JP2012209292A - Positive thermistor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lead-free positive thermistor with good reliability which can suppress temporal deterioration in resistance value without spoiling thermistor characteristics in spite of long-time power supply.SOLUTION: The positive thermistor includes lead-free semiconductor ceramic which contains substantially no lead as a component element assembly 1, and has external electrodes 2a, 2b formed at both end parts of the component element assembly 1. The semiconductor ceramic principally comprises a BaTiO-based composition, and part of Ba is replaced with alkali metal and Bi. The semiconductor ceramic has a measured sintered density of 70-90% for a logical sintered density. The semiconductor ceramic preferably has part of the Ba replaced with a rare earth element, and at least one kind among Na, K, and Li is useable as the alkali metal.

Description

  The present invention relates to a positive temperature coefficient thermistor, and more specifically, has a positive resistance temperature coefficient (hereinafter referred to as “PTC”) and is used for a heater application or the like (hereinafter referred to as “PTC thermistor”). .)

Barium titanate (BaTiO ₃ ) -based semiconductor ceramics have PTC characteristics that generate heat when a voltage is applied and the resistance value increases rapidly when the Curie point Tc at which phase transition from tetragonal to cubic is exceeded.

  As described above, the semiconductor ceramic having PTC characteristics has a resistance value that increases when the Curie point Tc is exceeded due to heat generation by voltage application, and current hardly flows, and the temperature decreases. And if temperature falls and resistance value becomes small, an electric current will flow easily again and temperature will rise. Semiconductor ceramics are widely used as a thermistor for a heater or a thermistor for starting a motor because it converges to a constant temperature or current by repeating the above-described process.

By the way, since the PTC thermistor used for a heater use is used at high temperature, it needs to have a high Curie point Tc. For this reason, conventionally, a part of Ba in BaTiO ₃ has been replaced with Pb to raise the Curie point Tc.

  However, since Pb is an environmentally hazardous substance, the realization of a lead-free semiconductor ceramic that substantially does not contain Pb is required in consideration of the environment.

Therefore, for example, in Patent Document 1, in a structure of Ba _1-2X (BiNa) _x TiO ₃ (where 0 <x ≦ 0.15) in which a part of Ba of BaTiO ₃ is substituted with Bi—Na, Nb There has been proposed a method for producing a BaTiO ₃ -based semiconductor ceramic that is sintered in nitrogen after adding one or more of tantalum, Ta, and rare earth elements, and then heat-treated in an oxidizing atmosphere.

In Patent Document 1, a BaTiO ₃ semiconductor ceramic having a Curie point Tc as high as 140 to 255 ° C. and a resistance temperature coefficient of 16 to 20% / ° C. is obtained although it is lead-free.

Patent Document 2 discloses that the composition formula is [(Al _{0.5 A} 2 _0.5 ) _x (Ba _1-y Q _y ) _1-x ] TiO ₃ (where A1 is one or more of Na, K, and Li). , A2 represents Bi, Q represents one or more of La, Dy, Eu, and Gd), and the x and y satisfy 0 <x ≦ 0.2 and 0.002 ≦ y ≦ 0.01. Semiconductor porcelain compositions have been proposed.

  This Patent Document 2 also obtains a composition having a Curie point Tc of 130 ° C. or higher while being a lead-free semiconductor ceramic.

JP 56-169301 A JP 2005-255493 A

  However, in PTC thermistors containing alkali metal and Bi as in Patent Document 1 and Patent Document 2, since alkali metal and Bi have volatility, they volatilize during firing, which may cause a composition shift.

  In order to avoid such a composition shift, it is conceivable that the alkali metal and Bi are volatilized and the alkali metal and Bi are included in advance in excess of the design value and fired.

  However, since Bi and alkali metal are more easily volatilized at the outer peripheral portion than inside the component element body, the composition of the inner part of the component element body and the outer periphery is different, and there is a fear of lowering reliability.

  The present invention has been made in view of such circumstances, and provides a lead-free PTC thermistor with good reliability capable of suppressing deterioration with time of resistance value without deteriorating PTC characteristics even when energized for a long time. For the purpose.

