Background of the invention

The present invention relates to sensors for monitoring oxygen partial pressure (hereinafter referred to as "pdV) in industrial ambients and/or processes where pθ ₂ and temperature play a relevant role, said sensors comprising a chemical composition containing one or more ceramic materials sensitive to the oxygen present in the surrounding atmosphere wherein the sensitivity of the said composition is shown by the colour assumed by said composition when it is exposed to oxygen containing atmosphere and temperature of said industrial ambients and/or processes.

In the description of the invention and in the claims, the product (s) of the invention is/are identified as "Chromatic Ceramic Oxygen Sensor (s)s or CCOS (s)". In the area of gas sensing, oxygen detection in gaseous mixtures, usually at high temperature, is very important. Many industrial processes are heavily biased by the solid-gas interaction and several different types of oxygen sensors have been developed for this kind of application. The main interest is for the sensors that can be used "in situ", accomplishing the measurement in the same ambient where the gas is produced or flows. They are most commonly applied for industrial process monitoring (steelworks, kilns) and to control combustion emissions, included the ^' monitoring of automotive exhaust pollutant emissions (e.g.: lambda-probes).

Nowadays, the most diffused pθ ₂ sensors involve the measurement of an electromotive force that is correlated, by the Nernst law, to the amount of oxygen present into the examined medium (gas or liquid) . Ionic and electronic conductivity is the most important property for the way these sensors operate and in most cases they are made by a solid electrolyte based on yttrium doped zirconium oxide (see e.g.: US5122487) . Other kinds of sensors exploit the oxidization

state of metallic compounds but in all cases with the purpose of inducing a change of the ionic and electronic conductivity.

GB 2224105 A describes a method for controlling the oxygen partial pressure in a furnace wherein the oxygen measuring instrument used is based on commonly known electrochemical sensors whereby the pθ ₂ is determined through voltage changes in a cell feed with the gas to be measured, This document does not describe or suggest any use of the change of colour of ceramic materials exposed to temperature and oxygen atmosphere as a means for monitoring the oxygen partial pressure.

A different type of sensor exploits the property of metal oxides doped with alkaline-earth metals, having generally the structure of a perovskite, to change their resistivity when exposed to O ₂ atmosphere (see e.g.: US5843858). Beside pθ ₂ sensors based on the measurement of electric properties of the sensor material, other devices are known which exploit the chromatic changes induced by the reactions of oxygen with the sensor. For instance, GB2298273 describes an oxygen indicating composition comprising a ferrous component, a ferrous ferrocyanide additive and L-absorbic acid utilized for O ₂ detection in food packages by a colour change of the composition.

The above described sensors present several disadvantages, which detrimentally influence their applicability. For example, the sensors based on the measurement of the electromotive force, because of the necessity of being electrically connected with the external environment, cannot be placed in all appropriate positions of an industrial kiln or of a system where a combustion process occurs, as it would be required for obtaining a pθ ₂ map inside the kiln or the system, which is a highly desirable feature in several industrial processes.

To the best knowledge of the applicants there are no industrial products or descriptions of devices in the scientific literature whereby the reactions between solids and

gas containing oxygen or between solids, liquids and gas containing oxygen induce changes in their phase compositions producing chromatic variations which are utilized to measure the oxygen partial pressure. A. Muan: "Reaction between Iron Oxides and Alumina-Silica Refractories", in Journal of the American Ceramic Society, 1958, Vol. 41, No. 8, 275-285, describes phase equilibrium data for the system FeO-Fe ₂ O ₃ -Al ₂ Os-SiO ₂ to evaluate the extent of attack of iron oxide on alumina-silica refractories at various pθ ₂ . This paper, although discussing the phase equilibria in the above system, does not give any indication about the possibility of using the colour changes of a ceramic material based on said system for measuring the oxygen partial pressure in industrial ambients or processes where both pθ ₂ and the temperature are relevant factors.

US 4,447,473 discloses a method of producing decorative objects forming a metal thin film on a glazed an baked surface of a base body and treating said base body at high temperature in oxygen atmosphere. Said treatment causes superficial oxidation of the metal thin film to form fine ruggedness or unevenness on the surface of the object causing light interference patterns layers and providing ornamental effects. In this case the colour effect is due to a different mechanism. This document does not teach or suggest using the change of colour due to phase transitions or other variations occurring within the same phase assemblage, caused by oxygen at certain temperatures to monitor the oxygen partial pressure in industrial ambients or processes where pθ ₂ and temperature play a relevant role. A. J. Faber "Optical properties and redox state of silicate glass melts", CR. Chimie 5 (2002) 705-712 describes some properties of silicate glass melts including high temperature optical absorption spectra influenced by the presence of transition metals like Fe and Cr and by OH groups and the redox states characterized by the equilibrium partial pressure

of oxygen dissolved in the melt. Although these properties are suggested for potential use for control of glass quality and colour of the final industrial product, the paper does not provide any indication or suggestion that colour changes occurring in a wide group of ceramic materials due to phase transitions or other variations in the same phase assemblage promoted by pθ ₂ and temperature may be used for the manufacture of sensors for monitoring the pθ ₂ in industrial ambients and processes .

