Gas Sensing Performance of Pure and Modified Nanostructured Screen Printed Zirconia Thick Films

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Gas Sensing Performance of Pure and Modified Nanostructured Screen Printed Zirconia Thick Films

Dr. G. B. Shelke*

Dept. of Physics

Nanasaheb Y. N. Chavan College, Chalisgaon, MHS, India

Dr. D. R. Patil

Dept. of Physics

Rani Laxmibai College, Parola, MHS, India

Abstract: Metal oxide based solid state gas sensors are the best selection to the development of commercial gas sensors for a wide range of applications. The great interest of industrial and domestic solid state gas sensors comes from their versatile advantages like portable size, high sensitivity in detecting very low concentrations (ppm, ppb or sub ppb level). Zirconium Oxide (Zirconia) powder is one such challenging material for the fabrication of gas sensors. Bulk Zirconium Oxide (ZrO2) powder in the form of thick film was observed to be less sensitive to the polluting gases. So, synthesized nanostructured ZrO2 powder was fabricated by simple screen printing technique followed by the calcinations at 500oC for 1 hr. Thick films of pure nanostructured ZrO2 powder were surface activated by dipping them into 0.01 M aqueous solution of Strontium Chloride and Bismuth Chloride separately for different intervals of time followed by calcinations at 500oC for 30 min. The films exhibit the semiconducting nature due to non-stoichiometry and respond to various gases. Optimizing the particular conditions, the thick films can be used for gas sensing to detect polluting and hazardous gases, viz. H2, H2S, Cl2, NH3, C2H5OH, LPG, etc. The surface morphology, chemical composition, crystal structure, electrical and thermal properties of the bulk and nanostructured ZrO2 have been investigated by Field Emission Scanning Electron Microscope (FE-SEM), Energy Dispersive Analysis by X-rays (E-DAX), X- Ray Diffraction (XRD), etc.

Keywords – Bulk and Nanostructured ZrO2, Synthesis, Thick Films, Polluting Gases, etc.

  1. INTRODUCTION

    An oxygen sensor is an electronic device that measures the concentration of oxygen (O2) in the medium to be analyzed. Oxygen sensors are used to measure the respiratory conditions, at the production of oxygen as well as oxygen analyzers which find a lot of use in medical applications such as anesthesia monitors, respirators and oxygen concentrators in the gases or liquids. Oxygen sensors are also used in hypoxic air fire prevention systems to monitor continuously the oxygen concentration inside the protected volumes [1-11]. Hydrogen sulfide has characteristic of rotten egg like odor and also it is a colourless gas. It appears naturally as a byproduct of decomposition. Hydrogen sulfide is a highly toxic gas [12- 18]. It reacts with the enzymes in the blood stream which inhibit cell respiration. In other words, high concentrations of hydrogen sulfide can shut off the lungs. Even low concentration exposure of the H2S gas can burn the respiratory track and cause swelling around the eyes [19-24]. Prolonged exposure of H2S gas renders the sense of smell

    inoperative. Many gas sensors are already available by Figaro-Engg. Inc., Sierra Monitors Inc., IST, etc. However, many problems persist till today. These problems can be minimized by varying different parameters in the measurement of gas sensing. The different parameters, viz. operating temperature, type of gas, gas concentrations, ageing (long term), type of materials, crystallite size, film thickness, dopants, activators, calcination temperature, activation time, etc. affect the gas response of the sensor. However, the devotion is paid in fabricating the ZrO2 and its compositions based low cost gas sensors. Hence, the effects of all above mentioned parameters are studied on the gas sensing performance of the nanostructured base materials and their compositions. The present work explains the comparative study of the sensors fabricated in the form of thick films.

  2. OBJECTIVES

    1. To synthesize the nanostructured ZrO2 by one of the simplest and cheapest process known as disc type ultrasonicated microwave assisted centrifuge technique.

