Thermal Analysis of Phosphor Raw Materials R. C. R O P P and M. A. AIA Chemical and Metallurgical Division, Sylvania Electric Products Inc., Towada, Pa.
b The thermal analyses of several phosphor raw materials are given to illustrate the methods as well as to show limitations and means of circumventing experimental difficulties. A combination of differential thermal analysis and thermogravimetric analysis is often needed to establish reaction temperatures and products of formation of a solid-state reaction.
S
oLrD-state reactions have been studied by thermal analysis methods. A complete discussion of differential thermal analysis (DTA) is given by Smothers and Chiang (7). West (10) discusses it particularly from the aspect of instrumentation, while Kissinger (2) has summarized the effect of the heating parameters on the general shape of the thermogram. Thermogravimetric analysis (TGA) techniques such as dynamic thermogravimetry and isothermal gravimetry may also be employed. Dynamic thermogravimetry is particularly attractive in the early stages of an investigation, because the complete thermal behavior of many systems can be observed rapidly. An excellent critique on thermogravimetry is given by Newkirk (3). Measurement of exothermic or endothermic changes by DTA, and changes in weight by TGA, coupled with x-ray diffraction data and chemical analyses of the products, can provide a quantitative estimation of the solid-state reactions. I n this paper the thermal analyses of several diverse solids are given to illustrate limitations of the methods employed, as well as to show methods or means of circumventing differences in the reactivities of solids undergoing thermal decomposition. The data show that a combination of methods is often needed to define the thermochemical behavior of the material being investigated. APPAilATUS A N D EXPERIMENTAL TECHNIQUE
X-ray diffraction patterns were obtained on a Philips Norelco unit using nirkel-filtered CuKa radiation of 1.5418 A. Chemical analyses were made using standard procedures. Differential thermal analyses were made on an apparatus similar to that of Stone (8). -2heating rate of 10" C. per minute was used. Sample size was changed by varying the size of holes in the nickel block, 1288
ANALYTICAL CHEMISTRY
one side of which was occupied by the reference material, a-alumina, and the other side by the unknown material, each having one side of the differential thermocouple (Pt-10% Rh us. Pt) embedded within. The DTA thermograms are direct tracings reduced in size for reproduction. Thermogravimetric data were obtained with a Stanton recording thermobalance, using a heating rate of 6" C. per minute in most cases. The compounds analyzed were of high purity and of interest &s phosphor raw materials (6,6). They were chosen to illustrate increasing complexity and difficulty of resolution of the step reactions. Resolution of Solid-state Reactions. COMPLEX REACTIONS EASILYRESOLVED. The decomposition of Sr(H*P04)2is a n example of a complex degradation which can be resolved easily. T h e DTA-TGA thermograms given in Figure 1 show t h a t the changes were easily separated and discernible, particularly the reactions causing the weight losses. From these data, the reactions can be written: Sr(HZP04)z
-+ 190-2100
160' C .
H 2 0 (1)
320-330O C.
+
r-Sr(POd2
850' C .
8-~3r(PO~)~
130' C.
Cd( HzPOC)~. 2Hz0 -+
c.