Summary of the invention

This invention relates to sensors for monitoring oxygen partial pressure (pθ ₂ ) in industrial ambients and/or processes where the oxygen partial pressure and temperature play an important role comprising a chemical composition containing one or more ceramic materials sensitive to the oxygen present in the surrounding atmosphere wherein the sensitivity of said compositions to the oxygen partial pressure is shown by the colour assumed by said composition when it is exposed to the oxygen containing atmosphere and temperature of said industrial ambients and/or processes (Chromatic Ceramic Oxygen Sensor or CCOS), characterized in that said ceramic material (s) comprise (s) (a) one or more chemical element (s) able to present at least two positive oxidation states besides the zero oxidation state, at least one of said chemical elements being present in the ceramic material (s) in at least one positive oxidation state (essential element (s) ) , and, optionally, (b) one or more chemical element (s) being present in the ceramic material (s) in a positive oxidation state which does not change when exposed to the oxygen containing atmosphere and temperature of said industrial ambients and/or processes (subsidiary element (s) ).

Another aspect of the present invention relates to a new application or use of chemical compositions containing one or more ceramic material (s) comprising one or more essential

element (s) and, optionally, one or more subsidiary element (s) as components of the above mentioned Chromatic Ceramic Oxygen Sensors utilized for monitoring pθ ₂ in industrial ambients or processes where pθ ₂ and temperature play a relevant role. A further aspect of this invention relates to a method of monitoring pθ ₂ in industrial ambients and/or processes where pθ ₂ and temperature play a relevant role by using the above mentioned Chromatic Ceramic Oxygen Sensors.

The sensor disclosed in the present invention comprises a chemical composition containing one or more materials sensitive to the oxygen present in the surrounding atmosphere and to the temperature. The raw materials used as components of the sensor can be set by cohesion to form a compact product with a defined shape and size. Otherwise, in case of raw materials that undergo complete melting during the sensor operation, the raw materials can be set and pressed into a container (i.e. a small container made by inert components). In both cases the obtained product is hereinafter referred to as "green ceramic body" The green ceramic body, when heated in oxygen containing atmosphere produces, through reactive sintering, or melting or combination of the two processes, a transformation of the green ceramic body into a sintered or fused ceramic body wherein a phase assemblage change occurs caused by the concomitant action of oxygen partial pressure and temperature (hereinafter with the term "sintered ceramic body" a sintered body as well as a partially sintered and partially fused or totally fused and finally solidified body is intended) . In the sintered ceramic body a new arrangement of crystalline and glassy phases ("phase assemblage") is recognized and identified simply by its colour. Said new arrangement may be due to a change either in the phases composition, or in their mutual proportions or to the presence of different oxidation states of the same essential element in the same phase or to a variation of the grain size in the crystalline and/or glass

phases. The correlation between PO2, phase assemblage and colour is pre-investigated and indexed for a wide range of compositions, of temperatures and pθ ₂ values. On the base of these correlations the characteristic chromatic variations assumed by the sintered ceramic body can be immediately evaluated and connected with the pθ ₂ value. It is possible and it may be convenient to use simultaneously a series of sensors of different composition in order to have an accurate determination of temperature and pθ ₂ values. In this case, the evaluation of the different p0 ₂ conditions the sensors have undergone inside the kiln or the combustion system can be achieved through the colour combinations assumed by each sensor.

Colours can be recognized without any particular analytic support. Tables where the chromatic variations are summarized, can be useful for this purpose. The analysis of chromatic variation can anyway be automated through the software recognition of the colorimetric index or of the spectral shapes of absorbed light. The CCOSs are intended as throwaway sensors for pθ ₂ monitoring since, after exposure to temperature in atmospheres of industrial ambients and/or processes containing oxygen, they undergo irreversible transformations and may not be reused for further pθ ₂ monitoring cycles. With the term CCOS as used herein it is intended the green ceramic body as well as the sintered ceramic body which forms after exposure to temperature and pθ ₂ .

Advantages of the invention The use of a green ceramic body as throwaway CCOS presents the notable advantage to make possible a space mapping of pθ ₂ variation in industrial ambients or processes, where oxygen containing gas is employed and/or emitted, for example, in kilns for firing and sintering ceramic materials or in combustion process. In fact the sensors of the present

invention can be positioned inside the atmosphere which is to be monitored according to a variety of different ways, since they can be moulded in different shapes and sizes according to specific requirements. This constitutes a great advantage as the recording of the value of pθ ₂ may be carried out at all the desired positions. The methods of using the present invention are analogous to the methods of using pyrometric cones, commonly applied in the ceramics industry to control the thermal path of the product during the firing cycle. Moreover, according to this invention, the use of ceramic compositions activated by different pθ ₂ and temperature values permits to broaden the set of information available for any step of a process, allowing a control of pθ ₂ value extended over space and time. As mentioned above the CCOS are placed inside the kiln or the system where a combustion process occurs and, therefore, they may control the whole thermal cycle during which the value of pθ ₂ is exposed to drastic changes. In particular, in said cases it may be useful to place inside the kiln or the system several CCOSs, each of which is sensitive to a specific temperature and pθ ₂ range. This will allow a better knowledge of the pθ ₂ value on certain peculiar stages of the process. Further advantages of the invention are the low cost, the independence on any electric measurement system, the independence on reference samples or systems commonly adopted by conventional sensors.

