Preparation method and application of Yb3+-doped high temperature thermistor materials

Abstract

A thermistor material composed of Ca.sub.1-xYb.sub.xCeNbWO.sub.8(0≤x≤0.2) can be used in a wide temperature range from 25 to 800° C. It is made from high-pure CaCO.sub.3, CeO.sub.2, NbO.sub.5, WO.sub.3 and Yb.sub.2O.sub.3. These ceramic materials with a scheelite structure can be obtained after mixing, grinding, calcination, pressing, cold isostatic pressing and high-temperature sintering, etc. The values of material constant B.sub.300° C./600° C. and ρ.sub.25° C. of thermistor materials are in the range of 6465K-6732K, 4.06×10.sup.7Ω.cm-8.63×10.sup.7Ω.cm. The thermistor material has a good thermostability and significant negative temperature coefficient (NTC) characteristic in the temperature range of 25° C. to 800° C., could be used as a potential for fabricating high-temperature thermistor sensors.

Claims

1. A Yb.sup.3+-doped high temperature thermistor ceramic material is a composite oxide that comprises Ca, Yb, Ce, W and Nb; wherein the thermistor material is prepared by CaCO.sub.3, CeO.sub.2, Nb.sub.2O.sub.5, WO.sub.3 and Yb.sub.2O.sub.3.

2. The Yb.sup.3+-doped high temperature thermistor material according to claim 1, wherein the thermistor material has a scheelite structure having chemical formula shown as Ca.sub.1-xYb.sub.xCeNbWO.sub.8, wherein 0<x≤0.2.

3. The Yb.sup.3+-doped high temperature thermistor material according to claim 1, wherein the thermistor material shows a significant negative temperature coefficient (NTC) characteristic in the temperature range of 25° C. to 800° C.

4. A process for preparing the Yb.sup.3+-doped high temperature thermistor material according to claim 2 comprises the following steps: a. weigh, mix and grind CaCO.sub.3, CeO.sub.2, Nb.sub.2O.sub.5, WO.sub.3 and Yb.sub.2O.sub.3 based on the chemical formula, Ca.sub.1-xYb.sub.xCeNbWO.sub.8; obtain a mixing powder; b. calcine the mixing powder, and further grind to obtain Ca.sub.1-xYb.sub.xCeNbWO.sub.8 powder; c. compress the Ca.sub.1-xYb.sub.xCeNbWO.sub.8 powder into a disk; d. cold isostatic press the disk, sinter the disk at high temperature to obtain a high-temperature thermistor ceramic after cooling to room temperature; e. coat the high-temperature thermistor ceramic with platinum paste electrode on both sides, and then anneal to obtain NTC thermistor ceramics after cooling to room temperature.

5. The process according to claim 4, wherein in step b, calcine the mixing powder at 1000 to 1200° C. for 2 to 6 hours, and then grind for 6 to 10 hours to obtain the Ca.sub.1-xYb.sub.xCeNbWO.sub.8 powder.

6. The process according to claim 4, wherein in step c, compress the Ca.sub.1-xYb.sub.xCeNbWO.sub.8powder at a pressure of 5-10 Kg/cm.sup.2 for 0.2 to 0.5 minutes to obtain the disk.

7. The process according to claim 4, wherein in step d, cold isostatic press the disk at 200 to 300 MPa for 1 to 3 minutes, sinter the disk at 1200 to 1400° C. for 2 to 6 hours to obtain the high-temperature thermistor ceramic.

8. The process according to claim 4, wherein in step e, coat the high-temperature thermistor ceramic with a thin layer non-fluxed Pt paste at 800 to 900° C. for 30 to 60 minutes, thus obtaining the NTC thermistor ceramic.

9. A method for manufacturing high-temperature thermistors with the thermistor material comprising a step of mixing the thermistor material with essential materials of the high-temperature thermistors.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0027] FIG. 1 is the XRD patterns of the ceramic materials.

[0028] FIG. 2 is the relationship between Inp and I/T for the NTC thermistors.

DETAILED DESCRIPTION OF INVENTION

[0029] The high-temperature thermistors according to the invention are prepared as follow.

