EP0810611A1 - High temperature thermistor containing rare earth metals - Google Patents

Abstract

A semiconductor ceramic, consisting of a rare earth metal oxide solid solution of composition (I), is new. (YaGdbSmcTbd)2O3 (I), in which a = 0 to 0.995, b = 0 to 0.995, c = 0 to 0.995, d = 0.01 to 0.995 and a = greater than 0, when b = 0, or b = greater than 0, when a = 0. Preferably, the oxide solid solution has a cubic crystalline structure of C-M2O3 type and may contain further doping elements selected from Nd, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu. Also claimed is a thermistor with a semiconductor ceramic of the above composition (I).

Description

The present invention relates to a high-temperature thermistor with a semiconductor ceramic made of a mixed crystal oxide of rare earth oxides, in particular a thermistor that can be used over the entire temperature range from room temperature to 1100 ° C.

Thermistors for high temperatures have become increasingly important in recent years due to new areas of application in immissions protection. They are used, for example, as a temperature sensor for industrial exhaust gas temperature measurements or for temperature control and overtemperature protection for catalytic exhaust gas combustion in cars. The typical application temperatures in cars are between 600 ° C and 1100 ° C, only at these elevated temperatures does the catalytic exhaust gas combustion work optimally. Thermistors made of oxidic semiconductor ceramics offer the advantage over thermocouples in this temperature range that they have a much larger output signal, so that a simpler circuit technology is sufficient for signal processing.

, wobei ρ und ρ₀ die jeweiligen spezifischen Widerstände bei den absoluten Temperaturen T und T₀ sind, B ein thermische Konstante und T die Temperatur in Kelvin ist. Für einen Thermistoren ist es besonders günstig, wenn die Widerstands- Temperatur- Kennlinie möglichst steil ist. Diese Steilheit wird durch die Konstante B bestimmt.Thermistors are also called NTC resistors because their resistance has a negative temperature coefficient (NTC). The specific electrical resistance of the NTC resistors decreases approximately exponentially with increased temperature according to the equation ${\text{ρ = <figure-callout id="0" label="ρ" filenames="imgf0002.png,imgf0003.png" state="{{state}}">ρ</figure-callout>}}_{\text{0}}{\text{exp B (1 / T - 1 / T}}_{\text{O}}\text{)}$

, where ρ and ρ _{0 are} the respective resistivities at the absolute temperatures T and T ₀ , B is a thermal constant and T is the temperature in Kelvin. It is particularly favorable for a thermistor if the resistance-temperature characteristic is as steep as possible. This slope is determined by the constant B.

Known technical solutions for thermistors are based on oxidic semiconductor ceramics, which are based on oxidic compounds of the transition metals of the spinel or perovskite type. Multi-phase systems are often used, in which the base material is modified by additional components. Today's NTC components consist almost exclusively of mixed crystals with a spinel structure, which are composed of 2 to 4 cations from the group Mn, Ni, Co, Fe, Cu and Ti. For such multiphase systems, the nominal resistance R ₂₅ and the B constant relevant for temperature sensitivity are set to variable values by appropriate reaction control during manufacture, so that the production of a certain range of thermistors is possible with a given offset. This procedure generally involves a considerable spread of the data of the individual specimens and from batch to batch, since the electrical parameters characterizing the thermistor take on different values depending on the structural structure of the ceramic that has been achieved. A sufficiently narrowly tolerated range of long-term stable thermistors therefore requires various forms of thermal and electrical aftertreatment as well as sorting and separating as separate work steps.

The production spread of NTC thermistors is quite critical because the contamination content in the sintered material is difficult to control. In addition, the ceramic compounds and their crystal structures that form during manufacture can change over time, especially at high temperatures. At high temperatures, there can also be a slow reaction with the oxygen in the atmosphere, which causes a permanent change in the resistance value and the temperature characteristic.

Therefore, mixed crystal oxides of the spinel or perovskite type can only be used up to about 500 ° C. At higher temperatures, their long-term stability is too low and their specific resistance is too low for many areas of application.

From A.J. Moulson and J.M. Herbert, "Electroceramics", Chapman and Hall, London, p.141 (1990) already discloses mixtures of rare earth metal oxides, e.g. a mixture of 70 cat. % Sm and 30 cat% Tb to use. This mixture can be used up to temperatures of 1000 ° C because it shows no tendency to react with the oxygen in the atmosphere.

At very high temperatures above 1000 ° C, however, instabilities in the resistance value also occur with this high-temperature thermistor material.

It is therefore the object of the present invention to provide a high-temperature thermistor which has narrow tolerances and is long-term stable even at very high temperatures.

