Since 2011, the refrigerant used in automobile HVAC systems (heating, ventilation, air conditioning) has been carbon dioxide instead of Freon gas. When the refrigerants were changed to carbon dioxide, it was discovered that the concentration of carbon dioxide inside the vehicle rapidly increased. To prevent drowsy driving due to an increased concentration of carbon dioxide in vehicles, an inside air quality monitoring system is therefore required. The need for a carbon dioxide sensor for these automatic ventilation systems will continue to grow, and at the same time, the enhancement of mobile air-conditioning systems demands low cost and high stability carbon dioxide sensors.
Traditionally, Non-Dispersive Infrared (NDIR) type sensors were widely used for measuring the concentra-tion of carbon dioxide. NDIR type sensors have high levels of selectivity, stability and durability, but they require a closed space for measurement, which requires a device with a specific volume and size. Also, such detecting devices are relatively expensive, and cannot make real time measurements. They are especially vulnerable and easy to contaminate in a hostile environment. Therefore, tremendous effort has been made to develop a low-cost, small sized and reliable sensor alternative. The solid state electrochemical sensor was developed to solve these drawbacks. However, commercial electrochemical sensors are still using a reference electrode material which requires a long initialization time.
In this thesis study, a highly sensitive solid state electrochemical carbon dioxide sensor was developed whose reference electrode was changed to YSZ/Pt from the traditional reference materials. Firstly, the carbon dioxide sensor was designed in a new form using a heterojunction of solid electrolytes, and it was prepared with sensing materials such as the Na-beta-Alumina sodium ion conductor and the YSZ oxygen ion conduc-tor. Then, the designed carbon dioxide sensor was fabricated by tape casting method for mass production.
The tape casting method is one of the most effective techniques for fabricating small sized ceramic devices. In this method a large number of samples can be prepared at a time and the reproducibility of the samples is better than with conventional pressing and sintering techniques. An attempt was made in this investigation to fabricate a miniature CO2 sensor by the tape casting technique for improved sensor application.
The responses of the developed sensors to various concentrations of CO2 were tested in a gas atmosphere at temperatures ranging from 550oC to 650oC. The EMF of the sensors exhibited Nernstian behavior with sensi-tivity greater than 99% of the theoretical value at 650oC. The activity of Na2O at the bi-electrolyte interface remained almost constant at the CO2 partial pressures tested. Lastly, the solid state electrochemical sensor and a resistivity sensor were extensively used in gas sensor applications.
These sensors offer advantages in the form of simple construction, durability to severe conditions and operating capability. Distinctively, the devices are sensitive to specific gases at elevated temperatures, between 300°C - 350°C for the resistivity sensor and 400°C - 500°C for the solid electrolyte gas sensor. Because of the high temperature, however, packaging was a separate challenge to be addressed.
One well-known structure for mounting a sensor is the air-floating structure. For example, hanging structures like micro bridges and cantilever structures have been used to reduce heat losses through substrates. Nevertheless, air-floating structures which use wire have many basic problems, such as reliability, inconvenience in processing and a resulting bulky device. An air-floating structure requires space for connecting a wire around the sensor. Also, due to the thinness of the wire, the control process is difficult. Moreover; repetitive operation in high temperatures causes thermal fatigue which breaks the wire down easily. Not only this, but situations where the product is actually used typically has vibration, so an air-floating structure has difficultly standing.
In order to solve these problems, the sensor should be directly attached to the substrate without using wire. This requires a new model with stable processing that can be directly applied to a printed circuit board (PCB) base. However, when a sensor is strongly attached to a substrate, heat spreads over the entire substrate, creat-ing high power consuming and problem in PCB by heat transfer. On such a background, heat should be dis-tributed asymmetrically by isolating the heat near the sensor and cooling the substrate.
