Sol-Gel PTCR Resistor Composition for High-Temperature Self-Limiting Heaters
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Solution Overview
Problem
Existing PTCR materials, such as BaTiO3 and bismuth alkaline titanates, have limitations including low Curie temperatures, high room temperature resistivity, and instability due to cation vacancies, making them unsuitable for high-temperature applications. Lead-doped materials are toxic, and alternative lead-free solutions exhibit undesirable TCR behavior and poor resistor stability.
Innovation Solution
A sol-gel paste composition incorporating doped semiconductor particles with perovskite or tungsten-bronze crystal structures, combined with conductive nanowires or nanorods, is sintered in a reducing environment to create a stable, high Curie temperature PTCR resistor with self-limiting behavior, using materials like BaNb2O6 and Sr2KNb5O15, and decorated with metallic particles to reduce contact resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If BaTiO3 is doped with lead to increase Curie temperature, then the Curie temperature is substantially increased, but lead is highly toxic and prohibited in many jurisdictions
Solution Approach 1:
The patent changes the chemical composition parameters by replacing lead with non-toxic dopants such as bismuth, sodium, potassium, and rare earth elements. This substitution maintains the ability to achieve high Curie temperatures (above 260°C) while eliminating the toxic harmful factor associated with lead doping.
Solution Approach 2:
The patent employs composite material systems combining barium titanate with bismuth alkaline titanates and various dopants. This composite approach enables achieving the desired high Curie temperature and electrical properties without relying on toxic lead, thus resolving the contradiction between temperature enhancement and toxicity elimination.
2Object-affected harmful factors
If bismuth alkaline titanates are used as lead-free PTCR materials, then toxicity is eliminated, but the Curie temperature is typically less than 260°C which limits application to temperatures below ca 250°C
Solution Approach 1:
The patent modifies the compositional parameters of bismuth alkaline titanates by incorporating specific dopants and adjusting the ratio of barium titanate to bismuth alkaline titanate. This parameter optimization enables achieving Curie temperatures above 260°C while maintaining the non-toxic characteristic of lead-free materials.
3Object-affected harmful factors
If bismuth alkaline titanates are used, then lead-free composition is achieved, but room temperature resistivity is high and TCR behavior is unacceptable prior to Curie temperature
Solution Approach 1:
The patent adjusts critical parameters including dopant concentration, sintering temperature, and atmospheric conditions to optimize the electrical properties. These parameter changes reduce room temperature resistivity and improve TCR behavior in the sub-Curie temperature range while maintaining lead-free composition and high Curie temperature.
4Reliability
If bismuth alkaline titanates are sintered at high temperature, then PTCR properties are developed, but evaporative loss of Bi and other elements creates cation vacancies causing poor resistor stability
Solution Approach 1:
The patent employs an inert or controlled atmosphere during sintering to prevent evaporative loss of bismuth and other volatile elements. This environmental control minimizes cation vacancy formation and maintains compositional stability, thereby achieving both PTCR properties and long-term resistor stability.
Solution Approach 2:
The patent incorporates excess dopant elements or compensating additives in the initial composition to compensate for expected evaporative losses during sintering. This beforehand cushioning ensures that sufficient dopant remains after sintering to maintain electrical stability and prevent degradation from cation vacancies.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution provides a stable, lead-free PTCR resistor with consistent power draw and inherent over-temperature protection, suitable for high-temperature applications, maintaining resistance to humidity and oxidation, and enabling screen-printable thick film heaters with self-limiting behavior.
Implementation Method 1
A sol-gel paste composition incorporating doped semiconductor particles with perovskite or tungsten-bronze crystal structures, combined with conductive nanowires or nanorods, is sintered in a reducing environment to create a stable, high Curie temperature PTCR resistor
Implementation Method 2
The present disclose pertains to sol-gel paste compositions of matter that when cured or fired exhibit a positive temperature coefficient of resistivity (PTCR)
Implementation Method 3
decorated with metallic particles to reduce contact resistance
Implementation Method 4
The expressions 'resistive' and 'conductive' in association with coatings are herein used to refer to coatings which will pass electrical currents
Data Source
Figure 1a~1b
Figure 2~3
Figure 4~5
AI summary
The present disclose provides a sol-gel paste composition of matter that is provided for application to a substrate to form an electrically conductive coating which can be used, for example, as a resistor in a thick-film resistive heater, which a positive temperature coefficient of resistivity (PTCR) as a consequence of the electronic properties of the electrically conductive semiconductors used in the resistor layer and which exhibits self-limiting behavior at elevated temperatures. The composition includes a sol gel solution in which up to 90% by weight (wt.%) of the solution is comprised of conductive and insulative powders in a uniform stable solution.