세라믹가스센서 참고문헌야나기다히로아키, 세라믹센서, 전파과학사 1992 전자세라믹스, 이즈미한경 1994 http://www.ngk.de/en/products-technologies/lambda-sensors/lambda-sensor-technologies/zirconium-dioxide-lambda-sensor/ http://www.ngkntk.co.jp/product/sensors/nox/index.html
가스센서 다공질세라믹스 ; 개기공 (open pore) 을통하여수증기나가스를세라믹스내에확산시키고결정표면에흡착시킨다 반도체세라믹스는표면층과의관계가크고, 온도나흡착에의하여그전기전도도가크게변화한다 검토되는조건 ; 신뢰성, 양산성, 가격, 형상및구조설계
가스센서의구조
가스흡착 : 물리흡착과화학흡착 물리흡착 : 저온도영역에서일어나며 5-10 kcal/mol 화학흡착 : 보다높은온도에서일어나며높은활성화에너지 (10-100 kal/mol). 고온에서현저하며화학반응으로고체표면에서전자이동이일어난다. 활성화에너지를넘을정도의충분히높은온도에서는가역적인화학흡착이있다. 흡착양은온도상승과함께감소.
가스센싱메카니즘 CO + O 2- CO 2 + 2e - ½ O 2 + 2e - O 2- 야나기다히로아키, 세라믹센서, p.68
SnO 2 gas sensor SnO 2 표면에흡착되어있던산소는감소되어서포텐셜장벽이낮아지므로전자가움직이기쉬워진다. 즉전기저항이저하. 대기중에포함된가스농도를전기저항변화로검출가능.
습도센서 MgO-Cr2O3-TiO2 계세라믹스, 기공률 25% 의 p 형반도체 200C 이하의온도에서수증기에의한흡탈착으로전기저항변화 소량의수증기로표면에수산기의형성, Cr 의 3 가가 4 가로산화되어생성된 H+( 프로톤 ) 의호핑에의해전기전도도변화 사용된 RuO2 는다공질전극으로가스분자흡착에자유롭다. ZrO2-MgO 계세라믹스, 수증기화학흡착전자전도의 n 형반도체 -10-700C 의광범위한온도에서화학흡착형으로작동. 활성화가스중수증기에의해큰전기저항의변화가있었다.
습도센서응용
SnO2 가스센서 300-400C 의작동온도의 n 형반도성가연성가스센서. Pt, Pd 의첨가강력한산화제로탄화수소의 C-H 결합의해리를촉진한다.
촉매를통한가스센서활성화 야나기다히로아키, 세라믹센서
Fe 2 O 3 계가스센서 Fe2O3 는가연성가스와반응하여, Fe3O4 로환원된다. Fe3O4 의전기저항은낮기때문에센서소자의전기저항이내려간다. 가연성가스가없어지면다시 Fe2O3 로복귀한다. Fe/ FeO / Fe3O4 / Fe2O3 LPG 가스성분을센싱 촉매없이사용가능
센서구조와물성
이온센서 선택적전극 ; 특정이온의농도에따라전위가선택적으로변한다. 세라믹전극사용. 이온선택성전극은용액중의이온농도를측정하는데 Nernst 의식을사용하여전위측정으로이온농도를구한다.
이온전도체를이용한 CO 감지센서 Pt 촉매상에서공기중의수증기와 CO 는반응하여서 CO + H 2 O CO 2 + 2H + + 2e - (1) proton H + 는이온전도체를통과하고생성된전자 e - 는전선을흘러서반대편극에서공기중의산소와반응하여서다음반응진행 (1/2)O 2 + 2H + + 2e - H 2 O (2) 1, 2 식을합하여표기하면 O + (1/2)O 2 CO 2 (3)
결함표기
Defect Chemistry
ZrO2
ZrO 2 센서 CaO 등을첨가한 ZrO 2 는고온에서산소이온이움직이는고체전해질형산소센서, ZrO 2 내에서산소이온만이쉽게이동. 작동온도가 400-800C 로고온이므로전극이나패키지등에특별한배려가있어야한다. 산소이온의양전극사이를이동하면서기전력이발생된다. 자동차용센서, 제철산업센서용으로응용.
ZrO 2 센서
ZrO 2 센서메카니즘
자동차용 ZrO 2 센서
Lambda sensor http://ngkntk.co.uk/index.php/technical-centre/lambda-sensors/how-does-the-lambda-sensorwork/ The ideal ratio of air and fuel to achieve this is around 14.7:1; this means that for 14.7 kg of air 1 kg of fuel would be used. This chemically correct air fuel ratio is known as a stoichiometric ratio or Lambda (λ)1.0. A fuel rich mixture would have a lower value e.g. 0.8 and a fuel lean mixture would have a higher value e.g. 1.2 The voltage produced in the fuel lean position should be approximately 0.1 volt and in the fuel rich position approximately 0.9 volt. The very useful part of this function is that at around the stoichiometric point there is a relatively large change in voltage. This allows the sensor to keep the engine emissions within strict limits by constantly bringing the fuelling system back from a fuel lean or fuel rich position to retain the stoichiometric mixture. The time taken to switch from fuel lean to fuel rich is approximately 300 milliseconds.
