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    مقالة علمية عن حساسات الرطوبة
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.مقالة علمية عن حساسات الرطوبة


issamnasser

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06-02-2011 05:37 PM




TiO2 doped with SnO2 and studing its structurail and electrical properties
ISSam Mohammed Nasser
        Aleppo University, Faculty of Science, Department of  Physics.                                                                                                                 
Post-Graduated Student (Master)
Dr. Mustafa Afuoni , Dr. Ghassan Nashed
Abstract
        In this research we prepared a ceramic sample of (TiO2) doped with different ratios of (SnO2) to produce a humidity sensor , that is applied in many important electronic and industrial fields. As well , to get better  electrical conductance . The work was done according to the diagram (1) . The samples were prepared by (Sol-Gel) technology , during three stages : (solution , gel , powder). The samples were studied by (XRD) to determine the crystal structure, the prepared samples of (TiO2) doped with (SnO2) showed poly-crystal structure , and grain size has nano-dimensions between  (40 nm and 65 nm) ,The electrical properties of the samples were studied  too ,The study of the prepared samples showed electrical resistance (6x108 Ω - 1x106  Ω) , its conductivity increased (Ωcm)when humidity increase (%RH),  Whenever the ratio of (SnO2) increased against (TiO2) , the electrical conductivity would increase . By using Mott-Schotkly equation the concentration of free electrons (ND) in (TiO2) , and this showed that not only the shallow charges contributed in conductivity , but the deep charges contributed , too , when the voltage increased (V) .                             
1 – Introduction :
        Sensors got a big interest and became a basical demand in the industrial and the electronic systems . The humidity sensor is one of these sensors , which can be designed of ceramic materials or compound materials . The ceramic sensors are preferred owing to their stable physical and chemical structure when they rare used in the environments ,Generally the designed sensors have excellent resistance as the ceramic materials have good dielectric.
The following oxides [Zn0-TiO2-ZrO-SnO2-SiO2] and others are good compound to produce and develop humidity sensors [1] . TiO2  among these oxides has good sensitivity toward humidity . Titanium Dioxide has many applications , including humidity sensors that are used in for removing systems on the rear windows ,and white wood coating[2] ,Titanium Dioxide has three phases: (Anatas, Rutile, brocket)  [3-4-7] . The study focused on the Anatas phase. It is n-type semiconductor. When it is heated to high degree (1000Cº nearly) , Anatas turns automatically into Rutile structure , as Rutile is the most common type of( TiO2 ), while Anatas is very rare in nature at about( 600 Cº) , Anatas is n-type semiconductor while Rutile is (p-type) [3]. It is recorded that the thin film sensors has lower sensitivity than ones showed by their analogues of porous ceramic . The sensitivity response of TiO2 toward recessive gases like (H2 and OH-) and vapor (H2O) is in all directions of absorption and adsorption . But humidity sensitivity come by adsorbed water layers which transferred protons in the porous structure at room temperature  (25 Cº) and illumination  intensity  (436 Lux) . So , both phases (Anatas , Rutile) show , nearly , the same changes in resistance or capacity , and the humidity sensitivity mechanism is connected basically with adsorption and absorption , hence the mechanism of ceramic is connected with the surface area of granules and their distribution , and this is very important factor . In this research we will prepare a sensor of TiO2 of Anatas phase , and study its electrical properties . Then we will prepare samples of ceramic material consists of (TiO2–SnO2) and study its structure and its electrical properties , and the changing in these properties when it is exposed to humidity .
2 – Importance and goals of the research :  This research aims to :
2-1–Preparing ceramic samples of (TiO2) doped with different ratios of SnO2. The prepared samples denoted with (Z1, Z2, Z3) .
2-2–Studying the changing in capacity and resistance for the prepared samples of (TiO2) doped with (SnO2) of the samples (Z1, Z2, Z3) ,when they are exposed to humidity , besides to the electrical conductance changing .
2-3–Comparing the results via relative humidity gauge and its relation with resistance for the samples (Z1, Z2, Z3).
3  - Experimental work :
3-1 Samples preparation : Beacher was cleaned by chemical solution to remove any suspended fats and impurities on its surface . The samples prepared according to certain criteria as shown by the scheme within the physical and chemicals conditions . TiCl4 solution with high purity (99%) from (PROLABO) and SnCl4 from  (PROLABO)  were weighed with different ration , as in table 1 . materials were put in a container which big enough for the mixture , in room temperature  (25 Cº) and illumination intensity (436 Lux) . The materials were mixed by magnetic mixer for two hours at 250 rpm . After that the samples were dried and donated with (Z1, Z2, Z3).
Sample Z1 Z2 Z3
TiO2 ratio 10 gr 8.8 gr 8 gr
SnO2 ratio 0 gr 1.2 gr 2 gr
Table 1: shows the preparation of ceramic samples of TiO2 doped with different ratios of SnO2 .
It was noticed that they turned from Solution gel (Sol-Gel) , which was one  of the forming phases of these sensors . Then the samples (Z1, Z2, Z3)were put in thermal flasks and were cured in (المرمدة) to (600 Cº) when the gel turned into powder . The resulted powder was put , after adding some drops of water to get a good structure for the sample , in special module and pressed ,Scheme 1 shows the way of working that is previously explained to get disks of ceramic for the three samples with various ratios. The used materials are TiCl4 , SnCl4 and organic solution (methanol)+ dimethyl acetone. Taking to notice the periods between each phase (solution– gel– powder) (granules) .                      3-2 working scheme :    the scheme(1) shows how to prepare the samples : 