SrHZP207
SrHzP2Or
with little influence from the preceding or following reactions. THERMALLY RESOLVABLE INTERACTING REACTIONS.The agreement between TGA and DTA is often less precise than in the above example. For instance, strong endothermic ab, sorptions occur between 100" and 200 C. for the compound Cd(HzP04)~. 2H20 (Figure 2, A ) . However, unlike Sr(HZP0&, the TGA thermogram (Figure 2, B ) does not show well defined plateaus and hence the location of the inflection points required for quantitative determination of the weight losses of the reactions is more difficult to fix. Each reaction appeared to affect the other members in the reaction series. Consequently, the reactions had to be rerun isothermally to obtain the data shown in Figure 3. The temperature employed had some effect on the rate of weight loss. Data from isothermal TGA and DTA thermograms, substantiated by x-ray diffraction and chemical analysis data, allowed an evaluation of the solid-state reactions:
+ H2O
&3r(P03)1
(2)
(4)
The TGA data (Figure 1, B ) show only the weight losses due to dehydration (Reactions 1 and 2) and the reaction can be followed as far as the formation of ySr(PO?)z. However, the DTA thermogram (Figure 1, A ) s h o w the thermal effects due to dehydration as well as the effects due to change in crystal structure (Reactions 3 and 4). The change to ~ ~ - S I - ( P Ois~ gradual, )~ whereas the change to CX-S~(PO,)~ is abrupt. These effects were substantiated by x-ray diffraction patterns and chemical analyses of the products. While the structure of these metaphosphates has not been established, it is probable that some of the thermal effects are attributable to changes in structure such as the formation of a ring structure, the trimetaphosphate, here designated as Y - S ~ ( P O ~a) ~grad, ual change to fi-Sr(P03)2, and a final change to a cross-linked polyphosphate of high molecular weight, a-Sr(P03)2. The data show that these particular reactions appear to proceed stepwise
2100
c.
+ H20 410' C. CdHzP207 fi-Cd(P03)z + HzO
Cd(HzP0d)z -+
__+
CdHzPz0,
(6)
(7)
The TGA thermogram in Figure 2 can be followed up to the formation of &Cd(P03)2, whereas the DTA thermogram shows both of the endothermic dehydrations, the gradual formation of P-Cd(POa)2,and the weak endothermic change corresponding to the formation of C Y - C ~ ( P O ~It) ~ is . probable that fi-Cd(P03)2 corresponds to the tetrametaphosphate, Cdz(PO&, whereas C X - C ~ ( P Ocorresponds ~)~ to the tetrapolyphosphate, Cda(P401a), of Thilo and Grunze (9). In this system there mere several reactions taking place a t the same time, and a combination of DT-4 and isothermal gravimetry was required to separate the reactions. REACTIONS DIFFICULTTO SEPARATE. Many complex reactions are difficult to separate and proceed a t a rate depending upon the nature and geometry of the immediate surroundings. DTA and dynamic TGA data may not be directly comparable, and assignment of specific reactions may be difficult. Garn and Kessler (1) showed that several well known reactions proceed according to a different mode if the immediate atmosphere surrounding the
800
t
/I
A . E
I
600
W
a
I-
w
-
-
3
a
sh 400I u
1
1000,
a
-
200 -
1
ENDOTHERMIC Sr H,P,O,
ENDOTHERMIC
M-Sr(P03)2
0
'E
6
1.50
U-J
1.00
u)
0
J
0
I
I I.lOmok H20
C
IO0
200
300
400
500
TEMPERATURE. ' C 200
0
Figure 1.
Reactions easily resolved
Thermal analysis of
Figure 2.
A. Differential thermal analysis 8. Thermogravimetric analysis
sample is changed. They reported a useful technique termed "reaction in a self-generated atmosphere." We applied this technique to study Cd5H2(P04)4.(H20)4,which reacts stepwise t o form a system in which the reactions are easily separated by D T A (Figure 4,A ) . Well defined endothermic transitions were obtained, each presumably corresponding to the loss of water. The interpretation of the reaction is given by successive numbering of the endothermic changes. However, dynamic TGA data (Figure 4,B ) showed one continuous change with the total loss of 5.05 moles of water per mole of starting material. Isothermal gravimetry gave the results listed in Table I, which were inconclusive, since they were not consistent within themselves. X-ray data showed that no changes in structure occurred until at least 4 moles of water had been lost. In this respect, Cd5H2(P04)4.(H?0)4 behaves like the well known calcium hydroxyapatite, which can lose water of constitution without changing structure. However, a change in the sample geometry so that a self-generated atmosphere (water vapor) was in equilibrium with the decomposing material (Figure 4,B ) gave evidence that a specific degradation mechanism occurred, depending upon the immediate atmosphere. The reactions resolved by this technique are:
600
400
TEMPERATURE,
Thermal analysis of Sr(H2POh
800
*c. Cd(H2P04)~.2H20
Interacting reactions A. Differential thermal analysis B . Thermogravimetric analysis
4, .4), the endothermic changes were
340' C.