  In order to achieve the above-mentioned object, the present inventors have conducted intensive research on lead-free semiconductor ceramics by varying the measured sintered density relative to the theoretical sintered density (hereinafter simply referred to as “sintered density”). However, by setting the sintered density of the semiconductor ceramic to 70 to 90%, it is possible to suppress the resistance value of the semiconductor ceramic after the reliability test from becoming higher than that before the reliability test. Thus, it was found that the reliability of the PTC thermistor can be improved without impairing the PTC characteristics even when a voltage is applied continuously for a long time.

The present invention has been made based on such knowledge, and a PTC in which a lead-free semiconductor ceramic that does not substantially contain lead is used as a component body, and external electrodes are formed at both ends of the component body. A thermistor, wherein the semiconductor ceramic is mainly composed of a BaTiO ₃ composition, a part of Ba is substituted with an alkali metal and Bi, and the sintered density of the semiconductor ceramic is 70 to 90%. It is characterized by.

  In the above description, “substantially free of lead” means that Pb is not intentionally added, and such a composition system in which Pb is not intentionally added is referred to as a non-lead type in the present invention.

  In the PTC thermistor of the present invention, it is preferable that a part of the Ba is replaced with a rare earth element in the semiconductor ceramic.

  In the PTC thermistor of the present invention, it is preferable that the alkali metal includes at least one of Na, K, and Li.

According to the PTC thermistor of the present invention, a PTC thermistor in which a lead-free semiconductor ceramic that does not substantially contain lead is used as a component body, and external electrodes are formed at both ends of the component body. The ceramic is mainly composed of a BaTiO ₃ -based composition, a part of Ba is substituted with alkali metals (Na, K, Li) and Bi, and the sintered density of the semiconductor ceramic is 70 to 90%. The resistance value after the reliability test before the reliability test can be prevented from increasing, and the reliability can be improved. That is, by reducing the sintered density of the semiconductor ceramic to 70 to 90%, the alkali metal is effectively volatilized also from the inside of the component body, so that unstable alkali metal remains at the crystal grain boundary. Can be suppressed. As a result, the resistance value of the semiconductor ceramic after the reliability test before the reliability test can be prevented from increasing, and even if a voltage is applied continuously for a long time, the resistance value is deteriorated over time without damaging the PTC characteristics. It is possible to obtain a PTC thermistor with good reliability that can suppress the above.

1 is a perspective view showing an embodiment (first embodiment) of a PTC thermistor according to the present invention. It is AA sectional drawing of FIG. It is sectional drawing of the PTC thermistor for demonstrating the subject of the conventional PTC thermistor. It is a figure which shows the relationship between the sintering density and resistance change rate in an Example.

  Next, an embodiment of the present invention will be described in detail.

  FIG. 1 is a perspective view schematically showing an embodiment of a PTC thermistor according to the present invention, and FIG. 2 is a cross-sectional view taken along line AA of FIG.

  The PTC thermistor includes a component body 1 and a pair of external electrodes 2 a and 2 b formed on both ends (surfaces) of the component body 1. The external electrodes 2a and 2b are formed of a single layer structure or a multilayer structure made of a conductive material such as Cu, Ni, Al, Cr, Ag, Ni—Cr alloy, Ni—Cu or the like.

  The component body 1 is mainly formed of a semiconductor ceramic represented by the general formula (A).

_{(Ba 1-uvw M u Bi} v Ln w) m TiO 3 ... (A)
Here, M is an alkali metal typified by Li, Na, and K, and Ln is a rare earth element. The rare earth element Ln is not particularly limited as long as it acts as a semiconducting agent. For example, at least one selected from the group of Y, Sm, Nd, Dy, and Gd is used. Can do.

As described above, the semiconductor ceramic is mainly composed of a BaTiO ₃ -based composition, and a part of Ba is substituted with alkali metals M and Bi and the rare earth element Ln.

  In this embodiment, the semiconductor ceramic is sintered so that the sintered density is 70 to 90%.