According to the invention, the object is achieved by using a thermistor with a semiconductor ceramic made from a mixed crystal oxide of rare earth metals of the composition [Y _a Gd _b Sm _c Tb _d ] ₂ O ₃ with 0 ≤ a ≤ 0.995; 0 ≤ b ≤ 0.995; 0 ≤ c ≤ 0.995; 0.01 ≤ d ≤ 0.995 and a> 0 if b = 0 or b> 0 if a = 0 is available. Such a thermistor is suitable as a temperature sensor for temperatures up to 1100 ° C. It is characterized by its particular stability at very high operating temperatures above 1000 ° C. It is therefore particularly suitable as a sensor in the hot area of catalytic exhaust gas cleaning or for temperature control for engine control.

In the context of the present invention, it is particularly preferred that the mixed crystal oxide has a cubic crystal structure of the CM ₂ O ₃ type. Thermistors with a semiconductor ceramic made of such mixed crystal oxides are characterized by a special high temperature stability.

It can also be preferred that the mixed crystal oxide contains, as further dopants, an element from the group of neodymium, europium, gadolinium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium.

It is preferred that 0.5 ≤ a ≤ 0.99; b = 0, c = 0 and 0.01 ≤ d ≤ 0.5.
It is further preferred that 0.65 a a 0,7 0.75, b = 0, c = 0, 0.25 d d 0,3 0.35.
It is particularly preferred that a = 0 and 0.1 ≤ b ≤ 0.7, c = 0 and 0.3 ≤ d ≤ 0.9.
It is also preferred that 0 ≤ a ≤ 0.30, b = 0 and 0.2 ≤ c ≤ 0.5 and 0.2 ≤ d ≤ 0.6.

The invention further relates to a semiconductor ceramic made of a mixed crystal oxide of the composition [Y _a Gd _b Sm _c Tb _d ] ₂ O ₃ with 0 ≤ a ≤ 0.995; 0 ≤ b ≤ 0.995; 0 ≤ c ≤ 0.995; 0.01 ≤ d ≤ 0.995 and a> 0 if b = 0 or b> 0 if a = 0.
A semiconductor ceramic which is characterized in that the mixed crystal oxide has a cubic crystal structure of the CM ₂ O ₃ type is particularly preferred.

• Fig. 1: Arrhenius-Kurve für Halbleiterkeramik aus Yttrium-Terbium-Oxid-Mischkristallen
• Fig. 2: Arrhenius-Kurve für Halbleiterkeramik aus Yttrium-Samarium-Terbium-Oxid-Mischkristallen
• Fig. 3: Arrhenius-Kurve für Halbleiterkeramik aus Gadolinium-Terbium-Oxid-Mischkristallen im Vergleich mit Arrhenius-Kurven gemäß Fig. 1 und 2.

The invention is explained in more detail below with the aid of examples and three figures.

• Fig. 1: Arrhenius curve for semiconductor ceramics made of yttrium-terbium-oxide mixed crystals
• Fig. 2: Arrhenius curve for semiconductor ceramics made of yttrium-samarium-terbium-oxide mixed crystals
• 3: Arrhenius curve for semiconductor ceramics made of gadolinium-terbium oxide mixed crystals in comparison with Arrhenius curves according to FIGS. 1 and 2.

The semiconductor ceramic with a mixed crystal oxide of the rare earth metals according to the invention contains binary, ternary, quaternary, etc. generally multiple mixed crystal oxides, the essential component of which is terbium and at least one further rare earth metal oxide from the group yttrium, samarium, gadolinium. The mixed crystal oxide can also contain neodymium, europium, dyspprosium, holmium, erbium, thulium, ytterbium or lutetium as further dopants.

Due to the terbium content in the structure, the semiconductor ceramic contains movable electrons, which make a significant contribution to the conductivity of the semiconductor ceramic.

The composition of the mixed crystal oxide is preferably chosen so that a cubic CM ₂ O ₃ type crystal structure is obtained. The prerequisite for this is that the average ionic radius of the cations is based on that of RD Shannon, Acta Cryst. A32 (1976) 751 values are less than 1.06 angstroms. These semiconductor ceramics are monomorphic, ie they do not change their crystal structure at higher temperatures.

Mixed crystal oxides of rare earth metals with a larger average ion radius, such as pure terbium sesquioxide, crystallize in the less symmetrical AM ₂ O ₃ type or BM ₂ O ₃ type. They are polymorphic, at medium and high temperatures their crystal structure changes to the CM ₂ O ₃ type (see AF Wells, Structural Inorganic Chemistry 4th Edition, Clarendon Press, Oxford, pp. 450 ff. (1975). Terbium sesquioxide itself converts to this cubic CM ₂ O ₃ structure at about 1000 ° C. Surprisingly, it was found that the mixed crystal oxides according to the invention which crystallize in the CM ₂ O ₃ type have an outstandingly improved stability at very high temperatures, because in the mixed crystal oxides according to the invention with cations according to the definition given, the crystal structure does not change at higher temperatures.