Detailed description of the invention

Phase diagrams of chemical systems represent the starting point on which the invention is based. With the term "chemical system" it is herein intended to mean a chemical composition consisting of inorganic phases which form ceramic material (s) comprising one or more chemical element (s) able to present at least two oxidation states besides the zero oxidation state, at least one of said chemical elements being present in the ceramic material (s) in at least one positive oxidation state

(essential element (s) )

The positive oxidation state (s) assumed by said essential element (s) in the ceramic material (s) are related to the pθ ₂ and temperature conditions to which they are exposed. In other words, an oxidation state of an essential element in a green ceramic body may change to another oxidation state of the same essential element in the corresponding sintered ceramic body, i.e. after exposure of the green ceramic body to the pU2 and temperature conditions of the industrial ambients and/or processes where the pθ ₂ is to be monitored.

Examples of said chemical elements may be selected from the transition elements. Typical examples of said chemical elements which can be advantageously utilized as essential elements according to this invention are the following: Fe, Cu, Mn, Cr, Ti, Ni and Co, preferably, Fe, Cu, Mn and Cr.

The ceramic material (s) comprise (s) also an amount of oxygen in its negative oxidation state (0 ^~2 ) which is sufficient to electrostatically balance the positive oxidation state(s) presented by said essential element(s). Accordingly, the chemical systems Fe-O, Cu-O, Mn-O, Cr-O, Ti-O, Ni-O and Co-C, preferably, Fe-O, Cu-O, Mn-O and Cr-O, may be employed to exploit the properties of the essential elmenent(s) according to this invention. The oxygen amount may be partially replaced, in particular in the green ceramic body prior to the exposure to the oxygen containing atmosphere and temperature, by some equivalent amounts of other chemical element (s) presenting a negative oxidation states or forming with oxygen negative ions. Examples of said elements are carbon, nitrogen and phosphor which, can be introduced in the green ceramic body under the form of carbides, nitrides, phosphates or carbonates.

Preferably, the above said chemical systems contain also one or more chemical element (s) being present in the ceramic material (s) in a positive oxidation state which does/do not change when exposed to the oxygen containing atmosphere and

temperature of the industrial ambients and/of processes where the pC> ₂ is to be monitored (subsidiary element (s) ) .

The oxidation state (s) of the subsidiary elements in the ceramic material is/are balanced by the presence of an equivalent oxygen amount in the negative oxidation state (Cf ² ) .

Preferably, the chemical elements that can be utilized as subsidiary elements, which, although not essential, are important for the embodiment of the invention, are selected from Al, Mg and Si. In such case, the chemical systems Al-O, Mg-O and Si-O is/are employed to introduce the subsidiary element (s) into the ceramic material (s). Such subsidiary elements, possibly in minor amounts, allow the occurrence of crystalline phases that, otherwise, may not take place. For instance, in the system Fe-Al-Si-O the phase fayalite appears only if Si is available in the 'System which is able to incorporate Fe2+, leading to unique colour features. Moreover, the presence of one or more of said subsidiary elements in the respective oxidation states (Al ⁺³ , Mg ⁺² , and Si ⁺⁴ ) may extend the stability of a crystalline phase even at more severe conditions of pθ ₂ and temperature, due to the property of crystals to form solid solutions. E.g. the spinel magnetite FeFe ₂ O ₄ , in the chemical system FeO-Fe ₂ O ₃ -SiO ₂ is not stable at pθ ₂ lower than 10 ^~10 atm O ₂ at liquid temperatures, while it persists at extreme reducing conditions (pθ ₂ lower than ICT ¹³ atm O ₂ ) if Al ⁺³ is added to the system, thus allowing the formation of a spinel phase enriched in the final form as hercynite FeAl ₂ O ₄ , again leading to unique colour features.

According to this invention where CCOSs are used, the redox reactions may occur in both directions. Therefore, in the manufacture of the green ceramic body, in addition to or in the place of the materials that according to the language commonly utilized in the ceramics art are defined as oxides, can be employed as raw materials also other phases, such as the phases that in the common language of the ceramics art are defined as non-oxides, such as carbides and/or nitrides and/or

phosphates, metals or alloys, or mineral phases such as carbonates. Usually, these raw materials during the sintering phase or melting or combination of the two processes totally or in large part decompose and/or bind to oxygen becaming part of an oxide or silicate phase when the green ceramic body is submitted to the pθ ₂ and temperature conditions to be measured. The redox reactions occurring in the ceramic material (s) containing the essential element (s) generate a chromatic change in the CCOS (s) which is taken as a measure of the pθ ₂ in the industrial ambient (s) or processes where pθ ₂ and temperature play an important role.

As this process can occur only at certain temperatures and pθ ₂ values, the above said chromatic variations can be observed only if the above said conditions are reached. In the manufacture of the green ceramic body according to this invention also organic materials which do no have any functions on the redox reaction, and that do not form or became part of the inorganic phases in the sintered ceramic body but, may act, for instance, as binders (e.g. carboxymethylcellulose) , can be added to the other raw materials.

According to this invention, when the above described CCOSs (in the form of green ceramic bodies) , are exposed to variable range of pθ ₂ and temperature phase assemblage changes are promoted. With the term "phase assemblage change" or "change in the phase assemblage" is herein intended a different association of chemical phases (i.e. crystalline solids, amorphous solids, liquids, gas) , which form from the green ceramic body via reactive sintering or melting or combination of the two processes. This process occurs in accordance with the thermodynamic properties of the relative chemical system, which are illustrated in the relevant phase diagrams.