    2. To prepare the thick films of nanostructured ZrO2 by screen printing technique. This is one of the simplest and low cost technique.

    3. To ensure longer life by maintaining proper thixotropy and rheology of the thick films.

    4. To achieve a suitable surface activation by dipping the thick films of ZrO2 into SrO2 and Bi2O3 for enhancing the gas response and selectivity.

    5. To analyze the synthesized pure and modified materials by different characterization techniques.

    6. To investigate the electrical and gas sensing performance of pure and surface activated ZrO2 thick films.

    7. To investigate and modify the response and recovery profile of pure and surface activated ZrO2 thick films.

    Sr. No

    .

    Material

    Technique

    Gas / Investigation

    Yea r

    Ref

    .

    No.

    01

    ZrO2

    Thick films

    H2

    2009

    25

    02

    HfO2, ZrO2

    Thin films

    N2

    2006

    26

    03

    ZrO2-Y2O3- TiO2

    Thick films

    Synthesis

    1999

    27

    Sr. No

    .

    Material

    Technique

    Gas / Investigation

    Yea r

    Ref

    .

    No.

    01

    ZrO2

    Thick films

    H2

    2009

    25

    02

    HfO2, ZrO2

    Thin films

    N2

    2006

    26

    03

    ZrO2-Y2O3- TiO2

    Thick films

    Synthesis

    1999

    27

  3. LITERATURE REVIEW Table 1: Literature Review

    04

    Ta2O5, ZrO2

    Thin films

    Synthesis

    2006

    28

    05

    TiO2, ZrO2

    Sol gel method

    Humidity

    2008

    29

    06

    (ZrO2)0.8-

    (-Fe2O3)0.2

    Thick films

    O2

    2000

    30

    07

    ZrO2

    Thin films

    Synthesis

    2002

    31

    08

    TiO2, ZrO2

    Sol gel method

    Characterization s

    2010

    32

    09

    CeO2, ZrO2

    Thick films

    O2

    2008

    33

    10

    Pt, ZrO2

    Thick films

    O2

    2000

    34

    11

    ZrO2, Fe2O3

    Thick films

    O2

    2003

    35

    12

    Ceria, Yttria, Zirconia

    Thick films

    O2

    2009

    36

    13

    ZrO2

    Thin films

    Synthesis

    2003

    37

    14

    ZrO2

    nanopowde r

    Wet chemical route

    Synthesis

    /td>

    2006

    38

    15

    ZrO2

    nanopowde r

    Precipitatio n methods

    Synthesis and characterizations

    2008

    39

    16

    Al2O3-ZrO2

    Sol gel method

    Synthesis and characterizations

    2000

    40

    04

    Ta2O5, ZrO2

    Thin films

    Synthesis

    2006

    28

    05

    TiO2, ZrO2

    Sol gel method

    Humidity

    2008

    29

    06

    (ZrO2)0.8-

    (-Fe2O3)0.2

    Thick films

    O2

    2000

    30

    07

    ZrO2

    Thin films

    Synthesis

    2002

    31

    08

    TiO2, ZrO2

    Sol gel method

    Characterization s

    2010

    32

    09

    CeO2, ZrO2

    Thick films

    O2

    2008

    33

    10

    Pt, ZrO2

    Thick films

    O2

    2000

    34

    11

    ZrO2, Fe2O3

    Thick films

    O2

    2003

    35

    12

    Ceria, Yttria, Zirconia

    Thick films

    O2

    2009

    36

    13

    ZrO2

    Thin films

    Synthesis

    2003

    37

    14

    ZrO2

    nanopowde r

    Wet chemical route

    Synthesis

    2006

    38

    15

    ZrO2

    nanopowde r

    Precipitatio n methods

    Synthesis and characterizations

    2008

    39

    16

    Al2O3-ZrO2

    Sol gel method

    Synthesis and characterizations

    2000

    40

    C. Bismuth Oxide (Bi2O3)

