Title of Invention

A NOVEL COMPOSITION USEFUL FOR MAKING MOISTURE SENSORS

Abstract

The present invention provides a process for making moisture sensors. This invention more particularly relates to relaxor ferroelectric lead magnesium niobate based novel composition showing very high humidity dependence of dielectric constant, dissipation factor and AC resistivity and a process for making moisture sensors moisture sensor made from the novel composition of the present invention. Such moisture sensors have a wide use in control systems for different industries like paper, tea, electronics, chemicals and for human comfort, e.g., air-conditioning.

Full Text

The present invention relates to a novel composition useful for making moisture sensors, a process for making moisture sensors there from and a moisture sensing device made thereof. This invention more particularly relates to relaxor ferroelectric lead magnesium niobate based novel composition showing very high humidity dependence of dielectric constant, dissipation factor and AC resistivity and a process for making moisture sensors moisture sensor made from the novel composition of the present invention. Such moisture sensors have a wide use in control systems for different industries like paper, tea, electronics, chemicals and for human comfort, e.g., air-conditioning. Since long, ceramic materials have been in use for moisture sensing. In this context, reference may be made to a research paper of R.K.Nahar and V.K.Khanna in International Journal of Electronics, 52 (6) (1982) p 557 where porous A12O3 was used as a capacitive moisture sensor. Such porous materials are easily contaminated in the atmosphere leading to drift and fatigue of the sensor characteristics. Also such sensors are mechanically weak due to presence of pores. Further reference may be made to a review paper of B.M. Kulwicki, Journal of the American Ceramic Society 7 (4) 41 (1991) p 697 where, other than capacitive moisture sensors, resistive (DC) sensors based on porous MgCr2O4-TiO2, LiF - A12O3 and also resistive (AC) sensors based on porous SiO2 have been discussed. Also reference may be made to a research paper of F. Gill et al., Materials Chemistry & Physics 56 (1998) pl40 where nanocrystalline BaTiO3 has been used as moisture sensors and to a US Patent No. 4, 752, 855 of June 21, 1998 on a ceramic mass formed of BaRuO3, V2O5 and Na2WO4 by Horst Fedter, Werner Grunwald, Peter Nolting, Claudio De La Prieta and Kurt Schmid. The drawbacks of all such moisture sensors are i) the necessity of a porous structure because water vapour absorption and capillary condensation in the pores are essential for moisture sensing which leads to drift and fatigue of the sensor characteristics with time and ii) as discussed earlier, such porous materials are also mechanically weak. The main object of the present invention is to provide a novel composition useful for making moisture sensors which obviates the drawbacks as detailed above.Another object of the present invention is to provide a novel composition based on relaxor ferroelectric lead magnesium niobate [Pb (Mg1/3Nb2/3)O3] useful for making moisture sensors. Still another object of the present invention is to provide a process of making dense moisture sensors from the said novel composition, which obviates the drawbacks as detailed above. Yet another object of the present invention is to provide dense moisture sensors with a measurement range of 1 ppm of moisture to 100% relative humidity. Still another object of the present invention is to provide a moisture sensing device incorporating the moisture sensor made from the novel composition. Accordingly, the present invention provides a novel composition useful for making moisture sensors which comprises: Lead magnesium niobate[Pb (Mg1/3Nb2/3)O3] - 80-98 wt% Lead oxide (PbO) - 2-10wt% Magnesium oxide (MgO) - 0-10 wt% The composition of the present invention is not a mere admixture but a synergistic mixture having properties, which are different from the mere aggregated properties of the individual ingredients. In an embodiment of the present invention, the lead magnesium niobate powder may be prepared by