LITHIUM OXIDE (Li2O)

Lithium oxide (Li2O) with its high tritium breeding potential is an attractive candidate for the solid breeder material of a D-T fusion power plant, because of its high lithium atom density (compared to other lithium ceramics or metallic lithium) and its relatively high thermal conductivity.

Li2O will be exposed to high temperatures under neutron irradiation during the operation of fusion blankets. Under these circumstances, a huge number of irradiation defects will be produced in Li2O, such as helium-induced swelling, relatively high thermal expansion, grain growth, LiOH(T) formation and precipitation at low temperatures, and LiOH(T) mass transport at high temperatures. Furthermore, Li2O will be subjected to stresses arising from differences of thermal expansion between Li2O and structural materials. These characteristics of Li2O lead to challenging engineering problems both in fabrication and blanket design.

GENERAL PROPERTIES - LITHIUM OXIDE [1]

Pure lithium oxide has a face-centered cubic structure . It melts at 1432 C and exhibits no phase changes from room temperature to the melting point.

Lithium oxide readily reacts with water vapour to form hydroxide, and with carbon dioxide to form carbonate; consequently, it must be stored and handled in a clean and dry atmosphere.

Physical properties

Density @ 25 C : 2.013 g/cm3
Melting Point : 1432 +/- 6 C

Thermal properties

Specific Heat @ 25 C : 2049 J/kg-K
Thermal Conductivity @ 100 oC : 11.29 W/m-K

Mechanical properties

There is very little or no data in the literature, on the mechanical properties of Lithium Oxide. Some results of the measurements made at NRL-S , ref [2], are as follows:

 TD* %   Elastic modulus GPa             Compressive strength MPa       Poisson ratio       
                                                                        [[nu]]              
         Static       Dynamic                                                            
 81.4    76           72 +/- 5        132                            0.18                
 91.5    ---          114 +/- 10      139                            0.13
*TD=Theoretical density

DATA AND CORRELATIONS

The effect of porosity on thermal and physical properties of ceramics is very pronounced and reflected in the available data. Thermal conductivity k (W/m-K) is affected by irradiation, porosity and morphology and has been expressed as follows, refs [3, 4]:

(1)

with p the porosity. Another expression from ref [5] is as follows:

(2)

with D = Bulk density / Theoretical density. All temperatures in Eqs 1-2 are in degrees K.

Table 1 presents the thermal and structural properties for lithium oxide as a function of temperature, refs [3, 4, 6, 7]. Polynomial correlations of the thermal and structural properties as functions of temperature in degrees K, are as follows:

(2)

(4)

(5)

Equations (1) to (4) are valid in the temperature range 300-1100 K.

Figures 1-2 show the variation of properties with temperature and Figs 3-4 show the effect of porosity on the thermal conductivity. Fig 3 shows a comparison between experimental data and the semi-empirical expressions of Eq 1, from refs [2, 3, 5].

             Table 1 Thermal and structural properties of lithium oxide                                     
   T K     [[rho]]    E GPa       [[nu]  k W/m-K     c J/kg-K    [[sigma]]y  [[alpha]]   
           kg/m3                  ]                              MPa         (10-6)      
                                                                             m/m-K       
   300     2010.0     140.822     0.2    14.526      1686.344                25.910      
   350                139.553            12.823      1950.142                26.770      
   400                138.284            11.478      2127.200                27.630      
   450                137.015            10.388      2253.825                28.490      
   500                135.746            9.487       2349.140                29.350      
   550                134.477            8.730       2423.993                30.210      
   600                133.208            8.085       2484.911                31.070      
   650                131.939            7.528       2536.012                31.930      
   700                130.670            7.044       2579.998                32.790      
   750                129.401            6.618       2618.701                33.650      
   800                128.132            6.240       2653.400                34.510      
   850                126.863            5.903       2685.009                35.370      
   900                125.594            5.601       2714.194                36.230      
   950                124.325            5.328       2741.453                37.090      
  1000                123.056            5.081       2767.160                37.950      
  1050                121.787            4.855       2791.604                38.810      
  1100                120.518            4.649       2815.011                39.670      
  1150                119.249            4.459       2837.555                40.530      
  1200                117.980            4.285       2859.378                41.390      


k (W/m-K)                            c (J/kg-K)

Temperature (K)

Figure 1: Thermal conductivity and specific heat of lithium oxide.





E (GPa)                            [[alpha]] (10-6 m/m-K)

Temperature (K)

Figure 2: Elastic modulus and thermal expansion coefficient of lithium oxide as a function of temperature.





k (W/m-K)

Temperature (K)

Figure 3: Comparison of theoretical and measured (thick lines) thermal conductivity of lithium oxide.





E (GPa)

Temperature (K)

Figure 4: The elastic modulus of lithium oxide with the effect of porosity.





References

  1. Goodfellow. Metals, Alloys, Compounds, Ceramics, Polymers, Composites. Catalogue 1993/94.

  2. Risely Technical Service Report, The use of lithium oxide as the breeder in fusion reactors. July 1989, pg 30.

  3. Modelling Analysis and Experiments for fusion Nuclear Technology. FNT Progress Report : Modelling and Finesse. January 1987, Chapter 2.2.

  4. M. Akiyama,. Desigh Technology of Fusion Reactors pg 470-472.

  5. Journal of Nuclear Materials, vol. 91, No 1, pp. 93-102, 19***.

  6. Thermophysical Properties of High Temperature Solid Materials. Chapter 4 part 1, pg 236 -238.

  7. Solid Tritium Breeder Materials-Li2O and LiAlO2 : A Data Base Review, April 4, 1985, pg 1976 .