    Bismuth occurs as silver, pink and white in its original form as with an iridescent oxide tarnish showing many colors from yellow to blue. The Bismuth oxide has base centered monoclinic crystal structure. Bismuth burns with a blue flame and its oxide forms yellow fumes, when burnt in oxygen. It is less toxic as compared with its other neighboring elements in the periodic table, viz. lead, antimony and polonium. Bismuth occurs as the native metal, and in ores such as bismuthinite and bismite. Bismuth oxide is a high-density metal. As Bismuth is a brittle metal, it is usually mixed with other metals to make it useful. Bismuth can be used in fire detectors and extinguishers, solders and electric fuses when it is mixed with low melting point metals, viz. tin or cadmium. Bismuth oxide is also used as cosmetics and paints.

  4. MATERIALS USED

    1. Zirconium Oxide (Zirconia, ZrO2)

      Zirconium dioxide (ZrO2) is also known as zirconia. It is a white crystalline oxide of zirconium. Its most occurring form, with a monoclinic crystalline structure having wide band gap of 5.8 eV. The band gap energy of ZrO2 is dependent on the phase (cubic, tetragonal, monoclinic or amorphous) and preparation methods with typical estimates from 5-7 eV. Zirconium dioxide is one of the very important ceramic materials. Pure ZrO2 has a monoclinic crystal structure at room temperature and found transitions to tetragonal and cubic at increasing temperatures. Zirconia is used in the production of ceramics with other uses including protective coating on particles of TiO2 pigments, as a refractory material, abrasives, enamels and insulations. Zirconium dioxide is also used in fuel cell membranes and oxygen sensors since; it has the ability to allow oxygen ions to move freely through the crystal structure at high temperatures. The high ionic conductivity makes it one of the most useful electro-ceramics.

    2. Strontium Oxide (SrO2)

    Strontium is a chemical element with symbol Sr and atomic number 38. Strontium is an alkaline earth soft metal in silver white or yellowish color. When it is exposed to air, the metal forms a dark oxide layer. Strontium occurs naturally found mainly in the minerals celestite and strontianite. The properties of strontium are intermediate between and similar to those of its group neighbors, calcium and barium. It is harder than barium and softer than calcium. The melting point of strontium is 777oC and boiling point is 1377oC. The density of strontium is 2.64 gm/cc. When Strontium metal burns in air, it produces strontium oxide as well as strontium nitride. However, it does not react with nitrogen below 380oC. It spontaneously forms only oxide at the room temperature. Strontium is widely used in fireworks and flares. It is also used for refining zinc and manufacturing of ferrite magnets.

  5. EXPERIMENTAL

    1. Synthesis of Nanostructured ZrO2 Powder

      Fig. 1: Disc type ultrasonicated microwave assisted centrifuge technique

      Nanostructured ZrO2 powder was synthesized by disc type ultrasonicated microwave assisted centrifuge technique [41- 46], by hydrolysis of AR grade zirconium oxychloride in aqueous-alcohol solution (Fig. 1). An aqueous-alcohol solution was prepared in the ratio of 1:1 from distilled water and propylene glycol. This solution was then mixed with 1M aqueous solution of zirconium oxychloride in the ratio 1:1. The special arrangement was made to add drop wise aqueous ammonia (0.1 ml / min.) with constant stirring until the optimum pH of solution becomes 7.9. After complete precipitation and centrifugation, the hydroxide was washed with distilled water until chloride ions were not detected by AgNO3 solution. Then the precipitate was allowed for ultrasonication and then kept in a microwave oven for 10 minutes with continuous on-off cycles, periodically, followed by calcination at 500oC for 2 hrs in muffle furnace. The dried precipitate was ground by agate pestle-mortar to ensure sufficiently fine particle size and re-calcined in a muffle furnace at 500°C for 2 hrs, to eliminate the organic impurities, if present. Thus, the dry white powder of nanostructured ZrO2 has been prepared to use.