For the embodiment of the invention it will be necessary to know the effect of the pθ ₂ on the nature of the phases constituting the sintered ceramic body, extending the

knowledge over a variable range of temperatures and chemical components. In this way it is possible to define the appropriate composition of the raw material mixtures which enable to identify, through the colour assumed by the above said reactive sintered ceramic body, any changes in the pθ ₂ during a variety of industrial processes.

The main chemical systems that have been found to be of relevance for this purpose are Fe-Al-Si-O (Muan A & Gee CL 1956, J. Am. Ceram. Soc, 39:207-215; Muan A 1957a, J. Am. Ceram. Soc, 40:121-133; Muan A 1957b, J. Am. Ceram. Soc, 40:420-431), Mn-Al-Si-O (Snow RB 1943 J. Am. Ceram. Soc, 26:11-20; Morris AE & Muan A 1966, J. Met., 18:957-960), Cu- Al-Si-O (Gadalla AMM & White J 1964, Trans. Br. Ceram. Soc, 63:39-62), Mg-Ca-Cr-Al-Si-O (Decterov SA & Pelton AD 1996, J. Phase Equilibria 17:476-487; Decterov & Pelton 1996, J. Phase Equilibria 17:488-494; Jung IH, Decterov SA, Pelton AD, J. Phase Equilibria 25 : 329-345) . Some other elements such as Co, Ti, Ni, might be considered as well.

As an example of the chemical system suitable for carrying out this invention, the system Fe-Al-Si-O is considered in more detail here below. The system Fe-Al-Si-O has been experimentally investigated and reviewed by Muan (1958, J. Am. Ceram. Soc, 41:275-283 and references therein). At pθ ₂ of 1 atm the condensed phases in equilibrium with a liquid are: silica (tridymite or cristobalite depending on temperature) , mullite with variable Fe ₂ O ₃ ZAl ₂ O ₃ and SiO ₂ / (Al ₂ θ ₃ +Fe ₂ θ ₃ ) ratio, spinel solid solution (essentially solid solution between magnetite and hercynite) , hematite with some Al ₂ O ₃ in solid solution, corundum that can incorporate some Fe ₂ O ₃ in solid solution and the so called phase 1:1, a mixture of Fe ₂ O ₃ and Al ₂ O ₃ in variable ratio (Majzlan J, Navrotsky A & Evans BJ 2002, Phys. Chem. Miner. 29, 515-526).

It is known that reducing conditions favour mineral phases containing Fe ²⁺ , while oxidizing conditions favour crystal structures able to accommodate Fe ³⁺ . Intermediate conditions

would therefore be appropriate to stabilize phases that incorporate both Fe ²⁺ and Fe ³⁺ such as spinel solid solution

(spinel s.s.)- Therefore, as hematite and the 1:1 phase can accommodate only Fe ³⁺ , they are stable only at pθ ₂ values which represent relatively high oxidizing conditions (the phase 1:1 is stable at pθ ₂ higher than 0.4 atm, hematite is stable at pC> ₂ higher than 0.2 atm). On the contrary, spinel solid solution is strongly favoured at intermediate pθ ₂ values. When the pθ ₂ values denote more and more reducing conditions (pC> ₂ values from 10 ^"10 to 10 ^"13 atm) Fe-cordierite, fayalite and wϋstite appear together with spinel solid solution at liquids temperatures in chemical systems with low Al content. Under the most extreme conditions (pC> ₂ = 10 ^~13 atm) spinel solid solution can coexist with metallic Fe. The chemical composition of spinel moves toward Al-rich endmembers as pθ ₂ decreases, with all iron oxide occurring in the ferrous state and all Fe ³⁺ being substituted by Al ³⁺ .

The sensor of this invention is therefore based on the promotion of phase assemblages produced in a sintered ceramic body. Oxidizing conditions (high pθ ₂ , arbitrarily considered equal to or higher than air oxygen partial pressure, i.e. 0.21 atm. O ₂ ) favour higher oxidation states, whereas reducing conditions (lower pθ ₂ , arbitrarily considered in the range of 10 ^~3 to 10 ^~13 atm) favour lower oxidation states. Variations in such mineral phase assemblage result in variations of the colour of the sintered ceramic body, as a direct function of the pθ ₂ present in the combustion process or kiln atmosphere.

Liquid composition contributes to define the final colour, if fast cooling rate prevails after the peak temperature of the sintering process (or melting or combination of the two processes) is reached. The glass derived by quenching the liquid may have different colours depending on the oxidation state of Fe. For instance, in the Fe-Al-Si-O-system, as pθ ₂ decreases from 0.9 to 0.4 atm, the content of FeO in the glass phase quenched from temperatures above the liquid phases

increases from 15 wt.% to 17.1 wt.% at the expense of Fe ₂ Os, that decreases from 40.3 to 37.9 wt.% (Muan A 1957, J. Am.

Ceram. Soc, 40:420-431). At more extreme pθ ₂ conditions, and in more complex alkali-bearing systems, as pθ ₂ value decreases from about 10 ^"1 to 10 ^"10 atm O ₂ (at T=1458°C), FeO content in a mafic (basaltic) silicate melts varies from 3.00 to 8.44 wt.% and Fe ₂ O ₃ from 10.02 to 0.52 wt.%. Such a variations do not appear to be dependent on the chemical composition of the melt

(Kress VC & Carmichael ISE 1988, Am. Mineral., 73, 1267-1274). Because in industrial applications the composition of the gas phase might be either at a constant value (e.g. fixed by combustion) or might evolve with temperature evolution, as buffered by the ceramic materials present in the furnace, two different scenarios have to be considered in applying phase diagrams to the present invention. In the first case, the composition of the gas phase is kept constant, the system has lower degree of freedom and as a result the liquid phase cannot move out from an isobaric surface. In the second case, the pO2 is varying and the system has a higher degree of freedom.

