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External View |
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Internal View |
The High Temperature Kelvin Probe (HTKP-2000) may be applied for the determination of work function (WF) and its changes as a function of time.
The HTKP-2000 is a unique surface sensitive equipment that may be used for investigation of materials at elevated temperatures and under controlled gas phase environment of atmospheric pressure (it does not require vacuum).
The HTKP-2000 may also be applied for in situ surface monitoring of materials during their processing. The probe is sensitive in an atomic scale with respect of the outermost surface layer.
Contents
The probe may be applied in several areas for the following determinations:
AREA |
PROPERTIES |
Solid-State Chemistry |
Defect chemistry, non-stoichiometry, chemical diffusion, gas/solid reactions |
Solid-State Physics |
Charge transport, semiconducting properties, bi-dimensional structural transitions |
Materials Science |
Equilibration kinetics, phase transitions in the boundary layer, electronic structure |
Metallurgy |
Mechanism and kinetics of oxidation and reduction processes |
Catalysis |
Mechanism and kinetics of chemisorption and catalytic processes |
Sensorics |
Determination of the sensing signal at the gas/solid interface |
Temperature |
RT - 1200 K |
Gas Environment |
Static/Dynamic |
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SIZE |
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Height |
1000 mm |
Base |
400x400 mm |
Weight |
21 kG |
Specimen's Size |
Approx 10x10x2mm |
Vibration Frequency |
30 Hz - 100 KHz |
Accuracy |
± 0.5 mV |
The HTKP-2000 and the ancillary equipment are shown, schematically, in Figure 1. The probe incorporates the following integral components: (1) vibrating system, (2) vibrating capacitor, and (3) tuning system (Figures 2 and Figures 3). The vibrating capacitor is formed of a specimen located on support and upper reference electrode. The electrode is put into vibrations using piezoceramic element. The distance between the capacitor plates (about 0.1 - 0.2 mm) is tuned using a micrometer attached to the sample support.
A tube-type furnace is located in the middle part (at the sample level). Upper and lower water coolers aim at preventing the upper and lower parts against heating. Gas inlet and outlet are connected to a gas flow system. The probe is gas tightened by two fringes. The specimen is located on a support. A rack allows to remove the lower part of the probe and to replace the specimen. Performance of the probe and its applications were reported in refs. [1-3].
Additional equipment items required for HTKP-2000 include: high voltage amplifier, voltmeter, frequency generator, integrator, oscilloscope, chart recorder, personal computer, temperature controllers, lock-in amplifier, scanner, gas-flow system, and oxygen sensor.
Figure 1. The HTKP-2000 and ancillary equipment
Figure 2. The HTKP-2000. Internal view (schematic)
WHY SURFACE PROPERTIES AT ELEVATED TEMPERATURES ARE IMPORTANT.
Properties of materials interfaces, such as surfaces and grain boundaries, are entirely different from those of the bulk phase [4,5]. It appears that most of materials properties are strongly influenced or even determined by interfaces. Concordantly, interfaces hold the key to tailoring materials properties for specific industrial applications.
There is an increasing need to understand interface properties and phenomena in situ during processing of materials at elevated temperatures and controlled gas phase composition. Application of the most of surface sensitive tools based on electron and ion spectroscopy, such as XPS, SIMS, ISS and LEED, is limited to room temperature (when materials are quenched) and high vacuum (when surface composition of compounds are different than those under atmospheric pressure).
The HTKP-2000 is a very powerful tool in hands of solid state chemists and materials scientists that allows to have an insight into the unknown world of surface properties at elevated temperatures under atmospheric pressure.
PRINCIPLE
Performance principle of HTKP is based on the determination of contact potential difference (CPD) between studied surface and a reference surface of known WF:
Eq(1)
where 
is WF of the studied specimen and reference surface, respectively.
While absolute WF of compounds has a complex meaning, the WF changes can be used for monitoring of several properties, such as chemisorption, description, catalytic processes, gas-solid equilibration, diffusion in the surface layer, structural transitions within the surface layer, redox processes and related charge transfer. According to Eq (1) we have:
Eq(2)
where 
denotes changes. Therefore, knowledge of the 
component is required for the determination of 
. The electronic circuitry for the determination of CPD using the HTKP-2000 is shown in Figure 4.
REFERENCE ELECTRODE
The HTKP-2000 is equipped with a Pt reference electrode. Figure 5 shows the WF changes of Pt, 
Pt, at elevated temperatures vs oxygen partial pressure, p(O2), [6]. These data may be used for the determination of using Eq(2). The determination of WF of the reference electrode in reducive atmospheres requires its calibration.
BASIC TERMS
WF is the work required to remove an electron from the EF level to the level just outside the surface where the electron is beyond electrostatic interactions [3]. Concordantly, the WF changes are equal to the EF changes
Eq(3)
Figure 6 shows WF of n-type semiconducting materials (in terms of a flat band model) where 
are WF values of non-stoichiometric oxides exhibiting low and high oxygen content.
Figure 6. Effect of p(O2) on Fermi energy at gas/solid equilibrium
The WF changes of oxide materials involve mainly the following two components [2,3]:
Eq(4)
where 
denote the WF changes related to (1) lattice nonstoichiometry and related concentration of defects, and (2) potential drop across space charge caused by chemisorption.
At high temperatures, when gas/solid equilibrium can be established rapidly, the WF changes are determined by the 
component. At moderate temperatures, when changes gas phase composition, such as p(O2), result in changes in the concentration of chemisorbed species (while the bulk remains quenched) the WF changes are determined by the 
component (Figure 7).
SENSITIVITY
The HTKP-2000 is extremely sensitive to the outermost surface layer. Figure 8 shows the changes of both WF, 
, and electrical conductivity, 
, of TiO2 during adsorption (A) and desorption (D) of oxygen at 300 K [7]. As seen, A results in WF increase by 0.7 eV and D leads to WF decrease by 0.4 eV (
denote reversible and irreversible WF change, respectively). As also seen, D results in a WF change by 
while 
remains constant.
These data indicate that WF is substantially more sensitive to the changes in the adsorbed layer while is not.
Figure 8. Effect of oxygen adsorption (A) and desorption (D) on WF of TiO2 at roomtemperature