mixing magnesium hydroxycarbonate and niobium pentoxide in a ball mill, calcining the mixture at a temperature in the range of 1100 to 1300°C for 15-20 hours to get magnesium niobate [MgNb2O6], grinding the magnesium niobate powder hi a ball mill, mixing the powder with PbO in the molar ratio of 1:3, heating the mixture at a temperature in the range of 750 to 800°C for 10-15 hours to get lead magnesium niobate followed by grinding in a ball mill. In another embodiment of the present invention, lead oxide may be obtained from materials such as lead oxide, lead carbonate, lead acetate, lead nitrate, lead oxalate and lead citrate. In yet another embodiment of the present invention, magnesium oxide may be obtained from materials such as magnesium carbonate, magnesium hydroxycarbonate, magnesium acetate, magnesium oxalate and magnesium citrate.Accordingly, the present invention provides a process for making moisture sensors from the said composition which comprises mixing of lead magnesium niobate with 2-10 wt% of lead oxide, 0-10 wt% of magnesium oxide and 1-3 wt% of a binder such as polyvinyl alcohol, ethyl cellulose, shaping the powder by pressing, sintering the green compacts at a temperature in the range of 900-950°C followed by soaking for 10-60 min, annealing the sintered samples at a temperature in the range of 800-850°C for a period of 5-15 h followed by electroding, curing and lead attachement. In an embodiment of the present invention the sintering may be effected at a rate in the range of 300-900°C/min and cooling effected at a rate in the range of 150-200°C/h. In another embodiment of the present invention the electroding may be done in the interdigited comb form by a known process such as screen printing using Ag-Pd paste. In still another embodiment of the present invention the curing may be effected at a temperature in the range of 600-800°C for a period in the range of 10-30 minutes. In yet another embodiment of the present invention the change in capacitance and dissipation factor of the electroded and leaded samples made from the said compositions in presence of moisture may be monitored by using tuned detector circuits where the sensor of the present invention, being capacitive elements of the detector circuits, work as highly reliable moisture sensors. Accordingly, the present invention provides a moisture sensing device which comprises connecting the novel sensor to known tuned detector circuits. The process steps of the present invention are as follows:- i) Lead magnesium niobate (PMN) powder was prepared by solid state synthesis route in which a mixture of magnesium hydroxyl carbonate (MHC) and niobium pentoxide (Nb2O5) was calcined at 1100-1300°C for 15-20 h to get MgNb2O6 (MN) followed by grinding in a ball mill for 20 h and its subsequent reaction with PbO at 750-800°C for 10-15 h. The prepare PMN powder was ground in a ball mill for 20 h. ii) The above powder was mixed with 2-10 wt% PbO, 0-10 wt% MgO and 1-3 wt% of a binder such as polyvinyl alcohol or ethyl cellulose, followed by pressing to form the green compacts. iii) The green compacts were sintered at a temperature in the range of 900-950°C at a rate 300-900°C/min followed by 10-60 minutes soaking and then cooling inside the furnace at a rate of50-200°C/h. iv) The samples were annealed at 800-850°C for 5-15 h. v) The sintered samples were electroded in the interdigited comb form by a known process such as screen printing using Ag-Pd paste. vi) The electrodes were cured at a temperature in the range of 600-800°C for 10-30 minutes followed by lead attachment by a known process such as soldering. In conventional ceramic humidity or moisture sensors, water vapour adsorption and capillary condensation in the pores give rise to change in electrical properties like dielectric constant, dissipation factor and AC resistivity which are exploited to monitor the moisture content. Thus the novelty of the present invention lies in the fact that unlike conventional porous ceramic humidity, the humidity sensors made from the novel composition does not require any pore for moisture detection. The increase in capacitance and dissipation factor of the sensors