Scheme(1) to prepare samples according to different ratios

4 – Experimental measurements
4-1 structure measurements :
To study the ceramic samples (Z1, Z2, Z3)structure which were prepared by using the X-ray deviation meter (XRD) , Philips Model , to measure  the spectrum of X-ray deviation for the samples (Z1, Z2, Z3). Figure 1 represents the spectrum of X-ray deviation for the three samples .

Figure 1 : the spectrum of X-ray deviation for the three samples.
From the figure 1 it is clear that :
A– Figure 1 represents the spectrum of X-ray deviation for pure SnO2 at 2θ about : {26.5 – 34 – 38 – 51.8 – 54.5 – 62} degree which meet the following levels : (110 – 101 – 200 – 211 – 220 – 310) , respectively , and which conform with the crystal structure of pure SnO2 .
B- The scheme 2 represents the spectrum of x-ray deviation for pure TiO2 at 2θ  , about {25.2 – 37.9 – 47.8 – 53.8 – 55 – 63} degree which meet the indexes of the following levels : 101 – 004 – 200 – 105 – 211 – 204 , respectively , and which conform the crystal structure of TiO2 (Anatas). The samples were processed at 600 Cº .
C- The scheme 3 represents the spectrum of x-ray deviation for the sample Z2 . From the study of the x-ray deviation spectrum it is clear that the formed ceramic material (TiO2 – SnO2) is poly crystalline owing to the linkage between the elements of the prepared material , and the lines of XRD conform the ones that recorded for (TiO2 – SnO2) [5] .  It is noticed that TiO2 is formed with solid liquid with SnO2 and that Titanium Dioxide TiO2 is multi crystallization structure , and it is of Anatas type . Adding SnO2 , with very low ratio to TiO2 affects the crystallization volume and molecular size of the prepared ceramic compound , hence the area of the active surface . As well , the molecules size reduce with increasing the concentration of SnO2 . Also , it is noticed that there is a structural looping from SnO2 molecules with TiO2 molecules . To know the size of granules that forming the ceramic material which includes TiO2 doped with SnO2 with different ratios , the following relation of (Seherer Equation) is applied [1] , which is given in the following relation (1) :
D = k λ/(δw cos θ)              (1)
Where : D is the size of granule , δw the wideness of peck opening at the middle of intensity , measured by Radian , k is constant its value 1 ,  λ  wave length used by XRD apparatus from Philips and its value λ=0.154 nm ,                θ deviation angle . The following chart gives the size of the formed granules :
Sample Granule size (nm)
Z1 60.484
Z2 42.394
Table 2 : the size of the formed granules
6- Measurement of DC :
To study the electrical characters (DC) of the three samples (Z1, Z2, Z3)of Titanium Dioxide doped with different ratios of SnO2 , an ohm (resistance) connection was made for the samples via a mask of Aluminum sheet graved by laser (Nd- yag Laser) on the surface of the sample , as in the figure :
                Prepared sample                    connecting poles

                                                        Ohm connecting   
Figure 2 : The used mask I the Ohm connecting
The aspects (I , V) for the three samples (Z1, Z2, Z3), were taken , and the figures (3 , 4 and 5) sow the aspect (I , V) for the ceramic samples (Z1, Z2, Z3), respectively , at different values of the relative humidity (36% , 50% , 70%) :