According to this invention the phase diagrams allow the expert in the field to predict the stable and metastable phase assemblages at given pθ ₂ and temperature values. This implies that the observation of the phase assemblage in a specifically conceived sintered ceramic body allow the determination of the pθ ₂ attained by this body, once temperature evolution is known. Because the sintered ceramic body displays a colour which is function of the amount and chemical composition of the crystalline phase and glass phase present, chromatic determinations can be used as a direct monitoring of pθ ₂ attained by the sintered ceramic body.

As indicated above, chemical systems which have been demonstrated to be sensitive to changes in pθ ₂ value and which display their sensitivity through colour changes are those that incorporate at least one essential element presenting at

least two oxidation states besides the zero oxidation state. Typically the oxidation states of the essential elements utilized in the sensitive ceramic material of the CCOS include the following: Fe ⁰ ' ⁺² ' ⁺³ , Cu ⁰ ' ⁺¹ - ⁺² , Mn ⁰ ' ⁺² ' ⁺³ ' ⁺ \ Cr ⁰ ' ⁺² ' ⁺³ ' ⁺ \ _τi o, +2, +3, ^ _{1 Ni} o, +i, +2, ₊ 3^ _Co o, +i, +2, +3^ _pre ferably Fe ⁰ ' ⁺² ' ⁺³ , _Mn o, +2, +3, +4^ _Cu o, +i. ₊ 2 _{f Cr} o, +2, +3, +^ _{most pre} ferably Fe ⁰ ' ⁺² ' ⁺³ . As specified above, at least one of the essential elements utilized in the oxygen sensitive ceramic material (s) must be present in at least one positive oxidation state. Although a large number of elements are able to present different oxidation states, the statistic distribution of each oxidation state for each element shows that some systems should be preferred for the embodiment of this invention. For example, Ni has 3 different oxidation states besides the zero oxidation state but out of 301 complexes, 294 present Ni ⁺² , 4 Ni ⁺¹ and 3 Ni ⁺³ (based on coordination complexes retrieved from the Cambridge Structural Database by Moore and co-workers; Venkataraman D, Du Y, Wilson SR, Hirsch KA, Zhang P & Moore JS 1997, J. Chem. Ed., 74:915-919). On the basis of such statistical distribution, there are not many possibilities of attaining chemical and environmental conditions suitable to stabilize Ni-bearing compounds with variable Ni oxidation states. On the contrary, the more widespread occurrence of Fe- bearing compounds with variable oxidations state makes the system Fe-Al-Si-O particularly suitable for the embodiment of the invention.

According to the invention, the above said essential elements (and preferably also the subsidiary elements) are incorporated into the sintered ceramic material as components of crystalline phases and glass phases, which are produced by exposure to the conditions (pθ ₂ and temperature) existing in the areas where the pθ ₂ values are to be determined. The crystalline phases usually involved in the sintered ceramics body of the CCOS preferably comprise metal Fe (Fe ⁰ ) , silicates, oxides of the essential elements as well as other oxides

and/or silicates of the subsidiary elements e.g. AI2O3. Furthermore, depending on temperatures, a liquid phase might develop during the firing cycle and, if the process implies fast cooling rates, a glass might result among the produced crystalline phases as a quench product of a liquid stable at higher temperatures. As a result, not only the crystalline phases but the glass phases as well, if present, contributes in defining the colour of the whole product. Isothermal planes of phase relations at different O ₂ pressure conditions are useful in predicting the nature and compositions of the phases, the quenched liquid included. If the process implies a slow cooling rate, the final assemblage of the sintered ceramic body of CCOS contains an arrangement and an amount of crystalline phases that depends on the crystallization paths it has undergone; said features can be determined through the known experimentally derived phase diagrams. In both cases of fast cooling and slow cooling rates the relative proportions of final phases, the composition of liquid and crystalline phases and the variations in the composition of the gas phase are deducible by known phase equilibrium diagrams.

The essential parameter for determining the pθ ₂ value is the colour attained by the sintered ceramic body sensor. The colour which is observed at the end of the technical process is the sum of the colour assumed by each of the phase constituting the sintered ceramic body. In Table 1 are represented the colours of some materials taken as examples of the expected condensed phases, i.e. both crystals and glass. The colour observed in the sintered ceramic body sensor used as indicator of variations of the pθ ₂ value might be therefore considered as related mainly to:

1) a change in the phase composition of phase assemblage; as a first example of change in the phase assemblage a starting composition in the Fe-Al-Si-O system composed (by weight) of 20% Al ₂ O ₃ , 15 % SiO ₂ , 65% Fe ₃ O ₄ may be considered. If

it is assumed that the starting composition is treated at a temperature of about 1400 ⁰ C and then submitted to a fast cooling rate, the following picture might be drawn:

As a second example of change in the phase assemblage, it may be considered the starting compositions in the Fe-Al-Si-O system composed (by weight) of 30-40% Al ₂ O ₃ , 5-10% SiO ₂ , 50-60% Fe ₃ O ₄ (to total 100%), with hematite, corundum and clay as raw materials. These compositions are very sensitive in the range of 0,2-1 atm O ₂ and 1500-1700 ⁰ C. In this example for all compositions there are two possible phase assemblages; corundum s.s. -liquid eventually with metastable hematite s.s. (reddish coloured) and spinel s.s. -liquid (brownish coloured), that take place depending on the value of temperature, pθ ₂ and composition. By using simultaneously a set of compositions in the range described above and observing in which way the colours change with the phase assemblages changes, it is possible to determine precisely the pθ ₂ value of the process. For example, at temperature of 165O ⁰ C, the following compositions A, B and C may be used:

Composition

A 37% Al ₂ O ₃ 5% SiO ₂ 58% Fe ₃ O ₄

B 40% Al ₂ O ₃ 5% SiO ₂ 55% Fe ₃ O ₄

C 38% Al ₂ O ₃ 5% SiO ₂ 57% Fe ₃ O ₄

In such case , if :

- A, B and C assume a reddish colour, the pθ ₂ is higher of 1 atm; only B assumes a reddish colour, the pOa is in the range 0,8-1 atm;

B and C assume a reddish colour, the pθ ₂ is in the range0,2-0,8 atm.

2) Compositional variations of solid solutions within the same phase assemblage; in this case, due to the occurrence of solid solutions involving elements at different oxidation states, changes in colour related to variations in pθ ₂ are not necessarily related to changes in the phase assemblages, but might be governed by continuous increases or decreases of an oxidation state at the expense of the other. As an example it may be considered a Fe-Al-Si-O system composed (by weight) of 20% Al ₂ O ₃ , 70% SiO ₂ , 10% Fe ₃ θ ₄ . With the same assumptions of fast cooling rate, and T=1400°C, the same phase assemblage composition (tridymite + mullite + liquid) appears differently coloured depending on the pθ ₂ value, because of the different Fe ² VFe ³⁺ ratio in the liquid phase. Indeed, although phase proportions are similar (57-55% of tridymite; 21-22 % of mullite s.s. and 22-17% of liquid phase at pθ ₂ varying from 1 atm to extremely reducing conditions) , the final colour varies from a bluish green to a yellowish green tint due to variations in the Fe ² VFe ³⁺ ratio in the glass, showing a yellowish-green tint when Fe ²⁺ prevails and bluish-green colour when Fe ³⁺ prevails.

3) grain-size variations of crystal structures and glass phases; besides the colour that characterizes the single crystal of a chemical phase, i.e. the colour that one can expect when coarse grain size prevails (the "streak" colour) , also the colour due to the presence of fine grained materials of the same chemical phase, resulting from sluggish crystallization

or slow kinetics, must be taken into account. With the term streak it is intended the colour of the powdered mineral, which may differ from the body colour of the bulk mineral. The streak (as indicated in Table 1) is determined by drawing the mineral across a piece of unglazed, white porcelain (streak plate) . It is worth mentioning that in some cases the same element at the same oxidation state might present different colours, due to the difference in the crystal structure where it is incorporated. As an example, Cr ³⁺ causes both red and green colours depending on the crystal structure where it is hosted, as observed in pyroxene (green) and ruby (red) .

Nonetheless, the thermodynamically favoured crystal structure

(although containing the same element at the same oxidation state) may vary as a function of the value of pC> ₂ , providing further ground for the embodiment of this invention. As mentioned before, presence of glass also contribute to the final colour. Different oxidation states gives different colour in glass. As an example Fe ²⁺ gives a bluish-green tint while Fe ³⁺ a yellowish-green colour. Other examples of colours of glass phases are indicated in Table 1.

In order to enhance and control differences in colour of the sintered ceramic body sensor it is possible to profit from chromatic variations related to grain size (Table 1) through the use of selected starting raw materials in the green ceramic body. As an example, the usage of crystal seeds of the expected stable phases in the sintered ceramic body depresses the nucleation rate, promotes the growth rate and favour the colour of the large single crystal. On the contrary finegrained, strongly metastable raw materials in the green body promote fast nucleation during sintering (or melting or combination of the two processes) and, therefore, the formation of fine-grained phases in the sintered ceramic body, whose colour approaches the streak of the constituents.

TABLE 1

Preparation of the sensor

In a preferred embodiment, the preparation of the CCOS includes the following steps: a) weighing of the quantities of raw materials selected as components of the green ceramic body. The raw materials are selected among natural and/or synthetic materials. In particular, the materials usually employed in the field of ceramics are preferably chosen. In a preferred embodiment the raw materials which are selected to implement the invention are: oxides and silicates (typically: corundum, quartz, cristobalite, periclase, spinel, rutile, hematite, andalusite, clay, kaolinite, talc); carbonates ; non-oxides (e.g. carbides or nitrides); metallic phases (e.g. silicon, alluminum, alloys); glasses and sol gel. Organic compounds may optionally be added as powder binders (e.g. carboxymethylcellulose) . In some cases raw materials such as oxides and/or silicates can be advantageusly used, at least in