CdjHz(Pod),. (H20) -+ Cda(POc)2.Cd*P207
+ 2H20
(11)
In the DTA thermogram (Figure
seen as a series of five distinct steps, whereas the TGA thermogram shows only a continuous loss of water, with no separable steps. We did not obtain the same reaction mechanism by DTA as that obtained gravimetrically using the "self-generated atmosphere" technique (Reactions 9, 10, and ll), indicating that one, or possibly two, of the changes indicated by DTA may have been crystallographic in nature.
950
"C.
vi
0
25
50
100
150
200
250
300
350
TIME, MINUTES
Figure 3.
Isothermal gravimetry of Cd(H2POd)Z.2H20 VOL 34, NO. 10, SEPTEMBER 1962
1289
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A.EL
060
a
0 20-
l 0
.
l
,
200
100
l
,
l
:
400
300
500
600
100
SO0
100
TEMPERATURE, *C.
Figure 5. P-SrHPOa
Thermogravimetric analysis of or-SrHPO, and Similar compounds with different structures
c
400
0
100
300
200
400
500
600
EXOTUERMIC
TEMPERAfURE,'C.
Figure 4.
r
J
Thermal analysis of CdsHz(P04)4.(H20h
360
0
Reactions difficult to separate Differential thermal analysis 8. Thermogravimetric analysis
w
A.
a
3
B
COMPARISON OF SIMILAR COMPOUNDS
Thermal analysis methods are often used to compare the properties of similar compounds. Selection of the best method depends on the specific compounds and is, again, an exploratory process. Some examples are described below, Polymorphs (Differing Structure). With similar compounds such as fl-SrHP04 and a-SrHP04, the DTA (not shown) and TGA thermograms (Figure 5) gave nearly identical results and showed that each compound follows a different reaction path. The reaction mwhanism for O-SrHPO4 indicated by the TGA thermograms and validated by x-ray diffraction includes a compound having the stoichiometry of a hemih$&te of the pyrophosphate. The reactions are:
+
"r IO0
I
Figure 6. ZnOz
Table 1.
Compound Cd6Ho(POi)a.(HiO)r
Separation of products other than by isothermal gravimetry proved difficult and chemical analysis was of little value in defining the products, because of the
Theoretical for conversion to C d r (Po4)2. CdzPzO7
1290
s
+
ANALYTICAL CHEMISTRY
Differential thermal analysis of CdOz and Similar compounds with some structure
2( a-SrHP04)-+ SrzP207 H,O (12) 2(p-SrHPO4) -+ SrzP2071/2H20t L / 2 H ~ 0(13) SrzPzO? '/iH?O SrpP207 '/&O (14) -+
I f /
-+nnL - - - ENDOTHERMIC
Isothermal Gravimetric Analysis of CdbH2(PO&.(H20)r
Temp., O
c.
200 240 290 530
yo 2.28 5.54 6.61 8.92 8.85
H20 LOSS
Moles/mole 1.29 3.12 3.73 5.03 5.00
X-Ray
Identification CdsH2(POn)r.(HzOh Cda(P04h. CdrPaOi
Figure 7.
Differ
entia1 therma
a n a l y s i s o f Ca-
HP04.2HQO Reactions dependent on surrounding atmosphere
A.
J
6.
Ambient air? Air jdried with Ca-
C.