Figure 3 : The aspect (I,V) for the samples (Z1, Z2, Z3)at    36% relative humidity

Figure 4 : The aspect (I,V) for the samples (Z1, Z2, Z3) at 50% relative humidity

Figure 5 : The aspect (I,V) for the samples (Z1, Z2, Z3)  at 70% relative humidity                                                          We noticed from studying the diagrams in figures (3 , 4 and 5) for the (I.V) aspect changing at the room temperature with stable illumination intensity and stable relative humidity that the relation is non-linear , as its structure is granular and multi-crystallization . The electrical resistance of the ceramic materials is subjected to the relation (2) : where the non-linear resistance was evaluated from the relation (1): [9]
I=(V/R )^α              (2)
a : the non-linearity factor . if we take the logarithm of the tow sides , we find that , for the current I1 :
a: non-linearity factor is calculated by the following relation (3):
α=(Ln I2 –Ln I1)/(Ln V2 –LnV1)          (3)   
From the figures (3 , 4 and 5) at different RH% we conclude the following : we calculate the non-linearity factor a of the studied ceramic samples (Z1, Z2, Z3), and the following table shows the values of the non-linearity factor with the changing in the level of he relative humidity of the studied samples at various concentrations . 
Sample a non-linearity factor at 36% RH a non-linearity factor at 50% RH a non-linearity factor at 70% RH
Z1 2.417 1.5 509 1.353
Z2 2.2098 1.3765 1.201
Z3 1.659953 1.3571 1.012
Table 3 : the changing in the non-linearity factor according to                                    the changing in the doping ratio at stable relative humidity  It is noticed that the values of the non-linearity factor are within (2.417 – 1.012) and it is more than one , which means that the structure of the samples is granular structure , and that the resistance of these samples integrate with the surface character of these samples when there is  changing in the non-linearity factor a . Table 4 shows this .
sample R(Ω)
at RH = 36% R(Ω)
at RH = 50% R(Ω)
at RH = 70%
Z1 6.65 x 108 1.56 x 108 8.23 x 107
Z2 2.11 x 108 9.6 x 107 2.34 x 107
Z3 2 x 108 4.23 x 107 7.33 x 106
Table 4 : the R values at different non-linearity factor (a) for the three samples .
The tables (3 , 4) show that non-linearity factor (a) decreases when the relative humidity increases , also when the doping ratio with SnO2 increases , so we get non-linearity relation . And that the resistances R(Ω) decreases when the relative humidity of the three samples  (Z1, Z2, Z3)increases . By electrometer from (Kaittvel) we measure the resistance R(Ω) of the prepared samples at different relative humidity levels . The figure shows the way of measuring :

Diagram 2 show how to connect the prepared samples and the humidity meter at the room temperature and illumination intensity (436 Lux) and the other apparatus , and the studying of the samples characters at different relative humidity levels , and the control opening for the humidity values , increasing or decreasing , and after taking the resistance values of the three samples at different relative humidity levels and drawing the aspect (R(Ω),%RH) .
Figure 6 shows the resistance R(Ω) changing according to the relative humidity level of the samples Z1 and Z2 .

Figure 6 : the resistance R(Ω) changing according to the relative humidity level.
Notice from the figure 6 that the resistance changes from 6x106(Ω)  to 1x106(Ω)  , when the relative humidity changes from RH = 1% to RH = 70% , for the prepared samples . It is found that the relation between R(Ω)  and RH% is a linear relation from (RH = 10% to RH = 70%) .
That means we can use the samples as humidity sensors from (RH=10% to RH=70%). Besides, the increasing in the doping ratio increases the conductivity of the formed sensor at certain value , where we notice a gluttony to OH- on the porosity of the conductor surface . Also , when the relative humidity increases RH% at each samples there will be decreasing in the resistance values . This leads us to a conclusion that the conductivity increases when the humidity increases [1] . So we can conclude that the humidity sensor formed from (TiO2–SnO2) is a good one, as the addition of SnO2 improves the porosity of the surface. With the repeating of humidity we conclude that Titanium Dioxide has a little slowing curve , which does affect its sensitivity towards humidity, as the addition of SnO2 records a little slowing in humidity sensitivity for Titanium Dioxide as in the figure (6 , 7 and 8). So , SnO2 is a good conductor and an ideal compound to increase the sensitivity of the sensor [1] .

