim 1, characterized in that after admixture of the sintering aid, press molding into tablets and sintering at temperatures below 1100 ⁰ C in an oxygen atmosphere and subsequent heat treatment at 65O ⁰ C in oxygen atmosphere, the uniform spinel phase is reformed. Field of application of the invention The invention relates to a method for producing a sintered ceramic for thermistors improved thermal stability at the same time increased sensitivity. Characteristic of the known state of the art Known technical solutions are based on semiconducting oxides of the transition elements and their combinations, for example in spinels and perovskites. (US ^Q at 3219480 GB 992,491th) can be modified, without the benefit of, taking often get multi-phase systems, e.g., B. cobalt Manganoxidsysteme (GB 1226789 PL 48148) for application by other components such as copper oxide, nickel oxide or lithium oxide Formation of a single phase. The nominal resistance R ₂ * (electrical resistance of the thermistor at T = 25 ⁰ C) and the relevant for the sensitivity of the temperature measurement B-constant according to the relationship R (T) = R _o exp (B / T) = R ₂₅ exp (1 / T-1/298) is set on the basis of such multiphase systems by a corresponding reaction in the sintering process to variable values, so that at a given offset, the production of a certain assortment of thermistors is possible. This procedure generally involves a considerable spread of the data of the individual specimens and in particular from batch to batch, since the electrical parameters characterizing the thermistor assume different values depending on the achieved structure structure of the ceramic. It has become known that thermistors z. B. on the basis of the system Ni _x Mn ₃ _ _X O _{4 are} manufactured (H. Walch, Siemens magazine 47 [1973], 65, Siemens Data Thermistor 1980/81). For the range of the composition 0 <χ <1.275 results in a largely uniform phase, which no longer has the said disadvantages of a large spread, provided that a ceramic structure is achieved, which is associated with the use of a special sintering aid (DD 266670 A1) , In the management of the sintering process for Thermistorfertigung a decomposition into a heterogeneous mixture when falling below the temperature of 720 ° C by sufficiently rapid cooling must be avoided in these oxide semiconductors with uniform phase inventory, and the scope is limited to temperatures up to 15O ⁰ C. Spin-line compounds in the system Ni _x Mn ₃ - _X O - where an improvement in the thermodynamic stability is achieved by a gradual substitution of manganese by magnesium on the basis of the principle of steered valence, have not hitherto been described in the literature. Object of the invention The object of the invention is to counteract the decomposition into different semiconducting oxide phases when a certain temperature is undershot and to control the process of sintering compaction to form a uniform structure for a reproducible adjustment of the Thermistorparameter in an advantageous manner and at the same time the B-constant in order to increase the sensitivity without increasing the specific resistance at the same time unfavorably.