Ceramic Electrodes Based on Magneli Phases of Titanium Oxides

Monophase and polyphase ceramic materials based on Magneli phases of titanium oxides of composition Ti3O5, Ti4O7, Ti5O9, and Ti6O11 were synthesized. The materials were obtained by mechanical activation of rutile with titanium and additives of niobium, vanadium, and iron, with subsequent sintering both in reductive atmosphere (hydrogen) and in neutral atmosphere (argon) in the temperature interval of 1060–1080°C. The dependences of the potentials of the obtained ceramic samples on time and composition during anodic polarization at current density of 5A/dm in a 1M solution of sulfuric acid were investigated. We developed a technique for manufacturing anodes in the form of hollow cylinders 60 mm in diameter with a wall 5 mm thick, and flat discs more than 60 mm in diameter.


Introduction
Magneli phases are compounds with the composition Ti n O 2n-1 , where n=4÷10.Such phases are known not only for the titanium-oxygen system.They have electronic conductivity and can be used as materials for electrochemical industry because they are rather resistant to aggressive media and function well as anodes in the range of potentials of oxygen evolution [1,2].
Besides, three compounds of composition Ti 4 O 7 , Ti 5 O 9 , and Ti 6 O 11 have the highest conductivity, and, correspondingly, they are most interesting.All these phases possess a triclinic system and very similar lattice parameters.This tells about the difficulty of obtaining a monophase product and performing identification of obtained compounds.We have worked on the synthesis of these phases before [3,4].We developed a procedure for obtaining the necessary compounds from initial mixtures of different compositions, with preliminary mechanical activation of these mixtures and annealing in a wide interval of temperatures using both reductive (hydrogen) and neutral atmosphere (argon).As a result, only some monophase samples were obtained, but most of the samples were still two-phase and threephase.In Fig. 1 a diagram of the conductivity of titanium oxides is shown depending on their oxygen content [5].The diagram is seen to have a rather complex configuration.Range 1 is the conductivity of most of the obtained samples which we have learned to synthesize well.The present work is dedicated to investigation of the dependence of ceramics potential on time during anodic polarization.It is also deals with the making of an actual electrode for a practicable electrochemical process by annealing products in large industrial furnaces.The electrode should be shaped like a ring about 60 mm in diameter and with a wall thickness no more than 5 mm.

Results and discussion
The usage of ceramics as an anodic material presupposes its tolerance for oxidation over a long period and invariability in its mechanical strength, specific resistance, and reaction surface.For qualitative estimation of the last two characteristics we used measurement of the potential of the studied sample in a 1M sulfuric acid solution with time during its polarization with direct anodic current with the density of 5 A/dm 2 .The values of anodic currents in actual electrochemical processes, as a rule, do not exceed this one.
Apparently, if during anodic polarization of the samples considerable changes in the abovementioned characteristics take place, it will cause changes in the value of anodic polarization included in the anodic potential which is being measured by us.
Fig. 2 shows the dependences of the potentials of the ceramic samples on time at their anodic polarization.The anodic potentials of samples 1-4 and 5-8 which correspond to the polarization current 5 A/dm 2 are rather close to each other.However, the second group of samples has two special features: 1.The potential increases by aproximately 1.5 times in the initial region of the curves.