[0030] a. The Ca.sub.1-xYb.sub.xCeNbWO.sub.8(0≤x≤0.2) polycrystalline powders are prepared by conventional solid-state reactions.

[0031] b. Appropriate amounts of high-purity Yb.sub.2O.sub.3(99.99%), CaCO.sub.3(99%), CeO.sub.2(99.99%), Nb.sub.2O.sub.5(99.99%), and WO.sub.3(99.99%) are well mixed using an agate mortar for 6 to 8 hours to obtain mixed powder.

[0032] c. The mixed powders obtained in the step b are calcined at 1000° C. to 1200° C. for 2 to 6 hours and then ground 6 to 10 hours to obtain Ca.sub.1-xYb.sub.xCeNbWO.sub.8 powder.

[0033] d. The calcined powders obtained in the step c are pressed into disks at a pressure of 5-10 Kg/cm.sup.2for 0.2 to 0.5 minutes. Cold isostatic pressing at 200 to 300 MPa for 1 to 3 minutes is used to enhance their green densities. The sintering is carried out using a conventional method at 1200 to 1400° C. for 2 to 6 hours to obtain thermistor ceramics.

[0034] e. For the characterization of electrical properties, the sintered pellets obtained in the step d are polished, coated with a thin layer of 0.1 mm thick non-fluxed Pt paste, and heated at 800 to 900° C. for 30 to 60 minutes. Then the thermistor ceramics composed of Yb.sub.2O.sub.3 doped Ca.sub.1-xYb.sub.xCeNbWO.sub.8 can be obtained. The temperature range of these thermistor materials is 25-800° C., the B.sub.300° C./600° C. constant is in the range of 6465K-6732K. The resistivity at 25° C. is in the range of 4.06×10.sup.7 Ω.cm-8.63×10.sup.7 Ω.cm.

Example 1

[0035] According to the composition of Ca.sub.0.95Yb.sub.0.05CeNbWO.sub.8, the raw materials of CaCO.sub.3, CeO.sub.2, Nb.sub.2O.sub.5, WO.sub.3 and Yb.sub.2O.sub.3 are respectively weighted and put into an agate mortar to mix and grind for 6 hours.

[0036] The mixed powders obtained in the step a are calcined at 1200° C. for 2 hours and then ground 6 hours to obtain Ca.sub.0.95Yb.sub.0.05CeNbWO.sub.8 powder.

[0037] The calcined powders obtained in the step b are pressed into disks at a pressure of 10 Kg/cm.sup.2 for 0.2 minutes.

[0038] The disks obtained in the step c are enhanced by cold isostatic pressing at 200 MPa for 3 minutes. The sintering is carried out using a conventional method at 1400° C. for 2 hours to obtain thermistor ceramics.

[0039] For the characterization of electrical properties, the sintered pellets obtained in the step d are polished, coated with a thin layer of 0.1 mm thick non-fluxed Pt paste, and heated at 900° C. for 30 min. Then the thermistor composed of Yb.sub.2O.sub.3 doped Ca.sub.0.95Yb.sub.0.05CeNbWO.sub.8 can be obtained. The material constant is B.sub.300/600° C. =6465 K, and the resistivity at 25° C. is 4.06×10.sup.7 Ω.cm.

Example 2

[0040] According to the composition of Ca.sub.0.9Yb.sub.0.1CeNbWO.sub.8, the raw materials of CaCO.sub.3, CeO.sub.2, Nb.sub.2O.sub.5, WO.sub.3 and Yb.sub.2O.sub.3 are respectively weighted and put into an agate mortar to mix and grind for 8 hours.

[0041] The mixed powders obtained in the step a are calcined at 1100° C. for 4 hours and then ground 4 hours to obtain Ca.sub.0.9Yb.sub.0.1CeNbWO.sub.8 powder.

[0042] The calcined powders obtained in the step b are pressed into disks at a pressure of 5 Kg/cm.sup.2 for 0.5 minutes.

[0043] The disks obtained in the step c are enhanced by cold isostatic pressing at 250 MPa for 2 minutes. The sintering is carried out using a conventional method at 1300° C. for 4 hours to obtain thermistor ceramics.