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21 part, in sintered form and/or may be transformed into a glass before their use as components of the green ceramic body. b) milling of different solid raw materials in order to achieve the required granulometry for each component. The granulometry of the compositions ranges from nanometric to very fine, with preferred values between 1 and 50 micron and for a coarse grain between 50 e 2000 micron. The same composition can include several components with different granulometry in order to promote mechanisms of compositional drift due to the partial, but progressive, re-absorption at the equilibrium of the material with coarser grain. On the other hand, fractioned crystallization paths can be set and activated by the use of highly reactive powders, promoting the early crystallization of crystalline phases that, in the subsequent stages of the process, remains segregated and inactive, keeping trace of the intermediate conditions. Both these mechanisms are useful to preserve information of all temperature-pθ ₂ path. c) mixing the components dry or wet for the time required to assure complete homogeneity. It is preferred to perform milling and mixing of dry components. It is further preferred, in case one of the components is in the liquid state, to add it at a second time respect to the dry powders, together with the amount of water requested to achieve the mixture consistency appropriated for the selected moulding process. d) moulding the green ceramic body according to the techniques well known to the experts in the ceramics art. In a preferred embodiment the moulding process is performed by pressing or extrusion in order to mould the mixture into simple shapes with different values of the surface/volume ratio (pastille, ring, cylinder, rod) . The surface/volume ratio is directly connected to the CCOS sensitivity, meaning that maximum sensitivity is achieved with a wide surface while sensors with large volume and limited surface present greater inertia. The CCOS sensitivity, that for the same composition

P2006/001631

22 is determined by the solid (detector) -gas (atmosphere) exchange surface, is obviously also connected to the compactness and open porosity of the material. Compactness and porosity are easily controlled acting on the parameters of the moulding process (i.e. drawing and pressing pressure), according to the well known techniques. The CCOS shape can be specifically adapted on demand, in order to dispose the sensor into the specific position of the industrial ambient and/or process where the monitoring of pθ ₂ is required (i.e. a hook shape to ease the hung up to an object) .

In case of raw materials that undergo complete melting during the sensor operation, the mixture of raw materials is set and pressed into a container that, when the materials are melted, avoids the liquid to flow away. Said container can be a small container or die made by inert materials that do not react with the sensor components.

According to the invention the CCOS can be optionally composed of an ensemble of different separated green ceramic bodies each of which is sensitive to specific pθ ₂ and/or temperature conditions.

Examples of applications

Further properties and advantages of the present invention will appear more evident through the description of the following preferred embodiments, exclusively intended for purpose of examples which are not limiting the invention and its applications. Example 1

According to a particularly preferred embodiment of the invention, CCOS are designed for application in the field of production of technical ceramics.

Titanium oxide is an excellent material to produce thread- guide for fine yarns in that the surfaces of the parts made by titanium oxide are lightly aggressive toward the thread and presents very low friction coefficients, although wear

resistance is still very high.

Titanium oxide can be modified to match the specification of good electrical conductivity, promoting the applications where the dissipation of the electrostatic charges accumulated on the thread is required. In fact, titanium oxide presents the oxidized (TiO ₂ ) and reduced (Ti ₂ O ₃ ) forms that can coexist in the same material generating a metastable state that permits the electric conduction, exploiting the deficiency of charges at the grain boundary. The two forms present completely different colorations: TiO ₂ is straw- coloured while Ti ₂ Os is black-bluish. The transformation from TiO ₂ to Ti ₂ O ₃ is achieved by a second thermal process which follows the first step of firing the Tiθ ₂ and is developed at a relatively low temperature (usually <1000°C) and atmosphere of Po ₂ = 10 ^~10 bar (usually with flux of nitrogen and 5% vol. hydrogen) .

In both processes, different oxygen partial pressure can lead to the generation of wastes:

- in the synterization of yellow titanate (Tiθ ₂ ) , variation of the colour due to different partial pressure cause waste

(even though for purely esthetical purposes) .

- in the partial reduction of black titanate (Ti ₂ Os) , a too high oxygen pressure produces a solid mixture having an electric conductivity value not sufficient to allow a proper application.

Usually the presence of these defects is related to inhomogeneity of pθ ₂ inside the kilns, due to the generation of fluxes inside the plans, that can be monitored very efficiently through CCOS produced by chemical systems containing Fe°' ^+2/+3 , which is much more sensitive to little variation of the oxygen pressure than the material undergoing the firing process. For this application can be used a CCOS composed by (by weight) 20% Al ₂ O ₃ , 15% SiO ₂ , 65% Fe ₃ O ₄ as previously described. With this CCOS an increasing reddish- brownish tonality is observed as the pθ ₂ grows up.

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Example 2

According to a particularly preferred embodiment of the invention, CCOS are designed for application in the field of production of traditional ceramics as porcelains.

Also for these materials the atmosphere of the kiln greatly affects the quality of the final product since it determines the final colour of the material, due to the presence of particular elements that change the colour depending on their oxidation state. It holds both for the base material and for surface decorations.

In the first case it occurs because of the presence of contaminants, like iron, that bind to oxygen in several different ways depending on oxygen concentration and lead to compounds having different tonality, that are not acceptable for high quality products.

On the contrary, for decorations, oxides mixture are used (based on iron, chromium, manganese, etc.), that naturally generate different colours depending on their oxidation state. Production plants usually have large size and therefore gas flows, which are influenced by the positions of burners and chimney, can easily determine local partial pressures which are quite different among themselves. Determination of pθ ₂ at various zones inside the kiln can not be achieved by usual probes available on the market because of the difficulty of their positioning inside the kilns. Said sensors allow only measurements of the average values of pθ ₂ at definite points into the plant. On the contrary the CCOSs, due to their versatility of shapes and to the low production cost, can be easily positioned at several selected points. In this way CCOS allow mapping the whole volume of the kilns with regard to pθ ₂ and temperature. This fact permit to easily identify the locations where the partial pressures are different with respect to the rest of the kiln and consequently to reduce the quantity of wastes.