  That is, as described in the section [Problems to be Solved by the Invention], in the PTC thermistor containing alkali metals M and Bi, these alkali metals M and Bi are volatilized during firing, There is a risk of causing compositional deviation.

  In addition, the alkali metal M volatilizes preferentially from the outer peripheral portion of the component element body 1 rather than inside the component element body 1. Therefore, a concentration gradient of the alkali metal M is generated in the component element body 1, and the concentration of alkali metal ions is lower in the outer peripheral portion B shown in FIG. In this case, the alkali metal ion that is unstable and easily moved tends to move in a direction that cancels the concentration gradient. Therefore, when a voltage is applied to the external electrodes 2a and 2b in such a state, the unstable alkali metal ions are transferred from the central portion A to the outer peripheral portion B through the crystal grain boundaries so as to cancel the concentration gradient. Move in the direction. Then, after the energization is finished, the alkali metal ions remain at the crystal grain boundaries, and as a result, the grain boundary resistance increases to increase the resistance, and the resistance value deteriorates with time, leading to a decrease in reliability.

  Therefore, in the present embodiment, the raw material powder is adjusted to moderately reduce the sintered density so that the concentration of the alkali metal M in the component body 1 after firing is uniform or substantially uniform. The alkali metal M is also volatilized effectively from the central part A. As a result, the concentration of the alkali metal M can be made substantially equal between the central portion A and the outer peripheral portion B of the component body 1, and the alkali metal ions are about to move from the central portion A to the outer peripheral portion B. Suppressed. As a result, it is possible to suppress an increase in grain boundary resistance by staying at the grain boundary at the end of energization, and reliability that can suppress the deterioration of the resistance value with time without deteriorating the PTC characteristics even when energized for a long time. A good lead-free PTC thermistor can be obtained.

  In order to make the volatilization during firing substantially equal between the central part A and the outer peripheral part B of the component body 1 as described above, it is necessary to reduce the sintered density to 90% or less. That is, if the sintered density exceeds 90%, the sintered density is too high, so that the alkali metal M is preferentially vaporized from the outer peripheral portion B and the concentration gradient of the alkali metal M in the component body 1 as in the conventional case. And alkali metal ions easily move. On the other hand, when the sintered density is less than 70%, the sinterability is lowered, and there is a possibility that the crystal particles are not sufficiently joined to become a semiconductor.

  Therefore, it is necessary to adjust the sintered density to 70 to 90%.

  As described later, the sintered density can be easily adjusted by including in the ceramic raw material a resin material that disappears during the firing process, such as polymethyl methacrylate resin (PMMA) or polystyrene.

  The substituted molar amount u of the alkali metal M and the substituted molar amount v of Bi are not particularly limited, but the total molar amount (u + v) is preferably 0.02 to 0.20. This can raise the Curie point Tc by substituting a part of Ba with alkali metals M and Bi. However, when the total molar ratio (u + v) is less than 0.02, the Curie point Tc can be sufficiently raised. On the other hand, if the total molar ratio (u + v) exceeds 0.20, the volatilization amounts of the alkali metals M and Bi increase, and a composition deviation from the theoretical composition of the sintered body tends to occur.

  Further, the molar ratio w of the rare earth element Ln is not particularly limited, but 0.0005 to 0.015 is preferable in order to obtain a desired semiconductor.

  Further, the molar ratio m of the Ba site and Ti site is 1.000 in the stoichiometric composition, but is not limited to this, and is appropriately selected within the range of 0.992 to 1.004 as required. be able to.

  Next, a method for manufacturing the PTC thermistor will be described.

  First, a Ba compound, a Ti compound, an M compound containing an alkali metal M, a Bi compound, and an Ln compound containing a predetermined rare earth element Ln are prepared as ceramic raw materials. Then, these ceramic raw materials are weighed and mixed to obtain a mixed powder so that the component composition of the semiconductor ceramic becomes a predetermined ratio.

  Next, an organic solvent and a polymeric dispersant are added to the mixed powder, and the mixture is thoroughly mixed and pulverized in a ball mill together with a pulverizing medium such as PSZ (partially stabilized zirconia) balls, and then the organic solvent is dried. Then, sizing using a mesh with a predetermined opening. Then, it heat-processes in 800-1000 degreeC for 2 hours, and obtains calcining powder.