[0044] For the characterization of electrical properties, the sintered pellets obtained in the step d are polished, coated with a thin layer of 0.1 mm thick non-fluxed Pt paste, and heated at 900° C. for 30 min. Then the thermistor composed of Yb.sub.2O.sub.3 doped Ca.sub.0.9Yb.sub.0.1CeNbWO.sub.8 can be obtained. The material constant is B.sub.300/600° C. =6470 K, and the resistivity at 25° C. is 4.39×10.sup.7 Ω.cm.

Example 3

[0045] According to the composition of Ca.sub.0.85Yb.sub.0.15CeNbWO.sub.8, the raw materials of CaCO.sub.3, CeO.sub.2, Nb.sub.2O.sub.5, WO.sub.3 and Yb.sub.2O.sub.3 are respectively weighted and put into an agate mortar to mix and grind for 8 hours.

[0046] The mixed powders obtained in the step a are calcined at 1000° C. for 6 hours and then ground 10 hours to obtain Ca.sub.0.85Yb.sub.0.15CeNbWO.sub.8 powder.

[0047] The calcined powders obtained in the step b are pressed into disks at a pressure of 8 Kg/cm.sup.2 for 0.3 minutes.

[0048] The disks obtained in the step c are enhanced by cold isostatic pressing at 200 MPa for 3 minutes. The sintering is carried out using a conventional method at 1200° C. for 6 hours to obtain thermistor ceramics.

[0049] For the characterization of electrical properties, the sintered pellets obtained in the step d are polished, coated with a thin layer of 0.1 mm thick non-fluxed Pt paste, and heated at 900° C. for 30 min. Then the thermistor composed of Yb.sub.2O.sub.3 doped Ca.sub.0.85Yb.sub.0.15CeNbWO.sub.8 can be obtained. The material constant is B.sub.300/600° C. =6580 K, and the resistivity at 25° C. is 6.33×10.sup.7 Ω.cm.

Example 4

[0050] According to the composition of Ca.sub.0.8Yb.sub.0.2CeNbWO.sub.8, the raw materials of CaCO.sub.3, CeO.sub.2, Nb.sub.2O.sub.5, WO.sub.3 and Yb.sub.2O.sub.3 are respectively weighted and put into an agate mortar to mix and grind for 8 hours.

[0051] The mixed powders obtained in the step a are calcined at 1100° C. for 3 hours and then ground 8 hours to obtain Ca.sub.0.8Yb.sub.0.2CeNbWO.sub.8 powder.

[0052] The calcined powders obtained in the step b are pressed into disks at a pressure of 10 Kg/cm.sup.2 for 0.5 minutes.

[0053] The disks obtained in the step c are enhanced by cold isostatic pressing at 300 MPa for 3 minutes. The sintering is carried out using a conventional method at 1350° C. for 4 hours to obtain thermistor ceramics.

[0054] For the characterization of electrical properties, the sintered pellets obtained in the step d are polished, coated with a thin layer of 0.1 mm thick non-fluxed Pt paste, and heated at 900° C. for 30 min. Then the thermistor composed of Yb.sub.2O.sub.3 doped Ca.sub.0.8Yb.sub.0.2CeNbWO.sub.8 can be obtained. The material constant is B.sub.300/600° C. =6732 K, and the resistivity at 25° C. is 8.63×10.sup.7 Ω.cm.

Contrasting Example 1

[0055] The Contrasting example 1 and example 4 have the same preparation method, the difference is as follows: x=0, the material constant is B.sub.300/600 C. =6707 K, and the resistivity at 25° C. is 4.28×10.sup.7 Ω.cm.

[0056] Drawing Illustration

[0057] XRD patterns of the ceramic materials are shown in FIG. 1. It can be seen that the structure of as-sintered ceramics is single scheelite structure, and no secondary phase.

[0058] Using the method of example 4, the relationship between Inp and I/T for the NTC thermistors is shown in FIG. 2. The thermistor material according to the invention has a good thermostability and significant negative temperature coefficient (NTC) characteristic in the temperature range of 25° C. to 800° C., could be used as a potential for fabricating high-temperature thermistor sensors.