CZ25362U1 - Thermoelectric material of Bi2-xSr2Co1,82O7+d type p of very high density - Google Patents

Description

Technical field

The technical solution relates to a new thermoelectric material based on strontium and bismuth mixed cobalt oxide suitable for the construction of high temperature thermoelectric batteries for converting waste heat into electrical energy.

Background Art

Due to the lack of electricity, in addition to new resources, there is a need to find ways to reduce the amount of heat loss in the industry.

The use of thermoelectric materials seems to be a suitable technology for direct energy regeneration.

Electric energy regeneration is based on the Seebeck effect, where electrical energy is generated as a result of the temperature gradient between the two ends of the conductor.

Mixed oxides Ca3Co3,93 _{± x} O _{9 + d} and _x BÍ2- Sr2Co, _i82 O7d (both PS positive thermopower) and Lai. _y Ca _y MnO3 (type n with negative thermosilane) belongs to the group of high temperature oxide materials for thermoelectric conversion.

Parameter x expresses the degree of deviation of composition from ideal stoichiometry, parameter d expresses the amount of oxygen in the structure (varies with increasing temperature) and parameter y expresses the degree of substitution of calcium for lanthanum.

A thermoelectric battery is produced by serial connection of these materials with the opposite sign of thermosilane (type p with type n; ie with different major charge carriers) placed in the temperature gradient.

The main advantage of thermoelectric cells is their simplicity: easy construction and absence of moving parts and working medium.

In order to maximize the efficiency of sample conversion, it must have the highest thermal conductivity (Seebeck coefficient) and the lowest possible thermal conductivity and specific electrical resistance.

The coefficient of thermoelectric efficiency ZT is a dimensionless quantity describing the efficiency of thermoelectric conversion. It is defined as: ZT = S ² * T / (Z * p), where S is a thermoset, λ is a thermal conductivity, p is a specific electrical resistance and T is the thermodynamic temperature.

The current thermoelectric batteries are constructed from intermetallic compounds: they have a high coefficient of thermoelectric conversion, but the application of these materials is complex: these substances are easily oxidized or decomposed at elevated temperature in air and often contain highly toxic elements (antimony, selenium, lead, etc. ).

These batteries consist of blocks of several millimeters of respective thermoelectrics connected in series so that their size ratio is inversely proportional to the ratio of their thermoelectric efficiency coefficient (the lower the efficiency, the greater the area these blocks must have).

These blocks of high-temperature thermoelectrics are made from finely ground powders of individual oxide materials by pressing on hydraulic presses and subsequent molding to the desired shape.

The blocks (beams) thus prepared reach about 65% of the maximum theoretical density, which is due to the special layered structure characteristic of this material (100% of the theoretical density corresponds to 6.81 g / cm ³ ).

The thermoelectric efficiency coefficient ZT does not reach such values in order to manufacture these materials industrially.

CZ 25362 Ul

The essence of the technical solution

The aforementioned drawbacks are remedied by a thermoelectric material of the p type whose composition is Bi ₂ . _x Sr ₂ Coi ₈₂ O _{7 + d} , where x ranges from 0 to 0.3 d, expressing oxygen stoichiometry, ranges from 0 to 1 according to the technical solution, which is that its density exceeds 90% of the maximum theoretical density, ie 6.13 g / cm ³ .

For the material with very high density Bi ₂ Sr ₂ Coi _{> 82} O _{7 + d} (x = 0), the physical properties (density, thermoset, thermal conductivity, specific electrical resistance) were measured and the coefficient of thermal efficiency ZT was determined.

A material with a density exceeding 97% of maximum theoretical density exhibits almost double the efficiency of heat to electrical conversion at 727 ° C (ie 1000 K) than was the case with material with 65% of theoretical density. (Figures 1 and 2).

It can be prepared by pressing at elevated temperature, whereby a density of up to 99% can be achieved, possibly by plasma sintering at elevated pressure.

In addition to the higher efficiency, this material also has much better mechanical properties and thus the lifetime of thermoelectric batteries can be expected to increase.

Clarifying drawings

Figure 1 shows a comparison of the microstructure of Bi ₂ Sr ₂ Coi _{j 82 O 7 + d} prepared by the classical method (density 65%), elevated temperature compression (98% density) and SPS (density 99%). (from Spark Plasma Sintering, ie plasma pressure sintering at elevated pressure). Taken on an electron microscope.