  To this calcined powder, an organic binder such as vinyl acetate or polyvinyl alcohol, resin beads such as PMMA and polystyrene, pure water, and additives such as Mn compound as necessary are added, and wet enough with the grinding media again. To obtain a ceramic slurry.

  Next, this ceramic slurry is pressed to obtain a molded body having a predetermined shape.

  Next, the molded body is heated at 500 to 600 ° C. in a predetermined atmosphere (for example, in an air atmosphere, a nitrogen atmosphere, or a mixed gas stream) to perform a binder removal treatment, and then in a predetermined atmosphere ( For example, in a nitrogen atmosphere or a mixed air stream in a reducing atmosphere), firing is performed for a predetermined time at a temperature at which the semiconductor is formed, for example, a maximum temperature of 1250 to 1450 ° C., to obtain a component body 1 that is a sintered body.

  Thereafter, external electrodes 2a and 2b are formed at both ends of the component body 1 by plating, sputtering, coating baking, or the like, thereby producing a PTC thermistor.

The present invention is not limited to the above embodiment. For example, in the above-described semiconductor ceramic, it is only necessary that BaTiO ₃ is a main component and a part of Ba is replaced with at least an alkali metal and Bi, and a part of Ba is replaced with Ca or Sr according to required PTC characteristics. It is also preferable.

  Moreover, if the said semiconductor ceramic has as a main component the composition represented by the said general formula (A), it may contain various additives as needed, for example, to increase a resistance temperature coefficient. It is also preferable to contain Mn which contributes.

  Further, even if inevitable impurities are mixed in the semiconductor ceramic, the characteristics are not affected. For example, there is a possibility that PSZ balls used as a grinding medium at the time of wet mixing and grinding may be mixed by about 0.2 to 0.3% by weight, but this does not affect the characteristics. Similarly, trace amounts of Fe, Si, and Cu of about 10 ppm by weight may be mixed in the ceramic raw material, but this does not affect the characteristics.

  Further, the semiconductor ceramic of the present invention is lead-free, but as described in the section of [Means for Solving the Problems], it should be substantially free of Pb and does not affect the characteristics. However, it does not exclude even Pb that is inevitably mixed in the range of 10 ppm by weight or less.

  Further, the external shape of the PTC thermistor is not limited, and any shape can be used.

  Next, examples of the present invention will be specifically described.

BaCO ₃ , Na ₂ CO ₃ , Bi ₂ O ₃ , TiO ₂ , and Y ₂ O ₃ are prepared as ceramic raw materials so that the composition after sintering becomes (Ba _0.898 Bi _0.05 Na _0.05 Y _0.002 ) TiO _3. The ceramic raw materials were weighed and mixed to obtain a mixed powder.

  Next, ethanol as an organic solvent and a polymer-type dispersant (maleic anhydride and ethylene oxide / propylene oxide derivative) are added to the mixed powder, and mixed and pulverized in a ball mill for 24 hours together with PSZ balls, Thereafter, ethanol was dried, and sized with a mesh having an opening of 300 μm. Then, it heat-processed for 2 hours at the temperature of 1000 degreeC, and calcined powder was obtained.

Next, vinyl acetate organic binder, Mn ₃ O ₄ , PMMA resin beads, and pure water are added to the calcined powder, and again mixed and pulverized with a PSZ ball in a ball mill for 16 hours to produce a ceramic slurry. did. Mn ₃ O ₄ was added so that the content was 0.05 mol%.

  Next, the ceramic slurry was dried and rectified using a mesh having an opening of 300 μm to obtain a ceramic raw material powder.

  And this ceramic raw material powder was shape-processed by the pressure of 98 MPa with the uniaxial press, and the disk-shaped molded object was obtained.

  The molded body was subjected to a binder removal treatment at a temperature of 600 ° C. for 2 hours in an air atmosphere and fired at a maximum temperature of 1300 ° C. for 2 hours in a nitrogen atmosphere having an oxygen concentration of 10,000 ppm by volume. Obtained.