made from the said composition in humid atmosphere is very high which gives rise to very good sensitivity. Such behaviour arises from an unusual mechanism (based on low frequency dispersion phenomenon) behind the humidity induced enhancement of dielectric constant and dissipation factor where surface adsorption of polar water molecules lowers the surface field of the relaxor ferroelectric lead magnesium niobate based material provided the fixing of the composition and the processing have been done in a desired manner. Being almost free of pores, the present moisture sensors are relatively immune to contamination and should show very slow drift and fatigue with time and also the present sensors should be mechanically strong and robust. The inventive steps lie in the formulation of such a novel composition which after proper processing gives a nonporous body to be used as a moisture sensor. The following examples illustrate the invention and the manner in which it may be carried out in practice; however, this should not be construed to limit the scope of the present invention. Example 1 8.85 g of magnesium hydroxycarbonate (MHC) was mixed thoroughly with 23.91 NbzOs.The mixture was calcined at 1100°C for 2 h in a ceramic crucible followed by furnace cooling (300°C/h). Thus prepared MgNb2O6 (MN) powder was ground in a ball mill for 20 h using agate grinding media. 9 g of MN powder and 20.1 g of PbO were mixed for 1 h. The mixture was calcined in an alumina crucible at 800°C for 10 h to get lead magnesium niobate (PMN) powder. The PMN powder after calculation was ground in a ball mill for 20 h using agate grinding media. 0.2 g of PbO along with 0.2 g of polyvinyl alcohol as a binder was added to 10 g of ground PMN powder. Pellets of 25 mm diameter 2 mm thickness were made by uniaxial pressing (pressure 50 MPa) using the above powder. The samples were kept on a dense alumina plate and were directly introduced inside the furnace at 950°C at a rate of 300°C/min and kept there for 15 min and finally cooled at a rate of 300°C/h. The samples were annealed at 800°C for 10 h. Interdigited electrodes of Ag-Pd were screen printed on one of the surfaces of the pellets and the electrodes were cured at 600°C for 15 minutes. Copper leads were attached at the two ends and the samples work as moisture sensors within a range of 1 ppm moisture to 100% relative humidity when attached to tuned detector circuits. Example 2 8.85 g of MHC was mixed thoroughly with 23.91 g of Nb2O5. The mixture was calcined to 1150° C for 2 h in a ceramic crucible followed by furnace cooling (300°C/h). Thus prepared MN powder was ground in a ball mill for 20 h using agate grinding media. 9 g of MN powder and 20.1 g of PbO were mixed for 1 h. The mixture was calcined hi an alumina crucible at 800°C for 10 h to get PMN powder. The PMN powder after calcination was ground in a ball mill for 20 h using agate grinding method. 0.4 g of PbO along with 0.2 g of polyvinyl alcohol as a binder was added to 10 g of ground PMN powder. Pellets of 25 mm dia & 2-3 mm thickness were made by uniaxial pressing (pressure 50 MPa) using the above powder. The samples were kept on a dense alumina plate and were introduced inside the furnace at 950°C at a rate of 300°C/min and kept there for 60 min and finally cooled at a rate of 300°C/h. The samples were annealed at 800°C for 10 h. Interdigited electrodes of Ag-Pd were screen printed on one of the surfaces of the pellets and the electrodes were cured at 950°C for 15 min. Copper lead wires were soldered and the samples work as moisture sensors with a range of 1 ppm moisture to 100% relative humidity when attached to tuned detector circuits. Example 3 8.85 g of MHC was mixed thoroughly with 23.91g NbaOs. The mixture was calcined at 1100°C for 6 h in a ceramic crucible followed by furnace cooling (300°C/h). Thus prepared MN powder was ground in a ball mill for 20 h using agate grinding media. 9 g of MN powder and 20.1 g of PbO were mixed for 1 h. The mixture was calcined in an alumina crucible at 800°C for 10 h to get PMN powder. The PMN powder after calcination was ground in a ball mill for 20 h using agate grinding method. 