    2. Thick Film Fabrication Technique

      Fig. 2: Screen printing set-up board

      Fig. 3: Sensor patterns printed on the glass substrate

      Thick film fabrication technique using screen printing mechanism is the most suitable, simple and economical method in the fabrication of sensors. The screen is held at about few millimeters above the substrate on a printing set up board (Fig. 2). The required thixotropic paste is kept on the top surface of stencil and squeegee pushes the paste and poured through the pores while it moves from one end to the other. The sensor patterns printed on the glass substrates are in rectangular form. The length of the sensor pattern kept was 9 mm and the width 4 mm (Fig. 3). The desired amounts of samples are prepared by repeating the above procedure. These samples are then subjected to a temperature in the range of 70-130oC for drying under an IR lamp. Next, curing is done in order to avoid the blistering of the film. It has two steps: removal of organic binder and high temperature curing / firing. In the first step, the residues of organic binder are eliminated by heating at low temperature. In the second step, the film is subjected to a furnace under controlled conditions. The chemical reactions that take place, give the required electrical and physical properties of the film.

      The thick film technique using screen printing offers a good control over the thickness and microstructure. The life time of thick films is expcted to be larger. The use of thick film technology in the production of chemical sensors has opened up the possibility of manufacturing sensors in a cost effective manner. Such properties of a thick film sensor are highly desirable for chemical applications. Furthermore, thick film technology has the ability to produce the sensors from nano-scaled materials. This is an advantage because minute sample volume is required. Also, the portability of the interface instrument for the sensor can be realized. The thixotropic paste was prepared by mixing the synthesized nanostructured powder of pure ZrO2 with a solution of ethyl cellulose in a mixture of butyl cellulose, butyl carbitol acetate and turpineol. While in formulating the paste, the ratio of inorganic to organic part was kept as 80:20. The thixotropic paste was screen printed on the glass substrates

      and the thick films of desired patterns were obtained [43- 46]. Films prepared were dried at 80oC under an IR lamp, followed by firing at 500oC for 30 min. in ambient air. Silver contacts were made by vacuum evaporation for electrical measurements and monitoring the gas sensing performance of thick films. Thus, the thick films of pure ZrO2 are now ready to use in the desired applications.

    3. Surface Activation of ZrO2 Thick Films

      Surface activation of as prepared thick films of pure ZrO2 was achieved by dipping them into a 0.01 M aqueous solution of strontium chloride and bismuth chloride, separately, for different intervals of time, viz. 5 min., 15 min., 30 min. and 45 min. and dried at 80oC under an IR lamp, followed by firing at 500oC for 30 min. in ambient air. The particles of strontium chloride and bismuth chloride dispersed on the film surface would be transformed to strontium oxide (SrO2) and bismuth oxide (Bi2O3) respectively, upon firing process. Thus, the sensor elements with different mass % of SrO2 and Bi2O3 incorporated in to thick films of pure ZrO2 were prepared.

    4. Static Gas Sensing System

    Fig. 4: Static gas sensing system

    The sensor element, heating unit, dc power supply, gas inject unit, temperature measuring unit, current meter (pico- ammeter), glass dome and steel base plate are the major components of static gas sensing system. The static gas sensing system is built in the laboratory (Fig. 4). There are electrical feeds on the base plate. Heating unit is fixed on the base plate. It provides the desired temperature to sensor for its proper performance. Sensor sample to be tested was mounted above the heater. Cr-Al thermocouple is mounted to measure the temperature. The thermocouple is connected to digital temperature indicator. Inlet gas port was fitted at one of the ports of base plate. Gas concentration inside the static system is achieved by injecting a known volume of test gas by gas inject syringe. A d. c. voltage of 0

    – 30 V is applied to the sensor element constantly and current is measured by pico-ammeter.