There is considerable dispersion in the steady values of polarization potential (1 V).
The short initial potential increase was probably caused by oxidation of the products which were used for manufacturing samples and did not fully react.Dispersion in the values of polarization potential could appear because of different conductivity of the samples.Tab.I contains data on the sample conductivities, their phase composition, types of additives, and the sintering atmosphere.Samples 1-4 show the steadiest behavior with time.It may be due both to their greater homogeneity and to their lesser porosity.
A minimal shift of the potential from its steady value was recorded for samples 9-11.The shift is probably due to their higher initial conductivity.A small potential increase with time can be explained by various causes.The most likely of them is change in the area of the anodic reaction surface.It results from filling of anode micropores with oxygen gas, which is the main product of the anodic reaction.
From the obtained results it may be concluded that all of the above-listed synthesized materials can be used for manufacturing of anodes resistant to their own oxidation.
As remarked above, according to customer order, the anode should be shaped like a ring.In industry similar-sized ceramic sleeves are made on the basis of aluminum oxide by centrifugal casting.Paraffin serves as a binder.This mixture is stirred in thermostable barrels with blenders at 80°C.In so doing the amount of paraffin equals to 8-12 weight per cent.Then this rather fluidic mixture is poured into a rotating mold and cooled with water, and a ready detail is taken out.Later paraffin is burnt during heating up to 950°C, and after that the details are annealed at the necessary temperature.But we failed to use this technique.The reason is that in order to obtain a burden with the necessary casting characteristic we had to add up to 30-35 weight per cent of paraffin to our powders, and it is almost half the volume.If we did that we would obtain products of a suitable composition but with a high porosity.
To produce details with lesser porosity, we developed and made a press-mold which functions both with unilateral and bilateral compression.The compacting force was 500-1000 kg/cm 2 .Paraffin in the amount of 8 weight per cent, glycerin, and vegetable oil served as binders.
It is known that the addition of a small amount of pentavalent metals to titanium oxide burden improves the electrochemical properties of the produced materials.We studied addition of vanadium, niobium, and iron, and achieved good results.That is why we decided to make material details with the composition Ti 3.95 V 0.05 O 7 and Ti 3.95 Nb 0.05 O 7 .Several samples from the series of the carried-out experiments were taken to illustrate the obtained results.
Sample N70.Binder -paraffin, 8 weight per cent; bilateral compression with the force 500 kg/cm 2 .The first annealing: burning of the binder at 1040°C, the cycle was 11 hours in a charge of corundum in air.The second annealing was in hydrogen, and it took 4 hours at 1060-1080°C without taking out of the charge.As a result, according to the data of X-ray phase analysis, a two-phase sample formed, which contained the phases Ti 4 O 7 and Ti 3 O 5 and had the conductivity of 24 S cm -1 , density of 2.9 g/cm 3 , and shrinkage of 7.7 per cent over the outer diameter.
Sample N66.Binder -glycerin, 5 weight per cent.The pressing force was 1000 kg/cm 2 .The first annealing was at 950°C in a charge of corundum in air.The second annealing was in hydrogen without a charge at 1080°C and took 2 hours.As a result, a threephase sample formed, which contained Ti 4 O 7 , Ti 5 O 9 , and Ti 6 O 11 .Its conductivity came to 240 S cm -1 , the density was 4.3 g/cm 3 , and the shrinkage over the outer diameter was 8.3 per cent.This sample is seen to be considerably more dense than the previous one in similar annealing modes, and its conductivity is also considerably higher.However, the shrinkage is high in both cases and is within 8 per cent over the outer diameter.
To reduce product shrinkage, it was decided to first synthesize the material and then to make an electrode of it.We added pure titanium (powder) to the burden as a reductive agent for TiO 2 .But at temperatures within 1000°C, at which paraffin burning takes place, titanium can oxidize.So, the technique available was changed.Paraffin burning was carried out in furnaces at 240°C for 36 h in a charge of corundum.Paraffin which is burning out of a detail creates a kind of coat around it.This coat consists mainly of carbon and protects the material from oxidation.Afterwards the details were sent to reductive annealing without being taken out of the charge.Annealing was completed in hydrogen at 1000°C, and it took 4 h.According to the X-ray data, a two-phase product was obtained; it consisted of Ti 4 O 7 and Ti 3 O 5 .It was ground, and electrodes were pressed of the powder.During pressing paraffin, glycerin, and vegetable oils were used as binders.Annealing of the products was carried out in a corundum charge at 1060-1080°C in hydrogen, and it took 4 h.The obtained electrodes also had a two-phase structure with approximately the same correlation of phases as the initial product.The conductivity of the products was from 100 to 250 S cm -1 , the density was from 2.4 to 2.7 g/cm 3 , and shrinkage over the outer diameter was from 2.2 to 0.5 per cent.
As the obtained results indicate, with preliminary synthesis of the product and in the same modes of annealing, making of the products with considerably lesser shrinkage is a success.It allows us to hope that we will manage to introduce a wire grid or a wire spiral into the product, which can be used as a contact for the produced electrode.The first attempts to introduce such a spiral into a detail carved of initial raw material were not successful.During sample heating, the titanium spiral, which has a rather high coefficient of thermal expansion (at 300°C α=8.3⋅10 -6 K -1 ; at 1000°C α=12.8⋅10 - K -1 ), broke the product still unsintered.When sintering, the pieces even shrank, so the product had many cracks (see Fig. 3, sample 1).As it was mentioned above, the electrodes were sintered not only in hydrogen but also in argon.The given X-ray diagrams (Fig. 4) show that during annealing in hydrogen the composition of the samples shifts towards phases with more titanium whereas during annealing in argon it shifts towards phases with more oxygen.In so doing in all of the experiments the amount of titanium and rutile (TiO

Conclusion
We investigated the dependences of the potential of ceramic samples of various composition on time during their anodic polarization.It follows from the obtained dependences that these materials can be used for manufacturing insoluble anodes.
We developed a technique for pressing and sintering of electrodes under industrial conditions.Products of the needed shape and conductivity were made.To do it, we designed and made press-molds for manufacturing anodes in the form of hollow cylinders and discs large in diameter.

Fig. 2
Fig. 2 The potential-time dependence of samples during their anodic polarization.Anodic current with the density of 5 A/dm 2 in a 1M solution of sulfuric acid.a) Samples of composition Ti 3.95 V 0.05 O 7 (2 and 4) and Ti 3.95 Nb 0.05 O 7 (1 and 3) with the conductivity of 160÷340 S cm -1 .b) The initial ratio titanium vs titanium oxide corresponds to the stoichiometric compound Ti 4 O 7 .The conductivity is 150÷450 S cm -1 .c) Samples of the initial composition Ti+TiO (50:50 mol.%).The conductivity is 330÷500 S cm -1 .