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25

A CCOS apt to monitor the kilns used for the production of porcelain has been developed and tested. It is based on the system Iron Oxide-Al ₂ θ ₃ -Siθ ₂ . The raw materials are iron glass and Alumina-silica glass prepared by melting pure oxides in the following proportions:

The green ceramic body is made by mixing (by weight) 86% of Iron glass and 14t% of Alumina-silica glass, grinding the mix 30 minutes with alcohol and pressing it to form pastilles of 1.5 cm diameter and 0.5 cm thickness. The final composition of the green body contains (by weight) 8.3% FeO, 2.9% Fe ₂ O ₃ , 61.6% Al ₂ O ₃ , 27.2% SiO ₂ .

The green bodies were heated in a vertical tube furnace at a rate of 300°C/h up to 1200°C; the maximum temperature was hold for 24 h and then the pastilles were quenched to environment temperature .

In five different experiments the oxygen partial pressure inside the kiln was set at 0.4, 0.21, 0.1, 10 ^"5 and 10~ ⁸ atm by changing the gas composition of the ambient. After the sintering process, the image of the surface of the pastilles was analysed by a software (Image Analisys Software; Name: Image-Pro Plus; Producer: Media Cybernetics, Inc) that permits to establish the RGB indexes (RED-GREEN-BLUE: see the norm of the International Organization for Standardization: ISO 22028-1:2004 Photography and graphic technology - Extended colour encodings for digital image storage, manipulation and interchange) of the colour of the pastille. In the RGB Colour model pure white is represented by the maximum values 255-255- 255 and the minimum values 0-0-0 represent pure black. We have verified that the RGB indexes change linearly with the oxygen partial pressure from light grey to dark grey and finally to black as follows:

Example 3 According to a particularly preferred embodiment of the invention, CCOS are designed for application in the field of production of refractory ceramics .

A very important production of refractories relates to ceramic rollers for continuous firing kilns used in the industry of tiles (known as kiln rollers) . The production of this kind of refractories is commonly achieved by the Al-Si-O system in order to promote the crystallization of mullite (commercially named mullite rollers). The system may include additional phases, with the purpose to confer peculiar properties to the ceramics (i.e. zircon to improve the density, silicon carbide to improve thermal conductivity) . The fundamental raw materials required to crystallize mullite and used to produce mullite rollers are clays and caolins , allumina, quartz, andalusite, mullite. The production process involves a thermal treatment of about 24 hours and maximum firing temperature ranging from 1400 and 1600 ⁰ C, preferably performed by methane combustion kilns, wherein the operation is optimized in order to assure the best combustion reaction efficiency. It is important that mullite crystallization occurs completely because the remaining reaction products, like quartz, could easily transform during the process into the polymorphs cristobalite e tridymite, especially detrimental for the quality of the refractory. The possibility to fully develop the crystallization reaction of mullite depends on a multitude of variables regarding the maximum

temperature of the process, the firing thermal cycle, the selected raw materials and the reaction kinetic. The partial pressure of oxygen that the material undergoes during the thermal process is normally a highly neglected variable, also for the lack of appropriate instruments which would be required to establish its impact. Even without modifying the raw materials and the other process parameters, it is possible to modify the maximum temperature by the order of tenths of centigrade degrees simply by changing the oxygen partial pressure inside the kiln. Particularly, it is highly convenient to develop the firing process with a first step (destabilization of reacting phases) in highly reducing conditions and with a second step (formation of the reaction phases) in oxidizing conditions. In large methane kilns, that burns hundreds of m ³ /h of methane and combustive air, the modification of the atmosphere implies the risk of introducing inhomogeneity inside the material under treatment. It occurs especially when, in the attempt to develop a reducing atmosphere, the contribution of combustive air is decreased and, consequently, the yield of the kiln decreases. Conventional pθ ₂ measurement systems are inadequate to face this specific problem, because they allow only an average estimation of the firing atmosphere, even if the sampling zone of the gas is changed. By using CCOSs, placed at different zones inside the kiln and, optionally, also inside the material under process, it is possible to monitor the real conditions of pθ ₂ which the material is undergoing. This allow to understand the reasons why the quality of the final product is not homogeneous, not only when resulting from different firing cycles, but, also among the products resulting from the same firing cycle. CCOSs apt to specific application may be developed within the system Fe-Al-Si-O, starting from a mixing of clay, alumina and hematite in the proportion of 30%, 40% and 30%. The starting colour of the green ceramic body is heavy red (Pantone 804 2x; RGB 255,127,30), while the sintered

ceramic body, fired in a thermal cycle as described above at a maximum temperature of 1420-1440 ⁰ C, turn to colours ranging from metallic grey (Pantone 417; RGB 119, 114, 99) in case of reducing atmosphere (pθ ₂ =10 ^~5 ) , to heavy red (Pantone 180; RGB 193, 56, 40) for atmospheres from stechiometric combustion of methane with excess of air (pO ₂ =10~ ² -10 ^~3 ) .