The semiconductor ceramics are manufactured according to the usual ceramic manufacturing methods. The binary oxides of the rare earth metals mentioned or, for example, their oxalates, carbonates, hydroxides or the like are used as starting compounds. used. The starting mixtures are weighed, then mixed dry or wet and ground. This is preferably followed by a calcination process at 1000 ° C. for better chemical homogenization and for better compaction. After a further grinding process, the shaping process for the green body follows by pressing, film drawing, screen printing or the like. The shaped green bodies go through a binder burnout and are then sintered at 1250 ° C to 1400 ° C. The sintering process is not very susceptible to faults and is not dependent on the gas atmosphere or the cooling curve.

The connection electrodes, preferably made of platinum, can be burned in as wire electrodes during the sintering. However, platinum paste can also be applied and baked using the screen printing process. Other methods are also possible, such as application using vacuum evaporation technology.

To test the thermistors, the resistance and its temperature dependency were determined in the temperature range from 200 ° C to 1100 ° C. The thermal resistance of the thermistors was also measured at high temperatures.

EXAMPLE 1

Mixed crystal oxides are produced which contain Y ₂ O ₃ and 3, 10 and 30 at% terbium. The starting compounds Y ₂ O ₃ and Tb ₄ O ₇ are mixed in the appropriate mixing ratio and ground with zircon grinding balls for 16 hours. This premixed powder is granulated with a conventional binder preparation. Tablets with a diameter of 6 mm and a thickness of 1 mm are pressed from the granules. These tablets are sintered in the air for six hours at 1350 ° C. X-ray diffraction pictures show that the semiconductor ceramic thus obtained from mixed crystal oxides is a single-phase material with a CM ₂ O ₃ structure. The average ionic radius of the mixed crystal oxides is 1.016 Å, 1.018 Å and 1.023 Å, respectively. The relative density of the mixed crystal oxides is greater than 94% of the theoretical density.

EXAMPLE 2

Quaternary mixed crystal oxides of yttrium oxide, samarium oxide and terbium oxide with the composition Y _0.5 Sm _0.9 Tb _0.6 O ₃ and Y _0.5 Sm _0.5 Tb _1.0 O _{3 are produced} by the same method as in Example 1. X-ray diffraction images show that the material is single-phase and crystallizes in the CM ₂ O ₃ type. The average ionic radius of the mixed crystal oxides is 1.056 Å and 1.046 Å, respectively. The relative density is greater than 95% of the theoretical density.

EXAMPLE 3

A ternary mixed crystal oxide with the composition Gd _1.4 Tb _0.6 O _{3 is produced} by the same method as in Example 1. X-ray diffraction images show that the material is single-phase and crystallizes in the CM ₂ O ₃ type. The average ion radius of the mixed crystal oxide is 1.054 Å. The density is greater than 95% of the theoretical density.

TEST RESULTS Temperature resistance characteristics

To test the thermistors according to the invention, their temperature-resistance characteristics are measured.

berechnet. Für Thermistoren wird gefordert, daß zwischen Temperatur und elektrischer Ausgangsgröße ein linearer Zusammenhang besteht. Für den Temperaturbereich, in dem die Arrhenius-Kurve linear oder angenähert linear ist, kann die Halbleiterkeramik als Theremistor verwendet werden.For this purpose, tablets made from the semiconductor ceramic according to the invention are coated with platinum paste on both sides for contacting. The specific resistance is measured while the temperature is varied. The reciprocal temperature is plotted against the logarithm of the specific conductivity σ. You get according to the Arrhenius curve, the slope of which gives the coefficient of thermal resistance B according to the formula ${\text{B = (lnR}}_{\text{1}}{\text{-lnR}}_{\text{2nd}}{\text{) / (1 / T}}_{\text{1}}{\text{- 1 / T}}_{\text{2nd}}\text{)}$

calculated. Thermistors are required to have a linear relationship between temperature and electrical output. For the temperature range in which the Arrhenius curve is linear or approximately linear, the semiconductor ceramic can be used as a thermistor.

1 shows the Arrhenius curves for three yttrium-terbium mixed crystal oxides. The three curves are approximately linear over the entire temperature range from approximately 200 ° C to 1100 ° C. In this temperature range, the semiconductor ceramics can be used as thermistors. Yttrium-terbium mixed crystal oxides with a terbium content of more than 10 at% have particularly favorable properties. They can be used up to temperatures of 1100 ° C.