This thesis describes an optimized substrate that was designed by analyzing numerical thermal distributions while changing the substrate geometry. To monitor the substrate temperature, we used the COMSOL Multi-physics simulation program. Two major factors for the substrate design were proposed. The first was the shape of the contacting hole between the sensor and the substrate, and the second was the number of holes in the substrate. In this simulation, we evaluated the change in consumed power depending on the number of holes and contact shape with the substrate. Based on that research, a substrate was fabricated from LTCC by the tape casting method. Then the substrate was experimentally tested in combination with the carbon dioxide sensors fabricated by tape casting in the previous experiment.
자동차 업체의 평가 결과, 2011년 이후 자동차의 HVAC (heating, ventilation, air conditioning) 시스템에 대해 기존의 프레온 가스 대신 이산화탄소 냉매로 변경 됨에 따라 차량 내부 이산화탄소 농도의 증가가 확인되었다. 이에 따라 자동차 내의 이산화탄소 농도의 증가로 인한 졸음운전을 방지하기 위해서 차내 공기 질 관리 시스템이 주목 받고 있으며, 이산화탄소 농도를 확인 할 수 있는 저가의 이산화탄소 가스센서 시스템이 필요하게 되었다. 하지만 대표적으로 사용되고 있는 NDIR 방식의 이산화탄소 센서의 경우 부피가 크고 가격이 비싸며 실시간 측정이 불가능하다는 단점이 있다. 따라서 이러한 단점들을 탈피한 전기화학적 방식을 이용한 센서 구현이 필요하다.
본 연구에서는 나트륨이온전도체 Na-Beta-Alumina (NBA)와 산소이온전도체 Yttria Stabilized Zirconia (YSZ)의 이종접합을 이용한 새로운 형태의 전기화학식 이산화탄소 센서를 개발하였다. 기존의 전기화학식 센서의 경우 기준 전극 물질을 사용함으로써 긴 초기 구동시간 문제를 발생시켰지만 이 논문에서 개발되는 전기화학식 이산화탄소 센서는 고체전해질인 YSZ/Pt를 기준전극으로 변경하여 이산화탄소 센서를 제작함으로써 이를 해결했다.
논문은 먼저 NBA의 나트륨 함량과 YSZ의 Yttria 조성을 변화해 가면서 분말의 압축성형을 통해서 센서를 제작하고 특성을 평가하였다. 그리고 Tape casting 기술을 이용하여 원가를 절감하며 대량 생산이 가능한 방식으로 제작하여 테스트를 진행했다. 측정범위는 이산화탄소 농도를 1000ppm에서 5000ppm까지 확인해보았고 650도에서 이론적 감도와의 오차가 1%로 정확하게 나왔다. 또한 Tape casting 공법에서 문제가 되던 이종간의 계면 문제를 NBA의 조성변화를 통해 계면을 강화시켜서 해결하였다. 또한, 센서의 패키지의 경우 기존의 금속선에 의해서 공중에 고정되어 있던 방법에서 표면에 접착이 가능한 소형 패키지를 구현함으로써 진동이 심하거나 충격에 위험이 있는 차량용 실내 환기시스템에 어울리는 기판을 개발하였다. 이를 통해서 진동이나 충격에 강화된 특성을 바탕으로 차량용 환기시스템에 활용 될 가능성을 보았다.
현재 Tape casting 기법을 이용한 센서의 개발은 양산화 된지 얼마 되지 않았고, 더욱이 이종접합을 이용한 센서의 경우로는 첫 시도이다. Tape casting 기법을 이용한 이종접합 센서 기술이 개발이 된다면 이것이 시발점이 되어 다양한 전해질을 이용한 센서가 개발 될 것으로 예상된다. 또한 회로에 직접 연결이 가능한 소형 패키지(SMD) 구현으로 인해 센서의 다양한 활용이 가능하리라 판단된다. 예로서 차량용 대기모니터링을 벗어나 산업단지 및 대도시의 환경을 측정하고, 가정 및 회사 등 실내에서 발생할 수 있는 이산화탄소 가스에 대한 모니터링도 가능할 것으로 예상된다.