Steam a t TOO°C.
sod. 1 / 2 h 0
0
E W
a 3
c 4
a Id a
z
!-
Sewkirk (4). Both isothernlal a11d dynainic thermogravimetry are illconclusive, since only the gross change in weight can be shown, but D T h shows very clearly the formation of these unstable hydrates RS Q function of atniosphere, and yields further data on the effect of a large excess of the reactant product, water (steam). In steam atmosphere (Figure 7 , C) the gross rcaction becomes exothermic, in contrast to the endothermic peak noted a t lower atmospheric humidity (Figure 7 . -1). DISCUSSION
DTLIand TG,I data are valuable in establishing the reaction temperatures and products of formation of a solidstate reaction. However, the successful use of the methods depends to a great degree on the manner in which the data are obtained, in many cases being dependent upon the thermal behavior of the compound analyzed. Examples have been given showing the utility of thermal analysis methods in separating and defining reactions, and in differentiating similar compounds. Information obtained by these ineans often cannot be easily duplicated by any other method. Although the data in this paper have been limited to the decomposition of inorganic phosphates and peroxides, the DTA-TGh method can be applied to all reactions in which thermal changes or weight changes occur. .ipplication to solution chemistry involves only the selettion of the proper reference materials to yield useful results. ACKNOWLEDGMENT
large amount of absorbed water usually associated with B-SrHPO4, and the sluggishness of Reaction 13. Isomorphs (Same Structure). For compounds of similar structure but not identical composition, the D T A method of analysis may give results which cannot be obtained by the T G A method alone or b y other combinations of methods. The decomposition of Zn02, like t h a t of the alkaline-earth peroxides, is endothermic. However, the decomposition of Cd02, which is an isomorph of ZnOn, is a violent exothermic reaction, in contrast to what might be expected. The DTA method easily differentiates the net heat effect (Figure 6), whereas TGA data show only a simple transition. The reactions may be written: CdOe + CdO
+
Zn02 + ZnO
+
1/202
1/202
(exothermic) (15)
(endothermic) (16)
The basis for these differences in thermal behavior is not readily apparent,
since both peroxides have the same structure and produce oxides of the same structure. The DTA thermogram for Zn02 indicates some sort of additional reaction denoted by the second pip on the endothermic peak in contrast to the smooth exothermic peak of CdO2. The possibility of momentary formation of a hydrate was considered, but chemical analyses and x-ray diffraction data showed only ZnO as the end product. REACTIONS AS A FUNCTION O F ATMOSPHERE
The authors are indebted to C. W. W . Hoffman, who made the x-ray measurements, and to G. J. Meisenhelter, who made the chemical analyses. LITERATURE CITED
(1) Gam, P. D., Kessler, J. E., ANAL. CHEM.32, 1563 (1960). (2) Kissinger, H. E., J. Res. Satl. Bur. Std. 57, 217 (1956). (3) Newkirk, A. E., ANAL.CHEM. 32, 1558 (1960). ( 4 ) .Rabatin, J. G., Gale, R. H., Kewkirk, A. E., J. Phys. Chem. 64, 491 (1960). (5) Ropp, R. C., Aia, M. A, Hoffman, C. W. W., VeIeker, T. J., ~ o o n e y , R . W.. ANAL.CHEM.31, 116rf1959).
(6)-Ropp,R. C., Mooney, R. W., J. Am. The mode of reaction may depend on the surrounding atmosphere and also on whether the atmosphere is static or dynamic. Figure 7 shows the utility of the DTA method in elucidating the decomposition of CaHP04.2H20 t o CaHP04. In moist ambient air the.dihydrate decomposes directly to CaHP04 (Figure 7 , A ) , but in dry air forms a series of successive hydrates (Figure 7 3 ) as shown by Rabatin, Gale, and
Chem. SOC.82,4848 (1960). (7) Smothers, W. J., Chiang, Y., “Dif-
ferential Thermal Analysls,” Chemical Publishing Co., New York, 1958. (. 8.) Stone, R. L., Ohio State Univ., Eng. Series, Bull. 146 (1951). (9) Thilo, E., Grunze, L., 2. anorg. U .
allgem. Chem. 290, 209 (1957). (10) Wy$, R. R., “The Defect .Solid State, T. J. Gray, ed., Interscience, New York, 1957.
RECEIVEDfor review April 23, 1962. Accepted June 18, 1962. VOL 34, NO. 10, SEPTEMBER 1962
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