Fig. 2 shows the Arrhenius curve for Y _0.5 Sm _0.9 Tb _0.6 O ₃ (lower curve) and Y _0.5 Sm _0.5 Tb _1.0 O ₃ (upper curve). Due to the lower resistance and the non-linearity of the Arrhenius curves above 600 ° C, mixed crystal oxides can be used as a sensor at temperatures from 20 ° C to 600 ° C.

3 shows the Arrhenius curves for Gd _1.4 Tb _0.6 O ₃ together with the Arrhenius curves from FIGS. 1 and 2 for comparison. This material can also be used at temperatures from 200 ° C to 1100 ° C.

1 shows the values for the specific electrical conductivities and for the thermal constants B of the mixed crystal oxides from exemplary embodiments 1 to 3. Tab. 1 Specific electrical conductivities and B constants Composition log σ (300 ° C) (Ω ^-1 .cm ^-1 ) log σ (600 ° C) (Ω ^-1 .cm ^-1 ) log σ (900 ° C) (Ω ^-1 .cm ^-1 ) B _300/600 (K) B _600/900 (K) 97% Y ₂ O ₃ : 3% Tb -9,333 -7,386 - 7472 - 90% Y ₂ O ₃ : 10% Tb -7,225 -5,445 -4,483 6831 7570 70% Y ₂ O ₃ : 30% Tb -5,310 -3,553 -2,487 6743 6252 70% Gd ₂ O ₃ : 30% Tb -5,082 -3,215 -2,487 7165 5729 45% Sm ₂ O ₃ : 30% Tb: 25% Y -3,771 -2,262 - 5791 - 25% Sm ₂ O ₃ : 50% Tb: 25% Y -2,587 -1,430 - 4440 -

Aging

The temperature-resistance characteristic must be reliably reproducible even at high temperatures. Especially for applications in motor vehicle construction, the deviations in the temperature Δ T at 600 ° C to 1000 ° C should not exceed +/- 2%, i.e. 20 ° C at 1000 ° C.

Two identical thermistors are selected for each of these measurements. One thermistor is heated to 1000 ° C for 100 h. Then the resistance-temperature characteristics of both thermistors are measured. If the resistance as a function of temperature is plotted for both thermistors, two parallel curves are obtained which are shifted by Δt against each other. The result of the measurements is shown in Table 4.5. The results show that mixed crystal oxides based on yttrium oxide showed the best results. No aging effect was observed in 70% at% Y ₂ O ₃ with 30 at% terbium oxide. Tab. 2 High temperature reliability Composition ΔT (° C) 70% Sm ₂ O ₃ : 30% Tb 13 65% Sm ₂ O ₃ : 30% Tb: 5% Nd 10th 90% Y ₂ O ₃ : 10% Tb 4th 70% Y ₂ O ₃ : 30% Tb 0

Claims (10)

Thermistor with a semiconductor ceramic made of a mixed crystal oxide of the rare earth elements of the composition

[Y _a Gd _b Sm _c Tb _d ] ₂ O ₃

With 0 ≤ a ≤ 0.995 0 ≤ b ≤ 0.995 0 ≤ c ≤ 0.995 0.01 ≤ d ≤ 0.995 and a> 0 if b = 0 or b> 0 if a = 0. Thermistor according to claim 1,
characterized by
that the mixed crystal oxide has a cubic crystal structure of the CM ₂ O ₃ type. Thermistor according to claim 2,
characterized by
that the mixed crystal oxide contains as further doping an element from the group neodymium, europium, gadolinium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium. Thermistor according to claim 1,
characterized in that 0.5 ≤ a ≤ 0.99 b = 0 c = 0 0.01 ≤ d ≤ 0.5. Thermistor according to claim 1,
characterized by
that 0.65 ≤ a ≤ 0.75 b = 0 c = 0 0.25 ≤ d ≤ 0.35 Thermistor according to claim 1,
characterized by
that a = 0 0.1 ≤ b ≤ 0.7 c = 0 0.3 ≤ d ≤ 0.9 Thermistor according to claim 1,
characterized by
that 0 ≤ a ≤ 0.30 b = 0 0.3 ≤ c ≤ 0.5 0.2 ≤ d ≤ 0.60 Semiconductor ceramic from a mixed crystal oxide of the composition

[Y _a Gd _b Sm _c Tb _d ] ₂ O ₃

With 0 ≤ a ≤ 0.995 0 ≤ b ≤ 0.995 0 ≤ c ≤ 0.995 0.01 ≤ d ≤ 0.995 and a> 0 if b = 0 or b> 0 if a = 0. Semiconductor ceramic according to claim 8,
characterized by
that the mixed crystal oxide has a cubic crystal structure of the CM ₂ O ₃ type. Semiconductor ceramic according to claim 9,
characterized by
that the mixed crystal oxide contains as further doping an element from the group neodymium, europium, gadolinium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium.

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