  6. CHARACTERIZATION TECHNIQUES

    The studies on microstructures and surface morphology of thick film samples were carried out by using X- Ray Diffractometer (XRD) and Field Effect Scanning Electron Microscopic (FE-SEM) techniques, respectively. Also, electrical characteristics, viz. I-V characteristics and

    conductivity profile of all the samples were also studied in the laboratory [41-47].

  7. RESULTS AND DISCUSSION

    1. Oxygen Sensors

      Fig. 5: Oxygen response of pure and surface activated ZrO2 thick films

      Fig. 5 depicts the O2 response of pure and surface activated ZrO2 (15 min.) thick films. From Fig. 5, it is observed that, the 15 min. SrO2 activated ZrO2 thick film is most sensitive and selective to O2 (50 ppm) at room temperature. Upon exposure of O2, the n-typeness of ZrO2 decreases in the extent that ruptures the p-SrO2 / n-ZrO2 heterojunctions and the whole material exhibits p-type semiconductivity. The holes contribute the net flow of current in the surface of the material. This may be the reason of increase in conductivity of the sensor upon exposure of O2 at room temperature. But, as the temperature increases, the O2 response decreases in the range from 75oC to 100oC. Above 100oC, the oxygen response is negligibly small [47]. However, Bi2O3 activated ZrO2 thick films are insensitive to O2 at all temperature range.

    2. H2S Sensors

      behind the free electrons in the material and enhances the conductivity of the material. The high performance of this film in H2S sensing may be attributed to the enhancement of active surface to volume ratio [47]. But, as the temperature increases, the H2S response decreases. However, SrO2 activated ZrO2 thick films are comparatively less sensitive to H2S at room temperature and insensitive at high temperature range.

  8. CONCLUSIONS

    A review of the existing literature suggests that, the modified and unmodified ZrO2 thick films possess in several important areas of application of modern microelectronic techniques, because of their uses in production of advanced infrared detectors and sensors for sensing toxic gases and magneto-resistive sensors. The gas response of the sensor must be as high as possible to trace level (ppm / ppb) of the gas with very high selectivity and no cross sensitivities. The response and recovery times must be very low (~ few seconds). Pure semiconducting oxides are expected to be stoichiometric and hence are less sensitive and less selective to toxic, hazardous and polluting gases.

    It was studied that, the non-stoichiometric zirconium oxide behaves as semiconductors and act as gas-sensing varistors. Various techniques are employed to modify the stoichiometric zirconium oxide to disturb their stoichiometry. Few most suitable techniques viz. sintering at higher temperatures, surface functionalization, etc. can be used. Sintering at higher temperatures would cause the elimination of undesired impurities in the material. Also, it would create the oxygen vacancies in the pure materials, modifying the various characteristics of the materials. The nanostructured ZrO2 material has the great potential to fabricate the gas sensors which have most advanced and highly applicable features. The review also reveals that, very few researchers studied on ZrO2 thick films but almost no attempt has been made to study dipping time variation with the response of gas. Hence, it is the need to study the effect of surface activation on structural, electrical and gas sensing properties by changing dipping concentration. So, there is a great space to work for the development of ZrO2 based gas sensors.

  9. ACKNOWLEDGEMENT

Fig. 6: H2S response of pure and surface activated ZrO2 thick films

Fig. 6 depicts the H2S response of pure and surface activated ZrO2 (45 min.) thick films. From Fig. 6, it is observed that, the 45 min. Bi2O3 activated ZrO2 thick film is most sensitive and selective to H2S (5 ppm) at room temperature. Upon exposure, H2S gas gets oxidized by utilizing the molecular oxygen adsorbed on the surface of the thick film, trapping

Authors are grateful to the Chairman of Sahajivan Shikshan Prasarak Mandal Ltd. Tehu, Parola and Principal of the Rani Laxmibai College, Parola for providing laboratory facilities; and also to the Hon. Chairman, Secretary and Management of the R. S. S. P. M. Ltd. Chalisgaon as well as to the Principal and Colleagues of Nanasaheb Y. N. Chavan College, Chalisgaon.

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