  Next, this component body is lapped and then an external electrode having a three-layer structure of NiCr / NiCu / Ag is formed using a sputtering method, whereby a diameter of 12 mm, a thickness of 2.0 mm, and a high resistance layer are formed. Samples with sample numbers 1 to 5 having a thickness y of 0 to 500 μm were produced.

  Next, the sintered density was calculated from the bulk density for each sample of sample numbers 1-5.

Further, for each of the samples Nos. 1 to 5, the specific resistance ρ ₀ at the temperature of 25 ° C. (room temperature), the PTC digit number ΔR, and the resistance change rate Δρ / ρ ₀ were obtained.

Here, the specific resistance ρ ₀ was measured by applying a voltage of 1 V at a temperature of 25 ° C. using a DC four-terminal method.

  The PTC digit number ΔR indicates the capability of the PTC thermistor, and is defined by Expression (1).

ΔR = log (ρmax / ρmin) (1)
Here, ρmax is a maximum value of specific resistance, and ρmin is defined by a minimum value of specific resistance. And the characteristic (henceforth "(rho) -T characteristic") of temperature T and specific resistance (rho) is measured, and PTC digit number (DELTA) R was calculated | required from the maximum value and the minimum value.

The resistance change rate Δρ / ρ ₀ was obtained by conducting an energization test. That is, a DC voltage of 20 V was applied to each sample and left for 1000 hours. Then, the resistance change rate ρ ₀ before the test and the resistance change rate ρ ₁ after the test are measured at a temperature of 25 ° C., the difference Δρ (= ρ ₁ −ρ ₀ ) is obtained, and the resistance change rate Δρ / ρ ₀ is obtained. Calculated.

Table 1 shows the sintered density, the specific resistance ρ ₀ at 25 ° C. (room temperature), the PTC digit number ΔR, and the resistance change rate Δρ / ρ ₀ for each sample of sample numbers 1 to 5.

  Sample No. 1 had a sintered density as low as 60% and could not be sufficiently sintered, and the sample could not be made into a semiconductor.

On the other hand, Sample No. 5 had a high sintering density of 93%, and therefore the resistance change rate Δρ / ρ ₀ was as large as 67%. This seems to be because the amount of Na moving from the inside to the surface side in the energization test increased.

On the other hand, Sample Nos. 2 to 4 have a sintered density of 70 to 90%, which is within the range of the present invention. Therefore, the resistance change rate Δρ / ρ ₀ is smaller than that of Sample No. 5 and the PTC characteristics are improved. I understood.

FIG. 4 is a diagram showing the relationship between the sintered density and the resistance change rate, where the horizontal axis shows the sintered density (%) and the vertical axis shows the resistance change rate Δρ / ρ ₀ (%).

As is apparent from FIG. 4, when the sintered density exceeds 90%, the resistance change rate Δρ / ρ ₀ increases, whereas when the sintered density is within the range of 70 to 90%, the resistance is increased. It has been found that the rate of change Δρ / ρ ₀ can be less than 50%, deterioration of the resistance value with time can be suppressed, and desired good reliability can be obtained.

  A lead-free PTC thermistor with good reliability that can suppress the deterioration of the resistance value with time without impairing the PTC characteristics even when energized for a long time can be obtained, and is particularly useful for high-temperature applications such as in-vehicle heaters.

1 Component body 2a, 2b External electrode

Claims (3)

A positive temperature coefficient thermistor in which lead-free semiconductor ceramic that does not substantially contain lead is used as a component body, and external electrodes are formed at both ends of the component body,
The semiconductor ceramic is mainly composed of a BaTiO ₃ composition, and a part of Ba is substituted with an alkali metal and Bi.
A positive temperature coefficient thermistor characterized in that a measured sintered density with respect to a theoretical sintered density of the semiconductor ceramic is 70 to 90%.   The positive temperature coefficient thermistor according to claim 1, wherein a part of the Ba is substituted with a rare earth element.   The positive temperature coefficient thermistor according to claim 1, wherein the alkali metal includes at least one of Na, K, and Li.

Images