Resistance R(Ω) changing for the sample Z1 with he changing in the relative humidity (RH) .

Resistance R(Ω) changing for the sample Z2 with he changing in the relative humidity (RH) .

Resistance R(Ω) changing for the sample Z3 with he changing in the relative humidity (RH) .
Doping with Sn4+ causes decreasing in the molecular volume , hence increasing the surface area and increasing the sensitivity of the formed material of TiO2 for relative humidity .
Also, the increasing in doping ratio of SnO2 with TiO2 increases the conductivity of the formed sensor at certain value , where we notice a gluttony of TiO2 , that is the increasing in RH% against a remarkable relative changing in resistance R(Ω). At high humidity , the resistance decrease owing to adsorption of OH- on the surface of TiO2 which increase the conductivity .
Figure 9 displays the conductivity increasing when the humidity changes .

Figure 9 : conductivity changing when the humidity changes
It is notice that there is an increasing in the relative humidity which affect the conductivity, as conductivity increases remarkably of the prepared samples .



 
     






Figure 10 : aspect of conductivity and doping ratio
Figure 10 shows the aspect σ (Ω cm)-1 with the changing in the doping ratio of the ceramic samples (Z1, Z2, Z3). It is fond that the electrical conductivity increases whenever the doping ration increase . We can say that the sample Z3 has the high value of conductivity . 
5-3-2 Measuring the AC :
By the analyzer of gain and phase from (Solectron 1255) , when we apply a voltage value 5(V) and a resistance Ra = 220 Ω with a frequency range 10 Hz to 20000 Hz .   
The relation is drawn between the nodal part X(W) wit the real part R(W) of the nodal reluctant given by the following relation of the three samples (Z1, Z2, Z3). The representation of the reluctant is given by the nodal form the relation 4
Z=R(W)+JX(W)          (4)
Figure 11 shows the spectrum of nodal reluctant between the nodal part X(W) and the real part R(W) of the three samples at relative humidity level 35.5% 


Figure 11 : nodal reluctant spectrum of the samples (Z1, Z2, Z3) at RH = 35.5% .

Figure 12 : nodal reluctant spectrum of the samples (Z1, Z2, Z3) at RH = 50% .







Figure 13 : nodal reluctant spectrum of the samples(Z1, Z2, Z3) at RH = 70% .


Figure 14 : changing in nodal reluctant according to the changing in relative humidity .
It is found from the figure 14 that there is decreasing in the reluctant when we increase the humidity ,We notice from the figures (12 , 13 and 14) the following :
The studied samples are subjected to Dibay model where :
A- The spectrum of the nodal reluctant of the prepared ceramic samples are regular semi-circles . This implied that the granular structure of the samples is homogeneous structure .
B-  As the values of R(W) decrease whenever the frequency (Hz) increases , the  equal circuit between each two near granules is a resistance and a capacitor parallel connected . When we draw the relation between 1/(C_p^2 ) and V when applying a frequency 20 KHz of the three samples , we find the following lines , which we can determine via them the intensity of the atoms and the effect of doping by SnO2 with TiO2 .