자동차용 ZrO 2 센서 http://www.ngk.de/en/products-technologies/lambda-sensors/lambda-sensor-technologies/zirconium-dioxide-lambda-sensor/
Lambda sensor The most popular method used by vehicle manufacturers to reduce engine emissions is the threeway catalyst (catalytic converter). This device has the ability to take the three main toxic gases produced by an engine which are carbon monoxide (CO), oxides of nitrogen (NOx) and hydrocarbons (HC) and convert them to considerably less harmful, non-poisonous gases: carbon dioxide (CO2), water (H2O) and nitrogen (N2). To carry out this conversion of gases efficiently the catalyst must operate within a specified temperature range but also be provided with exhaust gases that are within certain very tight tolerances dependant predominantly on air/fuel ratio. The precise control required to operate this system is provided by the use of an exhaust gas oxygen (Lambda) sensor installed upstream of the catalyst. A Lambda sensor has the ability to precisely measure the air/fuel ratio present in exhaust gases. By sending a signal to the control unit it can initiate a change to keep the fuelling system operating within the very tight tolerances required. This is known as a closed-loop control system. To further improve control of exhaust emissions most vehicles produced after 2000 have an additional Lambda sensor fitted down-stream of the catalyst that monitors the performance of the catalyst itself.
厳しい排気ガス規制をクリアするためには 触媒の排気ガス浄化率をできるだけ高く維持する必要があります 排気ガス浄化率は右図に示したように 理論空燃比付近でもっとも高くなり エンジン燃焼を理論空燃比周辺 ( 右図ウィンドウ内 ) で制御すれば クリーンなガスが排出されます
上図はリッチ ( 燃料過多 酸素濃度低 ) 燃焼後の排気ガスにさらされた際の酸素センサの挙動となります ジルコニアを高温に保ち イオン伝導性を持たせることによって 酸素濃度の高い大気側から排気ガス側への酸素イオン流が発生します 酸素イオンは負の電荷となりますので ジルコニアの両端電極間に起電力が発生していることになります 反面 リーン ( 燃料希薄 酸素濃度高 ) 燃焼後の排気ガスの場合 両端電極間での酸素濃度勾配が小さいため 起電力は発生しません ( 右図 ) http://www.ngkntk.co.jp/english/product/sensors/zirconia/index.html
一方 前出の様にジルコニア酸素センサは排気ガス中の酸素濃度に応じて 0 1V の出力を発生します 酸素センサの信号に基づき燃料噴射コントロールを行うことで 理論空燃比付近での燃焼を可能にし クリーンな排気ガスを実現することができます 触媒下流にもセンサを取り付け 触媒劣化検知などの自己診断機能をサポートするためにも用いられます 一般的には 昨今の車両にはバンク毎に上下流各 1 本ずつのセンサが用いられています つまり V 型エンジンには 4 本 / 車両の酸素センサが装着されています
Ammonia Arsine Carbon Dioxide Carbon Monoxide Chlorine Chlorine dioxide Combustible gases Diborane Ethylene Oxide Fluorine Hydrazine Hydrides Hydrogen Hydrogen Chloride Hydrogen Cyanide Hydrogen Fluoride Hydrogen Selenide Hydrogen Sulphide Mercaptan Methane Nitric Oxide Nitrogen Dioxide Oxygen Ozone Phosgene Phosphine Silane Sulphur Dioxide Tetrahydrothiophene Gas Sensing
In-vehicle high-precision NOx sensor (Smart NOx-Sensor)
NOx sensor vs. CLD( 化学発光式 )NOx 分析計
http://www.ngkntk.co.jp/product/sensors/nox/index.html
Urea Selective Catalytic Reduction 尿素 SCR 시스템은배기가스정화기술의하나로질화산화물 (NO x ) 를정화하는기술이다. SCR 은 Se lective Catalytic Reduction 으로선택적촉매환원을의미한다. SCR( 選択触媒還元 Selective Catalytic Reduction) 이므로요소외의다른환원제를사용할수있으며, 그중에서요소수를사용하는것이비교적많다. 원리는암모니아 (NH 3 ) 가窒素酸化物 (NO x ) 과화학반응을하여窒素 (N 2 ) 와물 (H 2 O) 로환원되는것을이용한것. 화력발전소나선박의배기가수처리시스템에서힌트를얻음. 그러나암모니아를차량에적재하는것은위험하므로尿素水를탱크에탑재하여이것을대기중에분사하므로고온에서가수분해하여암모니아가스를얻는다. 이암모니아로 No x 환원하여서 N 2 ( 窒素가스 ) 와 H 2 O ( 水蒸気 ) 를발생시킨다 http://www.ngkntk.co.jp/product/sensors/nox/index.html
Air quality sensor By sensing the air quality around the vehicle, this sensor can control an automated ventilation system,which automatically switches between fresh air and recirculation modes.