Figure 2 shows the dependence of the thermoelectric conversion coefficient ZT on the temperature for the material of different density. The temperature is in the thermodynamic scale (K).

Example of a technical solution

Example 1

Preparation by elevated temperature compression by solid phase reaction.

A substance with a chemical composition of Bi ₂ Sr ₂ Coi _{> 82} O _{7 + d} , was prepared from Co ₂ O ₃ (cobalt oxide, purity 99.99%), from SrCO ₃ (strontium carbonate, purity 99.9%) and from Bi ₂ O ₃ (bismuth oxide, purity 99.99%).

The powders were mixed together and homogenized, further calcined twice in a platinum crucible at 750 ° C (24 hours) and 800 ° C (24 hours), compressed on a hydraulic press at 0.1 GPa.

The finished moldings were sintered at 880 ° C for 100 hours in air.

The compacts (tablets) were ground in a planetary ring mill to produce a finely ground powder containing the Bi ₂ Sr ₂ Coi, ₈₂ O 7 _d phase.

The powder was then compressed on a special high pressure press at 5 GPa at 880 ° C for 15 minutes.

Density was measured for finished moldings (tablets) and blocks (bars) were prepared on which physical properties were measured.

The material thus produced reaches 98% of the maximum theoretical density, ie 6.67 g / cm ³ .

The microstructure is evident from the picture. 1 and the thermoelectric efficiency coefficient ZT is shown in FIG. 2.

-2CZ 25362 Ul

At 727 ° C, the thermoelectric efficiency coefficient ZT of the material thus prepared reaches 65% higher.

Example 2

High pressure plasma sintering from sol-gel precursors (SPS, Spark Plasma Sintering).

Phase BÍ2Sr ₂ Coi, 8 ₂ O7-d was prepared from metal acetates Bi (CH ₃ COO) ₃ (acetate, bismuth, 99% purity), Sr (CH ₃ COO) _2? / 2 H ₂ O (hemihydrate strontium acetate, purity 99.9%) and Co (CH ₃ COO) ₂ H ₂ O (cobalt acetate monohydrate, 99.9% purity).

Bismuth acetate was first dissolved in dilute acetic acid (2: 3), followed by addition of EDTA complexing agents (ethylenediaminetetraacetic acid, 99% purity) and TEA (triethanolamine, 98% purity) in a molar metal: EDTA: TEA = 1 ratio: 1: 1 and pH was adjusted to pH = 6-6.5 with NH ₃ (ammonia, aqueous 25%). Subsequently, other acetates were added to the solution.

The solution was gelated with stirring at 80 ° C. The resulting gel was first decomposed in a stainless steel vessel at 250 ° C for 2 hours and then calcined twice in a corundum crucible at 500 ° C (2 hours) and 800 ° C (2 hours).

The powder was then compressed on an apparatus allowing plasma sintering at 80 MPa at 800 ° C for 15 minutes.

The finished moldings were sintered at 880 ° C for 100 hours.

Extreme temperature rise and local melting occur at the boundaries of the electric discharge, which will allow the individual grains to "slide" one after the other, resulting in nearly perfect material compaction.

Density was measured for finished moldings (tablets) and blocks (bars) were prepared on which physical properties were measured.

The material thus produced reaches 99% of the maximum theoretical density, ie 6.74 g / cm ³ .

The microstructure is shown in Figure 1 and the coefficient of thermal efficiency of ZT is shown in Figure 2.

At 727 ° C, the thermoelectric efficiency coefficient ZT of the material thus prepared reaches 85% higher.

Industrial usability

In industry, this type of material can be used wherever more waste heat is generated, but a high temperature gradient is required for sufficient efficiency.

The most suitable application is the use of car exhausts, where the exhaust gas behind the catalyst reaches temperatures in excess of 700 ° C, where low-temperature thermoelectric batteries cannot be used.

Claims (1)

PROTECTION REQUIREMENTS 1. Bi ₂ - _x Sr ₂ Coi, ₈₂ O7 + _d thermoelectric material, type p, wherein x is from 0 to 0.3 and d is from Odd, characterized in that its density after sintering exceeds 6.13 g / cm ³ .