0.3 g of PbO, O.lg MgO along with 0.2g of polyvinyl alcohol as a binder were added to 10 g of ground PMN powder. Tablets of 25 mm dia & 2-3 mm thickness were made by uniaxial pressing (pressure 50 MPa) using the above powder. The samples were kept on a dense alumina plate and were introduced inside the furnace at 950°C at a rate of 300°C/min and kept there for 30 min and finally cooled at a rate of 300°C/h. The samples were annealed at 800°C for 10 h. Interdigited electrodes of Ag-Pd were screen printed on one surface and the electrodes were cured at 850°C for 15 min. After lead attachment the samples work as moisture sensors within a range of 1 ppm moisture to 100% relative humidity when attached to tuned detector circuits. Example 4 8.85 g of MHC was mixed thoroughly with 23.91 g of NbiOs. The mixture was calcined at 1150°C for 4 h in a ceramic crucible followed by furnace cooling ( 300°C/h). Thus prepared MN powder was ground in a ball mill for 20 h using agate grinding media. 9 g of MN powder and 20.1 g of PbO were mixed for 1 h. The mixture was calcined in an alumina crucible at 800°C for 10 h to get PMN powder. The PMN powder after calcination was ground in a ball mill for 20 h using agate grinding method. 0.5g of PbO, O.lg MgO along with 0.2g of polyvinyl alcohol as a binder were added to 10 g of ground PMN powder. Pellets of 25 mm dia & 2-3 mm thickness were made by uniaxial pressing (pressure 50 MPa) using the above powder. The samples were kept on a dense alumina plate and were introduced inside the furnace at 950°C at a rate of 300°C/min and kept there for 60 min and finally cooled at a rate of 300°C/h. The samples were annealed at 800°C for 10 h. Interdigited electrodes of Ag-Pd were screen printed on one surface and the electrodes were cured at 850°C for 15 min. After lead attachment the samples work as moisture sensors within a range of 1 ppm moisture to 100% relative humidity when attached to tuned detector circuits. The main advantages of the present invention are as follows: 1) The present composition can sense humidity in dense form without the necessity of pores. 2) Being almost free of pores, the present moisture sensors are immune to contamination and should show very slow drift and fatigue of sensor characteristics with time and 3) At the same time, being almost free of pores, the present sensors are relatively mechanically strong and robust. We claim :- 1. A novel composition useful for making moisture sensors which comprises:-Lead magnesium niobate(Pb(Mg1/3Nb2/3)O3) - 80-98 wt% Lead oxide (PbO) - 2-10 wt% Magnesium oxide (MgO) - 0-10 wt% 2. A process for making moisture sensors form the novel composition which comprises mixing of 80-89 wt % lead magnesium niobate with 2-10 wt% of lead oxide, 0-10 wt% of magnesium oxide and 1-3 wt% of a binder such as polyvinyl alcohol, ethyl cellulose, shaping the powder by pressing, sintering the green compacts at a temperature in the range of 900-950°C followed by soaking for 10-60 min, annealing the sintered samples at a temperature in the range of 800-850°C for a period in the range of 5-15 h followed by electroding, curing and lead attachment. 3. A process as claimed in claim 2, wherein the sintering is effected at a rate in the range of 300-900°C/min and cooling is effected at a rate in the range of 150-200°C /hour. 4. A process as claimed in claim 2, wherein the electroding is done in the interdigited comb form by a known process such as screen printing using Ag-Pd paste. 5. A novel composition useful for making moisture sensors substantially as herein described with reference to the examples accompanying this specification.

Documents:

907-DEL-2000-Abstract-(21-12-2009).pdf

907-del-2000-abstract.pdf

907-DEL-2000-Claims-(21-12-2009).pdf

907-del-2000-claims.pdf

907-DEL-2000-Correspondence-Others-(21-12-2009).pdf

907-del-2000-correspondence-others.pdf

907-del-2000-correspondence-po.pdf

907-DEL-2000-Description (Complete)-(21-12-2009).pdf

907-del-2000-description (complete).pdf

907-DEL-2000-Form-1-(21-12-2009).pdf

907-del-2000-form-1.pdf

907-del-2000-form-19.pdf

907-DEL-2000-Form-2-(21-12-2009).pdf

907-del-2000-form-2.pdf

907-DEL-2000-Form-3-(21-12-2009).pdf