Figure 15 : The aspect 1/(C_p^2 )  according to V for the sample Z1

Figure 16 : The aspect 1/(C_p^2 ) according to V for the sample Z2

Figure 17 : The aspect 1/(C_p^2 ) according to V for the sample Z3
The intensity of the donor electrons are calculated from the relation (5) :
1/(C_p^2 )=2/(q N_D A^2 〖ε_0 ε〗_r )  V  .              (5) □(      ⇒┬.)
Where : The charge of electron : q=1.6x10 -19  C
The isolation constant for Titanium Dioxide at 600 C:
(ε_r=85) [8].
De-electricity constant (ε_r = 8.85 x 10-12 F/M)
The area of the ohm connecting surface of the sample
A = 4x10-6 (m2).
Concentration of electrons : ND
It is found that the value of ND for the samples (Z1 , Z2 , Z3) from the curves clarified by the figure (16 , 17 and 18) are :
Sample Concentration of charges (ND) cm-3
Z1 6.92367x1017
Z2 1.1869x1018 , 8.3084x1017
Z3 2.0771x1018
ND (Z2) = 1.1869x1018  cm-3: the concentration of the surface electrons and the deep ones .
So , the intensity of the free charge (electrons) which come from deepness and contribute in the conductivity of the second sample is :
ND(Z2)=3.5606x1017 cm-3 ,which contributes in increasing the electricity conductivity , too .   
ND (Z3) = 2.0771x1018 cm-3 increases whenever the doping ratio increases . It is noticed that there is an increasing in the electrons intensity ND when we increase the doping ration , hence the increasing in the electricity conductivity for the doped samples . 
It is noticed , too , that the index (1/(C_p^2 )) is increasing index with the applied voltage . This means that the formed semi-conductor is (n) type . This represents the fact that the sensor formed from Titanium Dioxide and processed to the temperature 600 C is Anatas phase .
6- Discussion and the most important conclusions :
From presenting the previous study and the experimental work and measurements, we can summarize the following conclusions when we want to prepare a humidity sensor with good doping with different ratios with SnO2 :
1- The prepared sensor has multi-crystallization structure and the granules have nano dimensions [1] . This increases the porosity of the active surface of the oxide and increases its sensitivity when it is used as a sensor for gases , especially for humidity .                                                                                      2- Doping with SnO2 changes the resistance of the sensor , hence it changes the porous structure of the surface molecules of the sample .
3- Determining the crystal structure of the samples when we measure the spectrum of X-ray deviation . The purity of the used solution (TiCl4) is 99.9% , and the purity of SnCl4 is 99.6% . 
  4- Doping TiO2 with SnO2 affects the volume of crystallization, hence the molecules volume of the prepared compound . This increases the surface area . Also we notice that the molecules volume decreases when we decrease the concentration of SnO2 . Also we notice a structural looping between SnO2 molecules and TiO2 molecules .
5- The prepared TiO2 is Anatas type . This is confirmed by the study of x-ray deviation for the three studied samples .         
  6- We can say , that when we increase the relative humidity (RH%) , also with the increasing of the molecular pressure , the porosity of the sample surface increases , which creates a balance in molecules distribution where the gaps between the sample granules are filled with suitable barrier .
7- Whenever the doping ratio with SnO2 increases , the electricity conductivity increases .
8- By using Shotki relation , and the calculating of the free electron of TiO2 , we find that not only the surface charges contribute in conductivity , but the electronic charges that comes from deep internal levels , too .
References:
1 Kim, H.K; Sathaye S. D; 2005, Humidity Sensing Properties of Nanoporous TiO2-SnO2 Ceramic Sensors; Bull. Korean Chem. Soc. 2005, Vol. 26, No. 11, Korea.                                                                2- Jia-zhen ,YAN; Qing-gong, S ;2005 Research on rutile nano-titanium dioxide for weatherability modifying of white wood coating; Journal of Functional Materials;vol.1-2, China.                                        3- Chen ,Z ; Lu C;  27 July 2005;  Humidity Sensors: A Review of Materials and Mechanisms; Sensor LETTERS ;Vol. 3, 274–295, 2005. USA.                                                                                                        4- Mineiro, S. L; Nono M C. A;2006; Humidity Sensitive Characteristics of ZnO2-TiO2-Ta2O5 Ceramic; São José dos Campos – SP; CEP 12245-970, CP 515. Brazil.                                                                    5- Li, K; Wang, Y; Wang S; 2009; A compar ative study of CuO/TiO2-SnO2 , CuO/TiO2 and  CuO/SnO2 catalysts for low- temperature CO oxidation; Journal of Natural Gas Chemistry 18[2009]; Vol. 18 No. 4 2009. China  .           
6-G. Garcia-Belmonte; T. Dittrich;2003; Effect of humidity on the ac conductivity of nanoporousTiO2; JOURNAL OF APPLIED PHYSICS; VOL 94, NUMBER 8 ,Spain, Germany.                                                   
7- Lee .K. R; Kim. S. J; Song .J. S; 2002- Photocatalytic Characteristics of Nanometer-Sized Titania Powders Fabricated by a Homogeneous-Precipitation Process; journal; Vol.2,341-345, Korea.
8-C.T. Dervosa; Ef. Thiriosa; J. Novacovicha,all. accepted 8 October 2003. Zografou 157 80 Athens, Greece. Permittivity properties of thermally treated TiO2. Materials Letters 58 (2004) 1502– 1507. received in revised form 7 September 2003.
9-METZ R.;KOUMIR D.;MOREL J.(2008).Electrical Barriers for            Motion at Grain Boundaries of Co-doped SnO2 Varistor Ceramics journal of European ceramic Society 28,829-835.   









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